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Mechanochemical scission of transition metal-ligand bonds incoordination polymersCitation for published version (APA):Balan, A. (2015). Mechanochemical scission of transition metal-ligand bonds in coordination polymers.Technische Universiteit Eindhoven.

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Mechanochemical Scission of Transition Metal-‐Ligand Bonds in Coordination Polymers

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de rector magnificus prof.dr.ir. F.P.T. Baaijens, voor een commissie aangewezen door het College voor Promoties, in het openbaar te verdedigen op woensdag 11 november 2015 om 16:00 uur

door

Abidin Balan

geboren te Malatya, Turkije

Dit proefschrift is goedgekeurd door de promotoren en de samenstelling van de promotiecommissie is als volgt: voorzitter: prof.dr.ir. J.C. Schouten 1e promotor: prof.dr. R.P. Sijbesma 2e promotor: prof.dr. E.W. Meijer leden: prof.dr. J.H.N. Reek (UvA) prof.dr. G. Cravotto (Politecnico di Torino) prof.dr.ir. E.J.M. Hensen prof.dr. A.P.H.J. Schenning

Het onderzoek of ontwerp dat in dit proefschrift wordt beschreven is uitgevoerd in overeenstemming met de TU/e Gedragscode Wetenschapsbeoefening.

To my wife,

Gizem’e

Imagination will often carry us to worlds that never were, but without it we go nowhere.

(Carl Sagan)

The highest activity a human being can attain is learning for understanding, because to understand is to be free.

(Baruch Spinoza)

Abidin Balan

Mechanochemical Scission of Transition Metal-‐Ligand Bonds in Coordination Polymers

Eindhoven University of Technology, 2015

Printing: Ridderprint BV, the Netherlands

Cover: ‘’Jardine Media’’, Ridderprint BV

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-‐90-‐386-‐3946-‐8

This work has been financially supported by the Netherlands Organization for Scientific Research, Chemical Sciences (NWO-‐CW).

Copyright © 2015 by Abidin Balan

Table of Contents

Chapter 1 Introduction: Polymer Mechanochemistry .............................................................................. 1

History and background ................................................................................................................. 2 Mechanophores ............................................................................................................................. 7 Mechanoresponsive materials ....................................................................................................... 9 Mechanical release of small molecules in polymer matrices ....................................................... 11 Mechanochemical catalysis .......................................................................................................... 13 Aim and outline of this thesis ....................................................................................................... 16 References .................................................................................................................................... 18

Chapter 2 Mechanochemical chain scission in NHC-‐Pd centered coordination polymers ....................... 21

Introduction .................................................................................................................................. 22 Results and Discussions ................................................................................................................ 24 Synthesis ....................................................................................................................................... 24 Chain Scission in Pd(NHC-‐pTHF)2Cl2 ............................................................................................. 26 Determination of limiting molecular weight Mlim ......................................................................... 29 Scission Rates for M(NHC-‐pTHF)2Cl2 ............................................................................................. 32 Chain scission mechanism ............................................................................................................ 39 Conclusions ................................................................................................................................... 43 Experimental ................................................................................................................................ 44 References .................................................................................................................................... 46

Chapter 3 Mechanical scission of Pd(NHC)2Cl2 complexes probed with chemiluminescence .................. 49

Introduction .................................................................................................................................. 50 2-‐Coumaranones .......................................................................................................................... 51 Results and discussions ................................................................................................................ 52 Chain scission in Pd(NHC-‐pTHF)2Cl2 complexes in the presence of coumaranone ...................... 52 Light emission ............................................................................................................................... 58 Conclusions ................................................................................................................................... 63 Experimental ................................................................................................................................ 64 References .................................................................................................................................... 66

Chapter 4 Determination of ligand exchange dynamics and sonication induced ligand exchange rate in Imidazole-‐Pd centered coordination polymers .................................................................. 69

Introduction .................................................................................................................................. 70 Results and Discussions ................................................................................................................ 73 Synthesis of Pd-‐Imidazole complexes ........................................................................................... 73 Ligand exchange between EtIm and Pd(EtIm)2Cl2 ........................................................................ 75 Sonication induced ligand exchange ............................................................................................ 79 Rate of sonication induced chain scission .................................................................................... 80 Conclusions ................................................................................................................................... 85 Experimental ................................................................................................................................ 86 References .................................................................................................................................... 88

Chapter 5 Mechanochemically induced, directed ligand exchange in polymeric Pd(II) complexes ......... 89

Introduction .................................................................................................................................. 90 Results and discussions ................................................................................................................ 92 Mechanochemical synthesis of hetero-‐complexes ...................................................................... 95 Mechanochemical synthesis of block copolymers ....................................................................... 96 Conclusions ................................................................................................................................... 99 Experimental .............................................................................................................................. 100 References .................................................................................................................................. 102

Chapter 6 Transition metal bearing supramolecular polymer networks: Towards self-‐healing applications ........................................................................................................................ 103

Introduction ................................................................................................................................ 104 Results and discussions .............................................................................................................. 106 Conclusions ................................................................................................................................. 109 Experimental .............................................................................................................................. 110 References .................................................................................................................................. 111

Summary ............................................................................................................................ 113

Curriculum Vitae ................................................................................................................. 117

Acknowledgements ............................................................................................................ 119

Chapter 1Introduction: Polymer Mechanochemistry

The past 10 years have seen a resurgence of interest in the field of polymer mechanochemis-

try. Whilst the destructive effects of mechanical force on polymer chains have been known

for decades, it was only recently that researchers tapped into these forces to realize more

useful chemical transformations. Current developments in mechanochemistry involve the

introduction of mechanophores in polymer chains that can undergo useful reactions upon

mechanical activation. History and fundamental aspects underlying mechanochemical chain

scission in polymers together with elegant examples from recent literatures were described

in this chapter. At the end the aim and outline of this thesis are given.

Chapter 4 Determination of ligand exchange dynamics and sonication induced ligand exchange rate in Imidazole-‐Pd centered coordination polymers .................................................................. 69

Introduction .................................................................................................................................. 70 Results and Discussions ................................................................................................................ 73 Synthesis of Pd-‐Imidazole complexes ........................................................................................... 73 Ligand exchange between EtIm and Pd(EtIm)2Cl2 ........................................................................ 75 Sonication induced ligand exchange ............................................................................................ 79 Rate of sonication induced chain scission .................................................................................... 80 Conclusions ................................................................................................................................... 85 Experimental ................................................................................................................................ 86 References .................................................................................................................................... 88

Chapter 5 Mechanochemically induced, directed ligand exchange in polymeric Pd(II) complexes ......... 89

Introduction .................................................................................................................................. 90 Results and discussions ................................................................................................................ 92 Mechanochemical synthesis of hetero-‐complexes ...................................................................... 95 Mechanochemical synthesis of block copolymers ....................................................................... 96 Conclusions ................................................................................................................................... 99 Experimental .............................................................................................................................. 100 References .................................................................................................................................. 102

Chapter 6 Transition metal bearing supramolecular polymer networks: Towards self-‐healing applications ........................................................................................................................ 103

Introduction ................................................................................................................................ 104 Results and discussions .............................................................................................................. 106 Conclusions ................................................................................................................................. 109 Experimental .............................................................................................................................. 110 References .................................................................................................................................. 111

Summary ............................................................................................................................ 113

Curriculum Vitae ................................................................................................................. 117

Acknowledgements ............................................................................................................ 119

History and background

Mechanical activation of chemical bonds offers an alternative to conventional processes

such as thermal or photochemical activation.1 As stated by Kauzmann and Eyring,2

mechanical work done by an external force (W(r) = F × r) modifies the Morse potential (U’(r)

= U(r) – W(r)) and lowers the energy barrier for bond dissociation to such an extent that

thermal fluctuations can exceed this barrier at room temperature, in contrast to the

unstretched bond.3,4

Figure 1: Morse energy potentials representing a chemical bond in unperturbed state (dash-‐dotted

line) and of a bond under stress (solid line) showing the mechanochemical activation of bond

according to the TABS theory. The energy input due to mechanical work is shown as a dashed line.4

Polymer mechanochemistry was first described by Staudinger in the 1930s, when he

interpreted the decrease in molecular weight upon mastication of a polymer as the

mechanical rupture of macromolecules.5 Since then, the use of force to activate chemical

bonds and to initiate chemical reactions in polymers has become of interest because the

reaction pathways and outcome of a mechanochemical reaction can be completely different

from its thermal analogue. For instance, thermal ring-‐opening reaction of cyclobutanes

(CBs), an orbital-‐controlled reaction, is only allowed via a conrotatory pathway, yielding a

different isomer of the ring-‐opened product (E,Z or E,E) depending on the starting isomer

(cis-‐ or trans-‐CB, respectively). However, when activated by mechanical force, regardless of

the starting isomer, the ring-‐opening always yields the E,E isomer as a product. This

intriguing difference between the outcomes of mechanical and thermal reactions is a result

of the fact that mechanical work involved with force, in contrast to thermal energy, is

anisotropic (i.e. force has a direction).6–10

Figure 2: Electrocyclic ring opening of benzocyclobutenes. The intermediate resulting from the

electrocyclic ring opening of benzocyclobutenes depends on the method of activation and on the

geometry of the molecule.6

Nature also uses mechanochemistry, for instance, in the regulation of the activity of some

enzymes and proteins by force-‐induced structural changes. Proteins fold in many different

conformations, which changes the activity of enzymes associated to them. The free energy

difference between folded and unfolded states is usually lower than energy of a covalent

bond and this makes it possible to ‘force’ the protein into a different conformation and alter

its enzymatic activity even with low stress loadings.11 Klibanov et al. have investigated the

effect of mechanical stretching on enzyme activity by covalently attaching chymotrypsin and

Chapter 1

2

History and background


such as thermal or photochemical activation.1 As stated by Kauzmann and Eyring,2

mechanical work done by an external force (W(r) = F × r) modifies the Morse potential (U’(r)

= U(r) – W(r)) and lowers the energy barrier for bond dissociation to such an extent that

thermal fluctuations can exceed this barrier at room temperature, in contrast to the

unstretched bond.3,4

Figure 1: Morse energy potentials representing a chemical bond in unperturbed state (dash-‐dotted

line) and of a bond under stress (solid line) showing the mechanochemical activation of bond

according to the TABS theory. The energy input due to mechanical work is shown as a dashed line.4

Polymer mechanochemistry was first described by Staudinger in the 1930s, when he

interpreted the decrease in molecular weight upon mastication of a polymer as the

mechanical rupture of macromolecules.5 Since then, the use of force to activate chemical

bonds and to initiate chemical reactions in polymers has become of interest because the

reaction pathways and outcome of a mechanochemical reaction can be completely different

from its thermal analogue. For instance, thermal ring-‐opening reaction of cyclobutanes

(CBs), an orbital-‐controlled reaction, is only allowed via a conrotatory pathway, yielding a

different isomer of the ring-‐opened product (E,Z or E,E) depending on the starting isomer

(cis-‐ or trans-‐CB, respectively). However, when activated by mechanical force, regardless of

the starting isomer, the ring-‐opening always yields the E,E isomer as a product. This

intriguing difference between the outcomes of mechanical and thermal reactions is a result

of the fact that mechanical work involved with force, in contrast to thermal energy, is

anisotropic (i.e. force has a direction).6–10

Figure 2: Electrocyclic ring opening of benzocyclobutenes. The intermediate resulting from the

electrocyclic ring opening of benzocyclobutenes depends on the method of activation and on the

geometry of the molecule.6

Nature also uses mechanochemistry, for instance, in the regulation of the activity of some

enzymes and proteins by force-‐induced structural changes. Proteins fold in many different

conformations, which changes the activity of enzymes associated to them. The free energy

difference between folded and unfolded states is usually lower than energy of a covalent

bond and this makes it possible to ‘force’ the protein into a different conformation and alter

its enzymatic activity even with low stress loadings.11 Klibanov et al. have investigated the

effect of mechanical stretching on enzyme activity by covalently attaching chymotrypsin and

Polymer Mechanochemistry

3

1

trypsin to nylon, human hair, and viscose fibers.12 They showed that stretching of these

polymer supports induced a deformation on protein molecules and so a 3-‐fold decrease in

enzyme activity. Upon relaxation, the initial level of activity was reached instantaneously

revealing that enzyme activity can be altered reversibly by stretch-‐relax process.

Figure 3: Dependence of relative activity of enzymes covalently bound to different elastic supports

on the degree of stretching of the supports. (a) chymotrypsin on protein coated nylon fiber; (b)

trypsin on human hair; (c) chymotrypsin on viscose (cellulose) fiber. Schematic representation of the

deformation of enzyme bound to a mechanically stretched elastic fiber.12

One of the most efficient ways to exert force on a polymer in solution is by the use of

sonication since the strain rates accessible with sonochemistry are greater than those with

other solution-‐based flow techniques such as opposed jets or cross slots, allowing

mechanical activation to be obtained in polymers of lower molecular weight and at greater

scission rates.13 Upon sonication of solution, cavitation leads to strong elongational stresses

around collapsing bubbles.14 The part of the polymer chain closest to the collapsing bubble

wall is pulled at a higher velocity than the far end, and this velocity gradient creates stress

along the backbone. This force is accumulated through the polymer backbone and reaches a

maximum value at the center of the chain because the flow field is centrosymmetric with

respect to the molecule. As a consequence chain scission occurs preferentially around the

chain midpoint.15

Figure 4: Mechanism for ultrasound-‐induced polymer chain scission: (a) gradual bubble formation

results from pressure variations induced by the acoustic field; (b) rapid bubble collapse generates

solvodynamic shear; (c) small molecules undergo pyrolytic cleavage to form radical byproducts upon

bubble collapse, while polymer chains do not undergo pyrolytic cleavage because they do not

penetrate the bubble interface.13

Implosion of cavitation bubbles is essentially an adiabatic process, which leads to formation

of local hotspots within the bubble in which temperature and pressure increase drastically.

The content of cavitation bubble pyrolyzes under these extreme conditions and results in

formation of reactive species, such as radicals and persistent, protic secondary

byproducts.16. Recent studies in our group have shown that heat capacity of gas dissolved in

solution influences the formation of sonochemical impurities. For instance, the use of

methane (CH4) instead of argon (Ar) decreases the production of radicals significantly.17

Higher heat capacity and possible energy dissipation due to increased degrees of freedom in

CH4, compared to those of monoatomic Ar, decrease the temperature in hotspots and

suppress the reactive impurity formation. On the other hand, solubility of the saturation gas

may also have a negative influence on ultrasound induced mechanical chain scission. Iso-‐

Chapter 1

4

trypsin to nylon, human hair, and viscose fibers.12 They showed that stretching of these

polymer supports induced a deformation on protein molecules and so a 3-‐fold decrease in

enzyme activity. Upon relaxation, the initial level of activity was reached instantaneously

revealing that enzyme activity can be altered reversibly by stretch-‐relax process.

Figure 3: Dependence of relative activity of enzymes covalently bound to different elastic supports

on the degree of stretching of the supports. (a) chymotrypsin on protein coated nylon fiber; (b)

trypsin on human hair; (c) chymotrypsin on viscose (cellulose) fiber. Schematic representation of the

deformation of enzyme bound to a mechanically stretched elastic fiber.12

One of the most efficient ways to exert force on a polymer in solution is by the use of

sonication since the strain rates accessible with sonochemistry are greater than those with

other solution-‐based flow techniques such as opposed jets or cross slots, allowing

mechanical activation to be obtained in polymers of lower molecular weight and at greater

scission rates.13 Upon sonication of solution, cavitation leads to strong elongational stresses



along the backbone. This force is accumulated through the polymer backbone and reaches a

maximum value at the center of the chain because the flow field is centrosymmetric with

respect to the molecule. As a consequence chain scission occurs preferentially around the

chain midpoint.15

Figure 4: Mechanism for ultrasound-‐induced polymer chain scission: (a) gradual bubble formation

results from pressure variations induced by the acoustic field; (b) rapid bubble collapse generates

solvodynamic shear; (c) small molecules undergo pyrolytic cleavage to form radical byproducts upon

bubble collapse, while polymer chains do not undergo pyrolytic cleavage because they do not

penetrate the bubble interface.13

Implosion of cavitation bubbles is essentially an adiabatic process, which leads to formation

of local hotspots within the bubble in which temperature and pressure increase drastically.

The content of cavitation bubble pyrolyzes under these extreme conditions and results in

formation of reactive species, such as radicals and persistent, protic secondary

byproducts.16. Recent studies in our group have shown that heat capacity of gas dissolved in

solution influences the formation of sonochemical impurities. For instance, the use of

methane (CH4) instead of argon (Ar) decreases the production of radicals significantly.17

Higher heat capacity and possible energy dissipation due to increased degrees of freedom in

CH4, compared to those of monoatomic Ar, decrease the temperature in hotspots and

suppress the reactive impurity formation. On the other hand, solubility of the saturation gas

may also have a negative influence on ultrasound induced mechanical chain scission. Iso-‐


5

1

butane, for instance, a gas with higher solubility decreases the intensity of cavitation effects

so it leads to lower scission rates. Therefore, the selection of saturation gas is crucial for

both scission rate and sonochemical impurity formation.

Figure 5: Radical production rate and percentage scission of supramolecular polymer complex

Ag(NHC7k)2PF6 after 5 min of sonication using argon, nitrogen, methane and isobutane as saturation

gases.17

It is well established that mechanochemical scission of polymers only occurs above a

molecular weight threshold (Mlim).18 Below Mlim, no scission takes place since polymer

chains are too short to accumulate the force required to break chemical bonds.19 The rate of

mechanically induced scission in covalent polymers has been shown to be proportional to

[MW -‐ Mlim] when the initial molecular weight is greater than Mlim.4,20 Madras recently

reported that the chain scission rate coefficient ksc is linearly dependent on the initial

molecular weight of polymers based on experimental data for ultrasonic chain scission of a

number of polymers, including poly(ethylene oxide) (PEO), polyacrylamide (PAa), poly(butyl

acrylate) (PBA), and poly(methyl acrylate) (PMA). ksc is assumed to follow empirical formula;

ksc = kd (MW -‐ Mlim )λ

where kd is the degradation coefficient, MW is the initial molecular weight of the polymer,

Mlim is the limiting molecular weight, and the exponent λ is a value between 0 and 3. When

λ = 1, the experimental data was fitted to the theory by linear regression.21

Mechanophores

Limiting molecular weight (Mlim) and the rate of chain scission (ksc) are influenced

significantly if a mechanically responsive labile bond, mechanophore, is incorporated into a

polymer chain. Encina et. al. found that the ksc of poly(vinylpyrrolidone) increased 10-‐fold

when peroxide linkages were randomly incorporated into the polymer backbone. The

peroxide bond with bond dissociation energy of 51 kcal/mol is much weaker than the C-‐C

(88 kcal/mol) and the C-‐O (91 kcal/mol) bonds and makes the polymer susceptible to

mechanical energy input.22

Furthermore, the polymer with a mechanophore can be degraded at specific locations since

mechanical energy is selective for the weakest bond on mechanophore. Moore and co-‐

workers synthesized a linear PEG with a single weak azo link positioned at the center of the

chain. They demonstrated that the mechanically induced cleavage was localized almost

exclusively to the homolytic extrusion of nitrogen from the azo group since the energy of

activation for this process is 24-‐ 30 kcal/mol, much lower than that of the C-‐C and C-‐O bonds

in the PEG backbone.23

Chapter 1

6

butane, for instance, a gas with higher solubility decreases the intensity of cavitation effects

so it leads to lower scission rates. Therefore, the selection of saturation gas is crucial for

both scission rate and sonochemical impurity formation.

Figure 5: Radical production rate and percentage scission of supramolecular polymer complex

Ag(NHC7k)2PF6 after 5 min of sonication using argon, nitrogen, methane and isobutane as saturation

gases.17



chains are too short to accumulate the force required to break chemical bonds.19 The rate of

mechanically induced scission in covalent polymers has been shown to be proportional to

[MW -‐ Mlim] when the initial molecular weight is greater than Mlim.4,20 Madras recently

reported that the chain scission rate coefficient ksc is linearly dependent on the initial

molecular weight of polymers based on experimental data for ultrasonic chain scission of a

number of polymers, including poly(ethylene oxide) (PEO), polyacrylamide (PAa), poly(butyl

acrylate) (PBA), and poly(methyl acrylate) (PMA). ksc is assumed to follow empirical formula;

ksc = kd (MW -‐ Mlim )λ

where kd is the degradation coefficient, MW is the initial molecular weight of the polymer,

Mlim is the limiting molecular weight, and the exponent λ is a value between 0 and 3. When

λ = 1, the experimental data was fitted to the theory by linear regression.21

Mechanophores

Limiting molecular weight (Mlim) and the rate of chain scission (ksc) are influenced

significantly if a mechanically responsive labile bond, mechanophore, is incorporated into a

polymer chain. Encina et. al. found that the ksc of poly(vinylpyrrolidone) increased 10-‐fold

when peroxide linkages were randomly incorporated into the polymer backbone. The

peroxide bond with bond dissociation energy of 51 kcal/mol is much weaker than the C-‐C

(88 kcal/mol) and the C-‐O (91 kcal/mol) bonds and makes the polymer susceptible to

mechanical energy input.22

Furthermore, the polymer with a mechanophore can be degraded at specific locations since

mechanical energy is selective for the weakest bond on mechanophore. Moore and co-‐

workers synthesized a linear PEG with a single weak azo link positioned at the center of the

chain. They demonstrated that the mechanically induced cleavage was localized almost

exclusively to the homolytic extrusion of nitrogen from the azo group since the energy of

activation for this process is 24-‐ 30 kcal/mol, much lower than that of the C-‐C and C-‐O bonds

in the PEG backbone.23


7

1

Figure 6: Examples of mechanophores incorporated into polymer chains. A) Poly(vinylpyrrolidone)

with random peroxide linkages.22 B) Chain scission of silver-‐carbene-‐based polymer upon sonication

and subsequent trapping of the carbene with water.24 C) Ultrasound-‐Induced cleavage of dicyano-‐

substituted cyclobutane ring yields cyanoacrylate-‐terminated polymers.25 D) Mechanochemical

scission of heteronuclear supramolecular polymers in the presence of a scavenger complex resulted

in the formation of Palladium and Platinum heterocomplexes.26

Coordination bonds, weaker than covalent bonds on polymer backbone break more easiliy

and result in even lower Mlim compared to their covalent counterparts.26 Recently in our

group, high-‐molecular-‐weight linear coordination polymers of diphenylphosphine telechelic

polytetrahydrofuran with palladium(II) dichloride were developed.27 Molecular weights of

these polymers could be altered reversibly by ultrasound and it has been shown that

polytetrahydrofuran chains remain intact during sonication.28 This implies that only the

reversible palladium–phosphorus bonds are broken and coordinatively unsaturated

palladium complexes were produced by the application of mechanical forces on these

coordination polymers.29 Furthermore, polymers which include both PdII and PtII were

OO

OO

N N NO O O

m n x

NN pTHF*OMe

Ag⁺0X*

N N pTHF*OMe

ultrasound

NN pTHF*OMe

NN pTHF*OMe

H₂O

H

+

+0AgX

X:0Cl*0or0PF₆*

X:0AgCl₂0or0PF₆Mn0(pTHF):06.70kDA

BrO

O

O

OO

Br

O O

O

OOOON N

n n

ultrasound

BrO

O

OO

nO

ON

CH₃CN,06*90KC

Pd Pt pTHFpTHFpTHF

Cl

Cl

Cl

Cly

n

C₁₂H₂₅ Pd C₁₂H₂₅

Cl

Cl

ultrasoundPt0Scission

Pd0Scission

Pt C₁₂H₂₅

Cl

Cl

C₁₂H₂₅ Pt

Cl

Cl

pTHF pTHF

Pd C₁₂H₂₅

Cl

Cl

C₁₂H₂₅ Pd

Cl

Cl

pTHF pTHF

A)

B)

C)

D)

x

sonicated and shown that force selectively breaks the weaker Pd-‐Phosphine bonds which

were randomly distributed along the polymer backbone.26

Following the work on metal–phosphine coordination polymers, our group started

investigating mechanical dissociation of silver(I)-‐coordination complexes with N-‐heterocyclic

carbene (NHC) functionalized polymers.24 Groote et al. investigated mechanochemical

scission of metal-‐ligand bonds in Ag(NHC)2 supramolecular polymer complexes by

ultrasound using viscosity measurements and molecular dynamics simulations (MD)

combined with constrained geometry optimization calculations (COGEF).4 Calculations

indicated that the force required to break metal−ligand bond is between 400 and 500 pN,

much lower than the force that is typically required to break covalent bonds (several nN). It

has been shown that polymers with a Ag(NHC)2 coordination complex in the pTHF main

chain have significantly lower Mlim values.16

Mechanoresponsive materials

Mechanical activation of chemical bonds on polymers promises to provide opportunities to

detect and repair damage in polymeric materials. Incorporation of mechanophores into

polymers results in materials sensitive to mechanical stimuli and leads to useful molecular

transformations under stress. In solution, mechanically induced chain scission has been

used to release reactive end groups such as cyanoacrylates25 and trifluorovinyl ethers30 from

mechanophore precursors. Furthermore, transition metal complexes located in the center

of polymer chains have been dissociated by breaking the coordination bonds mechanically,

as an alternative to thermal activation for latent catalysts.1,31,32

An inspiring example of mechanically induced transformations on polymeric materials was

reported recently for the mechanochromic spiropyran (SP) to merocyanine (MC) transition.

Initially by ultrasound and later by the tensile experiments on elastomeric materials, it was

shown that the colorless SP is ring-‐opened to form the red MC dye upon mechanical

activation.8,33 Therefore, linear or crosslinked SP-‐functionalized PMA matrix changes its

color before failure upon stretching. In later studies, SP-‐based mechanochromic probes

were embedded in polyurethanes and polycaprolactone to monitor the effect of

temperature and plasticizers.25,34,35

Chapter 1

8

Figure 6: Examples of mechanophores incorporated into polymer chains. A) Poly(vinylpyrrolidone)

with random peroxide linkages.22 B) Chain scission of silver-‐carbene-‐based polymer upon sonication

and subsequent trapping of the carbene with water.24 C) Ultrasound-‐Induced cleavage of dicyano-‐

substituted cyclobutane ring yields cyanoacrylate-‐terminated polymers.25 D) Mechanochemical

scission of heteronuclear supramolecular polymers in the presence of a scavenger complex resulted

in the formation of Palladium and Platinum heterocomplexes.26

Coordination bonds, weaker than covalent bonds on polymer backbone break more easiliy

and result in even lower Mlim compared to their covalent counterparts.26 Recently in our

group, high-‐molecular-‐weight linear coordination polymers of diphenylphosphine telechelic

polytetrahydrofuran with palladium(II) dichloride were developed.27 Molecular weights of

these polymers could be altered reversibly by ultrasound and it has been shown that



palladium complexes were produced by the application of mechanical forces on these

coordination polymers.29 Furthermore, polymers which include both PdII and PtII were

OO

OO

N N NO O O

m n x

NN pTHF*OMe

Ag⁺0X*

N N pTHF*OMe

ultrasound

NN pTHF*OMe

NN pTHF*OMe

H₂O

H

+

+0AgX

X:0Cl*0or0PF₆*

X:0AgCl₂0or0PF₆Mn0(pTHF):06.70kDA

BrO

O

O

OO

Br

O O

O

OOOON N

n n

ultrasound

BrO

O

OO

nO

ON

CH₃CN,06*90KC

Pd Pt pTHFpTHFpTHF

Cl

Cl

Cl

Cly

n

C₁₂H₂₅ Pd C₁₂H₂₅

Cl

Cl

ultrasoundPt0Scission

Pd0Scission

Pt C₁₂H₂₅

Cl

Cl

C₁₂H₂₅ Pt

Cl

Cl

pTHF pTHF

Pd C₁₂H₂₅

Cl

Cl

C₁₂H₂₅ Pd

Cl

Cl

pTHF pTHF

A)

B)

C)

D)

x

sonicated and shown that force selectively breaks the weaker Pd-‐Phosphine bonds which

were randomly distributed along the polymer backbone.26

Following the work on metal–phosphine coordination polymers, our group started

investigating mechanical dissociation of silver(I)-‐coordination complexes with N-‐heterocyclic

carbene (NHC) functionalized polymers.24 Groote et al. investigated mechanochemical

scission of metal-‐ligand bonds in Ag(NHC)2 supramolecular polymer complexes by

ultrasound using viscosity measurements and molecular dynamics simulations (MD)

combined with constrained geometry optimization calculations (COGEF).4 Calculations

indicated that the force required to break metal−ligand bond is between 400 and 500 pN,

much lower than the force that is typically required to break covalent bonds (several nN). It

has been shown that polymers with a Ag(NHC)2 coordination complex in the pTHF main

chain have significantly lower Mlim values.16

Mechanoresponsive materials

Mechanical activation of chemical bonds on polymers promises to provide opportunities to

detect and repair damage in polymeric materials. Incorporation of mechanophores into

polymers results in materials sensitive to mechanical stimuli and leads to useful molecular


used to release reactive end groups such as cyanoacrylates25 and trifluorovinyl ethers30 from

mechanophore precursors. Furthermore, transition metal complexes located in the center

of polymer chains have been dissociated by breaking the coordination bonds mechanically,

as an alternative to thermal activation for latent catalysts.1,31,32

An inspiring example of mechanically induced transformations on polymeric materials was

reported recently for the mechanochromic spiropyran (SP) to merocyanine (MC) transition.

Initially by ultrasound and later by the tensile experiments on elastomeric materials, it was

shown that the colorless SP is ring-‐opened to form the red MC dye upon mechanical

activation.8,33 Therefore, linear or crosslinked SP-‐functionalized PMA matrix changes its

color before failure upon stretching. In later studies, SP-‐based mechanochromic probes

were embedded in polyurethanes and polycaprolactone to monitor the effect of

temperature and plasticizers.25,34,35


9

1

Figure 7: Examples of mechanoresponsive materials in recent literature: A) Mechanochromism:

force-‐induced ring-‐opening of spyropyran to merocyanine;33 B) Mechanoluminescence: polymers

containing bis(adamantyl)-‐1,2-‐dioxetane mechanophores emit light upon stretching.36

Another elegant example as a potential stress reporter, a mechano-‐luminescent material

based on a polymer-‐functionalized 1,2-‐dioxetane moiety, has been developed recently in

our group.36 Dioxetanes are organic peroxides and efficient sources of electronically excited

products upon chemical or thermal treatment.37 Opening of a four-‐membered 1,2-‐

dioxetane ring with two adjacent oxygen atoms yields two carbonyl groups, one of which is

in an electronically excited state and emits a photon (at 420 nm) on relaxation.

Bis(adamantyl)-‐substituted 1,2-‐dioxetane is thermally stable at room temperature (up to

~200 C) with a scission barrier of 37 kcal/mol, 36,38 and was successfully activated

mechanically by covalent incorporation in poly(methylacrylate) chains. Experiments have

shown that dioxetanes can be used as mechanical-‐probe for spatiotemporal mapping and

chain scission in polymers.

Mechanical release of small molecules in polymer matrices

The proliferation of mechanophores capable of producing useful reactive intermediates

within the polymer main chain has prompted others to investigate different modes of

chemically productive mechanical reactivity. One such mode is the mechanically induced

release of small molecules, a recent addition to the mechanochemistry field.

Mechanophores are incorporated into elastomeric networks and an applied force leads to

conformational changes and subsequent scission of bonds that are not part of the polymer

backbone, whilst maintaining the overall mechanical integrity of the polymer matrix.39 The

range of accessible reactivities remains limited, but researchers in this fledgling area are

starting to make use of the small molecule products for further reactions, including

polymerisation. This type of activation displays potential for applications in

mechanochemical catalysis, mapping deformations and damages in polymer networks and

self-‐healing or self-‐reinforcing elastomers.40

The first example of this type of activation was the mechanochemical generation of an acid,

developed by Diesendruck et al. Inspired by Craig’s gDHC system, they incorporated gem-‐

dichlorotetrahydro cyclopropanated indene into polymethyl acrylate matrix (PMA) and

showed that compression resulted in ring opening of cyclopropane to give the elimination

product 2-‐ chloronaphthalene, with the release of HCl.41 Calorimetric analysis of the sample

before and after compression demonstrated that up to 20% mechanophore conversion was

achieved at a load of 352 MPa. Control polymer in which the mechanophore was not

covalently incorporated into the PMA matrix showed 6% conversion under the same

conditions. Although significantly high thermal background reaction observed with control

polymer might limit its practical applications, this work represented an important step

towards the realization of autonomous self-‐healing materials.

Reports have also emerged from the Boydston group describing the flex, or bond-‐bending,

activation of an oxanorbornadiene, a Diels–Alder adduct of furan and dimethyl

acetylenedicarboxylate.42 They incorporated the mechanophore into poly(methyl acrylate)

(PMA) matrix and showed that the furan derivative could be released via a retro-‐[4+2]

cycloaddition under stress applied to the bulk polymer. The mechanically initiated

cycloreversion converts main-‐chain alkene moieties into alkynes and results in linearization

Chapter 1

10

Figure 7: Examples of mechanoresponsive materials in recent literature: A) Mechanochromism:

force-‐induced ring-‐opening of spyropyran to merocyanine;33 B) Mechanoluminescence: polymers

containing bis(adamantyl)-‐1,2-‐dioxetane mechanophores emit light upon stretching.36

Another elegant example as a potential stress reporter, a mechano-‐luminescent material

based on a polymer-‐functionalized 1,2-‐dioxetane moiety, has been developed recently in

our group.36 Dioxetanes are organic peroxides and efficient sources of electronically excited

products upon chemical or thermal treatment.37 Opening of a four-‐membered 1,2-‐

dioxetane ring with two adjacent oxygen atoms yields two carbonyl groups, one of which is

in an electronically excited state and emits a photon (at 420 nm) on relaxation.

Bis(adamantyl)-‐substituted 1,2-‐dioxetane is thermally stable at room temperature (up to

~200 C) with a scission barrier of 37 kcal/mol, 36,38 and was successfully activated

mechanically by covalent incorporation in poly(methylacrylate) chains. Experiments have

shown that dioxetanes can be used as mechanical-‐probe for spatiotemporal mapping and

chain scission in polymers.

Mechanical release of small molecules in polymer matrices

The proliferation of mechanophores capable of producing useful reactive intermediates

within the polymer main chain has prompted others to investigate different modes of

chemically productive mechanical reactivity. One such mode is the mechanically induced

release of small molecules, a recent addition to the mechanochemistry field.

Mechanophores are incorporated into elastomeric networks and an applied force leads to

conformational changes and subsequent scission of bonds that are not part of the polymer

backbone, whilst maintaining the overall mechanical integrity of the polymer matrix.39 The

range of accessible reactivities remains limited, but researchers in this fledgling area are

starting to make use of the small molecule products for further reactions, including

polymerisation. This type of activation displays potential for applications in

mechanochemical catalysis, mapping deformations and damages in polymer networks and

self-‐healing or self-‐reinforcing elastomers.40

The first example of this type of activation was the mechanochemical generation of an acid,

developed by Diesendruck et al. Inspired by Craig’s gDHC system, they incorporated gem-‐

dichlorotetrahydro cyclopropanated indene into polymethyl acrylate matrix (PMA) and

showed that compression resulted in ring opening of cyclopropane to give the elimination

product 2-‐ chloronaphthalene, with the release of HCl.41 Calorimetric analysis of the sample

before and after compression demonstrated that up to 20% mechanophore conversion was

achieved at a load of 352 MPa. Control polymer in which the mechanophore was not

covalently incorporated into the PMA matrix showed 6% conversion under the same

conditions. Although significantly high thermal background reaction observed with control

polymer might limit its practical applications, this work represented an important step

towards the realization of autonomous self-‐healing materials.

Reports have also emerged from the Boydston group describing the flex, or bond-‐bending,

activation of an oxanorbornadiene, a Diels–Alder adduct of furan and dimethyl

acetylenedicarboxylate.42 They incorporated the mechanophore into poly(methyl acrylate)

(PMA) matrix and showed that the furan derivative could be released via a retro-‐[4+2]

cycloaddition under stress applied to the bulk polymer. The mechanically initiated

cycloreversion converts main-‐chain alkene moieties into alkynes and results in linearization


11

1

that occurs during polymer elongation. After compression the polymer was soaked in

dichloromethane and the small molecules released mechanically diffused out of the matrix;

they could then be identified by and monitored with GC-‐MS and NMR. However, the forces

required to activate the mechanophore caused failure in the PMA matrix, limiting the

number of loading cycles. In a second publication, the authors partly addressed this issue by

incorporating the same mechanophore in a segmented PU matrix which required lower

stress loadings for successive mechanophore activation.43 Nevertheless, as a result of

random scission of the chemical crosslinks and the destruction of physical crosslinks within

the hard domains of the segmented PU matrix, only a maximum of 7% mechanophore

activation could be reached after 15 compression cycles.

Figure 8: Mechanical release of small molecules in polymer matrices: A) Potential indole-‐based

mechanocatalyst for acid generation.41 B) Solid state mechanoactivation of oxanorbornadiene,

producing an alkyne in the polymer backbone and releasing a small molecule furan.42

A)

B)

Mechanochemical catalysis

Mechanocatalysts are catalysts of which the activity or specificity is modified under the

influence of mechanical force. Two distinct modes of activation can be envisioned: steric

modification or unblocking of active sites. In steric activation, the activity of the catalyst is

modified by a change in the steric environment of the active site, e.g. by changing the

relative position of catalytically active groups that form the active site or by changing the

configuration of the binding site. Recently, research investigating the effect of a

photochemical switch coupled to a chelating bisphosphine on the activity profile of the

catalyst was published. The photochemical switch, a biindane, changes the bite angle of the

chelating ligand, and influences the enantioselectivity of reactions catalysed by Pd

complexes of the photoresponsive ligand. It was found that the effect of switching was

largest for Heck arylation reactions.44

The second and the more common approach to mechanocatalysis in synthetic systems is to

activate a catalyst by modifying its electronic properties. Catalysts in a latent state because

of pairing of acidic and basic sites are well known for their capability to be activated

thermally, and are employed in a number of different reactions. Some of the most striking

examples of this kind of catalysts are N-‐heterocyclic carbenes (NHCs).45 These Lewis bases

have been used as catalysts in various organic transformations, including condensation, 1,2-‐

and 1,4-‐addition, transesterification and ring-‐opening reactions.46,47 Due to their

nucleophilicity they show high reactivity towards various substrates but they can also be

masked in thermally labile precursors.48 NHC−metal complexes have been applied as

thermally latent catalysts in the preparation of a number of polymers, such as

poly(urethane), poly(methyl methacrylate), poly(caprolactone), and poly(amide).49–53

By their nature, latent transition metal (TM) catalysts54,55 with strongly bound ligands can be

adapted to form mechanocatalysts by providing the ligands with ‘handles’ to transfer

mechanical force. These handles can be (linear) polymer chains, which provide drag in a

viscous system that is sheared or undergoes elongational strain, or the ligands can be

connected to a polymer network in an elastic system.

Much of the work done in recent years on polymer mechanochemistry has made use of the

high elongational strain rates observed around collapsing cavitation bubbles in sonicated

Chapter 1

12

that occurs during polymer elongation. After compression the polymer was soaked in

dichloromethane and the small molecules released mechanically diffused out of the matrix;

they could then be identified by and monitored with GC-‐MS and NMR. However, the forces

required to activate the mechanophore caused failure in the PMA matrix, limiting the

number of loading cycles. In a second publication, the authors partly addressed this issue by

incorporating the same mechanophore in a segmented PU matrix which required lower

stress loadings for successive mechanophore activation.43 Nevertheless, as a result of

random scission of the chemical crosslinks and the destruction of physical crosslinks within

the hard domains of the segmented PU matrix, only a maximum of 7% mechanophore

activation could be reached after 15 compression cycles.

Figure 8: Mechanical release of small molecules in polymer matrices: A) Potential indole-‐based

mechanocatalyst for acid generation.41 B) Solid state mechanoactivation of oxanorbornadiene,

producing an alkyne in the polymer backbone and releasing a small molecule furan.42

A)

B)

Mechanochemical catalysis

Mechanocatalysts are catalysts of which the activity or specificity is modified under the

influence of mechanical force. Two distinct modes of activation can be envisioned: steric

modification or unblocking of active sites. In steric activation, the activity of the catalyst is

modified by a change in the steric environment of the active site, e.g. by changing the

relative position of catalytically active groups that form the active site or by changing the

configuration of the binding site. Recently, research investigating the effect of a

photochemical switch coupled to a chelating bisphosphine on the activity profile of the

catalyst was published. The photochemical switch, a biindane, changes the bite angle of the

chelating ligand, and influences the enantioselectivity of reactions catalysed by Pd

complexes of the photoresponsive ligand. It was found that the effect of switching was

largest for Heck arylation reactions.44

The second and the more common approach to mechanocatalysis in synthetic systems is to

activate a catalyst by modifying its electronic properties. Catalysts in a latent state because

of pairing of acidic and basic sites are well known for their capability to be activated

thermally, and are employed in a number of different reactions. Some of the most striking

examples of this kind of catalysts are N-‐heterocyclic carbenes (NHCs).45 These Lewis bases

have been used as catalysts in various organic transformations, including condensation, 1,2-‐

and 1,4-‐addition, transesterification and ring-‐opening reactions.46,47 Due to their

nucleophilicity they show high reactivity towards various substrates but they can also be

masked in thermally labile precursors.48 NHC−metal complexes have been applied as

thermally latent catalysts in the preparation of a number of polymers, such as

poly(urethane), poly(methyl methacrylate), poly(caprolactone), and poly(amide).49–53

By their nature, latent transition metal (TM) catalysts54,55 with strongly bound ligands can be

adapted to form mechanocatalysts by providing the ligands with ‘handles’ to transfer

mechanical force. These handles can be (linear) polymer chains, which provide drag in a

viscous system that is sheared or undergoes elongational strain, or the ligands can be

connected to a polymer network in an elastic system.

Much of the work done in recent years on polymer mechanochemistry has made use of the

high elongational strain rates observed around collapsing cavitation bubbles in sonicated


13

1

solutions.13 In addition to the distinctive features of sonochemically induced mechanical

reactivity described above, further attention needs to be paid to the sonication conditions in

the case of mechanochemical catalysis, because catalyst lifetime and turnover number may

be reduced by sonochemical byproducts. Therefore, the mechanochemical catalysis should

be performed under a gas that increases the lifetime of the active catalyst while still leading

to strong cavitation as mentioned above.

The concept of mechanochemical activation of a latent catalyst by ultrasound is illustrated

by the mechanochemical scission of metal-‐ligand bonds in Ag(NHC)2 supramolecular

polymer complexes.1,56 External force selectively breaks Ag-‐NHC bonds and yields free NHC

which was used to catalyze the transesterification of benzyl alcohol and vinyl acetate under

sonication.56,57 The complex form of the carbene displayed no activity proving the latency of

the catalyst. Control experiments confirmed that the catalyst was activated mechanically.

After successful application of the concept of mechanocatalysis its generality was tested

with bis-‐NHC ruthenium–alkylidene complex.31 Mechanistic studies revealed that ligand

dissociation is a crucial step in catalyst activation for Ru mediated olefin metathesis

reactions to form coordinatively unsaturated reactive Ru species.58 Among several effective

Ru catalysts bis-‐NHC ruthenium–alkylidene complexes were shown to be latent at ambient

temperature since dissociation of strong Ru-‐NHC bond requires elevated temperatures.59

Piermattei showed that Ru catalysts with pTHF chains attached bis NHC ligands resulted in a

latent metathesis catalyst that can be activated by mechanical force.56 Sonicating a solution

of diethyl diallyl malonate (DEDAM) in the presence of mechanically responsive Ru catalysts

(36 kg mol-‐1) resulted in approximately 20% conversion after 1h. Control experiments were

conducted to prove that catalyst activation is mechanical rather than thermal in nature. A

lower MW analogue of the catalyst (18 kg mol-‐1) showed lower activity due to slower chain

scission rate that decreased the amount of active catalyst formed in the timespan of

sonication. Replacing polymer actuators by butyl chains attached to NHC resulted in a

mechanically silent latent catalyst, showed less than 0.2 % conversion in the presence of

DEDAM after 1h of sonication.

Figure 9: Mechanochemical catalysis: A) Photo-‐mechanoactivation of a palladium catalyst for Heck

arylation B) Mechanically activated catalysts and the corresponding catalytic reactions C)

Mechanoactivation of latent ruthenium catalyst in the solid state, initiating in situ polymerization of

norbornene monomer in response to stress

In a later study, bis-‐NHC ruthenium–alkylidene complex was activated under compressive

strain.60 In order to initiate Ru mediated polymerization of norbornene in solid state,

Chapter 1

14

solutions.13 In addition to the distinctive features of sonochemically induced mechanical

reactivity described above, further attention needs to be paid to the sonication conditions in

the case of mechanochemical catalysis, because catalyst lifetime and turnover number may

be reduced by sonochemical byproducts. Therefore, the mechanochemical catalysis should

be performed under a gas that increases the lifetime of the active catalyst while still leading

to strong cavitation as mentioned above.

The concept of mechanochemical activation of a latent catalyst by ultrasound is illustrated

by the mechanochemical scission of metal-‐ligand bonds in Ag(NHC)2 supramolecular

polymer complexes.1,56 External force selectively breaks Ag-‐NHC bonds and yields free NHC

which was used to catalyze the transesterification of benzyl alcohol and vinyl acetate under

sonication.56,57 The complex form of the carbene displayed no activity proving the latency of

the catalyst. Control experiments confirmed that the catalyst was activated mechanically.

After successful application of the concept of mechanocatalysis its generality was tested

with bis-‐NHC ruthenium–alkylidene complex.31 Mechanistic studies revealed that ligand

dissociation is a crucial step in catalyst activation for Ru mediated olefin metathesis

reactions to form coordinatively unsaturated reactive Ru species.58 Among several effective

Ru catalysts bis-‐NHC ruthenium–alkylidene complexes were shown to be latent at ambient

temperature since dissociation of strong Ru-‐NHC bond requires elevated temperatures.59

Piermattei showed that Ru catalysts with pTHF chains attached bis NHC ligands resulted in a

latent metathesis catalyst that can be activated by mechanical force.56 Sonicating a solution

of diethyl diallyl malonate (DEDAM) in the presence of mechanically responsive Ru catalysts

(36 kg mol-‐1) resulted in approximately 20% conversion after 1h. Control experiments were

conducted to prove that catalyst activation is mechanical rather than thermal in nature. A

lower MW analogue of the catalyst (18 kg mol-‐1) showed lower activity due to slower chain

scission rate that decreased the amount of active catalyst formed in the timespan of

sonication. Replacing polymer actuators by butyl chains attached to NHC resulted in a

mechanically silent latent catalyst, showed less than 0.2 % conversion in the presence of

DEDAM after 1h of sonication.

Figure 9: Mechanochemical catalysis: A) Photo-‐mechanoactivation of a palladium catalyst for Heck

arylation B) Mechanically activated catalysts and the corresponding catalytic reactions C)

Mechanoactivation of latent ruthenium catalyst in the solid state, initiating in situ polymerization of

norbornene monomer in response to stress

In a later study, bis-‐NHC ruthenium–alkylidene complex was activated under compressive

strain.60 In order to initiate Ru mediated polymerization of norbornene in solid state,


15

1

polymer catalyst (34 kg mol-‐1) and a norbornene monomer were incorporated in a high

molecular weight Poly(tetrahydrofuran) (pTHF) matrix (Mn = 170 kDa, PDI = 1.3) which

provided the physical cross-‐linking through the crystalline domains and allowed

macroscopic forces to be transferred to the metal–ligand bonds. Consecutive compressions

showed that up to 25% of norbornene monomer was polymerized after five loading cycles.

Aim and outline of this thesis

The main aim of this thesis is to gain a better understanding of the fundamental processes

and mechanisms underlying mechanochemical chain scission in organometallic complexes.

Throughout this work, we have used Palladium (II) complexes with N-‐heterocyclic carbene

(NHC) or imidazole ligands. These coordination complexes are embedded within polymer

chains and coordination bonds are broken by ultrasonication or by straining bulk polymer

samples.

In Chapter 2, ultrasound induced chain scission in coordination complexes of Palladium and

Platinum with polytetrahydrofuran functionalized N-‐heterocyclic carbene ligands is

reported. Reversibility of chain scission and molecular weight dependence of scission rate

were determined. Comparing scission of Palladium and Platinum containing polymer

showed the influence of ligand dissociation energy on mechanochemical response of the

coordination polymers.

Scission of the Pd -‐ NHC bond releases free NHC. In Chapter 3, this basic NHC, is used to

abstract a proton from a 2-‐coumaranone derivative, which decomposes via a

chemiluminescent pathway once deprotonated in the presence of oxygen. Rate of

ultrasound induced scission and molecular weight threshold (Mlim) for mechanochemical

chain scission were determined under the reaction conditions that coumaranone

decomposition could be visually monitored.

In Chapter 4, thermal ligand exchange between imidazole Palladium (II) complexes has been

investigated to have a better understanding on ligand exchange dynamics and to determine

the rate of sonication induced ligand exchange in Imidazole-‐Pd coordination polymers.

In Chapter 5, information on the mechanism of mechanochemical chain scission and

mechanochemically induced ligand exchange of Pd(II) complexes, obtained in previous

chapters was used to direct the formation of heterocomplexes. Symmetric complexes with

high and low molecular weight polymer-‐attached ligands were mixed in solution and

sonicated. When one of the complexes has a molecular weight higher than the threshold

(Mlim) for mechanochemical chain scission, while the other is smaller, sonication leads to the

directed formation of a heterocomplex with two different ligands.

In Chapter 6, initial attempts and preliminary results for self-‐healing property of polymer

films of poly(methyl acrylate)-‐vinyl imidazole copolymers, which were crosslinked by ligand

metal coordination were reported. Reversibility of mechanochemical chain scission in

coordination complexes was utilized in the solid state to regain the mechanical properties of

a cross-‐linked polymer film.

Chapter 1

16

polymer catalyst (34 kg mol-‐1) and a norbornene monomer were incorporated in a high

molecular weight Poly(tetrahydrofuran) (pTHF) matrix (Mn = 170 kDa, PDI = 1.3) which

provided the physical cross-‐linking through the crystalline domains and allowed

macroscopic forces to be transferred to the metal–ligand bonds. Consecutive compressions

showed that up to 25% of norbornene monomer was polymerized after five loading cycles.

Aim and outline of this thesis



Throughout this work, we have used Palladium (II) complexes with N-‐heterocyclic carbene

(NHC) or imidazole ligands. These coordination complexes are embedded within polymer

chains and coordination bonds are broken by ultrasonication or by straining bulk polymer

samples.

In Chapter 2, ultrasound induced chain scission in coordination complexes of Palladium and

Platinum with polytetrahydrofuran functionalized N-‐heterocyclic carbene ligands is

reported. Reversibility of chain scission and molecular weight dependence of scission rate

were determined. Comparing scission of Palladium and Platinum containing polymer

showed the influence of ligand dissociation energy on mechanochemical response of the

coordination polymers.

Scission of the Pd -‐ NHC bond releases free NHC. In Chapter 3, this basic NHC, is used to

abstract a proton from a 2-‐coumaranone derivative, which decomposes via a

chemiluminescent pathway once deprotonated in the presence of oxygen. Rate of


chain scission were determined under the reaction conditions that coumaranone

decomposition could be visually monitored.

In Chapter 4, thermal ligand exchange between imidazole Palladium (II) complexes has been

investigated to have a better understanding on ligand exchange dynamics and to determine

the rate of sonication induced ligand exchange in Imidazole-‐Pd coordination polymers.

In Chapter 5, information on the mechanism of mechanochemical chain scission and

mechanochemically induced ligand exchange of Pd(II) complexes, obtained in previous

chapters was used to direct the formation of heterocomplexes. Symmetric complexes with

high and low molecular weight polymer-‐attached ligands were mixed in solution and

sonicated. When one of the complexes has a molecular weight higher than the threshold

(Mlim) for mechanochemical chain scission, while the other is smaller, sonication leads to the

directed formation of a heterocomplex with two different ligands.

In Chapter 6, initial attempts and preliminary results for self-‐healing property of polymer

films of poly(methyl acrylate)-‐vinyl imidazole copolymers, which were crosslinked by ligand

metal coordination were reported. Reversibility of mechanochemical chain scission in

coordination complexes was utilized in the solid state to regain the mechanical properties of

a cross-‐linked polymer film.


17

1

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Braun, P. V.; Martínez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Nature 2009, 459 (7243),

68–72.

(34) Kingsbury, C. M.; May, P. A.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R. J. Mater.

Chem. 2011, 21 (23), 8381–8388.

(35) O’Bryan, G.; Wong, B. M.; McElhanon, J. R. ACS Appl. Mater. Interfaces 2010, 2 (6), 1594–

1600.

(36) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.; Meijer, E. W.; Sijbesma, R. P. Nat.

Chem. 2012, 4 (7), 559–562.

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Res. 1974, 7 (4), 97–105.

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(39) Sottos, N. R. Nat. Chem. 2014, 6 (5), 381–383.

(40) Wiggins, K. M.; Brantley, J. N.; Bielawski, C. W. Chem. Soc. Rev. 2013, 42 (17), 7130–7147.

(41) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.; Silberstein, M. N.; Sottos, N. R.; White, S. R.;

Braun, P. V.; Moore, J. S. J. Am. Chem. Soc. 2012, 134 (30), 12446–12449.

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1600.


Chem. 2012, 4 (7), 559–562.


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19

1

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Chapter 1

20

Chapter 2Mechanochemical chain scission in NHC-Pd centered coordination polymers

Ultrasound induced chain scission in coordination complexes of Palladium and Platinum

with polytetrahydrofuran functionalized N-heterocyclic carbene ligand is reported. Scission

is reversible when the polymer complex is sonicated in toluene under methane. The lowest

molecular weight complex that breaks was determined to be 20 kDa. Above this size the rate

of scission increases linearly with molecular weight of the polymer complex. Constrained

geometry simulations of external force (COGEF) calculations with DFT method provided

insight into the response of the NHC-Pd center to applied external force. Comparing scission

rates of Palladium and Platinum coordination polymers showed the influence of the force

needed for ligand dissociation.

(44) Kean, Z. S.; Akbulatov, S.; Tian, Y.; Widenhoefer, R. A.; Boulatov, R.; Craig, S. L. Angew. Chem.

Int. Ed. 2014, 53 (52), 14508–14511.


2172.





J. 2009, 15 (13), 3103–3109.


(9), 2731–2740.


4181.


Macromolecules 2013, 46 (21), 8426–8433.


47 (14), 4548–4556.


2813.

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2009.



4929–4935.

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(59) Van der Schaaf, P. A.; Kolly, R.; Kirner, H.-‐J.; Rime, F.; Mühlebach, A.; Hafner, A. J. Organomet.

Chem. 2000, 606 (1), 65–74.

(60) Jakobs, R. T. M.; Ma, S.; Sijbesma, R. P. ACS Macro Lett. 2013, 2 (7), 613–616.

Introduction

Mechanical activation of chemical bonds promises to provide opportunities to detect and

repair damage in polymeric materials. Incorporation of mechanophores into polymers

results in materials sensitive to mechanical stimuli and leads to useful molecular


used to release reactive end groups such as cyanoacrylates,1 trifluorovinyl ethers2 or azides3

from mechanophore precursors. Furthermore, transition metal complexes located in the

center of polymer chains have been dissociated by breaking the coordination bonds

mechanically, as an alternative to thermal activation for latent catalysts.4–6

One of the most efficient ways to exert force on a polymer in solution is the use of

sonication.7 Upon sonication of solution, cavitation leads to strong elongational stresses



along the backbone. This force reaches a maximum value at the center of the chain because

the flow field is centrosymmetric with respect to the molecule and as a consequence chain

scission occurs preferentially around the chain midpoint9 for polymers above a critical

molecular weight.

In our group, high-‐molecular-‐weight linear coordination polymers of diphenylphosphine

telechelic polytetrahydrofuran with palladium(II) dichloride have been studied.10 Molecular

weights of these polymers could be altered by ultrasound and it has been shown that



palladium complexes are transiently produced by the application of mechanical forces on

these coordination polymers.12 Furthermore, polymers which include both PdII and PtII,

were sonicated and showed that mechanical force selectively breaks the weaker Pd-‐

Phosphine bonds which are randomly distributed along the polymer backbone. Scission rate

is influenced by small changes in metal ligand coordination bond strength.13

Phosphine ligands have been intensively researched in Pd-‐mediated C-‐C and C-‐N bond

forming reactions, which are among the most versatile and powerful synthetic methods.14

However, in the last two decades the popularity of phosphines has been challenged by N-‐

heterocyclic carbenes (NHCs).15 Compared to tertiary phosphines, NHC ligands show

superior thermal stability and their synthetic availability provides the possibility to extensive

tuning of steric and electronic properties.16–18 Palladium-‐NHC complexes exhibit high

stability both in solid state and in solution due to strong metal ligand bonds, allowing for

easy storage and handling.19 Organ et. al. prepared well-‐defined Pd-‐PEPPSI-‐(NHC)

complexes with NHC and Pyridine (Py) ligands coordinated to Pd(II).18,20 They have proposed

that Py ligand serves as a ‘throw away’ ligand that dissociates from the Pd center in catalyst

activation process. Although the activation mechanism of these latent catalysts is not yet

fully understood, it has been shown that the Pd:ligand ratio, optimally 1:1,21,22 is crucial to

obtain high reactivity.23,24

This chapter presents the ultrasound induced chain scission in Pd(NHC)2 and Pt(NHC)2

complexes, functionalized with poly(tetrahydrofuran) as force accumulating arms. Scission

rates were calculated from the change in molecular weight distribution of polymers during

sonication. Molecular weight dependence of the scission rate was established and the effect

of bond strength on scission rates were determined with Constrained geometry

optimization (COGEF) calculations. These calculations also provided a more in depth

understanding on the structural change of the complex under the influence of an external

force that leads to chain scission.

Chapter 2

22

Introduction

Mechanical activation of chemical bonds promises to provide opportunities to detect and

repair damage in polymeric materials. Incorporation of mechanophores into polymers

results in materials sensitive to mechanical stimuli and leads to useful molecular


used to release reactive end groups such as cyanoacrylates,1 trifluorovinyl ethers2 or azides3

from mechanophore precursors. Furthermore, transition metal complexes located in the

center of polymer chains have been dissociated by breaking the coordination bonds

mechanically, as an alternative to thermal activation for latent catalysts.4–6

One of the most efficient ways to exert force on a polymer in solution is the use of

sonication.7 Upon sonication of solution, cavitation leads to strong elongational stresses



along the backbone. This force reaches a maximum value at the center of the chain because

the flow field is centrosymmetric with respect to the molecule and as a consequence chain

scission occurs preferentially around the chain midpoint9 for polymers above a critical

molecular weight.

In our group, high-‐molecular-‐weight linear coordination polymers of diphenylphosphine

telechelic polytetrahydrofuran with palladium(II) dichloride have been studied.10 Molecular

weights of these polymers could be altered by ultrasound and it has been shown that



palladium complexes are transiently produced by the application of mechanical forces on

these coordination polymers.12 Furthermore, polymers which include both PdII and PtII,

were sonicated and showed that mechanical force selectively breaks the weaker Pd-‐

Phosphine bonds which are randomly distributed along the polymer backbone. Scission rate

is influenced by small changes in metal ligand coordination bond strength.13

Phosphine ligands have been intensively researched in Pd-‐mediated C-‐C and C-‐N bond

forming reactions, which are among the most versatile and powerful synthetic methods.14

However, in the last two decades the popularity of phosphines has been challenged by N-‐

heterocyclic carbenes (NHCs).15 Compared to tertiary phosphines, NHC ligands show

superior thermal stability and their synthetic availability provides the possibility to extensive

tuning of steric and electronic properties.16–18 Palladium-‐NHC complexes exhibit high

stability both in solid state and in solution due to strong metal ligand bonds, allowing for

easy storage and handling.19 Organ et. al. prepared well-‐defined Pd-‐PEPPSI-‐(NHC)

complexes with NHC and Pyridine (Py) ligands coordinated to Pd(II).18,20 They have proposed

that Py ligand serves as a ‘throw away’ ligand that dissociates from the Pd center in catalyst

activation process. Although the activation mechanism of these latent catalysts is not yet

fully understood, it has been shown that the Pd:ligand ratio, optimally 1:1,21,22 is crucial to

obtain high reactivity.23,24

This chapter presents the ultrasound induced chain scission in Pd(NHC)2 and Pt(NHC)2

complexes, functionalized with poly(tetrahydrofuran) as force accumulating arms. Scission

rates were calculated from the change in molecular weight distribution of polymers during

sonication. Molecular weight dependence of the scission rate was established and the effect

of bond strength on scission rates were determined with Constrained geometry

optimization (COGEF) calculations. These calculations also provided a more in depth

understanding on the structural change of the complex under the influence of an external

force that leads to chain scission.

Mechanochemical chain scission in NHC-Pd

23

2

Results and Discussions

Synthesis

Scheme 1: Synthetic route to mechanically responsive NHC-‐Metal complexes (i) DTBP (ii) 1-‐Ethyl

imidazole, 20 min (iii) ion exchange resin, MeOH (iv) NaOtBu, THF – 4Å MS, 1h (v) M(PhCN)2Cl2 (M:

Pd or Pt), 8h, (vi) C4H9I, THF, (vii) ion exchange resin, MeOH, (viii) PdCl2, Cs2CO3, dioxane, 5h.

Polymeric ligands were prepared from imidazolium terminated precursor salts (Scheme 1),

which were obtained via a cationic ring opening polymerization of tetrahydrofuran (THF).

Polymerizations were initiated by methyl triflate and subsequently terminated by N-‐ethyl

imidazole to yield (EtIm-‐pTHF)Cl.25 Molecular weights (MW) of resulting polymers (Table 1)

depend on the polymerization time at room temperature, and were analyzed by 1H NMR,

MALDI-‐TOF and gel permeation chromatography (GPC). The triflate counterions were

exchanged by chloride over ion exchange resin in methanol.

OS CF3

OOH3C

O

i, ii, iii

O O N N

O O N NCl

M

OONN

ClCl

+

iv, v

(EtIm-pTHF)Cl

M: Pd Pd(NHC-pTHF)2Cl2M: Pt Pt(NHC-pTHF)2Cl2

(EtIm-pTHF)Cl

N N

Pd

NN

ClClN NCl

vi, vii viiiN N

(ButEtIm)ClEtImPd(NHC-ButEt)2Cl2

Table 1: Molecular weights of EtIm-‐pTHF determined by GPC, NMR and MALDI-‐TOF

Polymer

Time (h)a

Molecular Weight (kDa)

GPCb NMRc MALDId

(EtIm-‐pTHF8k)Cl 1 19 9.4 8

(EtIm-‐pTHF12k)Cl 1.5 28 16 12

(EtIm-‐pTHF18k)Cl 2 48 21 18

(EtIm-‐pTHF25k)Cl 3 63 29 25

(EtIm-‐pTHF33k)Cl 4 72 38 33

(EtIm-‐pTHF40k)Cl 5 86 53 40

(EtIm-‐pTHF50k)Cl 6 108 63 50

a) polymerizations were initated by CH3CF3SO3 and performed at 20oC for given time, b) 2mg/ml polymer

solutions in THF were submitted to GPC at 20oC, 1 ml/min flow rate, PS standards were used for calibration c) 1H NMR spectra were taken in CD2Cl2 and Molecular weights were determined by end group analysis d) 1

mg/ml polymer solutions in CHCl3 were submitted to MALDI-‐TOF and peak tops were reported as molecular

weight.

(EtIm-‐pTHF)Cl salts were deprotonated on the imidazolium C2 position to create a N-‐

heterocyclic carbene (NHC-‐pTHF) which subsequently was coordinated to Palladium or

Platinum via exchange of the benzonitrile (PhCN) ligand in M(PhCN)2Cl2. The desired

complexes were obtained in high yield since PhCN coordination is weak and allows ligand

exchange at room temperature. Coordination of two NHC-‐pTHF ligands resulted in doubling

of molecular weight compared to the bare ligands as monitored by GPC. After 8h, molecular

weight distributions did not change and reactions were considered as completed. MW

distribution of polymers reveals bimodal GPC traces due to small amounts of pTHF not

terminated by EtIm. In order to determine initial weight fraction of M(NHC-‐pTHF)2Cl2

complexes bimodal distributions obtained in GPC were simulated by double Gaussian

function. Areas under the peaks were determined and used to calculate weight fractions.

Ligand coordination was also evident from 1H NMR which showed that signals for H(1)

proton of imidazole heterocycle had disappeared completely and H(4,5) protons shifted

upfield upon coordination to metals.

Chapter 2

24


Synthesis

Scheme 1: Synthetic route to mechanically responsive NHC-‐Metal complexes (i) DTBP (ii) 1-‐Ethyl

imidazole, 20 min (iii) ion exchange resin, MeOH (iv) NaOtBu, THF – 4Å MS, 1h (v) M(PhCN)2Cl2 (M:

Pd or Pt), 8h, (vi) C4H9I, THF, (vii) ion exchange resin, MeOH, (viii) PdCl2, Cs2CO3, dioxane, 5h.

Polymeric ligands were prepared from imidazolium terminated precursor salts (Scheme 1),

which were obtained via a cationic ring opening polymerization of tetrahydrofuran (THF).

Polymerizations were initiated by methyl triflate and subsequently terminated by N-‐ethyl

imidazole to yield (EtIm-‐pTHF)Cl.25 Molecular weights (MW) of resulting polymers (Table 1)

depend on the polymerization time at room temperature, and were analyzed by 1H NMR,

MALDI-‐TOF and gel permeation chromatography (GPC). The triflate counterions were

exchanged by chloride over ion exchange resin in methanol.

OS CF3

OOH3C

O

i, ii, iii

O O N N

O O N NCl

M

OONN

ClCl

+

iv, v

(EtIm-pTHF)Cl

M: Pd Pd(NHC-pTHF)2Cl2M: Pt Pt(NHC-pTHF)2Cl2

(EtIm-pTHF)Cl

N N

Pd

NN

ClClN NCl

vi, vii viiiN N

(ButEtIm)ClEtImPd(NHC-ButEt)2Cl2

Table 1: Molecular weights of EtIm-‐pTHF determined by GPC, NMR and MALDI-‐TOF

Polymer

Time (h)a

Molecular Weight (kDa)

GPCb NMRc MALDId

(EtIm-‐pTHF8k)Cl 1 19 9.4 8

(EtIm-‐pTHF12k)Cl 1.5 28 16 12

(EtIm-‐pTHF18k)Cl 2 48 21 18

(EtIm-‐pTHF25k)Cl 3 63 29 25

(EtIm-‐pTHF33k)Cl 4 72 38 33

(EtIm-‐pTHF40k)Cl 5 86 53 40

(EtIm-‐pTHF50k)Cl 6 108 63 50

a) polymerizations were initated by CH3CF3SO3 and performed at 20oC for given time, b) 2mg/ml polymer

solutions in THF were submitted to GPC at 20oC, 1 ml/min flow rate, PS standards were used for calibration c) 1H NMR spectra were taken in CD2Cl2 and Molecular weights were determined by end group analysis d) 1

mg/ml polymer solutions in CHCl3 were submitted to MALDI-‐TOF and peak tops were reported as molecular

weight.

(EtIm-‐pTHF)Cl salts were deprotonated on the imidazolium C2 position to create a N-‐

heterocyclic carbene (NHC-‐pTHF) which subsequently was coordinated to Palladium or

Platinum via exchange of the benzonitrile (PhCN) ligand in M(PhCN)2Cl2. The desired

complexes were obtained in high yield since PhCN coordination is weak and allows ligand

exchange at room temperature. Coordination of two NHC-‐pTHF ligands resulted in doubling

of molecular weight compared to the bare ligands as monitored by GPC. After 8h, molecular

weight distributions did not change and reactions were considered as completed. MW

distribution of polymers reveals bimodal GPC traces due to small amounts of pTHF not

terminated by EtIm. In order to determine initial weight fraction of M(NHC-‐pTHF)2Cl2

complexes bimodal distributions obtained in GPC were simulated by double Gaussian

function. Areas under the peaks were determined and used to calculate weight fractions.

Ligand coordination was also evident from 1H NMR which showed that signals for H(1)

proton of imidazole heterocycle had disappeared completely and H(4,5) protons shifted

upfield upon coordination to metals.


25

2

Chain Scission in Pd(NHC-‐pTHF)2Cl2

Chain scission in polymers Pd(NHC-‐pTHF)2Cl2 was investigated in toluene solutions (10

mg/ml) which were sonicated for 60 min using a sonication probe. During sonication Ar or

CH4 was bubbled through the solution starting 15 minutes prior to sonication. The double-‐

jacketed sonication vessel was cooled down to 2oC by water circulation from a thermostat-‐

controlled bath. Temperature inside the sonication vessel was checked by a thermocouple

and was constant at 25 oC after thermal equilibration (~ 5 min).

Figure 1: Molecular weight change during sonication of Pd(NHC-‐pTHF)2Cl2 in the presence of

trapping agents AcOH and MeCN to block coordination, monitored by GPC. Polymer solutions

(10mg/ml) in dry toluene were subjected to continuous sonication for 1h, while the internal

temperature was constant at 25oC. 200 µL aliquots were collected at given times, solvent was

evaporated under reduced pressure; residue was dissolved in THF and submitted immediately to

GPC.

Bond rupture on mechano-‐responsive organometallic unit at the center of polymer chain

yields free NHC and coordinatively unsaturated Pd. Reversibility of the ligand-‐metal

interactions might then result in re-‐coordination of sonication products. Therefore,

sonication was performed in the presence of acetic acid (AcOH) (2.4 %, w/w) and

acetonitrile (MeCN) (1.8 %, w/w) as trapping agents, to stabilize the scission products and

prevent the re-‐coordination. The stability of the Pd-‐NHC coordination complexes in the

presence of these trapping agents was established by stirring in toluene in the presence of

AcOH and MeCN at room temperature. In the course of 1h, no change in MW was observed

with GPC.

Figure 2: Reversibility test of Pd(NHC-‐pTHF18k)2Cl2: a) under CH4 b) under Ar. c) Comparison of

change in molecular weight distribution of Pd(NHC-‐pTHF18k)2Cl2 as a function of sonication time

with and without trapping agents under CH4 and Ar. d) Ultrasound induced reversible chain scission

of the Pd-‐NHC bond. Samples with a concentration of 10 mg/ml were subjected to continuous

sonication in dry toluene for 1h. In the absence of trapping agents molecular weight distribution did

not change under CH4 in contrast to sonication under Ar. Solid and dashed lines represent GPC

traces for Pd(NHC-‐pTHF18k)2Cl2 before and after sonication respectively.

After sonication in the presence of the trapping agents, GPC traces showed the formation of

species with half of the initial MW, confirming that scission occurs selectively at the central

bonds (Figure 1). In the presence of trapping agents, 1h of continuous sonication resulted in

approximately 55% chain scission for Pd(NHC-‐pTHF18k)2Cl2. However, without any trapping

agent under CH4, no significant change in molecular weight was observed after 1h (Figure

Chapter 2

26

Chain Scission in Pd(NHC-‐pTHF)2Cl2

Chain scission in polymers Pd(NHC-‐pTHF)2Cl2 was investigated in toluene solutions (10

mg/ml) which were sonicated for 60 min using a sonication probe. During sonication Ar or

CH4 was bubbled through the solution starting 15 minutes prior to sonication. The double-‐

jacketed sonication vessel was cooled down to 2oC by water circulation from a thermostat-‐

controlled bath. Temperature inside the sonication vessel was checked by a thermocouple

and was constant at 25 oC after thermal equilibration (~ 5 min).

Figure 1: Molecular weight change during sonication of Pd(NHC-‐pTHF)2Cl2 in the presence of

trapping agents AcOH and MeCN to block coordination, monitored by GPC. Polymer solutions

(10mg/ml) in dry toluene were subjected to continuous sonication for 1h, while the internal

temperature was constant at 25oC. 200 µL aliquots were collected at given times, solvent was

evaporated under reduced pressure; residue was dissolved in THF and submitted immediately to

GPC.

Bond rupture on mechano-‐responsive organometallic unit at the center of polymer chain

yields free NHC and coordinatively unsaturated Pd. Reversibility of the ligand-‐metal

interactions might then result in re-‐coordination of sonication products. Therefore,

sonication was performed in the presence of acetic acid (AcOH) (2.4 %, w/w) and

acetonitrile (MeCN) (1.8 %, w/w) as trapping agents, to stabilize the scission products and

prevent the re-‐coordination. The stability of the Pd-‐NHC coordination complexes in the

presence of these trapping agents was established by stirring in toluene in the presence of

AcOH and MeCN at room temperature. In the course of 1h, no change in MW was observed

with GPC.

Figure 2: Reversibility test of Pd(NHC-‐pTHF18k)2Cl2: a) under CH4 b) under Ar. c) Comparison of

change in molecular weight distribution of Pd(NHC-‐pTHF18k)2Cl2 as a function of sonication time

with and without trapping agents under CH4 and Ar. d) Ultrasound induced reversible chain scission

of the Pd-‐NHC bond. Samples with a concentration of 10 mg/ml were subjected to continuous

sonication in dry toluene for 1h. In the absence of trapping agents molecular weight distribution did

not change under CH4 in contrast to sonication under Ar. Solid and dashed lines represent GPC

traces for Pd(NHC-‐pTHF18k)2Cl2 before and after sonication respectively.

After sonication in the presence of the trapping agents, GPC traces showed the formation of

species with half of the initial MW, confirming that scission occurs selectively at the central

bonds (Figure 1). In the presence of trapping agents, 1h of continuous sonication resulted in

approximately 55% chain scission for Pd(NHC-‐pTHF18k)2Cl2. However, without any trapping

agent under CH4, no significant change in molecular weight was observed after 1h (Figure


27

2

2). This indicates that chain scission is selective to NHC-‐metal coordination bond and

without trapping agents scission is completely reversible. Control experiments were

performed with low molecular weight complex Pd(NHC-‐But)2Cl2 to investigate the role of

thermal effects in scission. Ultrasound did not cause any changes when Pd(NHC-‐But)2Cl2 was

sonicated with trapping agents for 1h (under CH4 or Ar). The amount of total imidazolium

after 1h of sonication of the 36k polymer is the same regardless of saturation gas; however,

under Ar sonication yields up to 30% irreversible chain scission. Further reaction of the

scission products (free NHC and coordinatively unsaturated Pd center) with sonochemical

impurities which are produced due to possible solvent pyrolysis in hot spots,26 prevents

reversible recoordination under argon. The use of CH4 instead of Ar as the saturation gas led

to suppression of thermal effects during sonication and reversible scission on NHC-‐Pd

coordination bond with increased lifetime of mechanically released free NHC in solution.

1H NMR spectra, taken for Pd(NHC-‐pTHF25k)2Cl2 samples before and after sonication,

showed that the sonication revealed two new sets of peaks (Figure 3). These peaks are

assumed to be due to free NHC that is taken off and subsequently protonated by the

trapping agent, and Pd center with monoNHC. Since the NHC taken off by ultrasound is

prevented from re-‐coordination, the remaining monoNHC-‐Pd part should be coordinated by

another ligand presents in the solution. MeCN was used to serve as the stabilizing ligand

during the course of sonication. However, NMR spectra showed that MeCN was not

coordinated. It is known for hetero NHC complexes of Pd that a stronger donating co-‐ligand

leads to downfield shift in NMR spectrum of the NHC ligand.27 Since MeCN is a weaker

donor than NHC, shift in opposite direction would have been expected upon coordination of

MeCN. Thus, it is not trivial at this point to determine the exact chemical structure of the

scission product that contains Pd, since coordinatively unsaturated Pd center could

coordinate to any other ligand (i.e. acetate anion) or dimerize28 to stabilize its structure.

Figure 3: Stacked 1H NMR spectra for (a) EtIm-‐pTHF25k (b) Pd(NHC-‐pTHF25k)2Cl2 and (c) Pd(NHC-‐

pTHF25k)2Cl2 after sonication.

Determination of limiting molecular weight Mlim



chains are too short to accumulate the force required to break chemical bonds.30 Mlim

lowers significantly if a weak bond is incorporated into a polymer chain. Coordination

bonds, weaker than covalent bonds on polymer backbone break more easily and result in

lower Mlim compared to their covalent counterparts.13 In addition to bond strength,

experimental conditions such as ultrasound intensity, solvent composition, temperature,

and gas molecules dissolved in solution also influence Mlim.31 In order to compare

mechanochemical properties of different mechanophores, experimental conditions should

be the same. Therefore we decided to investigate Mlim and the molecular weight

dependence of scission rate for a specific set of sonication conditions, and to correlate

scissability with bond dissociation energies for the Metal-‐NHC (Pd, Pt) complexes.

Chapter 2

28

2). This indicates that chain scission is selective to NHC-‐metal coordination bond and

without trapping agents scission is completely reversible. Control experiments were

performed with low molecular weight complex Pd(NHC-‐But)2Cl2 to investigate the role of

thermal effects in scission. Ultrasound did not cause any changes when Pd(NHC-‐But)2Cl2 was

sonicated with trapping agents for 1h (under CH4 or Ar). The amount of total imidazolium

after 1h of sonication of the 36k polymer is the same regardless of saturation gas; however,

under Ar sonication yields up to 30% irreversible chain scission. Further reaction of the

scission products (free NHC and coordinatively unsaturated Pd center) with sonochemical

impurities which are produced due to possible solvent pyrolysis in hot spots,26 prevents

reversible recoordination under argon. The use of CH4 instead of Ar as the saturation gas led

to suppression of thermal effects during sonication and reversible scission on NHC-‐Pd

coordination bond with increased lifetime of mechanically released free NHC in solution.

1H NMR spectra, taken for Pd(NHC-‐pTHF25k)2Cl2 samples before and after sonication,

showed that the sonication revealed two new sets of peaks (Figure 3). These peaks are

assumed to be due to free NHC that is taken off and subsequently protonated by the

trapping agent, and Pd center with monoNHC. Since the NHC taken off by ultrasound is

prevented from re-‐coordination, the remaining monoNHC-‐Pd part should be coordinated by

another ligand presents in the solution. MeCN was used to serve as the stabilizing ligand

during the course of sonication. However, NMR spectra showed that MeCN was not

coordinated. It is known for hetero NHC complexes of Pd that a stronger donating co-‐ligand

leads to downfield shift in NMR spectrum of the NHC ligand.27 Since MeCN is a weaker

donor than NHC, shift in opposite direction would have been expected upon coordination of

MeCN. Thus, it is not trivial at this point to determine the exact chemical structure of the

scission product that contains Pd, since coordinatively unsaturated Pd center could

coordinate to any other ligand (i.e. acetate anion) or dimerize28 to stabilize its structure.

Figure 3: Stacked 1H NMR spectra for (a) EtIm-‐pTHF25k (b) Pd(NHC-‐pTHF25k)2Cl2 and (c) Pd(NHC-‐

pTHF25k)2Cl2 after sonication.

Determination of limiting molecular weight Mlim



chains are too short to accumulate the force required to break chemical bonds.30 Mlim

lowers significantly if a weak bond is incorporated into a polymer chain. Coordination

bonds, weaker than covalent bonds on polymer backbone break more easily and result in

lower Mlim compared to their covalent counterparts.13 In addition to bond strength,

experimental conditions such as ultrasound intensity, solvent composition, temperature,

and gas molecules dissolved in solution also influence Mlim.31 In order to compare

mechanochemical properties of different mechanophores, experimental conditions should

be the same. Therefore we decided to investigate Mlim and the molecular weight

dependence of scission rate for a specific set of sonication conditions, and to correlate

scissability with bond dissociation energies for the Metal-‐NHC (Pd, Pt) complexes.


29

2

In order to prevent complication of the analysis from scission of covalent bonds, the

polymeric ligand must have a molecular weight that is lower than Mlim for covalent bond

scission. Therefore, the limiting molecular weight of covalent pTHF was determined in a

separate set of experiments. EtIm-‐pTHF samples with Mw ranging between 8 and 50 kDa

were subjected to ultrasound at 20 kHz with a power density of 15.4 W/cm2 in toluene

solutions (10 mg/ml) under CH4. Internal temperature of the double-‐jacketed sonication

vessel was constant at 25oC (±2 oC) throughout sonication.

GPC traces for polymer samples before and after sonication showed that the molecular

weight distribution for EtIm-‐pTHF below 25 kDa did not change significantly by sonication.

However, polymer with a MW higher than 30 kDa degraded irreversibly as shown by both

GPC (Figure 4) and MALDI-‐TOF (Figure 5). This sets a limit (Mlim) to the MW of force-‐

accumulating ligands (EtIm-‐pTHF) below which they are not destroyed by ultrasound. Thus,

only ligands with a molecular weight lower than Mlim for pTHF (<30kDa) were used to

synthesize mechano-‐responsive polymers with a central Pd-‐NHC mechanophore.

Figure 4: GPC traces for polymer samples of EtIm-‐pTHF taken before and after sonication. 10 mg/ml

samples were sonicated (continuous) in toluene under CH4. Aliquots were taken and solvent was

evaporated, residue was dissolved in THF and directly submitted to GPC. Black and red lines

represented traces for aliquots taken before and after sonication samples. Curves were normalized

to peak areas.

Figure 5: MALDI-‐TOF spectra for EtIm-‐pTHF50k before (left) and after (right) 1h of sonication.

The GPC trace of Pd(NHC-‐pTHF8k)2Cl2 did not change after 1 h of continuous sonication

under CH4 in the presence of acetonitrile and acetic acid as trapping agents (Figure 6).

However, higher molecular weight complex Pd(NHC-‐pTHF25k)2Cl2 showed around %80

chain scission after 1h of continuous sonication. Mechanochemical fragmentation of

Pd(NHC-‐pTHF25k)2Cl2 revealed a product with a sharp peak corresponding to MW of 25 kDa,

that is consistent with the previously reported center specific chain scission for

mechanophore incorporated polymers.32 This observation confirms that Mlim for Pd(NHC-‐

pTHF)Cl2 complex is higher than 16 kDa under above mentioned conditions and central

coordination bond remains intact when polymer chain is not sufficiently long.

Chapter 2

30

In order to prevent complication of the analysis from scission of covalent bonds, the

polymeric ligand must have a molecular weight that is lower than Mlim for covalent bond

scission. Therefore, the limiting molecular weight of covalent pTHF was determined in a

separate set of experiments. EtIm-‐pTHF samples with Mw ranging between 8 and 50 kDa

were subjected to ultrasound at 20 kHz with a power density of 15.4 W/cm2 in toluene

solutions (10 mg/ml) under CH4. Internal temperature of the double-‐jacketed sonication

vessel was constant at 25oC (±2 oC) throughout sonication.

GPC traces for polymer samples before and after sonication showed that the molecular

weight distribution for EtIm-‐pTHF below 25 kDa did not change significantly by sonication.

However, polymer with a MW higher than 30 kDa degraded irreversibly as shown by both

GPC (Figure 4) and MALDI-‐TOF (Figure 5). This sets a limit (Mlim) to the MW of force-‐

accumulating ligands (EtIm-‐pTHF) below which they are not destroyed by ultrasound. Thus,

only ligands with a molecular weight lower than Mlim for pTHF (<30kDa) were used to

synthesize mechano-‐responsive polymers with a central Pd-‐NHC mechanophore.

Figure 4: GPC traces for polymer samples of EtIm-‐pTHF taken before and after sonication. 10 mg/ml

samples were sonicated (continuous) in toluene under CH4. Aliquots were taken and solvent was

evaporated, residue was dissolved in THF and directly submitted to GPC. Black and red lines

represented traces for aliquots taken before and after sonication samples. Curves were normalized

to peak areas.

Figure 5: MALDI-‐TOF spectra for EtIm-‐pTHF50k before (left) and after (right) 1h of sonication.

The GPC trace of Pd(NHC-‐pTHF8k)2Cl2 did not change after 1 h of continuous sonication

under CH4 in the presence of acetonitrile and acetic acid as trapping agents (Figure 6).

However, higher molecular weight complex Pd(NHC-‐pTHF25k)2Cl2 showed around %80

chain scission after 1h of continuous sonication. Mechanochemical fragmentation of

Pd(NHC-‐pTHF25k)2Cl2 revealed a product with a sharp peak corresponding to MW of 25 kDa,

that is consistent with the previously reported center specific chain scission for

mechanophore incorporated polymers.32 This observation confirms that Mlim for Pd(NHC-‐

pTHF)Cl2 complex is higher than 16 kDa under above mentioned conditions and central

coordination bond remains intact when polymer chain is not sufficiently long.


31

2

Figure 6: GPC traces for polymer samples taken before and after sonication. a) EtIm-‐pTHF18k, b)

EtIm-‐pTHF50k, c) Pd(NHC-‐pTHF8k)2Cl2 and d) Pd(NHC-‐pTHF25k)2Cl2. Palladium containing polymers,

Pd(NHC-‐pTHF)2Cl2, were sonicated in the presence of trapping agents.

Scission Rates for M(NHC-‐pTHF)2Cl2

The rate of mechanically induced scission in covalent polymers has been shown to be

proportional to [MW -‐ Mlim] when the initial molecular weight is greater than Mlim32,33 When

such a molecular weight dependence is observed, thermal decomposition can be excluded,

because it proves that bond rupture process is mechano-‐chemical and not the result of local

heating or reaction with solvent decomposition products.34 The increase of scission rate

with increasing molecular weight is caused by the larger accumulated elongational stress at

the center of long polymer chains, in combination with their longer relaxation time, which

reduces the rate of stress relaxation by chain motion.35

Figure 7: Real data for GPC traces and deconvolution curves represented with black and gray lines

respectively for initial MW distribution of (a) Pd(NHC-‐pTHF25k)2Cl2 and (b) Pt(NHC-‐pTHF25k)2Cl2.

Molecular weight dependence of the mechanochemical scission rate for M(NHC-‐pTHF)2Cl2

complexes were determined by sonicating samples separately. Molecular weight

distributions for collected aliquots throughout sonication were determined by GPC in THF

and peak areas were calculated by deconvolution of traces with a bimodal molecular weight

distribution (Figure 7). The concentration change of polymer complex during sonication was

estimated under the assumption that peak area is proportional to weight fraction in RI

detection. The scission rates of the Pd and Pt complexes were fitted with equation 3,

assuming first order reaction kinetics since chain scission is selective to chain mid-‐point.

𝑃𝑃 𝑥𝑥 → 2𝑃𝑃 𝑥𝑥 2 (1)

− ! ! !!"

= 𝑘𝑘!"[𝑃𝑃 𝑥𝑥 ] (2)

where P(x) is initial polymer and P(x/2) is the fragmentation product. Thus, degradation follows first-‐order kinetics and weight fraction of starting material C1 decays exponentially with the equation;

𝐶𝐶! 𝑡𝑡 = 𝐶𝐶! 0 ×𝑒𝑒 !!!"! (3)

where, t is sonication time, C1(0) is initial weight fraction of starting material, C1(t) is weight fraction of starting material at any time during sonication and ksc is mechanochemical scission rate coefficient.

Chapter 2

32

Figure 6: GPC traces for polymer samples taken before and after sonication. a) EtIm-‐pTHF18k, b)

EtIm-‐pTHF50k, c) Pd(NHC-‐pTHF8k)2Cl2 and d) Pd(NHC-‐pTHF25k)2Cl2. Palladium containing polymers,

Pd(NHC-‐pTHF)2Cl2, were sonicated in the presence of trapping agents.

Scission Rates for M(NHC-‐pTHF)2Cl2

The rate of mechanically induced scission in covalent polymers has been shown to be

proportional to [MW -‐ Mlim] when the initial molecular weight is greater than Mlim32,33 When

such a molecular weight dependence is observed, thermal decomposition can be excluded,

because it proves that bond rupture process is mechano-‐chemical and not the result of local

heating or reaction with solvent decomposition products.34 The increase of scission rate

with increasing molecular weight is caused by the larger accumulated elongational stress at

the center of long polymer chains, in combination with their longer relaxation time, which

reduces the rate of stress relaxation by chain motion.35

Figure 7: Real data for GPC traces and deconvolution curves represented with black and gray lines

respectively for initial MW distribution of (a) Pd(NHC-‐pTHF25k)2Cl2 and (b) Pt(NHC-‐pTHF25k)2Cl2.

Molecular weight dependence of the mechanochemical scission rate for M(NHC-‐pTHF)2Cl2

complexes were determined by sonicating samples separately. Molecular weight

distributions for collected aliquots throughout sonication were determined by GPC in THF

and peak areas were calculated by deconvolution of traces with a bimodal molecular weight

distribution (Figure 7). The concentration change of polymer complex during sonication was

estimated under the assumption that peak area is proportional to weight fraction in RI

detection. The scission rates of the Pd and Pt complexes were fitted with equation 3,

assuming first order reaction kinetics since chain scission is selective to chain mid-‐point.

𝑃𝑃 𝑥𝑥 → 2𝑃𝑃 𝑥𝑥 2 (1)

− ! ! !!"

= 𝑘𝑘!"[𝑃𝑃 𝑥𝑥 ] (2)

where P(x) is initial polymer and P(x/2) is the fragmentation product. Thus, degradation follows first-‐order kinetics and weight fraction of starting material C1 decays exponentially with the equation;

𝐶𝐶! 𝑡𝑡 = 𝐶𝐶! 0 ×𝑒𝑒 !!!"! (3)

where, t is sonication time, C1(0) is initial weight fraction of starting material, C1(t) is weight fraction of starting material at any time during sonication and ksc is mechanochemical scission rate coefficient.


33

2

Figure 8: Change in initial polymer concentration vs. sonication time for M(NHC-‐pTHF18k)2Cl2 (M:

Pd, and Pt).

M(NHC-‐pTHF)2Cl2 samples were sonicated separately and their scission rate constants (ksc)

were determined by exponential fitting. The ksc vs MW plot in Figure 8 shows that chain

scission in Pt containing polymers is slower than in Pd containing counterparts for each

initial MW. Although the calculated ksc’s are linearly dependent on MW for both metals, the

slopes of the lines are different. Madras et al. used the following equation to express ksc as a

function of MW:35

𝑘𝑘!" = 𝑘𝑘! (𝑀𝑀𝑀𝑀 − 𝑀𝑀!"#)ƛ (4)

In which 𝑘𝑘! is an empirical proportionality constant. In the Pd and Pt complexes, the

molecular weight dependent scission rates fit well to equation 4 with exponent λ equal to 1.

Mlim values for both mechanophores were estimated from linear extrapolation of the plots

of ksc vs MW (Figure 10) as 20 kDa and 22 kDa for Pd(NHC-‐pTHF)2Cl2 and for Pt(NHC-‐

pTHF)2Cl2 (Table 2) respectively.

Table 2: kd and Mlim values for Pd(NHC-‐pTHF)2Cl2, Pt(NHC-‐pTHF)2Cl2 and EtIm-‐pTHF

Polymer kd (mol x g-‐1 x min-‐1) Mlim (kDa)

Pd(NHC-‐pTHF)2Cl2 7.42×10!! ±3.63×10!! 20

Pt(NHC-‐pTHF)2Cl2 5.27×10!! ±2.23×10!! 22

EtIm-‐pTHF 3.12×10!! ±5.54×10!! 29

kd is the slope and Mlim is the intercept in the plot of ksc vs. MW

The different 𝑘𝑘! values for the Pd and Pt complexes are in contrast to the observed MW

dependencies for covalent bond scission of isomers of cyclobutane mechanophores,

reported by Moore et al (Figure 9).36 They observed that calculated Fmax (the maximum force

imposed on the model mechanophores for bond rupture to occur) and experimentally

determined Mlim values differ significantly for different mechanophores. However, slopes in

a plot of ksc vs MW are identical for all regardless of mechanophore’s chemical structure.

Figure 9: Structures of cis and trans dicyano-‐substituted cyclobutanes (DCC and DCT), cis and trans

monocyano-‐substituted cyclobutanes (MCC and MCT), and cis and trans cyclobutanes without cyano

substituents (NCC and NCT). R represents a poly(methylacrylate) chain. The table summarizes the

Fmax and Mlim values for related mechanophores. Adapted from ref 36.

Chapter 2

34

Figure 8: Change in initial polymer concentration vs. sonication time for M(NHC-‐pTHF18k)2Cl2 (M:

Pd, and Pt).

M(NHC-‐pTHF)2Cl2 samples were sonicated separately and their scission rate constants (ksc)

were determined by exponential fitting. The ksc vs MW plot in Figure 8 shows that chain

scission in Pt containing polymers is slower than in Pd containing counterparts for each

initial MW. Although the calculated ksc’s are linearly dependent on MW for both metals, the

slopes of the lines are different. Madras et al. used the following equation to express ksc as a

function of MW:35

𝑘𝑘!" = 𝑘𝑘! (𝑀𝑀𝑀𝑀 − 𝑀𝑀!"#)ƛ (4)

In which 𝑘𝑘! is an empirical proportionality constant. In the Pd and Pt complexes, the

molecular weight dependent scission rates fit well to equation 4 with exponent λ equal to 1.

Mlim values for both mechanophores were estimated from linear extrapolation of the plots

of ksc vs MW (Figure 10) as 20 kDa and 22 kDa for Pd(NHC-‐pTHF)2Cl2 and for Pt(NHC-‐

pTHF)2Cl2 (Table 2) respectively.

Table 2: kd and Mlim values for Pd(NHC-‐pTHF)2Cl2, Pt(NHC-‐pTHF)2Cl2 and EtIm-‐pTHF

Polymer kd (mol x g-‐1 x min-‐1) Mlim (kDa)

Pd(NHC-‐pTHF)2Cl2 7.42×10!! ±3.63×10!! 20

Pt(NHC-‐pTHF)2Cl2 5.27×10!! ±2.23×10!! 22

EtIm-‐pTHF 3.12×10!! ±5.54×10!! 29

kd is the slope and Mlim is the intercept in the plot of ksc vs. MW

The different 𝑘𝑘! values for the Pd and Pt complexes are in contrast to the observed MW

dependencies for covalent bond scission of isomers of cyclobutane mechanophores,

reported by Moore et al (Figure 9).36 They observed that calculated Fmax (the maximum force

imposed on the model mechanophores for bond rupture to occur) and experimentally

determined Mlim values differ significantly for different mechanophores. However, slopes in

a plot of ksc vs MW are identical for all regardless of mechanophore’s chemical structure.

Figure 9: Structures of cis and trans dicyano-‐substituted cyclobutanes (DCC and DCT), cis and trans

monocyano-‐substituted cyclobutanes (MCC and MCT), and cis and trans cyclobutanes without cyano

substituents (NCC and NCT). R represents a poly(methylacrylate) chain. The table summarizes the

Fmax and Mlim values for related mechanophores. Adapted from ref 36.


35

2

Figure 10: Molecular weight dependence of scission rates (ksc) for coordination polymers M(NHC-‐

pTHF)2Cl2 (M:Pd and Pt), and covalent polymer (EtIm-‐pTHF)Cl.

In order to verify the observed difference in kd and Mlim between Pd and Pt containing

polymers, we have first calculated the bond dissociation energy (BDE) for M-‐NHC bond for

Pd and Pt using DFT method. The PBE0 hybrid functional that accounts for 25% of Hartree-‐

Fock exchange has been employed together with the TZVP basis set reported by Ahlrichs et.

al. 37,38 Data represents the reaction ML2 → LM + L where ligand (L) was chosen as EtMetIm

for structural simplicity. BDEs for M-‐ligand coordination bonds were calculated as 195

kJ/mol and 245 kJ/mol for Pd-‐NHC and Pt-‐NHC respectively. Energies mentioned here is the

energy difference between coordinated and dissociated states. This indicates that the

energy needed to remove one NHC ligand coordinated to metal in M(NHC-‐pTHF)2Cl2

complexes is significantly lower than C-‐C or C-‐O bond dissociation energies (>350 kJ/mol).39

Therefore, the coordination bond in M-‐NHC incorporated polymers is the weakest bond on

the chain backbone and breaks selectively during sonication.

Mlim for Pd and Pt containing M(NHC-‐pTHF)2Cl2 complexes are higher than the one

previously reported for Ag(NHC-‐pTHF)2PF6 complex (<13 kDa33). That is also consistent with

the BDE for Ag-‐NHC bond, which was calculated as 141 kJ/mol using the same method as for

Pd and Pt.

Determining of BDEs for the complexes in concern showed that the higher BDE corresponds

to a higher Mlim. However, it does not explain the difference in the molecular weight

dependence of scission rates, i.e. the difference in slopes (𝑘𝑘!) in Figure 10.

An explanation of the difference in 𝑘𝑘! values for coordinative bonds versus covalent bonds,

and between different coordinative bonds must be sought in the relation between force and

scission rate, which is expressed in its most simple form as the Bell-‐Evans equation:40,41

𝑘𝑘 𝐹𝐹 = 𝑘𝑘!𝑒𝑒!∆!‡/!!!

This equation expresses the fact that the force dependence of scission rate k(F) can be very

different for even for bonds with equal k0 when there is a difference between the values of

Δx‡, the change in bond length between the force free equilibrium bond length, and the

transition state bond length under force. Given the differences in equilibrium bond lengths

and the shape of the potential well for coordinative bonds and covalent (C-‐C) bonds,

differences in the slope of the plots in Figure 10 are not surprising. Similar trend in force-‐

rate relationship was reported previously in literature as shown in Figure 11.42

Figure 11: Measured (points) and calculated (lines) rate–force correlations of three reactions.42

In order to determine the Fmax dependence of Mlim for M(NHC-‐pTHF)2Cl2 complexes the

pulling force was implemented by the COGEF method, where constrained geometries are

used to simulate the presence of a pulling force.43 Distance between carbene carbons of

different NHCs coordinated to the same Pd center (C1 and C2) was increased sequentially

and the energy of the system was calculated. BDE, in this case, was calculated as the relative

energy at very high-‐constrained distance (Figure 12a). Fmax values were calculated from the

derivative of relative energy vs constrained distance curve (Figure 12b).

Chapter 2

36

Figure 10: Molecular weight dependence of scission rates (ksc) for coordination polymers M(NHC-‐

pTHF)2Cl2 (M:Pd and Pt), and covalent polymer (EtIm-‐pTHF)Cl.

In order to verify the observed difference in kd and Mlim between Pd and Pt containing

polymers, we have first calculated the bond dissociation energy (BDE) for M-‐NHC bond for

Pd and Pt using DFT method. The PBE0 hybrid functional that accounts for 25% of Hartree-‐

Fock exchange has been employed together with the TZVP basis set reported by Ahlrichs et.

al. 37,38 Data represents the reaction ML2 → LM + L where ligand (L) was chosen as EtMetIm

for structural simplicity. BDEs for M-‐ligand coordination bonds were calculated as 195

kJ/mol and 245 kJ/mol for Pd-‐NHC and Pt-‐NHC respectively. Energies mentioned here is the

energy difference between coordinated and dissociated states. This indicates that the

energy needed to remove one NHC ligand coordinated to metal in M(NHC-‐pTHF)2Cl2

complexes is significantly lower than C-‐C or C-‐O bond dissociation energies (>350 kJ/mol).39

Therefore, the coordination bond in M-‐NHC incorporated polymers is the weakest bond on

the chain backbone and breaks selectively during sonication.

Mlim for Pd and Pt containing M(NHC-‐pTHF)2Cl2 complexes are higher than the one

previously reported for Ag(NHC-‐pTHF)2PF6 complex (<13 kDa33). That is also consistent with

the BDE for Ag-‐NHC bond, which was calculated as 141 kJ/mol using the same method as for

Pd and Pt.

Determining of BDEs for the complexes in concern showed that the higher BDE corresponds

to a higher Mlim. However, it does not explain the difference in the molecular weight

dependence of scission rates, i.e. the difference in slopes (𝑘𝑘!) in Figure 10.

An explanation of the difference in 𝑘𝑘! values for coordinative bonds versus covalent bonds,

and between different coordinative bonds must be sought in the relation between force and

scission rate, which is expressed in its most simple form as the Bell-‐Evans equation:40,41

𝑘𝑘 𝐹𝐹 = 𝑘𝑘!𝑒𝑒!∆!‡/!!!

This equation expresses the fact that the force dependence of scission rate k(F) can be very

different for even for bonds with equal k0 when there is a difference between the values of

Δx‡, the change in bond length between the force free equilibrium bond length, and the

transition state bond length under force. Given the differences in equilibrium bond lengths

and the shape of the potential well for coordinative bonds and covalent (C-‐C) bonds,

differences in the slope of the plots in Figure 10 are not surprising. Similar trend in force-‐

rate relationship was reported previously in literature as shown in Figure 11.42

Figure 11: Measured (points) and calculated (lines) rate–force correlations of three reactions.42

In order to determine the Fmax dependence of Mlim for M(NHC-‐pTHF)2Cl2 complexes the

pulling force was implemented by the COGEF method, where constrained geometries are

used to simulate the presence of a pulling force.43 Distance between carbene carbons of

different NHCs coordinated to the same Pd center (C1 and C2) was increased sequentially

and the energy of the system was calculated. BDE, in this case, was calculated as the relative

energy at very high-‐constrained distance (Figure 12a). Fmax values were calculated from the

derivative of relative energy vs constrained distance curve (Figure 12b).


37

2

Table 3: BDE, Mlim, Fmax for coordination polymers with Ag, Pd, Pt and covalent polymer pTHF.

Polymer Bond ∆E (kJ/mol) Mlim (kDa) Fmax (nN)

Ag(NHC-‐pTHF)2PF6 Ag-‐Ca,b 141 >13 0.5

Pd(NHC-‐pTHF)2Cl2 Pd-‐Ca 195 20 3.0

Pt(NHC-‐pTHF)2Cl2 Pt-‐Ca 245 22 4.2

pTHF C-‐Cc

C-‐Oc

370

344 29d

6.9

7.6 a C represents the carbene carbon of NHC that is coordinated to the metal center bref 25 cref 41 dthis work

As summarized in Table 3, application of an external force of 2.9 nN and 4.2 nN would be

sufficient to achieve scission of the Pd–C and Pt-‐C bonds in M-‐NHC complexes respectively

at room temperature. Since these values are significantly lower than the force required to

break bonds on pTHF chain,43 M-‐ligand bonds are the most susceptible bonds on polymer

backbone to mechanical rupture. Mlim values, which were found experimentally for Pd(NHC-‐

pTHF)2Cl2, Pt(NHC-‐pTHF)2Cl2 and pTHF are in good agreement with the trend in calculated

BDE and Fmax values.

Figure 12: Chemical structure of Pd(NHC)2Cl2 that was used in calculations and change in relative

energy (a) and force (b) as a function of distance between NHC-‐Pd. Arrows on chemical structure

indicate the pulling direction.

Chain scission mechanism

In the previous section COGEF calculations were done with the constraint C1-‐Pd-‐C2 distance

as constraint. In order to understand the chain scission mechanism, however, a second

series of simulations was carried out. In mechanochemical experiments, tensile force is

transduced through the polymer chain to the central mechanophore, and the applied force

is orthogonal to C-‐Pd bond (Figure 14c). Therefore, in the calculations, pulling was also

simulated as orthogonal to NHC-‐Pd coordination bond. Ethyl groups were used instead of

long pTHF chains to reduce computational effort. The distance between two terminal

carbon atoms on different NHCs coordinated to the same Pd center was increased

sequentially from 8 to 12 Å. (Figure 13).

Chapter 2

38

Table 3: BDE, Mlim, Fmax for coordination polymers with Ag, Pd, Pt and covalent polymer pTHF.

Polymer Bond ∆E (kJ/mol) Mlim (kDa) Fmax (nN)

Ag(NHC-‐pTHF)2PF6 Ag-‐Ca,b 141 >13 0.5

Pd(NHC-‐pTHF)2Cl2 Pd-‐Ca 195 20 3.0

Pt(NHC-‐pTHF)2Cl2 Pt-‐Ca 245 22 4.2

pTHF C-‐Cc

C-‐Oc

370

344 29d

6.9

7.6 a C represents the carbene carbon of NHC that is coordinated to the metal center bref 25 cref 41 dthis work

As summarized in Table 3, application of an external force of 2.9 nN and 4.2 nN would be

sufficient to achieve scission of the Pd–C and Pt-‐C bonds in M-‐NHC complexes respectively

at room temperature. Since these values are significantly lower than the force required to

break bonds on pTHF chain,43 M-‐ligand bonds are the most susceptible bonds on polymer

backbone to mechanical rupture. Mlim values, which were found experimentally for Pd(NHC-‐

pTHF)2Cl2, Pt(NHC-‐pTHF)2Cl2 and pTHF are in good agreement with the trend in calculated

BDE and Fmax values.

Figure 12: Chemical structure of Pd(NHC)2Cl2 that was used in calculations and change in relative

energy (a) and force (b) as a function of distance between NHC-‐Pd. Arrows on chemical structure

indicate the pulling direction.

Chain scission mechanism

In the previous section COGEF calculations were done with the constraint C1-‐Pd-‐C2 distance

as constraint. In order to understand the chain scission mechanism, however, a second

series of simulations was carried out. In mechanochemical experiments, tensile force is

transduced through the polymer chain to the central mechanophore, and the applied force

is orthogonal to C-‐Pd bond (Figure 14c). Therefore, in the calculations, pulling was also

simulated as orthogonal to NHC-‐Pd coordination bond. Ethyl groups were used instead of

long pTHF chains to reduce computational effort. The distance between two terminal

carbon atoms on different NHCs coordinated to the same Pd center was increased

sequentially from 8 to 12 Å. (Figure 13).


39

2

Figure 13: Energy optimized structures for Pd(NHC)2Cl2 while increasing the constrained distance

between the atoms indicated in black.

On the calculated curve of the energy vs. distance (Figure 14) three characteristic regions

can be seen. i) Small-‐constrained distance between the pulling ends. There is only a small

increase of the energy due to the relatively small torsional angle constraints. Here, the

distance between the central Pd and the carbon atoms does not increase.

ii) The strain in the system is sufficient to stretch the Pd-‐C bonds. This is observed as the

steep increase of the energy until the cleavage point. At this stage, the NHC and ethyl

groups lay in the same plane, and all strain imposed on the system is converted to

stretching of the Pd-‐C bond. The complex is still symmetric at this point and the distances

C1-‐Pd and C2-‐Pd are equal.

iii) At a constrained distance between terminal atoms of the complex (11.6 Å), the

symmetry of the system is broken and Pd settles much closer to the C2 atom.

Figure 14: a) Change in relative energy and b) distance between NHC-‐Pd as a function of constrained

distance. c) Chemical structure of Pd(NHC)2Cl2 that was used in calculations. d) Idealized potential

energy surface (PES) of the investigated system along the Pd-‐C1 and Pd-‐C2 bond length coordinates.

The geometry of the system has been frozen during the simulations, and the eventual distortion of

the imidazole rings is not included in the results. Arrows on chemical structure indicate the pulling

direction.

The C1-‐Pd-‐C2 bridge stretching proceeds via the path that is shown in Figure 14d. Both C-‐Pd

bonds stretch at the same rate and the energy rises along the steepest, diagonal path. Once

the energy rises above a threshold value, it is more energetically favorable to split along one

of the two directions to give C1-‐Pd or C2-‐Pd bond cleavage.

Comparison with calculations for silver carbene complex shows that scission of the Pd-‐NHC

has a higher activation barrier than scission of Ag-‐NHC bond. This explains the significant

difference in Mlims to break Ag-‐NHC and Pd-‐NHC bonds. Additionally, when the distance

between two terminal atoms increases, the most labile bond (Pd-‐C coordination bond)

breaks eventually on the chain.

Chapter 2

40

Figure 13: Energy optimized structures for Pd(NHC)2Cl2 while increasing the constrained distance

between the atoms indicated in black.

On the calculated curve of the energy vs. distance (Figure 14) three characteristic regions

can be seen. i) Small-‐constrained distance between the pulling ends. There is only a small

increase of the energy due to the relatively small torsional angle constraints. Here, the

distance between the central Pd and the carbon atoms does not increase.

ii) The strain in the system is sufficient to stretch the Pd-‐C bonds. This is observed as the

steep increase of the energy until the cleavage point. At this stage, the NHC and ethyl

groups lay in the same plane, and all strain imposed on the system is converted to

stretching of the Pd-‐C bond. The complex is still symmetric at this point and the distances

C1-‐Pd and C2-‐Pd are equal.

iii) At a constrained distance between terminal atoms of the complex (11.6 Å), the

symmetry of the system is broken and Pd settles much closer to the C2 atom.

Figure 14: a) Change in relative energy and b) distance between NHC-‐Pd as a function of constrained

distance. c) Chemical structure of Pd(NHC)2Cl2 that was used in calculations. d) Idealized potential

energy surface (PES) of the investigated system along the Pd-‐C1 and Pd-‐C2 bond length coordinates.

The geometry of the system has been frozen during the simulations, and the eventual distortion of

the imidazole rings is not included in the results. Arrows on chemical structure indicate the pulling

direction.

The C1-‐Pd-‐C2 bridge stretching proceeds via the path that is shown in Figure 14d. Both C-‐Pd

bonds stretch at the same rate and the energy rises along the steepest, diagonal path. Once

the energy rises above a threshold value, it is more energetically favorable to split along one

of the two directions to give C1-‐Pd or C2-‐Pd bond cleavage.

Comparison with calculations for silver carbene complex shows that scission of the Pd-‐NHC

has a higher activation barrier than scission of Ag-‐NHC bond. This explains the significant

difference in Mlims to break Ag-‐NHC and Pd-‐NHC bonds. Additionally, when the distance

between two terminal atoms increases, the most labile bond (Pd-‐C coordination bond)

breaks eventually on the chain.


41

2

Fmax was calculated for pulling on the terminal carbon atoms of the ethyl groups as 2.2 nN

that is significantly lower than that for pulling directly from carbene carbons (Figure 15). The

lower number indicates that although the trend in Table 3 is realistic, however, Fmax values

obtained from pulling on C1 and C2 are upper limits to break NHC-‐Pd bond mechanically and

overestimate the actual value of the force needed to break this bond.

Figure 15: Force as a function of distance between terminal atoms on the ethyl groups. Arrows on

chemical structure indicate the pulling direction. M:Pd.

Conclusions

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 complexes were investigated in

toluene. Reversibility of chain scission depends on the nature of gas molecules dissolved in

toluene and scission is reversible when the polymer complex is sonicated under CH4. Mlim for

Pd(NHC-‐pTHF)2Cl2 was calculated from the graph of ksc vs Mw graph as approximately 20

kDa. Critical molecular weight for the complex (Mc) determined as 60 kDa that is twice as

high as Mlim for EtIm-‐pTHF. Above Mc ultrasound induced degradation of ligands due to C-‐C

bond scission would prevent reversibility. 16 kDa complex Pd(NHC-‐pTHF18k)2Cl2 and small

molecule model complex Pd(NHC-‐ButEtIm)2Cl2 were not affected by sonication confirming

the stability of complexes when the force is lower than the threshold value to break Pd-‐NHC

bond. Scission rate is dependent on initial molecular weight of the polymer and this

indicates that chain scission has a true mechanical nature. Constrained geometry

optimization (COGEF) calculations showed that distance between NHC and Pd increases for

both NHCs coordinated to metal center and one of them breaks upon stepwise increase in

constrained distance. The results of the calculations also showed the importance of

choosing attachment points when determining the value of Fmax. This is also in agreement

with the findings on attachment point-‐reactivity relation reported by Craig et al.44–46

Results confirmed that ligands can be released from thermally stable Pd-‐NHC complexes

mechanically to produce free NHC and coordinatively unsaturated metal center, and further

use of these reactive mechano-‐chemical products is of interest.

Chapter 2

42

Fmax was calculated for pulling on the terminal carbon atoms of the ethyl groups as 2.2 nN

that is significantly lower than that for pulling directly from carbene carbons (Figure 15). The

lower number indicates that although the trend in Table 3 is realistic, however, Fmax values

obtained from pulling on C1 and C2 are upper limits to break NHC-‐Pd bond mechanically and

overestimate the actual value of the force needed to break this bond.

Figure 15: Force as a function of distance between terminal atoms on the ethyl groups. Arrows on

chemical structure indicate the pulling direction. M:Pd.

Conclusions

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 complexes were investigated in

toluene. Reversibility of chain scission depends on the nature of gas molecules dissolved in

toluene and scission is reversible when the polymer complex is sonicated under CH4. Mlim for

Pd(NHC-‐pTHF)2Cl2 was calculated from the graph of ksc vs Mw graph as approximately 20

kDa. Critical molecular weight for the complex (Mc) determined as 60 kDa that is twice as

high as Mlim for EtIm-‐pTHF. Above Mc ultrasound induced degradation of ligands due to C-‐C

bond scission would prevent reversibility. 16 kDa complex Pd(NHC-‐pTHF18k)2Cl2 and small

molecule model complex Pd(NHC-‐ButEtIm)2Cl2 were not affected by sonication confirming

the stability of complexes when the force is lower than the threshold value to break Pd-‐NHC

bond. Scission rate is dependent on initial molecular weight of the polymer and this

indicates that chain scission has a true mechanical nature. Constrained geometry

optimization (COGEF) calculations showed that distance between NHC and Pd increases for

both NHCs coordinated to metal center and one of them breaks upon stepwise increase in

constrained distance. The results of the calculations also showed the importance of

choosing attachment points when determining the value of Fmax. This is also in agreement

with the findings on attachment point-‐reactivity relation reported by Craig et al.44–46

Results confirmed that ligands can be released from thermally stable Pd-‐NHC complexes

mechanically to produce free NHC and coordinatively unsaturated metal center, and further

use of these reactive mechano-‐chemical products is of interest.


43

2

Experimental

General

All chemicals were purchased from commercial sources used without further purification, unless

specified otherwise. Dry tetrahydrofuran (THF, HPLC grade) was degassed with argon and purified by

passage through activated alumina solvent column prior to use. A Varian 400MR or a Varian Mercury

400 spectrometer was used to record 1H NMR (400 MHz) Chemical shifts are reported in ppm and

referenced to tetramethylsilane or solvent. Gel permeation chromatography (GPC) was performed

on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and MIXEDD columns

(linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector at 254 nm and

RID-‐10A refractive index detector, and THF as eluent at a flow rate of 1 mL/min (20 °C). Polystyrene

standards were used for calibration.

Sonication experiments

A homemade, double-‐jacketed glass reactor with a volume of 10 mL was used in the sonication

experiments. A Sonics and Materials 20 kHz, 0.5 in. diameter titanium alloy ultrasound probe with

half wave extension (parts 630-‐0220 and 630-‐0410) was operated using a Sonics and Materials

VC750 power supply. The temperature in the reactor was maintained with a Lauda E300 cooling

bath and measured using a 0.5 mm diameter thermocouple. Solutions were sonicated continuously,

temperature of the solution was checked by thermocouple and it was constant at 20oC after the

thermal equilibrium was achieved in the first 3-‐5 mins. During sonication saturation a gas (Ar or CH4)

was bubbled through solution via teflon tubing. Aliquots of 100 µL were taken at different time

intervals. Toluene was removed under reduced pressure and residues were dissolved in THF and

submitted to GPC. GPC results were analyzed and scission kinetics was determined by double

Gaussian de-‐convolution method.

Synthesis of α-‐(N-‐ethylimidazolium)-‐ω-‐methoxy poly(tetrahydrofuran)

Polymer salts precursor to ligand NHC-‐pTHF were synthesized via cationic ring-‐opening

polymerization of tetrahydrofuran (THF).47 THF (100 mL) and DTBP (200 µL, 0.92 mmol) were added

methyl triflate (100 µL, 0.91 mmol) inside a Schlenk round-‐bottom flask under Ar to initiate the

polymerization. After stirring for defined time (Figure S1), the polymerization was terminated by N-‐

ethylimidazole (200 µL, ca. 2.1 mmol). After 20 mins, solution was diluted to app ¼ of its initial

volume and precipitated in water (400 mL) overnight at ambient temperature. White polymer was

washed with water, dissolved in diethyl ether (200 mL), dried over MgSO4 and precipitated overnight

at –30 °C, white powder was filtered washed with cold Et2O and yielded ligands as white powder. Ion

exchange of the anion to chloride was carried out by stirring the polymer with Dowex® exchange

resin in methanol for 2–3 hours. Then, the resin was removed by filtration and the methanol was

evaporated in vacuo and the residue was precipitated in Et2O at -‐30oC again. In order to remove

traces of solvents, ligands were left under vacuum at ambient temperature overnight prior to use. 1H

NMR [EtIm-‐pTHF12kCl CD2Cl2, 400 MHz]: 11 ppm (s, NHC), 7.2 ppm (d, NHC), 4.4 ppm (t, N-‐CH2) 3.0-‐

3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2).

Synthesis of Pd(II)–NHC polymer complexes Pd(NHC-‐pTHF)2Cl2.

EtIm-‐pTHF (400 mg) was dissolved in THF (10 ml) and stirred over 4A molecular sieves for 30 mins

under Ar. NatOBu (2.5 eq.) was added in one portion and solution stirred for another 30 Å mins.

Then, Pd(PhCN)2Cl2 (0.5 eq.) was added and ligand exchange yielded desired polymer attached

mechanophores. Crude products were filtered over alumina (eluent: THF), and concentrated under

reduced pressure. Then, the complex was dissolved in DCM washed with water and dried over

MgSO4. Solution containing Pd(NHC-‐pTHF)2Cl2 in DCM passed through Celite. Solvent was

evaporated under reduced pressure and light yellow polymer was left under vacuum overnight to

remove all residual solvents. 1H NMR [Pd(NHC-‐pTHF12k)2Cl2 CD2Cl2, 400 MHz]: 6.9 ppm (s, NHC), 4.5

ppm (t, N-‐CH2), 3.0-‐3.5 ppm (br, O-‐CH2-‐), 1.1-‐1.9 ppm (br, OCH2-‐CH2-‐).

Synthesis of model complex Pd(NHC-‐EtBut)2Cl2

Ethyl imidazole (480 mg, 5 mmol) and iodobutane (1840 mg, 10 mmol) were refluxed in dry THF (25

ml) overnight. Then, two-‐phase liquid reaction mixture was cooled down to room temperature and

upper transparent part was removed with glass pipette. Yellow oily product was washed with THF

(10 ml x 3) and then residue was dissolved in MeOH (10 ml). Ion exchange resin was added to

exchange iodine counterions with chlorine. After stirring 2h at RT mixture was filtered over a filter

paper, solvent was evaporated under reduced pressure and light yellow oil was left under vacuum

overnight.

EtButIm (100 mg, 0.53 mmol), PdCl2 (46 mg, 0.26 mmol) and Cs2CO3 (814.5 mg, 2.5 mmol) were

dissolved in Dioxane (4.4 mL) and the reaction mixture was heated to 80 °C. After 5 hours, it was

allowed to cool down to room temperature. Volatiles were removed under reduced pressure.

Purification by column chromatography (CH2Cl2) afforded Pd(NHC-‐EtBut)2Cl2 as a light yellow solid.

Chapter 2

44

Experimental

General



passage through activated alumina solvent column prior to use. A Varian 400MR or a Varian Mercury

400 spectrometer was used to record 1H NMR (400 MHz) Chemical shifts are reported in ppm and

referenced to tetramethylsilane or solvent. Gel permeation chromatography (GPC) was performed

on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and MIXEDD columns

(linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector at 254 nm and

RID-‐10A refractive index detector, and THF as eluent at a flow rate of 1 mL/min (20 °C). Polystyrene

standards were used for calibration.







temperature of the solution was checked by thermocouple and it was constant at 20oC after the

thermal equilibrium was achieved in the first 3-‐5 mins. During sonication saturation a gas (Ar or CH4)

was bubbled through solution via teflon tubing. Aliquots of 100 µL were taken at different time

intervals. Toluene was removed under reduced pressure and residues were dissolved in THF and

submitted to GPC. GPC results were analyzed and scission kinetics was determined by double


Synthesis of α-‐(N-‐ethylimidazolium)-‐ω-‐methoxy poly(tetrahydrofuran)

Polymer salts precursor to ligand NHC-‐pTHF were synthesized via cationic ring-‐opening

polymerization of tetrahydrofuran (THF).47 THF (100 mL) and DTBP (200 µL, 0.92 mmol) were added

methyl triflate (100 µL, 0.91 mmol) inside a Schlenk round-‐bottom flask under Ar to initiate the

polymerization. After stirring for defined time (Figure S1), the polymerization was terminated by N-‐

ethylimidazole (200 µL, ca. 2.1 mmol). After 20 mins, solution was diluted to app ¼ of its initial

volume and precipitated in water (400 mL) overnight at ambient temperature. White polymer was

washed with water, dissolved in diethyl ether (200 mL), dried over MgSO4 and precipitated overnight

at –30 °C, white powder was filtered washed with cold Et2O and yielded ligands as white powder. Ion

exchange of the anion to chloride was carried out by stirring the polymer with Dowex® exchange

resin in methanol for 2–3 hours. Then, the resin was removed by filtration and the methanol was

evaporated in vacuo and the residue was precipitated in Et2O at -‐30oC again. In order to remove

traces of solvents, ligands were left under vacuum at ambient temperature overnight prior to use. 1H

NMR [EtIm-‐pTHF12kCl CD2Cl2, 400 MHz]: 11 ppm (s, NHC), 7.2 ppm (d, NHC), 4.4 ppm (t, N-‐CH2) 3.0-‐

3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2).

Synthesis of Pd(II)–NHC polymer complexes Pd(NHC-‐pTHF)2Cl2.

EtIm-‐pTHF (400 mg) was dissolved in THF (10 ml) and stirred over 4A molecular sieves for 30 mins

under Ar. NatOBu (2.5 eq.) was added in one portion and solution stirred for another 30 Å mins.

Then, Pd(PhCN)2Cl2 (0.5 eq.) was added and ligand exchange yielded desired polymer attached

mechanophores. Crude products were filtered over alumina (eluent: THF), and concentrated under

reduced pressure. Then, the complex was dissolved in DCM washed with water and dried over

MgSO4. Solution containing Pd(NHC-‐pTHF)2Cl2 in DCM passed through Celite. Solvent was

evaporated under reduced pressure and light yellow polymer was left under vacuum overnight to

remove all residual solvents. 1H NMR [Pd(NHC-‐pTHF12k)2Cl2 CD2Cl2, 400 MHz]: 6.9 ppm (s, NHC), 4.5

ppm (t, N-‐CH2), 3.0-‐3.5 ppm (br, O-‐CH2-‐), 1.1-‐1.9 ppm (br, OCH2-‐CH2-‐).

Synthesis of model complex Pd(NHC-‐EtBut)2Cl2

Ethyl imidazole (480 mg, 5 mmol) and iodobutane (1840 mg, 10 mmol) were refluxed in dry THF (25

ml) overnight. Then, two-‐phase liquid reaction mixture was cooled down to room temperature and

upper transparent part was removed with glass pipette. Yellow oily product was washed with THF

(10 ml x 3) and then residue was dissolved in MeOH (10 ml). Ion exchange resin was added to

exchange iodine counterions with chlorine. After stirring 2h at RT mixture was filtered over a filter

paper, solvent was evaporated under reduced pressure and light yellow oil was left under vacuum

overnight.

EtButIm (100 mg, 0.53 mmol), PdCl2 (46 mg, 0.26 mmol) and Cs2CO3 (814.5 mg, 2.5 mmol) were

dissolved in Dioxane (4.4 mL) and the reaction mixture was heated to 80 °C. After 5 hours, it was

allowed to cool down to room temperature. Volatiles were removed under reduced pressure.

Purification by column chromatography (CH2Cl2) afforded Pd(NHC-‐EtBut)2Cl2 as a light yellow solid.


45

2

1H NMR spectra for EtButIm and Pd(NHC-‐EtBut)2Cl2.

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Chapter 2

46

1H NMR spectra for EtButIm and Pd(NHC-‐EtBut)2Cl2.

References


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(42) Kucharski, T. J.; Boulatov, R. J. Mater. Chem. 2011, 21 (23), 8237–8255.


(44) Gossweiler, G. R.; Kouznetsova, T. B.; Craig, S. L. J. Am. Chem. Soc. 2015, 137 (19), 6148–6151.

(45) Wang, J.; Kouznetsova, T. B.; Kean, Z. S.; Fan, L.; Mar, B. D.; Martínez, T. J.; Craig, S. L. J. Am.

Chem. Soc. 2014, 136 (43), 15162–15165.

(46) Brown, C. L.; Craig, S. L. Chem. Sci. 2015, 6 (4), 2158–2165.

(47) Dubreuil, M. F.; Farcy, N. G.; Goethals, E. J. Macromol. Rapid Commun. 1999, 20 (7), 383–386.

Chapter 2

48

Chapter 3Mechanical scission of Pd(NHC)2Cl2 complexes probed with chemiluminescence

Polytetrahydrofuran (pTHF) functionalized N-heterocyclic carbene (NHC) ligand has been

dissociated mechanically from Pd(NHC-pTHF)2Cl2 complex. A 2-coumaranone derivative,

which decomposes via a chemiluminescent pathway once deprotonated, was used to moni-

tor scission event. Chemiluminescence of coumaranone involves the formation of thermally

labile 1,2-dioxetanone that requires molecular oxygen to form. Therefore, ultrasound was

applied in air-saturated toluene. Influence of saturation gas on the scission rate and molecu-

lar weight threshold (Mlim) for mechanochemical chain scission was determined.

(40) Bell, G. I. Science 1978, 200 (4342), 618–627.

(41) Evans, E.; Ritchie, K. Biophys. J. 1997, 72 (4), 1541–1555.

(42) Kucharski, T. J.; Boulatov, R. J. Mater. Chem. 2011, 21 (23), 8237–8255.


(44) Gossweiler, G. R.; Kouznetsova, T. B.; Craig, S. L. J. Am. Chem. Soc. 2015, 137 (19), 6148–6151.

(45) Wang, J.; Kouznetsova, T. B.; Kean, Z. S.; Fan, L.; Mar, B. D.; Martínez, T. J.; Craig, S. L. J. Am.

Chem. Soc. 2014, 136 (43), 15162–15165.

(46) Brown, C. L.; Craig, S. L. Chem. Sci. 2015, 6 (4), 2158–2165.

(47) Dubreuil, M. F.; Farcy, N. G.; Goethals, E. J. Macromol. Rapid Commun. 1999, 20 (7), 383–386.

Introduction

N-‐heterocyclic carbenes (NHCs)1 are nucleophilic and highly basic molecules, which have

been used in various organic transformations, including condensation, transesterification

and ring-‐opening reactions.2,3 Although they are highly reactive towards various substrates

in their free form, their reactivity can be masked in thermally labile precursors.4 NHC−metal

complexes have been applied as thermally latent catalysts in a number of polymerization

reactions.5–9


such as thermal or photochemical activation.12 Mechanical work done by an external force

lowers the energy barrier for bond dissociation to such an extent that thermal fluctuations

can exceed this barrier at room temperature.13 For efficient transduction of applied external

force onto mechanically labile bonds of molecules in solution, attachment of polymer chains

is required. Incorporation of functional groups with mechanically responsive labile bonds

(“mechanophores”) into polymers has provided materials that are sensitive to mechanical

stimuli and led to useful strain induced molecular transformations. Mechanoresponsive

materials have gained more interest over the last decade; e.g. materials that change color

upon strain or heal microcracks autonomously by use of proper mechanophores have been

developed.14,15

Polymer attached NHC−metal complexes, for instance Ag(NHC-‐pTHF)2PF6, have been utilized

in our group as latent mechanochemical catalysts.10,11 Coordination bonds in these

complexes were broken mechanically to release free NHC, which then catalyzed

transesterification reactions. In the previous chapter of this thesis, it was shown that

Pd(NHC-‐pTHF)2Cl2 complexes can also be used as thermally stable mechanoresponsive

coordination polymers to release NHC upon sonication.

Recently, our group reported the development of a mechano-‐luminescent material based

on a polymer-‐functionalized 1,2-‐dioxetane moiety.16 Dioxetanes are organic peroxides and

efficient sources of electronically excited products upon chemical or thermal treatments.17

Opening of a four-‐membered 1,2-‐dioxetane ring with two adjacent oxygen atoms yields two

carbonyl groups, one of which is in an electronically excited state and emits a photon (at

420 nm) on relaxation. Bis(adamantyl)-‐substituted 1,2-‐dioxetane is thermally stable at room

temperature and has been successfully activated mechanically by straining polymer samples

that covalently incorporate the luminescent unit in poly(methylacrylate) chains.

Experiments have shown that dioxetanes can be used as mechanochemical-‐probe for

spatiotemporal mapping and chain scission in polymers. A limitation of using the dioxetane

unit as a mechanophore is that for a given dioxetane, the barrier to scission is fixed. The

currently employed bisadamantyl dioxetane has a scission barrier of 155 kJ/mol, and is

thermally stable up to ~200 oC.16,18 Lowering the barrier for decomposition by reducing the

steric bulk of the dioxetane will also reduce its thermal stability. Therefore, it is of interest

to develop an alternative scheme for mechanoluminescene in which luminescence is

induced chemically by deprotonation of a chemiluminescent probe by a mechanochemically

released base, such as an NHC-‐group. Such a scheme was first explored in collaboration with

Jessica Clough in our group, by studying the release of NHC from polymeric Ag-‐carbene

complexes, to initiate base induced chemiluminescence of 2-‐Coumaranone. Although

luminescence was indeed observed, the thermal lability of the Ag complexes prevented

detailed analysis of the mechanochemical effects. In the current Chapter, the use of

Pd(NHC)2Cl2 complexes for mechanochemical release of NHC for the base induced

chemiluminescence of coumaranone is investigated.

2-‐Coumaranones

Lofthouse et al. reported the synthesis and chemiluminescence of 2-‐coumaranone

derivatives in 1979.19 They reported that dimethylformamide solutions of coumaranone

derivatives emit violet light in the presence of triethylamine, at room temperature under

air. Later, Schramm et al. reported a systematic study of the mechanism of this reaction. By

isolating the decomposition products they established that the chemiluminescent step

involves the formation and decomposition of an unstable 1,2-‐dioxetanone intermediate.20

Because the proposed 1,2-‐dioxetanone intermediate decomposes rapidly, it has not been

isolated so far, but indirect proof for its existence comes from the study of charge transfer

complexes with perylene. 21,22

Investigation of decomposition mechanisms for 1,2-‐dioxetanone under sonication

conditions is a subject of a separate study in our laboratories and beyond the scope of this

thesis.

Chapter 3

50

Introduction

N-‐heterocyclic carbenes (NHCs)1 are nucleophilic and highly basic molecules, which have

been used in various organic transformations, including condensation, transesterification

and ring-‐opening reactions.2,3 Although they are highly reactive towards various substrates

in their free form, their reactivity can be masked in thermally labile precursors.4 NHC−metal

complexes have been applied as thermally latent catalysts in a number of polymerization

reactions.5–9


such as thermal or photochemical activation.12 Mechanical work done by an external force

lowers the energy barrier for bond dissociation to such an extent that thermal fluctuations

can exceed this barrier at room temperature.13 For efficient transduction of applied external

force onto mechanically labile bonds of molecules in solution, attachment of polymer chains

is required. Incorporation of functional groups with mechanically responsive labile bonds

(“mechanophores”) into polymers has provided materials that are sensitive to mechanical

stimuli and led to useful strain induced molecular transformations. Mechanoresponsive

materials have gained more interest over the last decade; e.g. materials that change color

upon strain or heal microcracks autonomously by use of proper mechanophores have been

developed.14,15

Polymer attached NHC−metal complexes, for instance Ag(NHC-‐pTHF)2PF6, have been utilized

in our group as latent mechanochemical catalysts.10,11 Coordination bonds in these

complexes were broken mechanically to release free NHC, which then catalyzed

transesterification reactions. In the previous chapter of this thesis, it was shown that

Pd(NHC-‐pTHF)2Cl2 complexes can also be used as thermally stable mechanoresponsive

coordination polymers to release NHC upon sonication.

Recently, our group reported the development of a mechano-‐luminescent material based

on a polymer-‐functionalized 1,2-‐dioxetane moiety.16 Dioxetanes are organic peroxides and

efficient sources of electronically excited products upon chemical or thermal treatments.17

Opening of a four-‐membered 1,2-‐dioxetane ring with two adjacent oxygen atoms yields two

carbonyl groups, one of which is in an electronically excited state and emits a photon (at

420 nm) on relaxation. Bis(adamantyl)-‐substituted 1,2-‐dioxetane is thermally stable at room

temperature and has been successfully activated mechanically by straining polymer samples

that covalently incorporate the luminescent unit in poly(methylacrylate) chains.

Experiments have shown that dioxetanes can be used as mechanochemical-‐probe for

spatiotemporal mapping and chain scission in polymers. A limitation of using the dioxetane

unit as a mechanophore is that for a given dioxetane, the barrier to scission is fixed. The

currently employed bisadamantyl dioxetane has a scission barrier of 155 kJ/mol, and is

thermally stable up to ~200 oC.16,18 Lowering the barrier for decomposition by reducing the

steric bulk of the dioxetane will also reduce its thermal stability. Therefore, it is of interest

to develop an alternative scheme for mechanoluminescene in which luminescence is

induced chemically by deprotonation of a chemiluminescent probe by a mechanochemically

released base, such as an NHC-‐group. Such a scheme was first explored in collaboration with

Jessica Clough in our group, by studying the release of NHC from polymeric Ag-‐carbene

complexes, to initiate base induced chemiluminescence of 2-‐Coumaranone. Although

luminescence was indeed observed, the thermal lability of the Ag complexes prevented

detailed analysis of the mechanochemical effects. In the current Chapter, the use of

Pd(NHC)2Cl2 complexes for mechanochemical release of NHC for the base induced

chemiluminescence of coumaranone is investigated.

2-‐Coumaranones

Lofthouse et al. reported the synthesis and chemiluminescence of 2-‐coumaranone

derivatives in 1979.19 They reported that dimethylformamide solutions of coumaranone

derivatives emit violet light in the presence of triethylamine, at room temperature under

air. Later, Schramm et al. reported a systematic study of the mechanism of this reaction. By

isolating the decomposition products they established that the chemiluminescent step

involves the formation and decomposition of an unstable 1,2-‐dioxetanone intermediate.20

Because the proposed 1,2-‐dioxetanone intermediate decomposes rapidly, it has not been

isolated so far, but indirect proof for its existence comes from the study of charge transfer

complexes with perylene. 21,22

Investigation of decomposition mechanisms for 1,2-‐dioxetanone under sonication

conditions is a subject of a separate study in our laboratories and beyond the scope of this

thesis.

Mechanically induced chemiluminescence

51

3

Results and discussions

Chain scission in Pd(NHC-‐pTHF)2Cl2 complexes in the presence of coumaranone

In order to investigate chain scission in polymers, Pd(NHC-‐pTHF)2Cl2 (0.2 mM) and

coumaranone (0.2 mM) were sonicated in toluene for 60 min using a sonication probe. It

has been shown that chemiluminescent decomposition of coumaranone requires

deprotonation followed by a reaction with molecular oxygen.20 Therefore, dry air was

bubbled through the solution during sonication providing a constant stream of gas, which

contains both molecular oxygen required for dioxetanone formation and nitrogen as

cavitation gas. The double-‐jacketed sonication vessel was cooled down to 2oC by water

circulation from a thermostat-‐controlled bath. The temperature inside the sonication vessel

was monitored with a thermocouple and recorded as below 30 oC during sonication after

thermal equilibration for approximately 5 min.

Molecular weights of complexes were determined with MALDI-‐TOF as shown in the

previous chapter. Bimodal distributions obtained with GPC were fitted with double Gaussian

function, which gives the best fit for selective midpoint scission of a weak metal–ligand

coordination bonds, to determine initial weight fractions of Pd(NHC-‐pTHF)2Cl2 complexes.

The two peaks of the bimodal distribution were assumed to have the same polydispersity so

peak widths were fixed. Areas under GPC traces were calculated from the fits and were

normalized to total peak area (i.e. A1+A2=1). Concentration of starting material C1 at each

sonication time t is proportional to the relative peak area in RI detection:

𝐶𝐶! 𝑡𝑡 = 𝐴𝐴! 𝑡𝑡

Polymer degradation during sonication follows first-‐order kinetics and weight fraction of

starting material C1 decays exponentially with the equation; 𝐶𝐶! 𝑡𝑡 = 𝐶𝐶! 0 ×𝑒𝑒 !!!"! where

ksc is mechanochemical scission rate coefficient and t is sonication time. Exponential fitting

of C1(t)/C1(0) vs t graph reveals the scission rate coefficient ksc (R2=0.99) that ranges

between 0.005 and 0.065 min-‐1. Chain scission in Pd(NHC-‐pTHF)2Cl2 complexes under air in

the presence of 2-‐coumaranone increases from 20% to 98% for the initial MW of the

polymer complexes 16 kDa and 50kDa respectively.

GPC traces showed the formation of polymer species with half of the initial MW during

sonication (Figure 1), confirming that scission takes place and is not reversible, as is

expected when free NHC is protonated by the presence of 2-‐coumaranone. GPC traces

taken after different time intervals during sonication show two distinct peaks corresponding

to initial and broken polymer fractions. Such behavior is usually not observed for

polydisperse polymers without a weak bond (see for example ref 23) and suggests that

scission is selective for the mid-‐chain coordination bond.

In the presence of 2-‐coumaranone under air, Pd(NHC-‐pTHF18k)2Cl2 showed ≈90% chain

scission after 1h of continuous sonication, while 55% scission was observed after sonicating

for 1h under CH4 with AcOH and MeCN as trapping agents. The highest molecular weight

complex Pd(NHC-‐pTHF25k)2Cl2 was broken completely (≥ 98%) in 1h, with 25 kDa polymer

as the only polymeric product and complete disappearance of the 50kDa peak

corresponding to the complex (Figure 1d).

Chapter 3

52


Chain scission in Pd(NHC-‐pTHF)2Cl2 complexes in the presence of coumaranone

In order to investigate chain scission in polymers, Pd(NHC-‐pTHF)2Cl2 (0.2 mM) and

coumaranone (0.2 mM) were sonicated in toluene for 60 min using a sonication probe. It

has been shown that chemiluminescent decomposition of coumaranone requires

deprotonation followed by a reaction with molecular oxygen.20 Therefore, dry air was

bubbled through the solution during sonication providing a constant stream of gas, which

contains both molecular oxygen required for dioxetanone formation and nitrogen as

cavitation gas. The double-‐jacketed sonication vessel was cooled down to 2oC by water

circulation from a thermostat-‐controlled bath. The temperature inside the sonication vessel

was monitored with a thermocouple and recorded as below 30 oC during sonication after

thermal equilibration for approximately 5 min.

Molecular weights of complexes were determined with MALDI-‐TOF as shown in the

previous chapter. Bimodal distributions obtained with GPC were fitted with double Gaussian

function, which gives the best fit for selective midpoint scission of a weak metal–ligand

coordination bonds, to determine initial weight fractions of Pd(NHC-‐pTHF)2Cl2 complexes.

The two peaks of the bimodal distribution were assumed to have the same polydispersity so

peak widths were fixed. Areas under GPC traces were calculated from the fits and were

normalized to total peak area (i.e. A1+A2=1). Concentration of starting material C1 at each

sonication time t is proportional to the relative peak area in RI detection:

𝐶𝐶! 𝑡𝑡 = 𝐴𝐴! 𝑡𝑡

Polymer degradation during sonication follows first-‐order kinetics and weight fraction of

starting material C1 decays exponentially with the equation; 𝐶𝐶! 𝑡𝑡 = 𝐶𝐶! 0 ×𝑒𝑒 !!!"! where

ksc is mechanochemical scission rate coefficient and t is sonication time. Exponential fitting

of C1(t)/C1(0) vs t graph reveals the scission rate coefficient ksc (R2=0.99) that ranges

between 0.005 and 0.065 min-‐1. Chain scission in Pd(NHC-‐pTHF)2Cl2 complexes under air in

the presence of 2-‐coumaranone increases from 20% to 98% for the initial MW of the

polymer complexes 16 kDa and 50kDa respectively.

GPC traces showed the formation of polymer species with half of the initial MW during

sonication (Figure 1), confirming that scission takes place and is not reversible, as is

expected when free NHC is protonated by the presence of 2-‐coumaranone. GPC traces

taken after different time intervals during sonication show two distinct peaks corresponding

to initial and broken polymer fractions. Such behavior is usually not observed for

polydisperse polymers without a weak bond (see for example ref 23) and suggests that

scission is selective for the mid-‐chain coordination bond.

In the presence of 2-‐coumaranone under air, Pd(NHC-‐pTHF18k)2Cl2 showed ≈90% chain

scission after 1h of continuous sonication, while 55% scission was observed after sonicating

for 1h under CH4 with AcOH and MeCN as trapping agents. The highest molecular weight

complex Pd(NHC-‐pTHF25k)2Cl2 was broken completely (≥ 98%) in 1h, with 25 kDa polymer

as the only polymeric product and complete disappearance of the 50kDa peak

corresponding to the complex (Figure 1d).


53

3

Figure 1: GPC traces for Pd(NHC-‐pTHF)2Cl2 taken during sonication in air saturated toluene (10

mg/ml) in the presence of 2-‐coumaranone (0.2 mM). Internal temperature was kept below 30oC.

Aliquots were taken at given time intervals. Toluene was evaporated and residues were submitted

to GPC in THF (2 mg/ml). Initial molecular weight for polymers: a) 16 kDa b) 24 kDa c) 36 kDa d) 50

kDa

Table 1: Scission rates and total scission after 1h sonication for Pd(NHC-‐pTHF)2Cl2 polymers in the

presence of coumaranone under air.

MW (kg/mol) ksc (min-‐1) Scission (1 h)

16 0.0055 28 %

24 0.0150 60 %

36 0.0360 88 %

50 0.0600 98 %

Chain scission was determined from GPC traces using the method discussed above and is

plotted as a function of time (Figure 2). Scission rates ksc were determined by fitting first

order rate constants to the data in Figure 2a with the method presented in Chapter 2, and

are given in Table 1. A value of 13.5 kDa was determined for the limiting molecular weight

(Mlim) from linear extrapolation of a plot of ksc vs MW (Figure 2b). This value is significantly

lower than the Mlim determined previously for the same complex when sonicated under CH4

(20 kDa).

Empirically, ksc in sonication usually increases with MW following the equation 𝑘𝑘!" =

𝑘𝑘! (𝑀𝑀𝑀𝑀 − 𝑀𝑀!"#)ƛ 26 with kd corresponding to the slope of a plot of ksc vs MW. Figure 2b

shows that the kd for the system under air is significantly higher than for sonication under

methane (Table 2).

Table 2: kd (mol x g-‐1 x min-‐1) for Pd(NHC-‐pTHF)2Cl2 determined for the sonications under air and CH4.

𝒌𝒌𝒅𝒅𝑷𝑷𝑷𝑷 𝒂𝒂𝒂𝒂𝒂𝒂 𝒌𝒌𝒅𝒅𝑷𝑷𝑷𝑷(𝑪𝑪𝑪𝑪𝟒𝟒)

(1.75 ± 0.1)×10!! (7.4 ± 0.3)×10!!

In Chapter 2, the kinetics of ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 were

reported. It was shown that scission rates are strongly dependent on the heat capacity and

solubility of the saturation gas because these parameters influence the formation and

implosion of cavitation bubbles, which eventually is the source of elongational stresses in

solution.24,25 The molecular weight dependence of scission rate and limiting molecular

Chapter 3

54

Figure 1: GPC traces for Pd(NHC-‐pTHF)2Cl2 taken during sonication in air saturated toluene (10

mg/ml) in the presence of 2-‐coumaranone (0.2 mM). Internal temperature was kept below 30oC.

Aliquots were taken at given time intervals. Toluene was evaporated and residues were submitted

to GPC in THF (2 mg/ml). Initial molecular weight for polymers: a) 16 kDa b) 24 kDa c) 36 kDa d) 50

kDa

Table 1: Scission rates and total scission after 1h sonication for Pd(NHC-‐pTHF)2Cl2 polymers in the

presence of coumaranone under air.

MW (kg/mol) ksc (min-‐1) Scission (1 h)

16 0.0055 28 %

24 0.0150 60 %

36 0.0360 88 %

50 0.0600 98 %

Chain scission was determined from GPC traces using the method discussed above and is

plotted as a function of time (Figure 2). Scission rates ksc were determined by fitting first

order rate constants to the data in Figure 2a with the method presented in Chapter 2, and

are given in Table 1. A value of 13.5 kDa was determined for the limiting molecular weight

(Mlim) from linear extrapolation of a plot of ksc vs MW (Figure 2b). This value is significantly

lower than the Mlim determined previously for the same complex when sonicated under CH4

(20 kDa).

Empirically, ksc in sonication usually increases with MW following the equation 𝑘𝑘!" =

𝑘𝑘! (𝑀𝑀𝑀𝑀 − 𝑀𝑀!"#)ƛ 26 with kd corresponding to the slope of a plot of ksc vs MW. Figure 2b

shows that the kd for the system under air is significantly higher than for sonication under

methane (Table 2).

Table 2: kd (mol x g-‐1 x min-‐1) for Pd(NHC-‐pTHF)2Cl2 determined for the sonications under air and CH4.

𝒌𝒌𝒅𝒅𝑷𝑷𝑷𝑷 𝒂𝒂𝒂𝒂𝒂𝒂 𝒌𝒌𝒅𝒅𝑷𝑷𝑷𝑷(𝑪𝑪𝑪𝑪𝟒𝟒)

(1.75 ± 0.1)×10!! (7.4 ± 0.3)×10!!

In Chapter 2, the kinetics of ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 were

reported. It was shown that scission rates are strongly dependent on the heat capacity and

solubility of the saturation gas because these parameters influence the formation and

implosion of cavitation bubbles, which eventually is the source of elongational stresses in

solution.24,25 The molecular weight dependence of scission rate and limiting molecular


55

3

weight for Pd(NHC-‐pTHF)2Cl2 for sonication under CH4 were given in Chapter 2 are different

than those under air.

Groote has previously studied the effect of saturation gas on scission rate for Ag(NHC-‐

pTHF)2PF6 polymeric complex and reported that scission proceeds with comparable rates

under N2 and CH4.24 Therefore, it can be concluded that the presence of 20% of O2, has a

strong effect on scission rate. This may be due to the higher solubility of O2 in toluene

compared to N2 since this means cavitation bubbles involve mainly O2 rather than N2.27

Reactive sonochemical impurities produced due to pyrolysis of the content of cavitation

bubbles in hot spots28 might result in chemical breakage of polymer and increase the

scission rate.

Figure 2: a) Chain scission rates for Pd(NHC-‐pTHF)2Cl2 complexes under air in the presence of

coumaranone. b) Scission rate constants ksc plotted against initial molecular weights of polymers

under air (with coumaranone) and under CH4 (with CH3COOH and CH3CN). MW of polymer

complexes are 16 kDa, 24 kDa, 36 kDa and 50 kDa.

Chapter 3

56

weight for Pd(NHC-‐pTHF)2Cl2 for sonication under CH4 were given in Chapter 2 are different

than those under air.

Groote has previously studied the effect of saturation gas on scission rate for Ag(NHC-‐

pTHF)2PF6 polymeric complex and reported that scission proceeds with comparable rates

under N2 and CH4.24 Therefore, it can be concluded that the presence of 20% of O2, has a

strong effect on scission rate. This may be due to the higher solubility of O2 in toluene

compared to N2 since this means cavitation bubbles involve mainly O2 rather than N2.27

Reactive sonochemical impurities produced due to pyrolysis of the content of cavitation

bubbles in hot spots28 might result in chemical breakage of polymer and increase the

scission rate.

Figure 2: a) Chain scission rates for Pd(NHC-‐pTHF)2Cl2 complexes under air in the presence of

coumaranone. b) Scission rate constants ksc plotted against initial molecular weights of polymers

under air (with coumaranone) and under CH4 (with CH3COOH and CH3CN). MW of polymer

complexes are 16 kDa, 24 kDa, 36 kDa and 50 kDa.


57

3

Light emission

Mechanobase induced chemiluminescence was studied by sonication of air saturated

toluene solutions containing coumaranone 1 and Pd(NHC-‐pTHF)2Cl2 or small molecule

model complex Pd(NHC-‐EtBut)2Cl2, and monitoring light emission with a photodiode. During

experiments, the sonication flask was kept in the dark to prevent interference of

background light. The double-‐jacketed sonication vessel was cooled down to 2oC by water

circulation from a thermostat-‐ controlled bath to keep the temperature inside the

sonication vessel constant below 30 oC.

When a 10 mg/ml (0.2 mM) solution of Pd(NHC-‐pTHF25k)2Cl2 in toluene was sonicated in

the presence of 2-‐coumaranone (0.2 mM), light intensity increased slowly as shown in

Figure 3, indicating chemiluminescent decomposition of coumaranone. Light intensity

reached a maximum level after about 300 s., after which it slowly decreased. When

sonication was stopped after 850 s., the photodiode response dropped back to its

background value in approximately 70s.

Figure 3: A 0.2 mM solution of Pd(NHC-‐pTHF25k)2Cl2 in air saturated toluene was sonicated in the

presence of 2-‐coumaranone (0.2 mM). Temperature inside the sonication vessel was kept below 30 oC Luminescence was monitored with a photodiode (inset), which was placed under the sonication

flask during sonication. Light intensity was measured as the change in current.

In a control experiment (Scheme 1), a (0.2 mM) solution of low molecular weight analog

Pd(NHC-‐EtBut)2Cl2, model complex, was sonicated in the presence of coumaranone (0.2

mM) and methoxy-‐end-‐capped pTHF (30 kDa, 0.2 mM). Photodiode response did not rise

above the background level during sonication. Additonally, refluxing the mixture of Pd(NHC-‐

pTHF25k)2Cl2 (10 mg/ml) and coumaranone in toluene did not result in detectable

chemiluminescence. These observations rule out the contribution of thermal ligand-‐

dissociation to base induced chemiluminescence.

To confirm that the of presence of O2 is essential in mechano-‐chemiluminescence, a mixture

of Pd(NHC-‐pTHF25k)2Cl2 and coumaranone was sonicated under CH4 otherwise identical to

above mentioned sonication conditions. No chemiluminescence was observed.

In order to test its thermal stability, Pd(NHC-‐pTHF25k)2Cl2 (10 mg/ml) and coumaranone

(0.2 mM) were kept in a toluene solution at RT for 30 days. Then a 0.1 ml aliquot was

submitted to GPC and the Mw distribution was unchanged. When the stored solution was

sonicated, the maximum light intensity was identical to that of a freshly prepared solution

Scheme 1: Mechano-‐chemiluminescence a result of decomposition of coumaranone when sonicated

in the presence of Pd(NHC-‐pTHF)2Cl2. Light emission was not detected when PTHF and the Pd-‐NHC

complex were both present in the solution but not covalently attached to each other.

N N

Pd

NN

ClCl R¹

O

HNOR²

OO

hvN N

Pd

NN

ClCl

O

OR¹

O

HNOR²

OO

OOn

n

n

hvx

-Pd(NHC/pTHF12k)₂Cl₂--Pd(NHC/pTHF18k)₂Cl₂--Pd(NHC/pTHF25k)₂Cl₂

PTHF:-50-kDA

+

+ +

Chapter 3

58

Light emission

Mechanobase induced chemiluminescence was studied by sonication of air saturated

toluene solutions containing coumaranone 1 and Pd(NHC-‐pTHF)2Cl2 or small molecule

model complex Pd(NHC-‐EtBut)2Cl2, and monitoring light emission with a photodiode. During

experiments, the sonication flask was kept in the dark to prevent interference of

background light. The double-‐jacketed sonication vessel was cooled down to 2oC by water

circulation from a thermostat-‐ controlled bath to keep the temperature inside the

sonication vessel constant below 30 oC.

When a 10 mg/ml (0.2 mM) solution of Pd(NHC-‐pTHF25k)2Cl2 in toluene was sonicated in

the presence of 2-‐coumaranone (0.2 mM), light intensity increased slowly as shown in

Figure 3, indicating chemiluminescent decomposition of coumaranone. Light intensity

reached a maximum level after about 300 s., after which it slowly decreased. When

sonication was stopped after 850 s., the photodiode response dropped back to its

background value in approximately 70s.

Figure 3: A 0.2 mM solution of Pd(NHC-‐pTHF25k)2Cl2 in air saturated toluene was sonicated in the

presence of 2-‐coumaranone (0.2 mM). Temperature inside the sonication vessel was kept below 30 oC Luminescence was monitored with a photodiode (inset), which was placed under the sonication

flask during sonication. Light intensity was measured as the change in current.

In a control experiment (Scheme 1), a (0.2 mM) solution of low molecular weight analog

Pd(NHC-‐EtBut)2Cl2, model complex, was sonicated in the presence of coumaranone (0.2

mM) and methoxy-‐end-‐capped pTHF (30 kDa, 0.2 mM). Photodiode response did not rise

above the background level during sonication. Additonally, refluxing the mixture of Pd(NHC-‐

pTHF25k)2Cl2 (10 mg/ml) and coumaranone in toluene did not result in detectable

chemiluminescence. These observations rule out the contribution of thermal ligand-‐

dissociation to base induced chemiluminescence.

To confirm that the of presence of O2 is essential in mechano-‐chemiluminescence, a mixture

of Pd(NHC-‐pTHF25k)2Cl2 and coumaranone was sonicated under CH4 otherwise identical to

above mentioned sonication conditions. No chemiluminescence was observed.

In order to test its thermal stability, Pd(NHC-‐pTHF25k)2Cl2 (10 mg/ml) and coumaranone

(0.2 mM) were kept in a toluene solution at RT for 30 days. Then a 0.1 ml aliquot was

submitted to GPC and the Mw distribution was unchanged. When the stored solution was

sonicated, the maximum light intensity was identical to that of a freshly prepared solution

Scheme 1: Mechano-‐chemiluminescence a result of decomposition of coumaranone when sonicated

in the presence of Pd(NHC-‐pTHF)2Cl2. Light emission was not detected when PTHF and the Pd-‐NHC

complex were both present in the solution but not covalently attached to each other.

N N

Pd

NN

ClCl R¹

O

HNOR²

OO

hvN N

Pd

NN

ClCl

O

OR¹

O

HNOR²

OO

OOn

n

n

hvx

-Pd(NHC/pTHF12k)₂Cl₂--Pd(NHC/pTHF18k)₂Cl₂--Pd(NHC/pTHF25k)₂Cl₂

PTHF:-50-kDA

+

+ +


59

3

The changes in chemiluminescence intensity with time in Figure 3 reflect the kinetics of the

individual steps of the process, which consists of scission of the carbene metal complex,

deprotonation of coumaranone, oxidation of the anion to dioxetanone, and finally

decomposition of the dioxetanone under emission of a photon, with the light intensity being

proportional to the concentration of dioxetanone. If scission would be the slowest step in

the process, light intensity would be proportional to the rate at which the complex breaks,

and simple first order decay would be observed with a rate constant corresponding to the

scission rate constant of 1 x 10-‐3 s-‐1. The initial rise in intensity in Figure 3 therefore shows

that scission is not the slowest step, and reflects the accumulation of an intermediate,

either the anion or the 2-‐dioxetanone. The latter has a reported decomposition rate

constant kD = 1.08(±0.01) x10-‐2 s-‐1 in MeCN at 25oC.21 This rate constant matches reasonably

well with the fast decrease of signal when sonication was stopped. Since deprotonation of

the coumaranone may be assumed to be fast, we conclude that decomposition of

intermediate is the slowest step. Simulation of the process with kinetic modeling software,

using the parameters in Scheme 2 indeed reproduces qualitatively the observed rise and

decay in light intensity (Figure 4).

Scheme 2: Schematic representation of mechanochemiluminescence, estimated rate constants and

proposed chemiluminescent decomposition mechanism in the presence of base and oxygen.

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 yields free carbene which abstracts a proton

from 2-‐coumaranone and then it decomposes through a luminescent pathway.

F

O

HN

OBu

O

O O2

1-2 dioxetanone2-coumaranone

hv

F

O

HN

OBu

O

OF

O

OO

O

HN

BuO O

CO2

F

OH

NH

OBu

O O

+

1 32

3

N N

Pd

NN

ClCl

O

O

n

n

N NOn

Pd

NN

ClCl

On

Ultrasound ksc$:$0.001$s)¹

k₁$:$0.030$s)¹

k₃$>>$k)₃

k₁$:$0.011$s)¹

k₄$>>$k)₄

[PMP]₀6:60.26mM

[Coum]₀6:60.26mM

Chapter 3

60

The changes in chemiluminescence intensity with time in Figure 3 reflect the kinetics of the

individual steps of the process, which consists of scission of the carbene metal complex,

deprotonation of coumaranone, oxidation of the anion to dioxetanone, and finally

decomposition of the dioxetanone under emission of a photon, with the light intensity being

proportional to the concentration of dioxetanone. If scission would be the slowest step in

the process, light intensity would be proportional to the rate at which the complex breaks,

and simple first order decay would be observed with a rate constant corresponding to the

scission rate constant of 1 x 10-‐3 s-‐1. The initial rise in intensity in Figure 3 therefore shows

that scission is not the slowest step, and reflects the accumulation of an intermediate,

either the anion or the 2-‐dioxetanone. The latter has a reported decomposition rate

constant kD = 1.08(±0.01) x10-‐2 s-‐1 in MeCN at 25oC.21 This rate constant matches reasonably

well with the fast decrease of signal when sonication was stopped. Since deprotonation of

the coumaranone may be assumed to be fast, we conclude that decomposition of

intermediate is the slowest step. Simulation of the process with kinetic modeling software,

using the parameters in Scheme 2 indeed reproduces qualitatively the observed rise and

decay in light intensity (Figure 4).

Scheme 2: Schematic representation of mechanochemiluminescence, estimated rate constants and

proposed chemiluminescent decomposition mechanism in the presence of base and oxygen.

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 yields free carbene which abstracts a proton

from 2-‐coumaranone and then it decomposes through a luminescent pathway.

F

O

HN

OBu

O

O O2

1-2 dioxetanone2-coumaranone

hv

F

O

HN

OBu

O

OF

O

OO

O

HN

BuO O

CO2

F

OH

NH

OBu

O O

+

1 32

3

N N

Pd

NN

ClCl

O

O

n

n

N NOn

Pd

NN

ClCl

On

Ultrasound ksc$:$0.001$s)¹

k₁$:$0.030$s)¹

k₃$>>$k)₃

k₁$:$0.011$s)¹

k₄$>>$k)₄

[PMP]₀6:60.26mM

[Coum]₀6:60.26mM


61

3

Example of simulated light intensity (proportional to [Cox]) with following kinetic parameters:

PMP = PM + P k1 = 0.001 s-‐1 (ksc for Pd(NHC-‐pTHF25k)2Cl2 taken from Table 1), k-‐1 = 108 M-‐1s-‐1

(backward reaction was assumed to be diffusion controlled)

P + C = PH + Canion k2 = 108 s-‐1 (assumed to be diffusion controlled), k-‐2 = 10-‐4 M-‐1s-‐1

(assumption: k2>>k-‐2)

Canion + O2 = Cox k3 = 0.03 s-‐1 (assumption: k3>>k-‐3)

Cox = Cdecomp k4 = 0.011 s-‐1 (k4 is taken from ref 21, assumption: k4>>k-‐4,)

Backward rates are included because gepasi cannot handle zero reverse rates. Initial

concentrations were taken as [PMP] = 0.2 mM [C] = 0.2 mM [O2] = 1 M (value was set high

to keep [O2] constant).

Figure 4: Simulated change in light intensity (proportional to [Cox]) is compared with experimental

data. GEPASI21 was used for simulation with kinetic parameters in Scheme 1.

Figure 5: Comparison of the light intensity during sonication of Pd(NHC-‐pTHF)2Cl2 polymers in the

presence of 2-‐coumaranone. MW for polymers are 50 kDa, 36 kDa and 24 kDa.

Conclusions

Chain scission in the presence of coumaranone has been investigated. Base induced

chemiluminescent property of coumaranone provided a molecular probe to monitor chain

scission in situ. Kinetic studies revealed that the NHC-‐Pd bond is mechanically more labile

under air than under CH4. The lowest molecular weight for polymeric complex Pd(NHC-‐

pTHF)2Cl2 to break under ultrasound induced strain was calculated as 13.5 kDa. Above that

MW the rate of mechanochemical chain scission increases linearly with initial Mw.

Sonicating Pd(NHC-‐pTHF)2Cl2 complexes reveals free NHC-‐pTHF ligands which abstracts

proton from coumaranone that is followed by a dioxetanone and its subsequent

chemiluminescent decomposition. Mechanically induced release of a base to activate

chemiluminescence could be monitored by a photodiode. Control experiments showed that

the chemiluminescence is a result of mechanical activation NHC-‐Pd bond to yield free NHC.

It has been shown here that Pd(NHC-‐pTHF)2Cl2 complexes are thermally stable sources of

base that can be released under mechanical stress in due course.

These preliminary results represent the first promising example of mechano-‐

chemiluminescence and in the presence of 2-‐coumaranone, mechanochemical chain

Chapter 3

62

Example of simulated light intensity (proportional to [Cox]) with following kinetic parameters:

PMP = PM + P k1 = 0.001 s-‐1 (ksc for Pd(NHC-‐pTHF25k)2Cl2 taken from Table 1), k-‐1 = 108 M-‐1s-‐1

(backward reaction was assumed to be diffusion controlled)

P + C = PH + Canion k2 = 108 s-‐1 (assumed to be diffusion controlled), k-‐2 = 10-‐4 M-‐1s-‐1

(assumption: k2>>k-‐2)

Canion + O2 = Cox k3 = 0.03 s-‐1 (assumption: k3>>k-‐3)

Cox = Cdecomp k4 = 0.011 s-‐1 (k4 is taken from ref 21, assumption: k4>>k-‐4,)

Backward rates are included because gepasi cannot handle zero reverse rates. Initial

concentrations were taken as [PMP] = 0.2 mM [C] = 0.2 mM [O2] = 1 M (value was set high

to keep [O2] constant).

Figure 4: Simulated change in light intensity (proportional to [Cox]) is compared with experimental

data. GEPASI21 was used for simulation with kinetic parameters in Scheme 1.

Figure 5: Comparison of the light intensity during sonication of Pd(NHC-‐pTHF)2Cl2 polymers in the

presence of 2-‐coumaranone. MW for polymers are 50 kDa, 36 kDa and 24 kDa.

Conclusions

Chain scission in the presence of coumaranone has been investigated. Base induced

chemiluminescent property of coumaranone provided a molecular probe to monitor chain

scission in situ. Kinetic studies revealed that the NHC-‐Pd bond is mechanically more labile

under air than under CH4. The lowest molecular weight for polymeric complex Pd(NHC-‐

pTHF)2Cl2 to break under ultrasound induced strain was calculated as 13.5 kDa. Above that

MW the rate of mechanochemical chain scission increases linearly with initial Mw.

Sonicating Pd(NHC-‐pTHF)2Cl2 complexes reveals free NHC-‐pTHF ligands which abstracts

proton from coumaranone that is followed by a dioxetanone and its subsequent

chemiluminescent decomposition. Mechanically induced release of a base to activate

chemiluminescence could be monitored by a photodiode. Control experiments showed that

the chemiluminescence is a result of mechanical activation NHC-‐Pd bond to yield free NHC.

It has been shown here that Pd(NHC-‐pTHF)2Cl2 complexes are thermally stable sources of

base that can be released under mechanical stress in due course.

These preliminary results represent the first promising example of mechano-‐

chemiluminescence and in the presence of 2-‐coumaranone, mechanochemical chain


63

3

scission can be visually monitored, which could be used for mapping deformations in

polymer samples.

Experimental

General



passage through activated alumina solvent column. Toluene was dried over 4Å molecular sieves for

at least 12h prior to use. Gel permeation chromatography (GPC) was performed on a Shimadzu

LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and MIXEDD columns (linear range of

MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector at 254 nm and RID-‐10A refractive

index detector, and THF as eluent at a flow rate of 1 mL/min (20 °C). Polystyrene standards were

used for calibration.


A homemade, double-‐jacketed glass vessel with a volume of 10 mL was used in the sonication



VC750 power supply. A silicon photodiode (Hanamatsu, diameter of photosensitive area 7 mm)

positioned underneath the vessel during sonication for mechano-‐chemiluminescence experiments.

The set-‐up was covered to exclude background light. The temperature in the vessel was maintained

with a Lauda E300 cooling bath and measured using a 0.5 mm diameter thermocouple. Solutions

were sonicated continuously. Temperature of the solution was checked by thermocouple and

recorded as below 30oC after the thermal equilibrium was achieved in the first 5 mins. During

sonication air was bubbled through the solution via teflon tubing. Aliquots of 100 µL were taken at

different time intervals toluene was removed under reduced pressure and residues were dissolved

in THF and submitted to GPC. In order to calculate mechanochemical scission rates GPC traces for

samples taken during sonication were fitted using Origin 8.5 to following equation:

𝑰𝑰 𝒙𝒙 =𝑨𝑨𝟏𝟏

𝒘𝒘 𝝅𝝅 𝟐𝟐𝒆𝒆!𝟐𝟐

𝒙𝒙!𝒙𝒙𝒄𝒄,𝟏𝟏𝒘𝒘

𝟐𝟐

+𝑨𝑨𝟐𝟐


𝒙𝒙!𝒙𝒙𝒄𝒄,𝟐𝟐𝒘𝒘

𝟐𝟐

Where;

A1 = Area of peak 1, high molecular weight peak

A2 = Area of peak 2, low molecular weight peak

w = 2σ = Width of both peaks

X = Retention time

Xc,1 = Retention time at the center of peak 1


All peaks were normalized to total peak area (A1+A2=1). All GPC traces during sonication were fitted

with the indicated formula, fixing values for Xc,1, Xc,2, and w to the same values for each samples.

Chapter 3

64

scission can be visually monitored, which could be used for mapping deformations in

polymer samples.

Experimental

General



passage through activated alumina solvent column. Toluene was dried over 4Å molecular sieves for

at least 12h prior to use. Gel permeation chromatography (GPC) was performed on a Shimadzu

LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and MIXEDD columns (linear range of

MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector at 254 nm and RID-‐10A refractive

index detector, and THF as eluent at a flow rate of 1 mL/min (20 °C). Polystyrene standards were

used for calibration.


A homemade, double-‐jacketed glass vessel with a volume of 10 mL was used in the sonication



VC750 power supply. A silicon photodiode (Hanamatsu, diameter of photosensitive area 7 mm)

positioned underneath the vessel during sonication for mechano-‐chemiluminescence experiments.

The set-‐up was covered to exclude background light. The temperature in the vessel was maintained

with a Lauda E300 cooling bath and measured using a 0.5 mm diameter thermocouple. Solutions

were sonicated continuously. Temperature of the solution was checked by thermocouple and

recorded as below 30oC after the thermal equilibrium was achieved in the first 5 mins. During

sonication air was bubbled through the solution via teflon tubing. Aliquots of 100 µL were taken at

different time intervals toluene was removed under reduced pressure and residues were dissolved

in THF and submitted to GPC. In order to calculate mechanochemical scission rates GPC traces for

samples taken during sonication were fitted using Origin 8.5 to following equation:

𝑰𝑰 𝒙𝒙 =𝑨𝑨𝟏𝟏


𝒙𝒙!𝒙𝒙𝒄𝒄,𝟏𝟏𝒘𝒘

𝟐𝟐

+𝑨𝑨𝟐𝟐


𝒙𝒙!𝒙𝒙𝒄𝒄,𝟐𝟐𝒘𝒘

𝟐𝟐

Where;

A1 = Area of peak 1, high molecular weight peak

A2 = Area of peak 2, low molecular weight peak

w = 2σ = Width of both peaks

X = Retention time



All peaks were normalized to total peak area (A1+A2=1). All GPC traces during sonication were fitted

with the indicated formula, fixing values for Xc,1, Xc,2, and w to the same values for each samples.


65

3

References


2172.





J. 2009, 15 (13), 3103–3109.


(9), 2731–2740.


4181.


Macromolecules 2013, 46 (21), 8426–8433.


47 (14), 4548–4556.



14968–14969.





68–72.

(15) Black Ramirez, A. L.; Kean, Z. S.; Orlicki, J. A.; Champhekar, M.; Elsakr, S. M.; Krause, W. E.;

Craig, S. L. Nat. Chem. 2013, 5 (9), 757–761.


Chem. 2012, 4 (7), 559–562.


Res. 1974, 7 (4), 97–105.


Am. Chem. Soc. 1975, 97 (24), 7110–7118.

(19) Lofthouse, G. J.; Suschitzky, H.; Wakefield, B. J.; Whittaker, R. A.; Tuck, B. J. Chem. Soc. [Perkin

1] 1979, No. 0, 1634–1639.

(20) Schramm, S.; Weiss, D.; Navizet, I.; Roca-‐Sanjuán, D.; Brandl, W.; Beckert, R.; Görls, H.

ARKIVOC 2013, iii, 174–188.

(21) Ciscato, L. F. M. L.; Bartoloni, F. H.; Colavite, A. S.; Weiss, D.; Beckert, R.; Schramm, S.

Photochem. Photobiol. Sci. 2013, 13 (1), 32–37.

(22) Almeida de Oliveira, M.; Bartoloni, F. H.; Augusto, F. A.; Ciscato, L. F. M. L.; Bastos, E. L.;

Baader, W. J. J. Org. Chem. 2012, 77 (23), 10537–10544.

(23) Odell, J. A.; Keller, A. J. Polym. Sci. Part B Polym. Phys. 1986, 24 (9), 1889–1916.





(27) Field, L. R.; Wilhelm, E.; Battino, R. J. Chem. Thermodyn. 1974, 6 (3), 237–243.


Chapter 3

66

References


2172.





J. 2009, 15 (13), 3103–3109.


(9), 2731–2740.


4181.


Macromolecules 2013, 46 (21), 8426–8433.


47 (14), 4548–4556.



14968–14969.





68–72.

(15) Black Ramirez, A. L.; Kean, Z. S.; Orlicki, J. A.; Champhekar, M.; Elsakr, S. M.; Krause, W. E.;

Craig, S. L. Nat. Chem. 2013, 5 (9), 757–761.


Chem. 2012, 4 (7), 559–562.


Res. 1974, 7 (4), 97–105.


Am. Chem. Soc. 1975, 97 (24), 7110–7118.

(19) Lofthouse, G. J.; Suschitzky, H.; Wakefield, B. J.; Whittaker, R. A.; Tuck, B. J. Chem. Soc. [Perkin

1] 1979, No. 0, 1634–1639.

(20) Schramm, S.; Weiss, D.; Navizet, I.; Roca-‐Sanjuán, D.; Brandl, W.; Beckert, R.; Görls, H.

ARKIVOC 2013, iii, 174–188.

(21) Ciscato, L. F. M. L.; Bartoloni, F. H.; Colavite, A. S.; Weiss, D.; Beckert, R.; Schramm, S.

Photochem. Photobiol. Sci. 2013, 13 (1), 32–37.

(22) Almeida de Oliveira, M.; Bartoloni, F. H.; Augusto, F. A.; Ciscato, L. F. M. L.; Bastos, E. L.;

Baader, W. J. J. Org. Chem. 2012, 77 (23), 10537–10544.






(27) Field, L. R.; Wilhelm, E.; Battino, R. J. Chem. Thermodyn. 1974, 6 (3), 237–243.



67

3

Chapter 4Determination of ligand exchange dynamics and sonication induced ligand exchange rate in Imidazole-Pd centered coordination polymers

Thermal ligand exchange between imidazole Palladium (II) complexes has been investigated

in the presence of excess free imidazole ligand in CHCl3. Ligand exchange between bound

and free ligands in a mixture of Pd(EtIm)2Cl2 and EtIm was shown to occur through an asso-

ciative process via intermediate [Pd(EtIm)3Cl]Cl. NMR and MALDI-TOF established the struc-

ture of the intermediate. In contrast to the thermal process, ligand exchange was dissocia-

tive when mixtures of polymer functionalized complex Pd(Im-pTHF)2Cl2 and low molecular

weight complex Pd(DodecIm)2Cl2 were sonicated in solution. 40% of exchange was observed

after 1h of continuous sonication at 20 °C.

Introduction

Coordination polymers (CPs), with their dynamic reversible nature, find applications as

smart materials, for instance, as stimuli-‐responsive, self-‐healing and photoactive

polymers.1,2 Recent attention in this research area is focused on the characterization of the

dynamic properties of CPs by exploring their exchange kinetics,3 ring–chain equilibria,4

solvent interactions,5 and new metal– ligand combinations.6

We have shown that the coordination sphere of transition-‐metal complexes in the main

chain of CPs can be manipulated by means of mechanical force.7 As demonstrated by us and

other research groups, transition metal CPs can be used as mechanoresponsive polymers

and provide opportunity for mechanical release of reactive groups that can be used in

further reactions, e.g. as catalytically active sites.8

Ultrasound is one of the most efficient techniques to create forces that break bonds of

polymer backbones in solution.9 In order to effectively transduce elongational strain fields in

solution, high molecular weight polymers are required.10 Incorporation of weak bonds into a

polymer chain increases the sensitivity of the chain to mechanical force and thus decreases

the molecular weight threshold for mechanochemical chain scission. Groote et al.

investigated the scission of metal-‐ligand bonds in supramolecular polymer complexes by

ultrasound and showed that the force required to break metal−ligand bond is much lower

than the force that is typically required to break covalent bonds.11 Thus, ultrasonication of

reversible coordination polymers enables the specific rupture of metal ligand coordination

bonds since force selectively breaks the weakest bond on the chain.

One of the first reports on mechanochemistry of CP’s involved high-‐molecular-‐weight linear

polymers of alkyldicyclohexylphosphine (ADChP) telechelic polytetrahydrofuran with

palladium(II) dichloride.7,12 Molecular weights of these polymers were altered reversibly by

ultrasonic chain scission. In order to scavenge the reactive chain ends created by ultrasonic

scission, coordinatively unsaturated Pd(II) and phosphine, bisalkyldiphenylphosphine

(ADPhP) complex of palladium(II) dichloride was added. This low molecular weight complex

shows high reactivity towards ADChP ligands and coordinatively unsaturated Pd, but not

towards the coordination polymer. High nucleophilicity of ADChP resulted in complete

displacement of ADPhP ligand.4 These results aroused our curiosity towards understanding

the mechanisms and kinetics of ligand exchange triggered by ultrasound.

Insight in the dynamics of ligand exchange in coordination polymers is required for rational

use in diverse applications such as responsive materials, where the ligand exchange kinetics

determine mechanical properties, and mechano-‐catalysis, where relative rates of scission

and association determine the steady state concentration of active species.

Generally, ligand exchange occurs by one of two mechanisms: dissociative or associative.13–

16 In a dissociative mechanism one of the ligands coordinated to the metal center

dissociates, leaving an electron deficient complex. Then, a free ligand coordinates to metal,

resulting in ligand exchange. In transition metal complexes, dissociative ligand exchange is

favored in 18e-‐ systems to avoid the formation of an energetically unfavorable 20e-‐

intermediate. On the other hand, ligand exchange occurs through an associative mechanism

for 16e-‐ square planar complexes. The intermediate in this case is an 18e-‐ complex and

therefore provides a lower energy route to the product than the 14e-‐ intermediate, formed

via dissociative ligand exchange.

In the current Chapter, the thermal ligand exchange mechanism (Scheme 1) and rate of

ultrasound-‐induced scission of Pd-‐Imidazole CP’s are investigated. Pd-‐Imidazole

coordination complexes were selected to learn how ligand exchange could be initiated by

mechanical force. This knowledge can be used in further studies such as solid-‐state catalysis

and self-‐healing materials. Initial attempts and some preliminary results for the latter are

introduced in Chapter 6.

Chapter 4

70

Introduction

Coordination polymers (CPs), with their dynamic reversible nature, find applications as

smart materials, for instance, as stimuli-‐responsive, self-‐healing and photoactive

polymers.1,2 Recent attention in this research area is focused on the characterization of the

dynamic properties of CPs by exploring their exchange kinetics,3 ring–chain equilibria,4

solvent interactions,5 and new metal– ligand combinations.6

We have shown that the coordination sphere of transition-‐metal complexes in the main

chain of CPs can be manipulated by means of mechanical force.7 As demonstrated by us and

other research groups, transition metal CPs can be used as mechanoresponsive polymers

and provide opportunity for mechanical release of reactive groups that can be used in

further reactions, e.g. as catalytically active sites.8

Ultrasound is one of the most efficient techniques to create forces that break bonds of

polymer backbones in solution.9 In order to effectively transduce elongational strain fields in

solution, high molecular weight polymers are required.10 Incorporation of weak bonds into a

polymer chain increases the sensitivity of the chain to mechanical force and thus decreases

the molecular weight threshold for mechanochemical chain scission. Groote et al.

investigated the scission of metal-‐ligand bonds in supramolecular polymer complexes by

ultrasound and showed that the force required to break metal−ligand bond is much lower

than the force that is typically required to break covalent bonds.11 Thus, ultrasonication of

reversible coordination polymers enables the specific rupture of metal ligand coordination

bonds since force selectively breaks the weakest bond on the chain.

One of the first reports on mechanochemistry of CP’s involved high-‐molecular-‐weight linear

polymers of alkyldicyclohexylphosphine (ADChP) telechelic polytetrahydrofuran with

palladium(II) dichloride.7,12 Molecular weights of these polymers were altered reversibly by

ultrasonic chain scission. In order to scavenge the reactive chain ends created by ultrasonic

scission, coordinatively unsaturated Pd(II) and phosphine, bisalkyldiphenylphosphine

(ADPhP) complex of palladium(II) dichloride was added. This low molecular weight complex

shows high reactivity towards ADChP ligands and coordinatively unsaturated Pd, but not

towards the coordination polymer. High nucleophilicity of ADChP resulted in complete

displacement of ADPhP ligand.4 These results aroused our curiosity towards understanding

the mechanisms and kinetics of ligand exchange triggered by ultrasound.

Insight in the dynamics of ligand exchange in coordination polymers is required for rational

use in diverse applications such as responsive materials, where the ligand exchange kinetics

determine mechanical properties, and mechano-‐catalysis, where relative rates of scission

and association determine the steady state concentration of active species.

Generally, ligand exchange occurs by one of two mechanisms: dissociative or associative.13–

16 In a dissociative mechanism one of the ligands coordinated to the metal center

dissociates, leaving an electron deficient complex. Then, a free ligand coordinates to metal,

resulting in ligand exchange. In transition metal complexes, dissociative ligand exchange is

favored in 18e-‐ systems to avoid the formation of an energetically unfavorable 20e-‐

intermediate. On the other hand, ligand exchange occurs through an associative mechanism

for 16e-‐ square planar complexes. The intermediate in this case is an 18e-‐ complex and

therefore provides a lower energy route to the product than the 14e-‐ intermediate, formed

via dissociative ligand exchange.

In the current Chapter, the thermal ligand exchange mechanism (Scheme 1) and rate of

ultrasound-‐induced scission of Pd-‐Imidazole CP’s are investigated. Pd-‐Imidazole

coordination complexes were selected to learn how ligand exchange could be initiated by

mechanical force. This knowledge can be used in further studies such as solid-‐state catalysis

and self-‐healing materials. Initial attempts and some preliminary results for the latter are

introduced in Chapter 6.

Determination of ligand exchange dynamics

71

4

Scheme 1: Potential ligand exchange pathways between complex Pd(EtIm)2Cl2 and free ligand EtIm

NN Pd NCl

N

Cl+ N N

NN Pd NCl

N

Cl

+

N

N

NN Pd NCl

N

Cl

N

N

NN Pd NN

ClN

N+ Cl

'

NN Pd N

N

ClN

N

Cl

NN Pd NCl

N

Cl

+N

N

Dissociative

Associative


Synthesis of Pd-‐Imidazole complexes

Scheme 2: Synthesis of polymer attached complex and model complexes.

Im-‐pTHF ligands were prepared by terminating poly tetrahydrofuran (pTHF) with imidazole

(Im) (Scheme 2). Cationic ring opening polymerization of THF was performed as described in

Chapter 2. Polymerization was terminated by nucleophilic substitution of the sodium

imidazolate, obtained by deprotonation of 1H-‐imidazole with NaH in THF. Once Im-‐pTHF is

formed, the nucleophilic N(3) nitrogen can react further with cationic chain ends to form 1,3

pTHF-‐imidazolium salt. Therefore, sodium imidazolate end-‐capper was used five-‐fold in

excess relative to methyl triflate initiator. Molecular weights of resulting polymers were

analyzed by 1H NMR, MALDI-‐TOF and gel permeation chromatography (GPC). MW value

obtained in MALDI-‐TOF was used for the further calculations. Depending on the

polymerization time Im-‐pTHFs with MW of 18 kDa and 35 kDa were prepared.

Chapter 4

72

Scheme 1: Potential ligand exchange pathways between complex Pd(EtIm)2Cl2 and free ligand EtIm

NN Pd NCl

N

Cl+ N N

NN Pd NCl

N

Cl

+

N

N

NN Pd NCl

N

Cl

N

N

NN Pd NN

ClN

N+ Cl

'

NN Pd N

N

ClN

N

Cl

NN Pd NCl

N

Cl

+N

N

Dissociative

Associative


Synthesis of Pd-‐Imidazole complexes

Scheme 2: Synthesis of polymer attached complex and model complexes.

Im-‐pTHF ligands were prepared by terminating poly tetrahydrofuran (pTHF) with imidazole

(Im) (Scheme 2). Cationic ring opening polymerization of THF was performed as described in

Chapter 2. Polymerization was terminated by nucleophilic substitution of the sodium

imidazolate, obtained by deprotonation of 1H-‐imidazole with NaH in THF. Once Im-‐pTHF is

formed, the nucleophilic N(3) nitrogen can react further with cationic chain ends to form 1,3

pTHF-‐imidazolium salt. Therefore, sodium imidazolate end-‐capper was used five-‐fold in

excess relative to methyl triflate initiator. Molecular weights of resulting polymers were

analyzed by 1H NMR, MALDI-‐TOF and gel permeation chromatography (GPC). MW value

obtained in MALDI-‐TOF was used for the further calculations. Depending on the

polymerization time Im-‐pTHFs with MW of 18 kDa and 35 kDa were prepared.


73

4

Polymer functionalized imidazolyl ligands Im-‐pTHF, EtIm and DodecIm were coordinated to

Pd via exchange of the acetonitrile (MeCN) ligand in Pd(MeCN)2Cl2 to give

mechanoresponsive complex Pd(Im-‐pTHF)2Cl2 and low molecular weight complexes

Pd(EtIm)2Cl2 and Pd(DodecIm)2Cl2. MeCN-‐Pd coordination is sufficiently labile to allow ligand

exchange at room temperature in dichloromethane (DCM). Coordination of imidazole

ligands was evident from the change in chemical shifts for H(2), H(4) and H(5) protons on

the imidazole heterocycle. In the Pd(Im-‐pTHF)2Cl2 complex, chemical shifts for the imidazolyl

moiety were identical to those in the small molecule counterparts Pd(EtIm)2Cl2 and

Pd(DodecIm)2Cl2. 1H NMR spectra were in good agreement with literature data published

previously.17

Im-‐pTHF coordination reactions were completed after 2h as molecular weight distributions

monitored by GPC did not change after 2h. GPC also showed that reaction of polymer

functionalized ligand Im-‐pTHF (Mn = 18 kDa) resulted in doubling of the molecular weight to

36 kDa (Figure 1). This indicates that two imidazoles are coordinated to Pd although it has

been reported that coordination of three imidazole ligands to Pd (II) may also take place at

higher ligand to metal ratio and specific reaction conditions.17

Figure 1: 1H NMR spectra (left, in CD2Cl2) and GPC traces (right, in THF) for Im-‐pTHF18k and Pd(Im-‐

pTHF18k)2Cl2.

Ligand exchange between EtIm and Pd(EtIm)2Cl2

Structure of the intermediate

In order to determine the ligand exchange mechanism for palladium-‐imidazole complexes,

free ligand EtIm (L) and Pd(EtIm)2Cl2 (C) were mixed in a 1:1 molar ratio (0.09 M each) in

CDCl3. The 1H-‐NMR spectrum (Figure 2) revealed the coexistence of three magnetically non-‐

equivalent species, L, coordination complexes C and intermediate, [Pd(EtIm)3Cl]Cl (I).

MALDI-‐TOF spectrum indicated that tris-‐imidazole coordinated complex [Pd(EtIm)3Cl]Cl

formed when free ligand and Pd(EtIm)2Cl2 were mixed (Figure 3).

Figure 2: Stacked 1H NMR spectra for EtIm (b), Pd(EtIm)2Cl2 (c) and [Pd(EtIm)3Cl]Cl (a) in CDCl3 (400

MHz).

Chapter 4

74

Polymer functionalized imidazolyl ligands Im-‐pTHF, EtIm and DodecIm were coordinated to

Pd via exchange of the acetonitrile (MeCN) ligand in Pd(MeCN)2Cl2 to give

mechanoresponsive complex Pd(Im-‐pTHF)2Cl2 and low molecular weight complexes

Pd(EtIm)2Cl2 and Pd(DodecIm)2Cl2. MeCN-‐Pd coordination is sufficiently labile to allow ligand

exchange at room temperature in dichloromethane (DCM). Coordination of imidazole

ligands was evident from the change in chemical shifts for H(2), H(4) and H(5) protons on

the imidazole heterocycle. In the Pd(Im-‐pTHF)2Cl2 complex, chemical shifts for the imidazolyl

moiety were identical to those in the small molecule counterparts Pd(EtIm)2Cl2 and

Pd(DodecIm)2Cl2. 1H NMR spectra were in good agreement with literature data published

previously.17

Im-‐pTHF coordination reactions were completed after 2h as molecular weight distributions

monitored by GPC did not change after 2h. GPC also showed that reaction of polymer

functionalized ligand Im-‐pTHF (Mn = 18 kDa) resulted in doubling of the molecular weight to

36 kDa (Figure 1). This indicates that two imidazoles are coordinated to Pd although it has

been reported that coordination of three imidazole ligands to Pd (II) may also take place at

higher ligand to metal ratio and specific reaction conditions.17

Figure 1: 1H NMR spectra (left, in CD2Cl2) and GPC traces (right, in THF) for Im-‐pTHF18k and Pd(Im-‐

pTHF18k)2Cl2.

Ligand exchange between EtIm and Pd(EtIm)2Cl2

Structure of the intermediate

In order to determine the ligand exchange mechanism for palladium-‐imidazole complexes,

free ligand EtIm (L) and Pd(EtIm)2Cl2 (C) were mixed in a 1:1 molar ratio (0.09 M each) in

CDCl3. The 1H-‐NMR spectrum (Figure 2) revealed the coexistence of three magnetically non-‐

equivalent species, L, coordination complexes C and intermediate, [Pd(EtIm)3Cl]Cl (I).

MALDI-‐TOF spectrum indicated that tris-‐imidazole coordinated complex [Pd(EtIm)3Cl]Cl

formed when free ligand and Pd(EtIm)2Cl2 were mixed (Figure 3).

Figure 2: Stacked 1H NMR spectra for EtIm (b), Pd(EtIm)2Cl2 (c) and [Pd(EtIm)3Cl]Cl (a) in CDCl3 (400

MHz).


75

4

Figure 3: MALDI-‐TOF spectrum of [Pd(EtIm)3Cl]Cl taken in CHCl3, 1mg/ml. Inset: chemical structure

of [Pd(EtIm)3Cl]Cl and its simulated mass spectrum.

2D EXSY spectra

The mechanism of exchange between free and bound Im was investigated using 2D

exchange spectroscopy (EXSY18,19). To monitor the ligand exchange, complex 2 was mixed

with free ligand in deuterated chloroform (CDCl3). 2D EXSY spectra were taken in CDCl3 at RT

with several different mixing times (tmix). Spectra indicate that free Im (which is not

coordinated to Pd center, abbreviated as L) exchanges with Im coordinated to Pd on the

time scale of typical tmix of 100-‐500 ms. When tmix was lower than 200 ms, spectra showed

cross peaks for exchange of L with intermediate [Pd(EtIm)3Cl]Cl (I) and for Pd(EtIm)2Cl2 (C)

with I. However, only at higher mixing times (>200 ms) cross peaks for L-‐C exchange

becomes visible (Figure 4).

Figure 4: 2D EXSY spectra of 0.09 M solution of N-‐ethyl Imidazole and Pd(EtIm)2Cl2 in CDCl3 at tmix =

200 ms and tmix = 500 ms. Circles were used to highlight cross peaks appear when tmix is high (≥ 300

ms).

In order to investigate exchange rate constants, ratios of cross peaks to diagonal peaks were

calculated using absolute peak volumes at each tmix. C-‐L crosspeaks are absent at low

mixing times and only appear at mixing times above 200 ms. The absence of direct exchange

between C and L in the presence of C-‐I and L-‐I cross peaks demonstrates the intermediacy

of I in the ligand exchange process of Pd(EtIm)2Cl2. Dissociative ligand exchange rates are

too slow to give cross-‐peaks in the spectrum.

NN Pd NN

ClN

N

N NNN Pd N

ClN

Cl

L C I

+ Cl)

Chapter 4

76

Figure 3: MALDI-‐TOF spectrum of [Pd(EtIm)3Cl]Cl taken in CHCl3, 1mg/ml. Inset: chemical structure

of [Pd(EtIm)3Cl]Cl and its simulated mass spectrum.

2D EXSY spectra

The mechanism of exchange between free and bound Im was investigated using 2D

exchange spectroscopy (EXSY18,19). To monitor the ligand exchange, complex 2 was mixed

with free ligand in deuterated chloroform (CDCl3). 2D EXSY spectra were taken in CDCl3 at RT

with several different mixing times (tmix). Spectra indicate that free Im (which is not

coordinated to Pd center, abbreviated as L) exchanges with Im coordinated to Pd on the

time scale of typical tmix of 100-‐500 ms. When tmix was lower than 200 ms, spectra showed

cross peaks for exchange of L with intermediate [Pd(EtIm)3Cl]Cl (I) and for Pd(EtIm)2Cl2 (C)

with I. However, only at higher mixing times (>200 ms) cross peaks for L-‐C exchange

becomes visible (Figure 4).

Figure 4: 2D EXSY spectra of 0.09 M solution of N-‐ethyl Imidazole and Pd(EtIm)2Cl2 in CDCl3 at tmix =

200 ms and tmix = 500 ms. Circles were used to highlight cross peaks appear when tmix is high (≥ 300

ms).

In order to investigate exchange rate constants, ratios of cross peaks to diagonal peaks were

calculated using absolute peak volumes at each tmix. C-‐L crosspeaks are absent at low

mixing times and only appear at mixing times above 200 ms. The absence of direct exchange

between C and L in the presence of C-‐I and L-‐I cross peaks demonstrates the intermediacy

of I in the ligand exchange process of Pd(EtIm)2Cl2. Dissociative ligand exchange rates are

too slow to give cross-‐peaks in the spectrum.

NN Pd NN

ClN

N

N NNN Pd N

ClN

Cl

L C I

+ Cl)


77

4

Figure 5: Graph of cross-‐peak intensities relative to diagonal intensities vs tmix for exchange between

C-‐I, L-‐I (left) and L-‐C (right).

On the other hand, for short mixing times, the intensity of C-‐I and L-‐I cross peaks increases

linearly with tmix indicating that these are direct exchange processes. Rate constants for

these processes were calculated from the slope of the transfer function ф vs tmix (Figure 6)

as described by Perrin and Dwyer.18 The (pseudo) first order rate constants were found to

be kL-‐I = 3.7 s–1 (r2 = 0.999) and kC-‐I = 3.2 s–1 (r2 = 0.999).

Figure 6: In order to determine rate

constants for two site exchange between

L-‐I and C-‐I by the equation 𝑘𝑘 = !!!"#

×ф,

the transfer functions (ф) were calculated

using the expression:ф = ln !!!!!!

where

𝑟𝑟 = !!!!!!!!!"!!!"

. Rate constants were

obtained from the slope of a plot of ф vs

tmix.

Sonication induced ligand exchange

Sonication induced ligand exchange was studied in toluene using polymer functionalized

complex Pd(Im-‐pTHF18)2Cl2 and a small molecule model complex at a 1:100 mole ratio.

Dodecyl Imidazole complex Pd(DodecIm)2Cl2 was used because of its high solubility in

toluene. MWs of the complexes are sufficiently different to allow monitoring of the ligand

exchange with GPC. 0.1 mM solutions of polymers were sonicated using a sonication probe

and solutions were saturated with CH4 by bubbling the gas through the solution starting 15

minutes prior to sonication. The temperature inside the sonication vessel was constant at

25 oC to prevent thermal reactions. This was done by circulation of water at 2oC between

the walls of the double-‐jacketed sonication vessel.

Mechanochemically induced ligand exchange resulted in fragmentation of Pd(Im-‐

pTHF18)2Cl2. Emergence of a new peak in GPC at a MW of 18 kDa was monitored over time.

This new peak is consistent with the formation of product with two different imidazole

ligands coordinated to the same metal center. 17% of the initial Pd(Im-‐pTHF18k)2Cl2

experienced ligand exchange after 2h of continuous sonication as monitored by GPC. On the

other hand, without sonication, the MW distribution of the mixture did not change over the

course of 2 h at RT, indicating that thermal ligand exchange between the complexes is slow.

Figure 7: Ultrasound induced ligand exchange between Pd(Im-‐pTHF18k)2Cl2 (0.1 mM) and Pd(Im-‐

Dodec)2Cl2 without sonication (left) and with sonication (right). Molar ratio between polymer and

small molecule is 1:100.

Chapter 4

78

Figure 5: Graph of cross-‐peak intensities relative to diagonal intensities vs tmix for exchange between

C-‐I, L-‐I (left) and L-‐C (right).

On the other hand, for short mixing times, the intensity of C-‐I and L-‐I cross peaks increases

linearly with tmix indicating that these are direct exchange processes. Rate constants for

these processes were calculated from the slope of the transfer function ф vs tmix (Figure 6)

as described by Perrin and Dwyer.18 The (pseudo) first order rate constants were found to

be kL-‐I = 3.7 s–1 (r2 = 0.999) and kC-‐I = 3.2 s–1 (r2 = 0.999).

Figure 6: In order to determine rate

constants for two site exchange between

L-‐I and C-‐I by the equation 𝑘𝑘 = !!!"#

×ф,

the transfer functions (ф) were calculated

using the expression:ф = ln !!!!!!

where

𝑟𝑟 = !!!!!!!!!"!!!"

. Rate constants were

obtained from the slope of a plot of ф vs

tmix.

Sonication induced ligand exchange

Sonication induced ligand exchange was studied in toluene using polymer functionalized

complex Pd(Im-‐pTHF18)2Cl2 and a small molecule model complex at a 1:100 mole ratio.

Dodecyl Imidazole complex Pd(DodecIm)2Cl2 was used because of its high solubility in

toluene. MWs of the complexes are sufficiently different to allow monitoring of the ligand

exchange with GPC. 0.1 mM solutions of polymers were sonicated using a sonication probe

and solutions were saturated with CH4 by bubbling the gas through the solution starting 15

minutes prior to sonication. The temperature inside the sonication vessel was constant at

25 oC to prevent thermal reactions. This was done by circulation of water at 2oC between

the walls of the double-‐jacketed sonication vessel.

Mechanochemically induced ligand exchange resulted in fragmentation of Pd(Im-‐

pTHF18)2Cl2. Emergence of a new peak in GPC at a MW of 18 kDa was monitored over time.

This new peak is consistent with the formation of product with two different imidazole

ligands coordinated to the same metal center. 17% of the initial Pd(Im-‐pTHF18k)2Cl2

experienced ligand exchange after 2h of continuous sonication as monitored by GPC. On the

other hand, without sonication, the MW distribution of the mixture did not change over the

course of 2 h at RT, indicating that thermal ligand exchange between the complexes is slow.


Dodec)2Cl2 without sonication (left) and with sonication (right). Molar ratio between polymer and

small molecule is 1:100.


79

4


Dodec)2Cl2. Molar ratio between polymer and small molecule is 1:100.

Pd(Im-‐pTHF35k)2Cl2 polymer was also tested in sonication induced ligand exchange. After 1h

of continuous sonication, 40% of complexes were exchanged, a significantly higher fraction

than for the 36k complex. The molecular weight dependence of the rate demonstrates that

bond scission is a mechano-‐chemical process and not the result of local heating or reaction

with solvent decomposition products.

Rate of sonication induced chain scission

Figure 9: Concentration change of Pd(Im-‐pTHF)2Cl2 complexes during sonication in the presence of

Pd(DodecIm)2Cl2. Red lines represent linear fitting.

The rate of sonication induced chain scission was determined as described in Chapter 2.

Samples were taken during sonication and submitted to the GPC. Since area under the GPC

trace is proportional to the weight fraction of the polymer, change in polymer concentration

could be found by deconvolution of the GPC traces. Regardless of initial molecular weights

of polymer complex, the concentration of initial complex decreased linearly with sonication

time (Figure 9), suggesting that the consumption of complex is a zeroth order process, of

which the rate does not depend on the concentration of complex. This behavior is in stark

contrast to the experiments described in previous chapters, where the concentration of

polymer decayed exponentially during sonication, from which it was concluded that

mechanochemical chain scission follows first order reaction kinetics.

Thus, in the current experiments, the rate-‐determining step appears not to be the

mechanochemical dissociation of the polymer ligand, but a later step. In order to analyze

the process in more detail, rates of separate steps were calculated, and the overall process

was simulated.

Sonication induced ligand exchange between Pd(Im-‐pTHF)2Cl2 and Pd(Im-‐Dodec)2Cl2 involves

three main steps: i) dissociation of polymeric ligand under stress, ii) association of free

Chapter 4

80


Dodec)2Cl2. Molar ratio between polymer and small molecule is 1:100.

Pd(Im-‐pTHF35k)2Cl2 polymer was also tested in sonication induced ligand exchange. After 1h

of continuous sonication, 40% of complexes were exchanged, a significantly higher fraction

than for the 36k complex. The molecular weight dependence of the rate demonstrates that

bond scission is a mechano-‐chemical process and not the result of local heating or reaction

with solvent decomposition products.

Rate of sonication induced chain scission

Figure 9: Concentration change of Pd(Im-‐pTHF)2Cl2 complexes during sonication in the presence of

Pd(DodecIm)2Cl2. Red lines represent linear fitting.

The rate of sonication induced chain scission was determined as described in Chapter 2.

Samples were taken during sonication and submitted to the GPC. Since area under the GPC

trace is proportional to the weight fraction of the polymer, change in polymer concentration

could be found by deconvolution of the GPC traces. Regardless of initial molecular weights

of polymer complex, the concentration of initial complex decreased linearly with sonication

time (Figure 9), suggesting that the consumption of complex is a zeroth order process, of

which the rate does not depend on the concentration of complex. This behavior is in stark

contrast to the experiments described in previous chapters, where the concentration of

polymer decayed exponentially during sonication, from which it was concluded that

mechanochemical chain scission follows first order reaction kinetics.

Thus, in the current experiments, the rate-‐determining step appears not to be the

mechanochemical dissociation of the polymer ligand, but a later step. In order to analyze

the process in more detail, rates of separate steps were calculated, and the overall process

was simulated.

Sonication induced ligand exchange between Pd(Im-‐pTHF)2Cl2 and Pd(Im-‐Dodec)2Cl2 involves

three main steps: i) dissociation of polymeric ligand under stress, ii) association of free


81

4

ligand to Pd(Im-‐Dodec)2Cl2 to form the tris-‐imidazole intermediate as mentioned above, and

iii) dissociation of one of the three ligands.

Relevant reactions during sonication are (Scheme 3);

𝑃𝑃𝑃𝑃𝑃𝑃 ↔ 𝑃𝑃𝑃𝑃 + 𝑃𝑃 1

𝑃𝑃𝑃𝑃 + 𝐿𝐿 → 𝑃𝑃𝑃𝑃𝑃𝑃 2

𝐿𝐿𝐿𝐿𝐿𝐿 + 𝑃𝑃 ↔ 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 3

𝑃𝑃𝑃𝑃𝑃𝑃 + 𝐿𝐿 ↔ 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 4

𝐿𝐿𝐿𝐿𝐿𝐿 + 𝐿𝐿 ↔ 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 5


𝑃𝑃𝑃𝑃𝑃𝑃 + 𝑃𝑃 ↔ 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 7


𝑃𝑃:𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝐼𝐼𝐼𝐼 − 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝,𝑀𝑀:𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑐𝑐𝑐𝑐𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃! ,

𝐿𝐿: 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 (𝐼𝐼𝐼𝐼 − 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷)

Scheme 3: Plausible mechanism for ultrasound induced ligand exchange between Pd(Im-‐pTHF)2Cl2

and Pd(Im-‐Dodec)2Cl2.

NNPd N NCl

N

NC₁₂H₂₅

Cl*

NN Pd N NCl

ClO O N N C₁₂H₂₅ NN Pd N N

Cl

ClO C₁₂H₂₅+

n n n

On

On

NN Pd N NCl

ClO O +

n nNN Pd N N

Cl

ClC₁₂H₂₅ C₁₂H₂₅

N N O +n

NN Pd N NCl


NNPd N NCl

N

N

Cl*

C₁₂H₂₅ C₁₂H₂₅

O

n

NN PdCl

ClO

nN N C₁₂H₂₅

NN Pd N NCl

ClO C₁₂H₂₅+

PMP LML

P

L

PMLL

PML

PMPL

PM PMLL

Chapter 4

82

ligand to Pd(Im-‐Dodec)2Cl2 to form the tris-‐imidazole intermediate as mentioned above, and

iii) dissociation of one of the three ligands.

Relevant reactions during sonication are (Scheme 3);

𝑃𝑃𝑃𝑃𝑃𝑃 ↔ 𝑃𝑃𝑃𝑃 + 𝑃𝑃 1

𝑃𝑃𝑃𝑃 + 𝐿𝐿 → 𝑃𝑃𝑃𝑃𝑃𝑃 2


𝑃𝑃𝑃𝑃𝑃𝑃 + 𝐿𝐿 ↔ 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 4



𝑃𝑃𝑃𝑃𝑃𝑃 + 𝑃𝑃 ↔ 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 7


𝑃𝑃:𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝐼𝐼𝐼𝐼 − 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝,𝑀𝑀:𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑐𝑐𝑐𝑐𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃! ,

𝐿𝐿: 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 (𝐼𝐼𝐼𝐼 − 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷)

Scheme 3: Plausible mechanism for ultrasound induced ligand exchange between Pd(Im-‐pTHF)2Cl2

and Pd(Im-‐Dodec)2Cl2.

NNPd N NCl

N

NC₁₂H₂₅

Cl*

NN Pd N NCl

ClO O N N C₁₂H₂₅ NN Pd N N

Cl

ClO C₁₂H₂₅+

n n n

On

On

NN Pd N NCl

ClO O +

n nNN Pd N N

Cl


N N O +n

NN Pd N NCl


NNPd N NCl

N

N

Cl*

C₁₂H₂₅ C₁₂H₂₅

O

n

NN PdCl

ClO

nN N C₁₂H₂₅

NN Pd N NCl

ClO C₁₂H₂₅+

PMP LML

P

L

PMLL

PML

PMPL

PM PMLL


83

4

Relative peak areas in 1H NMR spectrum of the small molecule intermediate (Figure 2) give a

1:5:5 ratio of equilibrium concentrations of I:C:L. Since the initial concentrations of C and L

are 0.09 M,

𝐿𝐿 ! = 0.09𝑀𝑀, 𝐶𝐶 ! = 0.09𝑀𝑀, [𝐼𝐼] = [𝐶𝐶] 5

𝐼𝐼 = 0.015 𝑀𝑀 𝑎𝑎𝑎𝑎𝑎𝑎 𝐶𝐶 , 𝐿𝐿 = 0.075

the association constant can be calculated as

𝐾𝐾! = [𝐼𝐼]

[𝐶𝐶]×[𝐸𝐸] =0.0150.075! ≅ 2.7 𝑀𝑀!!

Rate constants in scission experiments can be written as:

Pd(Im-‐pTHF-‐35k)2Cl2 showed 40% chain scission after 1h sonication, so the scission rate

constant k1 should be higher than 10-‐4 s-‐1. The reverse reaction and reaction (2) are assumed

to be diffusion controlled. We used the diffusion controlled association constant calculated

previously for UPy dimers in toluene (k-‐1 = 108 M-‐1s-‐1)20

𝐾𝐾 =𝑘𝑘!!

𝑘𝑘! + 𝑘𝑘!=

𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃

where k2 is sum of rates for reactions that involves polymeric ligand, P. Those reactions are

(3), (6) and (7).

The steps in the exchange process were modeled in the metabolic modeling program

GEPASI21 in order to verify that the combination of elementary reaction rate constants

discussed above results in a linear decrease of the polymer concentration during sonication.

When the rate constants and equilibrium concentrations as calculated above were used in

simulation, the polymer concentration indeed decreased linearly in time (Figure 10). The

simulations showed that this is caused by the slow kinetics of the ligand association step

(i.e. third ligand coordinates to Pd(NIm)2 complex, reaction (3) in Scheme 3).

P M P P M +k₁

k&₁

P

Figure 10: Simulated plot for change in polymer concentration during sonication vs. time using

GEPASI compared to the data that was obtained experimentally.

Conclusions

The ligand exchange mechanism for imidazole Palladium complexes has been investigated in

the presence of excess free imidazole ligand in CHCl3 as an associative process. When

Pd(EtIm)2Cl2 and EtIm were mixed, a tris imidazolyl intermediate complex [Pd(EtIm)3Cl]Cl

was identified by NMR and MALDI-‐TOF. It has been shown with 2D EXSY that there is direct

magnetization transfer between Pd(EtIm)2Cl2 and the intermediate. However, exchange

between Pd(EtIm)2Cl2 and EtIm is indirect, and proceeds via the stable intermediate

[Pd(EtIm)3Cl]Cl.

Complexes with monotelechelic polymeric ligands Pd(Im-‐pTHF)2Cl2 were sonicated in the

presence of Pd(Im-‐Dodec)2Cl2 and GPC traces showed that during sonication polymeric

ligands dissociate from the central metal.

Previously, the rate of sonication induced chain scission in coordination complexes was

investigated for NHC bearing Pd(NHC-‐pTHF)2Cl2 in the presence of trapping agents (AcOH

and MeCN). These trapping agents coordinate to the metal center and protonate the free

carbene released when the bond between metal and ligand is broken. Although it has been

established in previous chapters that the bond scission observed during sonication of these

complexes is mechanically induced, it is still arguable that trapping agents play a role in

thermally induced chain scission by destabilizing the coordination bond. However, the

ultrasound induced ligand exchange presented in this chapter occurs in the absence of

Chapter 4

84

Relative peak areas in 1H NMR spectrum of the small molecule intermediate (Figure 2) give a

1:5:5 ratio of equilibrium concentrations of I:C:L. Since the initial concentrations of C and L

are 0.09 M,

𝐿𝐿 ! = 0.09𝑀𝑀, 𝐶𝐶 ! = 0.09𝑀𝑀, [𝐼𝐼] = [𝐶𝐶] 5

𝐼𝐼 = 0.015 𝑀𝑀 𝑎𝑎𝑎𝑎𝑎𝑎 𝐶𝐶 , 𝐿𝐿 = 0.075

the association constant can be calculated as

𝐾𝐾! = [𝐼𝐼]

[𝐶𝐶]×[𝐸𝐸] =0.0150.075! ≅ 2.7 𝑀𝑀!!

Rate constants in scission experiments can be written as:

Pd(Im-‐pTHF-‐35k)2Cl2 showed 40% chain scission after 1h sonication, so the scission rate

constant k1 should be higher than 10-‐4 s-‐1. The reverse reaction and reaction (2) are assumed

to be diffusion controlled. We used the diffusion controlled association constant calculated

previously for UPy dimers in toluene (k-‐1 = 108 M-‐1s-‐1)20

𝐾𝐾 =𝑘𝑘!!

𝑘𝑘! + 𝑘𝑘!=

𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃

where k2 is sum of rates for reactions that involves polymeric ligand, P. Those reactions are

(3), (6) and (7).

The steps in the exchange process were modeled in the metabolic modeling program

GEPASI21 in order to verify that the combination of elementary reaction rate constants

discussed above results in a linear decrease of the polymer concentration during sonication.

When the rate constants and equilibrium concentrations as calculated above were used in

simulation, the polymer concentration indeed decreased linearly in time (Figure 10). The

simulations showed that this is caused by the slow kinetics of the ligand association step

(i.e. third ligand coordinates to Pd(NIm)2 complex, reaction (3) in Scheme 3).

P M P P M +k₁

k&₁

P

Figure 10: Simulated plot for change in polymer concentration during sonication vs. time using

GEPASI compared to the data that was obtained experimentally.

Conclusions

The ligand exchange mechanism for imidazole Palladium complexes has been investigated in

the presence of excess free imidazole ligand in CHCl3 as an associative process. When

Pd(EtIm)2Cl2 and EtIm were mixed, a tris imidazolyl intermediate complex [Pd(EtIm)3Cl]Cl

was identified by NMR and MALDI-‐TOF. It has been shown with 2D EXSY that there is direct

magnetization transfer between Pd(EtIm)2Cl2 and the intermediate. However, exchange

between Pd(EtIm)2Cl2 and EtIm is indirect, and proceeds via the stable intermediate

[Pd(EtIm)3Cl]Cl.

Complexes with monotelechelic polymeric ligands Pd(Im-‐pTHF)2Cl2 were sonicated in the

presence of Pd(Im-‐Dodec)2Cl2 and GPC traces showed that during sonication polymeric

ligands dissociate from the central metal.

Previously, the rate of sonication induced chain scission in coordination complexes was

investigated for NHC bearing Pd(NHC-‐pTHF)2Cl2 in the presence of trapping agents (AcOH

and MeCN). These trapping agents coordinate to the metal center and protonate the free

carbene released when the bond between metal and ligand is broken. Although it has been

established in previous chapters that the bond scission observed during sonication of these

complexes is mechanically induced, it is still arguable that trapping agents play a role in

thermally induced chain scission by destabilizing the coordination bond. However, the

ultrasound induced ligand exchange presented in this chapter occurs in the absence of


85

4

trapping agents, and thermal processes would lead to the formation of an equilibrium

mixture of starting material and product Pd[(Im-‐pTHF)(Im-‐Dodec)]Cl2. The selective

formation of product therefore further proves that the scission is mechanochemical. The

direction of the reaction is governed by the fact that the coordination bond in the product

Pd[(Im-‐pTHF)(Im-‐Dodec)]Cl2 is not responsive to mechanical stress because it is at the end

of the polymer chain, where solvodynamic forces are too low to cause scission.

Experimental

General

All chemicals were purchased from commercial sources used without further purification unless

specified otherwise. Dry tetrahydrofuran (THF, HPLC grade) and dichloromethane (DCM) were

degassed with argon and purified by passage through activated alumina solvent column prior to use.

Sodium hydride was 60% dispersion in mineral oil as obtained from commercial source washed with

n-‐hexane under Ar in a Schlenk flask prior to use. A Varian 400MR or a Varian Mercury 400

spectrometer was used to record 1H NMR (400 MHz) Chemical shifts are reported in ppm and

referenced to chemical shifts for tetramethylsilane or residual solvents. 2D EXSY spectra were

recorded on Varian Inova 500 spectrometer with NOESY pulse sequence. Gel permeation

chromatography (GPC) was performed on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel

5μm MIXED-‐C and MIXEDD columns (linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐

M10A UV-‐vis detector at 254 nm and RID-‐10A refractive index detector, and THF as eluent at a flow

rate of 1 mL/min (20 °C). Polystyrene standards were used for calibration.







temperature of the solution were checked by thermocouple was constant at 25oC after the thermal

equilibrium was achieved in the first 3-‐5 mins. During sonication saturation gas (CH4) was bubbled

through solution via teflon tubing. Aliquots of 100 µL were taken at different time intervals. Toluene

was removed under reduced pressure and residues were dissolved in THF and submitted to GPC.

Results were analyzed and concentration change during sonication was determined by double


Synthesis of α-‐(N-‐imidazole)-‐ω-‐methoxy poly(tetrahydrofuran)

Polymer ligands (Im-‐pTHF) were synthesized via cationic ring-‐opening polymerization of

tetrahydrofuran (THF).22 THF (100 mL) and DTBP (200 µL, 0.92 mmol) were added methyl triflate

(100 µL, 0.91 mmol) inside a Schlenk round-‐bottom flask under Ar to initiate the polymerization. End

capper imidazolyl was prepared in a separate round bottom flask. Imidazole (306 mg, 4.5 mmol) was

added in portions onto NaH (400 mg, 10 mmol) dispersion in dry THF (10 ml) under Ar. After stirring

for defined time as described in chapter 2, the polymerization was terminated by adding imidazolyl

solution in one portion to polymerization flask. After 20 mins, 10 ml MeOH was added, the solution

was diluted to app ¼ of its initial volume under reduced pressure and precipitated in water (400 mL)

overnight at ambient temperature. White polymer was washed with water, dissolved in diethyl

ether (200 mL), dried over MgSO4 and precipitated overnight at –30 °C, white powder was filtered

washed with cold Et2O and yielded ligands as white powder. In order to remove traces of solvents,

ligands were left under vacuum at ambient temperature overnight prior to use. Molecular weights

were determined by MALDI-‐TOF as 18 kDa and 35 kDa depending on the polymerization time (2h

and 3.5h respectively). 1H NMR Im-‐pTHF18k [CD2Cl2, 400 MHz]: 7.84 ppm (s, Im), 7.17 ppm (s, Im),

7.06 ppm (s, Im), 4.08 ppm (t, N-‐CH2, J: 8Hz), 3.0-‐3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2-‐).

Synthesis of Pd(II)–imidazole polymer complexes Pd(Im-‐pTHF)2Cl2.

Im-‐pTHF (400 mg) was dissolved in DCM (10 ml) and added Pd(MeCN)2Cl2 (0.5 eq.). Ligand exchange

yielded desired polymer attached mechanophores after 2h. Solvent was evaporated under reduced

pressure. 1H NMR Pd(Im-‐pTHF18k)2Cl2 [CD2Cl2, 400 MHz]: 8.03 ppm (s, Im), 7.38 ppm (s, Im), 6.67

ppm (s, Im), 3.98 ppm (t, N-‐CH2, J: 8Hz), 3.0-‐3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2-‐).

Chapter 4

86

trapping agents, and thermal processes would lead to the formation of an equilibrium

mixture of starting material and product Pd[(Im-‐pTHF)(Im-‐Dodec)]Cl2. The selective

formation of product therefore further proves that the scission is mechanochemical. The

direction of the reaction is governed by the fact that the coordination bond in the product

Pd[(Im-‐pTHF)(Im-‐Dodec)]Cl2 is not responsive to mechanical stress because it is at the end

of the polymer chain, where solvodynamic forces are too low to cause scission.

Experimental

General

All chemicals were purchased from commercial sources used without further purification unless

specified otherwise. Dry tetrahydrofuran (THF, HPLC grade) and dichloromethane (DCM) were

degassed with argon and purified by passage through activated alumina solvent column prior to use.

Sodium hydride was 60% dispersion in mineral oil as obtained from commercial source washed with

n-‐hexane under Ar in a Schlenk flask prior to use. A Varian 400MR or a Varian Mercury 400

spectrometer was used to record 1H NMR (400 MHz) Chemical shifts are reported in ppm and

referenced to chemical shifts for tetramethylsilane or residual solvents. 2D EXSY spectra were

recorded on Varian Inova 500 spectrometer with NOESY pulse sequence. Gel permeation

chromatography (GPC) was performed on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel

5μm MIXED-‐C and MIXEDD columns (linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐

M10A UV-‐vis detector at 254 nm and RID-‐10A refractive index detector, and THF as eluent at a flow

rate of 1 mL/min (20 °C). Polystyrene standards were used for calibration.







temperature of the solution were checked by thermocouple was constant at 25oC after the thermal

equilibrium was achieved in the first 3-‐5 mins. During sonication saturation gas (CH4) was bubbled

through solution via teflon tubing. Aliquots of 100 µL were taken at different time intervals. Toluene

was removed under reduced pressure and residues were dissolved in THF and submitted to GPC.

Results were analyzed and concentration change during sonication was determined by double


Synthesis of α-‐(N-‐imidazole)-‐ω-‐methoxy poly(tetrahydrofuran)

Polymer ligands (Im-‐pTHF) were synthesized via cationic ring-‐opening polymerization of

tetrahydrofuran (THF).22 THF (100 mL) and DTBP (200 µL, 0.92 mmol) were added methyl triflate

(100 µL, 0.91 mmol) inside a Schlenk round-‐bottom flask under Ar to initiate the polymerization. End

capper imidazolyl was prepared in a separate round bottom flask. Imidazole (306 mg, 4.5 mmol) was

added in portions onto NaH (400 mg, 10 mmol) dispersion in dry THF (10 ml) under Ar. After stirring

for defined time as described in chapter 2, the polymerization was terminated by adding imidazolyl

solution in one portion to polymerization flask. After 20 mins, 10 ml MeOH was added, the solution

was diluted to app ¼ of its initial volume under reduced pressure and precipitated in water (400 mL)

overnight at ambient temperature. White polymer was washed with water, dissolved in diethyl

ether (200 mL), dried over MgSO4 and precipitated overnight at –30 °C, white powder was filtered

washed with cold Et2O and yielded ligands as white powder. In order to remove traces of solvents,

ligands were left under vacuum at ambient temperature overnight prior to use. Molecular weights

were determined by MALDI-‐TOF as 18 kDa and 35 kDa depending on the polymerization time (2h

and 3.5h respectively). 1H NMR Im-‐pTHF18k [CD2Cl2, 400 MHz]: 7.84 ppm (s, Im), 7.17 ppm (s, Im),

7.06 ppm (s, Im), 4.08 ppm (t, N-‐CH2, J: 8Hz), 3.0-‐3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2-‐).

Synthesis of Pd(II)–imidazole polymer complexes Pd(Im-‐pTHF)2Cl2.

Im-‐pTHF (400 mg) was dissolved in DCM (10 ml) and added Pd(MeCN)2Cl2 (0.5 eq.). Ligand exchange

yielded desired polymer attached mechanophores after 2h. Solvent was evaporated under reduced

pressure. 1H NMR Pd(Im-‐pTHF18k)2Cl2 [CD2Cl2, 400 MHz]: 8.03 ppm (s, Im), 7.38 ppm (s, Im), 6.67

ppm (s, Im), 3.98 ppm (t, N-‐CH2, J: 8Hz), 3.0-‐3.6 ppm (br O-‐CH2-‐), 1.3-‐2.2 ppm (br, OCH2-‐CH2-‐).


87

4

References

(1) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125 (46), 13922–13923.

(2) Dobrawa, R.; Würthner, F. J. Polym. Sci. Part Polym. Chem. 2005, 43 (21), 4981–4995.

(3) Yount, W. C.; Juwarker, H.; Craig, S. L. J. Am. Chem. Soc. 2003, 125 (50), 15302–15303.


(5) Vermonden, T.; van der Gucht, J.; de Waard, P.; Marcelis, A. T. M.; Besseling, N. A. M.;

Sudhölter, E. J. R.; Fleer, G. J.; Cohen Stuart, M. A. Macromolecules 2003, 36 (19), 7035–7044.

(6) Michelsen, U.; Hunter, C. A. Angew. Chem. Int. Ed. 2000, 39 (4), 764–767.

(7) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem.-‐Int. Ed. 2004, 43 (34), 4460–4462.


(9) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29 (1), 295–326.



4929–4935.


(13) Basolo, F.; Chatt, J.; Gray, H. B.; Pearson, R. G.; Shaw, B. L. J. Chem. Soc. Resumed 1961, No. 0,

2207–2215.

(14) Cramer, R. J. Am. Chem. Soc. 1972, 94 (16), 5681–5685.

(15) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117 (23), 6414–6415.

(16) Ghosh, P. K.; Mandal, H. K.; Mahapatra, A. Int. J. Chem. Kinet. 2011, 43 (3), 130–140.

(17) Szulmanowicz, M. S.; Zawartka, W.; Gniewek, A.; Trzeciak, A. M. Inorganica Chim. Acta 2010,

363 (15), 4346–4354.

(18) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90 (6), 935–967.

(19) Dwyer, T. J.; Norman, J. E.; Jasien, P. G. J. Chem. Educ. 1998, 75 (12), 1635.

(20) Söntjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc.

2000, 122 (31), 7487–7493.

(21) Mendes, P. Trends Biochem. Sci. 1997, 22 (9), 361–363.


2024.




Chapter 4

88

Chapter 5Mechanochemically induced, directed ligand exchange in polymeric Pd(II) complexes

Mechanochemically induced ligand exchange of Pd(II) complexes was used to direct the for-

mation of heterocomplexes. Symmetric complexes with high and low molecular weight po-

lymer-attached ligands were mixed in solution and sonicated. When one of the complexes

has a molecular weight higher than the threshold (Mlim) for mechanochemical chain scission,

while the other is smaller, sonication leads to the directed formation of a heterocomplex

with two different ligands.

References

(1) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125 (46), 13922–13923.

(2) Dobrawa, R.; Würthner, F. J. Polym. Sci. Part Polym. Chem. 2005, 43 (21), 4981–4995.



(5) Vermonden, T.; van der Gucht, J.; de Waard, P.; Marcelis, A. T. M.; Besseling, N. A. M.;

Sudhölter, E. J. R.; Fleer, G. J.; Cohen Stuart, M. A. Macromolecules 2003, 36 (19), 7035–7044.

(6) Michelsen, U.; Hunter, C. A. Angew. Chem. Int. Ed. 2000, 39 (4), 764–767.



(9) Suslick, K. S.; Price, G. J. Annu. Rev. Mater. Sci. 1999, 29 (1), 295–326.



4929–4935.


(13) Basolo, F.; Chatt, J.; Gray, H. B.; Pearson, R. G.; Shaw, B. L. J. Chem. Soc. Resumed 1961, No. 0,

2207–2215.

(14) Cramer, R. J. Am. Chem. Soc. 1972, 94 (16), 5681–5685.

(15) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc. 1995, 117 (23), 6414–6415.

(16) Ghosh, P. K.; Mandal, H. K.; Mahapatra, A. Int. J. Chem. Kinet. 2011, 43 (3), 130–140.

(17) Szulmanowicz, M. S.; Zawartka, W.; Gniewek, A.; Trzeciak, A. M. Inorganica Chim. Acta 2010,

363 (15), 4346–4354.

(18) Perrin, C. L.; Dwyer, T. J. Chem. Rev. 1990, 90 (6), 935–967.

(19) Dwyer, T. J.; Norman, J. E.; Jasien, P. G. J. Chem. Educ. 1998, 75 (12), 1635.

(20) Söntjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc.

2000, 122 (31), 7487–7493.

(21) Mendes, P. Trends Biochem. Sci. 1997, 22 (9), 361–363.


2024.




Introduction

Coordination polymers (CPs) have been used as mechanoresponsive polymers for

mechanical release of reactive groups, such as catalytically active sites, luminescent end

groups and small molecules.1–6 Our research group has shown that the coordination sphere

of transition-‐metal complexes in the main chain of CPs can be manipulated in solution by

ultrasound.7 The force required to break metal−ligand bond is much lower than the force

that is typically required to break covalent bonds.8 Thus, ultrasonication of CPs enables the

specific rupture of metal ligand coordination bonds. 9

Mechanical chain scission in reversible bonds drives dynamic systems away from

equilibrium. Paulusse showed, for instance, that the ring chain equilibrium in Pd-‐phosphine

coordination polymers is shifted reversibly by ultrasound.10 The equilibrium composition of

coordination polymer solutions can also be changed by ultrasound. As described in Chapter

4, Pd(Im)2Cl2 complexes in toluene do not show ligand exchange at room temperature.

However, when free imidazole is present, it replaces one of the coordinated imidazoles via

an associative mechanism, which results in ligand exchange. This offers a method to

selectively change the composition of a mixture of complexes by mechanochemical

activation.

Scheme 1: PdCl2-‐Imidazole complexes are kinetically stable against ligand exchange in the absence

of free ligand (top). Sonication dissociates one of the ligands of the polymeric complex by selectively

breaking the metal ligand bond. Then, released free ligand induces associative ligand exchange.

Provided that the molecular weight (MW) one of the complexes is above the limiting MW

(Mlim),11,12 to break the metal ligand bond by ultrasound, ligands can be exchanged and the

equilibrium will shift to the hetero-‐complex. Since mechanical force on the weak

coordinative bond increases with chain length, only polymer that has high MW, is

mechanically labile and breaks upon sonication. Free ligand binds to other complex in the

NN Pd N NCl

ClO O

NN PdCl

ClO

nN N O NN Pd N N

Cl

ClC12H25 C12H25

NN Pd N NCl

ClO

nC12H25

+

NN Pd N NCl

ClC12H25 C12H25+ x

solution and results in a solution with non-‐equilibrium composition (Scheme 1). In principle,

this feature of mechanochemical polymer scission allows formation of block copolymers

from a mixture of two symmetric coordination complexes (Figure 1).

Figure 1: Cartoon representation for the mechanochemically initiated block copolymerization.

In this chapter we investigate the potential of sonication-‐induced directed ligand exchange

towards copolymerization. Mechanical stress created in solution by implosion of cavitation

bubbles is used to transfer polymeric ligands from one metal center to another. This leads

to hetero-‐complexes with different ligands or different polymers attached to them. In order

to investigate mechanically directed ligand exchange three sets of experiments were

conducted. Firstly, two Pd(Im)2Cl2 complexes with different molecular weights were mixed

and the solution was subjected to ultrasound. Secondly, pTHF attached Pd complexes with

different ligands, NHC and Imidazole, were mixed to monitor the ultrasound-‐induced

formation of heterocomplex. Finally, Pd(Im-‐pTHF)2Cl2 was mixed with poly(methylacrylate)

attached pyridine (Py-‐pMA) complex of palladium, Pd(PyPMA)2Cl2, and sonicated to form a

block copolymer linked by Pd coordination. In all three cases change in MW distribution of

mixtures and formation of new species were monitored by GPC.

Chapter 5

90

Introduction

Coordination polymers (CPs) have been used as mechanoresponsive polymers for

mechanical release of reactive groups, such as catalytically active sites, luminescent end

groups and small molecules.1–6 Our research group has shown that the coordination sphere

of transition-‐metal complexes in the main chain of CPs can be manipulated in solution by

ultrasound.7 The force required to break metal−ligand bond is much lower than the force

that is typically required to break covalent bonds.8 Thus, ultrasonication of CPs enables the

specific rupture of metal ligand coordination bonds. 9

Mechanical chain scission in reversible bonds drives dynamic systems away from

equilibrium. Paulusse showed, for instance, that the ring chain equilibrium in Pd-‐phosphine

coordination polymers is shifted reversibly by ultrasound.10 The equilibrium composition of

coordination polymer solutions can also be changed by ultrasound. As described in Chapter

4, Pd(Im)2Cl2 complexes in toluene do not show ligand exchange at room temperature.

However, when free imidazole is present, it replaces one of the coordinated imidazoles via

an associative mechanism, which results in ligand exchange. This offers a method to

selectively change the composition of a mixture of complexes by mechanochemical

activation.

Scheme 1: PdCl2-‐Imidazole complexes are kinetically stable against ligand exchange in the absence

of free ligand (top). Sonication dissociates one of the ligands of the polymeric complex by selectively

breaking the metal ligand bond. Then, released free ligand induces associative ligand exchange.

Provided that the molecular weight (MW) one of the complexes is above the limiting MW

(Mlim),11,12 to break the metal ligand bond by ultrasound, ligands can be exchanged and the

equilibrium will shift to the hetero-‐complex. Since mechanical force on the weak

coordinative bond increases with chain length, only polymer that has high MW, is

mechanically labile and breaks upon sonication. Free ligand binds to other complex in the

NN Pd N NCl

ClO O

NN PdCl

ClO

nN N O NN Pd N N

Cl

ClC12H25 C12H25

NN Pd N NCl

ClO

nC12H25

+

NN Pd N NCl

ClC12H25 C12H25+ x

solution and results in a solution with non-‐equilibrium composition (Scheme 1). In principle,

this feature of mechanochemical polymer scission allows formation of block copolymers

from a mixture of two symmetric coordination complexes (Figure 1).

Figure 1: Cartoon representation for the mechanochemically initiated block copolymerization.

In this chapter we investigate the potential of sonication-‐induced directed ligand exchange

towards copolymerization. Mechanical stress created in solution by implosion of cavitation

bubbles is used to transfer polymeric ligands from one metal center to another. This leads

to hetero-‐complexes with different ligands or different polymers attached to them. In order

to investigate mechanically directed ligand exchange three sets of experiments were

conducted. Firstly, two Pd(Im)2Cl2 complexes with different molecular weights were mixed

and the solution was subjected to ultrasound. Secondly, pTHF attached Pd complexes with

different ligands, NHC and Imidazole, were mixed to monitor the ultrasound-‐induced

formation of heterocomplex. Finally, Pd(Im-‐pTHF)2Cl2 was mixed with poly(methylacrylate)

attached pyridine (Py-‐pMA) complex of palladium, Pd(PyPMA)2Cl2, and sonicated to form a

block copolymer linked by Pd coordination. In all three cases change in MW distribution of

mixtures and formation of new species were monitored by GPC.

Mechanochemically induced, directed ligand exchange

91

5


Polymer (pTHF) attached imidalozyl ligands (Im-‐pTHF) were prepared by terminating

cationic ring opening polymerization of THF sodium imidazolate. pTHF-‐Im ligands with

molecular weights of 30k and 6k were synthesized by adjusting polymerization time. 1H

NMR, GPC and MALDI-‐TOF were used to characterize the ligands. Molecular weights are

given as the peak MW determined by MALDI-‐TOF.

Figure 2: GPC traces of Pd(Im-‐pTHF-‐30k)2Cl2 (left) and Pd(Im-‐pTHF-‐6k)2Cl2 (right)

Symmetrical complexes Pd(Im-‐pTHF)2Cl2 were prepared by simply mixing polymer attached

Im-‐pTHF ligands with Pd(MeCN)2Cl2. GPC traces indicating a doubling of molecular weight

compared to the free ligands. (Figure 2) The peaks in the GPC traces of the 60 kDa and 12

kDa bis(imidazole)-‐Pd complexes have shoulders on the low MW side. 1H NMR did not

reveal peaks for un-‐complexed imidazole, which suggest that the shoulders correspond to

unfunctionalized pTHF not end-‐capped with imidazole.

Figure 3: GPC trace the mixture of 60 kDa and 12 kDa bis(imidazole)-‐Pd complexes after thermal

equilibrium has been reached by heating a mixture of Pd(Im-‐pTHF30k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 at

60 °C for 8 h.

When the two coordination polymers of 60 kDa and 12 kDa were mixed in toluene at a 1:1

ratio, no ligand exchange was observed after 2 h, thus the polymers are inert to each other.

However, after 8h at 60 oC the GPC trace showed the formation of a new species with

intermediate molecular weight (Figure 3).

A possible explanation for that could be at high temperatures complexes are more dynamic

and exchange their ligands, thus a new coordination polymer, which has one ligand from

each complex, was formed (monitored by GPC). When thermal equilibrium was established,

a statistical mixture was obtained containing three polymers with different MWs.

Chapter 5

92


Polymer (pTHF) attached imidalozyl ligands (Im-‐pTHF) were prepared by terminating

cationic ring opening polymerization of THF sodium imidazolate. pTHF-‐Im ligands with

molecular weights of 30k and 6k were synthesized by adjusting polymerization time. 1H

NMR, GPC and MALDI-‐TOF were used to characterize the ligands. Molecular weights are

given as the peak MW determined by MALDI-‐TOF.

Figure 2: GPC traces of Pd(Im-‐pTHF-‐30k)2Cl2 (left) and Pd(Im-‐pTHF-‐6k)2Cl2 (right)

Symmetrical complexes Pd(Im-‐pTHF)2Cl2 were prepared by simply mixing polymer attached

Im-‐pTHF ligands with Pd(MeCN)2Cl2. GPC traces indicating a doubling of molecular weight

compared to the free ligands. (Figure 2) The peaks in the GPC traces of the 60 kDa and 12

kDa bis(imidazole)-‐Pd complexes have shoulders on the low MW side. 1H NMR did not

reveal peaks for un-‐complexed imidazole, which suggest that the shoulders correspond to

unfunctionalized pTHF not end-‐capped with imidazole.

Figure 3: GPC trace the mixture of 60 kDa and 12 kDa bis(imidazole)-‐Pd complexes after thermal

equilibrium has been reached by heating a mixture of Pd(Im-‐pTHF30k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 at

60 °C for 8 h.

When the two coordination polymers of 60 kDa and 12 kDa were mixed in toluene at a 1:1

ratio, no ligand exchange was observed after 2 h, thus the polymers are inert to each other.

However, after 8h at 60 oC the GPC trace showed the formation of a new species with

intermediate molecular weight (Figure 3).

A possible explanation for that could be at high temperatures complexes are more dynamic

and exchange their ligands, thus a new coordination polymer, which has one ligand from

each complex, was formed (monitored by GPC). When thermal equilibrium was established,

a statistical mixture was obtained containing three polymers with different MWs.


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5

Figure 4: GPC traces for polymer sample after sonicating the mixture of Pd(Im-‐pTHF30k)2Cl2 and

Pd(Im-‐pTHF6k)2Cl2 for 2h. New polymer was obtained with 35 kDa which corresponds to the MW for

Pd(NHC-‐pTHF30k)(Im-‐pTHF6k)Cl2.

We anticipated that the product distribution could be biased by mechanical initiation of

exchange when the same mixture was sonicated instead of heated. To investigate this, a 1:1

mixture of coordination polymers Pd(Im-‐pTHF30k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 was sonicated

for 2h. After sonication the GPC trace revealed a single peak at 35 kDa with small shoulders

corresponding to initial polymers.

After ligand exchange, the metal center is not located in the middle of the polymer chain.

Since the solvodynamic force reaches a maximum value at the center due to the

centrosymmetric nature of the flow field with respect to the molecule,11,13,14 the product is

not responsive to the mechanical stress although its MW is above the Mlim to break Pd-‐

imidazole bond.

Mechanochemical synthesis of hetero-‐complexes

In order to investigate the directed mechanochemical formation of hetero-‐complexes two

structurally different ligands coordinated in the same metal center, Pd(NHC-‐pTHF)2Cl2 and

Pd(Im-‐pTHF)2Cl2, were mixed. Both complexes are kinetically inert at room temperature i.e.

ligand dissociation is very slow and the complexes don’t exchange ligands without heating.

Scheme 2: Route for synthesis of hetero-‐complex of Pd. Ultrasound initiates ligand exchange and

product possess two different ligands (NHC and imidazole) coordinated to the same Pd.

Figure 5: GPC traces for polymer samples taken before and after sonication, mixture of Pd(NHC-‐

pTHF18k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 (left), mixture of Pd(NHC-‐pTHF25k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2

(right).

In order to test this, Pd(NHC-‐pTHF)2Cl2 complex (36 kDa or 50 kDa, both above Mlim) and

Pd(Im-‐pTHF)2Cl2 (12kDa, below Mlim) were mixed in a 1:1 ratio in toluene and sonicated.

After sonicating for 2h, a significant amount of complex had formed with a MW

corresponding to the asymmetric complex (i.e., MWproduct = MWinitialP1/2 + MWinitialP2/2). The

polymer formed after sonication of 12 kDa Pd(Im-‐pTHF)2Cl2 with Pd(NHC-‐pTHF25k)2Cl2

(Figure 5) showed high conversion to product, with only small shoulders of the initial

Chapter 5

94

Figure 4: GPC traces for polymer sample after sonicating the mixture of Pd(Im-‐pTHF30k)2Cl2 and

Pd(Im-‐pTHF6k)2Cl2 for 2h. New polymer was obtained with 35 kDa which corresponds to the MW for

Pd(NHC-‐pTHF30k)(Im-‐pTHF6k)Cl2.

We anticipated that the product distribution could be biased by mechanical initiation of

exchange when the same mixture was sonicated instead of heated. To investigate this, a 1:1

mixture of coordination polymers Pd(Im-‐pTHF30k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 was sonicated

for 2h. After sonication the GPC trace revealed a single peak at 35 kDa with small shoulders

corresponding to initial polymers.

After ligand exchange, the metal center is not located in the middle of the polymer chain.

Since the solvodynamic force reaches a maximum value at the center due to the

centrosymmetric nature of the flow field with respect to the molecule,11,13,14 the product is

not responsive to the mechanical stress although its MW is above the Mlim to break Pd-‐

imidazole bond.

Mechanochemical synthesis of hetero-‐complexes

In order to investigate the directed mechanochemical formation of hetero-‐complexes two

structurally different ligands coordinated in the same metal center, Pd(NHC-‐pTHF)2Cl2 and

Pd(Im-‐pTHF)2Cl2, were mixed. Both complexes are kinetically inert at room temperature i.e.

ligand dissociation is very slow and the complexes don’t exchange ligands without heating.

Scheme 2: Route for synthesis of hetero-‐complex of Pd. Ultrasound initiates ligand exchange and

product possess two different ligands (NHC and imidazole) coordinated to the same Pd.

Figure 5: GPC traces for polymer samples taken before and after sonication, mixture of Pd(NHC-‐

pTHF18k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 (left), mixture of Pd(NHC-‐pTHF25k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2

(right).

In order to test this, Pd(NHC-‐pTHF)2Cl2 complex (36 kDa or 50 kDa, both above Mlim) and

Pd(Im-‐pTHF)2Cl2 (12kDa, below Mlim) were mixed in a 1:1 ratio in toluene and sonicated.

After sonicating for 2h, a significant amount of complex had formed with a MW

corresponding to the asymmetric complex (i.e., MWproduct = MWinitialP1/2 + MWinitialP2/2). The

polymer formed after sonication of 12 kDa Pd(Im-‐pTHF)2Cl2 with Pd(NHC-‐pTHF25k)2Cl2

(Figure 5) showed high conversion to product, with only small shoulders of the initial


95

5

polymers. When Pd(NHC-‐pTHF18k)2Cl2 was used, shoulders were higher in intensity after

sonicating for 2h indicating that the conversion was lower. Product distribution in both

experiments unequivocally shows that hetero-‐complex formation is a directed,

mechanochemical process rather than a thermal process, which would lead to a statistical

mixture of symmetric starting material and asymmetric complexes.

As a control experiment, Pd(NHC-‐pTHF18k)2Cl2 was sonicated in the presence of acetonitrile

and acetic acid to trap scission products. This resulted in products with MW around 18 kDa,

with a peak in GPC that is clearly distinguishable from the product peak when a mixture of

Pd(NHC-‐pTHF18k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 was sonicated (Figure 6).

Figure 6: Comparison of the GPC traces for Pd(NHC-‐pTHF18k)2Cl2 before and after sonication in the

presence of Pd(Im-‐pTHF6k)2Cl2 or the trapping agents acetonitrile and acetic acid.

Mechanochemical synthesis of block copolymers

In order to investigate the mechanochemical synthesis of block copolymers, pyridine-‐

capped poly methyl acrylate based polymeric ligand (PyPMA, Scheme 3) was synthesized via

SET-‐LRP of methyl acrylate15 using 4-‐pyridinyl-‐2-‐bromoisobutyrate as the initiator.

Combining PyPMA with 0.5 equiv of Pd(CH3CN)2Cl2 in DCM at room temperature for 2 h

resulted in a doubling of MW, as determined by GPC, that was consistent with the

displacement of thermally labile CH3CN by pyridine-‐based ligands.

Scheme 3: Synthesis of PyPMA ligand and Pd(PyPMA)2Cl2 i) 2-‐bromo-‐isobutyrylbromide, CH2Cl2, 0oC-‐

25oC, ii) MA, Me6TREN, Cu wire, RT, iii) Pd(CH3CN)2Cl2, CH2Cl2, RT.

Pd(PyPMA-‐45k)2Cl2 complex (90 kDa) and Pd(Im-‐pTHF)2Cl2 (10 kDa, below Mlim) were mixed

in a 1:1 ratio in toluene. GPC traces for polymers before mixing revealed monomodal

distribution. A new peak corresponding to a MW of 50 kDa appeared upon mixing that

suggests a ligand swapping between species without sonication. However, concentrations of

different species did not change during 2 h at room temperature (Figure 7) indicating that

equilibrium was achieved immediately. After sonicating the same mixture for 2 h, a

significant amount of complex had formed (Figure 8) with a MW corresponding to the

asymmetric complex (50 kDa) that consists of two different ligands (pyridine and imidazole)

and two different polymers (PTHF and PMA). Comparing the GPC traces of hetero-‐complex

that contains block copolymer and a partially dissociated complex showed that the

molecular weight of hetero-‐complex is indeed significantly higher than the dissociation

product of Pd(PyPMA)2Cl2 (Figure 8).

Chapter 5

96

polymers. When Pd(NHC-‐pTHF18k)2Cl2 was used, shoulders were higher in intensity after

sonicating for 2h indicating that the conversion was lower. Product distribution in both

experiments unequivocally shows that hetero-‐complex formation is a directed,

mechanochemical process rather than a thermal process, which would lead to a statistical

mixture of symmetric starting material and asymmetric complexes.

As a control experiment, Pd(NHC-‐pTHF18k)2Cl2 was sonicated in the presence of acetonitrile

and acetic acid to trap scission products. This resulted in products with MW around 18 kDa,

with a peak in GPC that is clearly distinguishable from the product peak when a mixture of

Pd(NHC-‐pTHF18k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 was sonicated (Figure 6).

Figure 6: Comparison of the GPC traces for Pd(NHC-‐pTHF18k)2Cl2 before and after sonication in the

presence of Pd(Im-‐pTHF6k)2Cl2 or the trapping agents acetonitrile and acetic acid.

Mechanochemical synthesis of block copolymers

In order to investigate the mechanochemical synthesis of block copolymers, pyridine-‐

capped poly methyl acrylate based polymeric ligand (PyPMA, Scheme 3) was synthesized via

SET-‐LRP of methyl acrylate15 using 4-‐pyridinyl-‐2-‐bromoisobutyrate as the initiator.

Combining PyPMA with 0.5 equiv of Pd(CH3CN)2Cl2 in DCM at room temperature for 2 h

resulted in a doubling of MW, as determined by GPC, that was consistent with the

displacement of thermally labile CH3CN by pyridine-‐based ligands.

Scheme 3: Synthesis of PyPMA ligand and Pd(PyPMA)2Cl2 i) 2-‐bromo-‐isobutyrylbromide, CH2Cl2, 0oC-‐

25oC, ii) MA, Me6TREN, Cu wire, RT, iii) Pd(CH3CN)2Cl2, CH2Cl2, RT.

Pd(PyPMA-‐45k)2Cl2 complex (90 kDa) and Pd(Im-‐pTHF)2Cl2 (10 kDa, below Mlim) were mixed

in a 1:1 ratio in toluene. GPC traces for polymers before mixing revealed monomodal

distribution. A new peak corresponding to a MW of 50 kDa appeared upon mixing that

suggests a ligand swapping between species without sonication. However, concentrations of

different species did not change during 2 h at room temperature (Figure 7) indicating that

equilibrium was achieved immediately. After sonicating the same mixture for 2 h, a

significant amount of complex had formed (Figure 8) with a MW corresponding to the

asymmetric complex (50 kDa) that consists of two different ligands (pyridine and imidazole)

and two different polymers (PTHF and PMA). Comparing the GPC traces of hetero-‐complex

that contains block copolymer and a partially dissociated complex showed that the

molecular weight of hetero-‐complex is indeed significantly higher than the dissociation

product of Pd(PyPMA)2Cl2 (Figure 8).


97

5

Figure 7: GPC traces for the mixture of Pd(PyPMA-‐45k)2Cl2 complex (90 kDa) and Pd(Im-‐pTHF6k)2Cl2

showed that the mixture is in equilibrium and concentration of different species did not change after

120 min.

As a control experiment, Pd(PyPMA-‐45k)2Cl2 was sonicated and scission products were

trapped with CH3CN and trifluoroacetic acid (TFA). Mid-‐chain scission yields polymers with

MW around 45 kDa. Comparing the GPC traces of hetero-‐complex that contains block

copolymer and a partially dissociated complex showed that the molecular weight of hetero-‐

complex is indeed significantly higher than the dissociation product of Pd(PyPMA)2Cl2 thus,

the new species is the hetero-‐complex rather than the fragments of Pd(PyPMA-‐45k)2Cl2.

(Figure 8).

Figure 8: GPC traces for the different species: a) Pd(PyPMA-‐45k)2Cl2 (black), PyPMA-‐45k (red), and

partially dissociated complex (blue). b) Mixture of Pd(PyPMA-‐45k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 after

sonicating for 120 min (red). Dashed line corresponds to the GPC trace for partially dissociated

complex Pd(PyPMA-‐45k)2Cl2.

Conclusions

In Chapters 2 and 4, ultrasonic scission in Pd(NHC-‐pTHF)2Cl2 and Pd(Im-‐pTHF)2Cl2 complexes

was analyzed in detail. In the current chapter, the information on ligand exchange kinetics,

limiting molecular weights and scission rates has been used to synthesize hetero-‐complexes.

Mechano-‐chemically induced hetero-‐complex formation is slower than mechanochemical

bond scission of Pd(NHC)2Cl2 complexes in the presence of trapping agents. After sonicating

the mixture for 2h, significant amounts of starting polymers were still present. An

explanation is that the formation of triscoordinated complexes, which are the intermediates

in the formation of heterocomplexes, is slow compared to trapping, while the concentration

of the second polymer is also lower than the trapping agents.

Results presented in this chapter show that mechanochemically induced block

copolymerization is possible by attaching desired polymers on easily modified ligands.

Chapter 5

98

Figure 7: GPC traces for the mixture of Pd(PyPMA-‐45k)2Cl2 complex (90 kDa) and Pd(Im-‐pTHF6k)2Cl2

showed that the mixture is in equilibrium and concentration of different species did not change after

120 min.

As a control experiment, Pd(PyPMA-‐45k)2Cl2 was sonicated and scission products were

trapped with CH3CN and trifluoroacetic acid (TFA). Mid-‐chain scission yields polymers with

MW around 45 kDa. Comparing the GPC traces of hetero-‐complex that contains block

copolymer and a partially dissociated complex showed that the molecular weight of hetero-‐

complex is indeed significantly higher than the dissociation product of Pd(PyPMA)2Cl2 thus,

the new species is the hetero-‐complex rather than the fragments of Pd(PyPMA-‐45k)2Cl2.

(Figure 8).

Figure 8: GPC traces for the different species: a) Pd(PyPMA-‐45k)2Cl2 (black), PyPMA-‐45k (red), and

partially dissociated complex (blue). b) Mixture of Pd(PyPMA-‐45k)2Cl2 and Pd(Im-‐pTHF6k)2Cl2 after

sonicating for 120 min (red). Dashed line corresponds to the GPC trace for partially dissociated

complex Pd(PyPMA-‐45k)2Cl2.

Conclusions

In Chapters 2 and 4, ultrasonic scission in Pd(NHC-‐pTHF)2Cl2 and Pd(Im-‐pTHF)2Cl2 complexes

was analyzed in detail. In the current chapter, the information on ligand exchange kinetics,

limiting molecular weights and scission rates has been used to synthesize hetero-‐complexes.

Mechano-‐chemically induced hetero-‐complex formation is slower than mechanochemical

bond scission of Pd(NHC)2Cl2 complexes in the presence of trapping agents. After sonicating

the mixture for 2h, significant amounts of starting polymers were still present. An

explanation is that the formation of triscoordinated complexes, which are the intermediates

in the formation of heterocomplexes, is slow compared to trapping, while the concentration

of the second polymer is also lower than the trapping agents.

Results presented in this chapter show that mechanochemically induced block

copolymerization is possible by attaching desired polymers on easily modified ligands.


99

5

Experimental

General

All chemicals were purchased from commercial sources and used without further purification unless

specified otherwise. Toluene was dried over 4A molecular sieves. Gel permeation chromatography

(GPC) was performed on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and

MIXEDD columns (linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector

at 254 nm and RID-‐10A refractive index detector, and THF as eluent at a flow rate of 1 mL/min (20

°C). Polystyrene standards were used for calibration. Pd(NHC-‐pTHF)2Cl2 and Pd(Im-‐pTHF)2Cl2 were

synthesized according to procedures presented in Chapter 2 and Chapter 4 respectively.






bath and measured using a 0.5 mm diameter thermocouple. 5 ml solutions were sonicated

continuously; temperature of the solution was kept constant by circulating water at 2oC. Prior to

sonication CH4 was bubbled through solution via teflon tubing. Aliquots of 100 µL were taken after

2h. Toluene was removed under reduced pressure; residues were dissolved in THF and submitted to

GPC.

Synthesis of 4-‐pyridinyl-‐2-‐bromo-‐isobutyrate (PyI)

To a solution of 4-‐hydroxypyridine (1.00 g, 10.5 mmol) in CH2Cl2 (dry, 15 mL), 2-‐bromo-‐

isobutyrylbromide (1.43 mL, 11.6 mmol) was added drop-‐wise at 0 °C. The resulting suspension was

allowed to warm to ambient temperature and then stirred overnight under argon. The reaction was

then diluted with CH2Cl2 (30 mL), washed with saturated NaHCO3 (3 × 50 mL) and water (3 × 50 mL),

dried over MgSO4 and then passed through a short column of neutral alumina (CH2Cl2). The solvent

was removed under reduced pressure to afford the desired product (2.18 g, 8.94 mmol) as a clear,

viscous liquid in 85% yield. 1H NMR (CDCl3, 400 MHz): δ 8.64 (d, 3J = 6.0 Hz, 2H), 7.14 (d, 3J = 6.8 Hz,

2H), 2.04 (s, 6H).

Synthesis of PyPMA

25 mL Schlenk flask was charged with methyl acrylate (MA) (4.0 mL, 44 mmol), 10 mM solution of

tris(2-‐[dimethyl]-‐aminoethyl)amine (Me6TREN, 4.0 mL, 40 μmol) in DMSO and 4-‐pyridinyl-‐2-‐bromo-‐

isobutyrate (1) (15.5 mg, 64 μmol). Solution was degassed by freeze-‐pump-‐thaw cycle (3 times). A

magnetic stir-‐bar wrapped with copper wire, and inserted into the solution. The solution was stirred

at ambient temperature for 1 h. Polymerization was terminated by THF (10 ml, stabilized by BHT).

The resulting deep blue solution was then added slowly to excess methanol (150 mL), which caused

a polymeric material to precipitate as a gummy solid. Centrifugation was performed to sediment out

the polymer and the supernatant was decanted. The residue was then washed with methanol (5 ×

20 mL) and dried under reduced pressure to afford the desired polymer. GPC (THF) Mn = 50 kDa,

MALDI-‐TOF: 45 kDA. 1H NMR (CD2Cl2, 400 MHz): δ 8.73 (d, 3J = 8 Hz), 7.80 (d, 3J =8 Hz), 3.51 (br), 2.30

(br), 1.91 (br), 1.68 (br), 1.45 (br)

Synthesis of Pd(PyPMA)2Cl2

PyPMA (400 mg, 8.90 μmol )was mixed with Pd(CH3CN)2Cl2 (1.15 mg, 4.45 μmol) in DCM and stirred

at room temperature for 2 h under argon. Resulting light yellow solution was filtered over filter

paper and solvent was evaporated under reduced pressure. 1H NMR (CD2Cl2, 400 MHz): δ 8.80 (d, 3J

= 8 Hz), 7.23 (d, 3J =8 Hz), 3.51 (br), 2.30 (br), 1.91 (br), 1.68 (br), 1.45 (br)

Chapter 5

100

Experimental

General

All chemicals were purchased from commercial sources and used without further purification unless

specified otherwise. Toluene was dried over 4A molecular sieves. Gel permeation chromatography

(GPC) was performed on a Shimadzu LC10-‐AD, using Polymer Laboratories PL Gel 5μm MIXED-‐C and

MIXEDD columns (linear range of MW: 200–2000000 g/mol), a Shimadzu SPD-‐M10A UV-‐vis detector

at 254 nm and RID-‐10A refractive index detector, and THF as eluent at a flow rate of 1 mL/min (20

°C). Polystyrene standards were used for calibration. Pd(NHC-‐pTHF)2Cl2 and Pd(Im-‐pTHF)2Cl2 were

synthesized according to procedures presented in Chapter 2 and Chapter 4 respectively.






bath and measured using a 0.5 mm diameter thermocouple. 5 ml solutions were sonicated

continuously; temperature of the solution was kept constant by circulating water at 2oC. Prior to

sonication CH4 was bubbled through solution via teflon tubing. Aliquots of 100 µL were taken after

2h. Toluene was removed under reduced pressure; residues were dissolved in THF and submitted to

GPC.

Synthesis of 4-‐pyridinyl-‐2-‐bromo-‐isobutyrate (PyI)

To a solution of 4-‐hydroxypyridine (1.00 g, 10.5 mmol) in CH2Cl2 (dry, 15 mL), 2-‐bromo-‐

isobutyrylbromide (1.43 mL, 11.6 mmol) was added drop-‐wise at 0 °C. The resulting suspension was

allowed to warm to ambient temperature and then stirred overnight under argon. The reaction was

then diluted with CH2Cl2 (30 mL), washed with saturated NaHCO3 (3 × 50 mL) and water (3 × 50 mL),

dried over MgSO4 and then passed through a short column of neutral alumina (CH2Cl2). The solvent

was removed under reduced pressure to afford the desired product (2.18 g, 8.94 mmol) as a clear,

viscous liquid in 85% yield. 1H NMR (CDCl3, 400 MHz): δ 8.64 (d, 3J = 6.0 Hz, 2H), 7.14 (d, 3J = 6.8 Hz,

2H), 2.04 (s, 6H).

Synthesis of PyPMA

25 mL Schlenk flask was charged with methyl acrylate (MA) (4.0 mL, 44 mmol), 10 mM solution of

tris(2-‐[dimethyl]-‐aminoethyl)amine (Me6TREN, 4.0 mL, 40 μmol) in DMSO and 4-‐pyridinyl-‐2-‐bromo-‐

isobutyrate (1) (15.5 mg, 64 μmol). Solution was degassed by freeze-‐pump-‐thaw cycle (3 times). A

magnetic stir-‐bar wrapped with copper wire, and inserted into the solution. The solution was stirred

at ambient temperature for 1 h. Polymerization was terminated by THF (10 ml, stabilized by BHT).

The resulting deep blue solution was then added slowly to excess methanol (150 mL), which caused

a polymeric material to precipitate as a gummy solid. Centrifugation was performed to sediment out

the polymer and the supernatant was decanted. The residue was then washed with methanol (5 ×

20 mL) and dried under reduced pressure to afford the desired polymer. GPC (THF) Mn = 50 kDa,

MALDI-‐TOF: 45 kDA. 1H NMR (CD2Cl2, 400 MHz): δ 8.73 (d, 3J = 8 Hz), 7.80 (d, 3J =8 Hz), 3.51 (br), 2.30

(br), 1.91 (br), 1.68 (br), 1.45 (br)

Synthesis of Pd(PyPMA)2Cl2

PyPMA (400 mg, 8.90 μmol )was mixed with Pd(CH3CN)2Cl2 (1.15 mg, 4.45 μmol) in DCM and stirred

at room temperature for 2 h under argon. Resulting light yellow solution was filtered over filter

paper and solvent was evaporated under reduced pressure. 1H NMR (CD2Cl2, 400 MHz): δ 8.80 (d, 3J

= 8 Hz), 7.23 (d, 3J =8 Hz), 3.51 (br), 2.30 (br), 1.91 (br), 1.68 (br), 1.45 (br)


101

5

References



Chem. 2012, 4 (7), 559–562.





(6) Vermonden, T.; van Steenbergen, M. J.; Besseling, N. A. M.; Marcelis, A. T. M.; Hennink, W. E.;

Sudhölter, E. J. R.; Cohen Stuart, M. A. J. Am. Chem. Soc. 2004, 126 (48), 15802–15808.



4929–4935.




(12) Ribas-‐Arino, J.; Shiga, M.; Marx, D. Angew. Chem. Int. Ed. 2009, 48 (23), 4190–4193.



(15) Percec, V.; Guliashvili, T.; Ladislaw, J. S.; Wistrand, A.; Stjerndahl, A.; Sienkowska, M. J.;

Monteiro, M. J.; Sahoo, S. J. Am. Chem. Soc. 2006, 128 (43), 14156–14165.

Chapter 5

102

Chapter 6Transition metal bearing supramolecular polymer networks: Towards self-healing applications

Synthesis of polymethyl acrylate (PMA) with 5% imidazole (VIm) groups and initial attempts

to prepare self-healing p(MA-co-VIm)-metal films are reported. Pendant VIm serves as a

ligand that leads to reversible cross-linking upon complexation to metal salts such as Copper

and Palladium.

References



Chem. 2012, 4 (7), 559–562.





(6) Vermonden, T.; van Steenbergen, M. J.; Besseling, N. A. M.; Marcelis, A. T. M.; Hennink, W. E.;

Sudhölter, E. J. R.; Cohen Stuart, M. A. J. Am. Chem. Soc. 2004, 126 (48), 15802–15808.



4929–4935.




(12) Ribas-‐Arino, J.; Shiga, M.; Marx, D. Angew. Chem. Int. Ed. 2009, 48 (23), 4190–4193.





Introduction

Supramolecular polymer networks are three-‐dimensional assemblies of macromolecules

connected by non-‐covalent bonds.1 These networks form a useful class of materials with the

potential utility caused by the reversibility of their constituent supramolecular bonds.2 Such

bonds formed through hydrogen bonding or transition metal complexation are quite strong

but under stress they are broken more easily than covalent bonds.3–5 Their reversibility

renders supramolecular polymer networks useful for applications as self-‐healing scaffolds,

or in shape-‐memory materials.6,7

Up to date, self-‐healing materials have been developed using various reversible bonds such

as Diels-‐Alder adducts,8 hydrogen bonding,9,10 ionic interactions,11 π-‐π interactions,12 or

host-‐guest interactions.13 In addition, reversible coordination bonds have been used to

obtain self-‐healing materials since metal-‐ligand interaction is less sensitive to moisture

compared to hydrogen bonds. For the application of metallo-‐supramolecular polymers in

self-‐healing materials, mostly nitrogen-‐based aromatic ligands were used (Figure 1). Metal-‐

ligand bond in these materials was either used for chain extension or as a cross-‐linker. 5,14–23

We have shown previously that bisimidazole-‐Pd complexes do not show any significant

ligand exchange between two different Pd centers at room temperature. However, excess

free imidazole, which is formed when Im-‐Pd bonds break under stress, initiates a ligand

exchange reaction and replaces one of the imidazoles coordinated to Pd via an associative

mechanism. We anticipated that polymer films, which possess such Im-‐metal coordination

as cross-‐linker, would be ideal candidates for autonomous self-‐healing materials. High

association affinity between imidazole and transition metals would make the material

strong while promoting a fast healing for a broken or partially deformed sample. In this

chapter, synthesis of polymethyl acrylate (PMA) with pendant imidazole (VIm) groups which

serve as the cross-‐linking sites, and preliminary results for self-‐healing property of p(MA-‐co-‐

VIm)-‐Metal films are reported.

Figure 1: Examples from recent literature for polymeric ligands and metals investigated in self-‐

healing metallosupramolecular polymers.5, 14-‐23

ON

NN

NN

O N

NN

NN

x yn

NH

NH

O O NH

NH

O NH

O O O O O

NH

O O NH

NH

O O O1 2 1 4 1 3 1

O4

1 OCNNCO

2N O NO₂

OHHO

3

4

NNN N N N

NHO

OH

HOO

H

NN N

OO ORn m

n m

/Zn²⁺/or/La³⁺ /Fe²⁺/or/Cd²⁺

/Zn²⁺/or/Eu³⁺

O O

OOO

O

/Fe²⁺

a) b) c)

d)

NR₂

NR₂R₂N

R₂N

NNN NO O

NH

R₂N

R₂N

NNO

O

HN

O

NR₂

NR₂

N NO

e)

/Pd²⁺/or/Pt²⁺

Chapter 6

104

Introduction

Supramolecular polymer networks are three-‐dimensional assemblies of macromolecules

connected by non-‐covalent bonds.1 These networks form a useful class of materials with the

potential utility caused by the reversibility of their constituent supramolecular bonds.2 Such

bonds formed through hydrogen bonding or transition metal complexation are quite strong

but under stress they are broken more easily than covalent bonds.3–5 Their reversibility

renders supramolecular polymer networks useful for applications as self-‐healing scaffolds,

or in shape-‐memory materials.6,7

Up to date, self-‐healing materials have been developed using various reversible bonds such

as Diels-‐Alder adducts,8 hydrogen bonding,9,10 ionic interactions,11 π-‐π interactions,12 or

host-‐guest interactions.13 In addition, reversible coordination bonds have been used to

obtain self-‐healing materials since metal-‐ligand interaction is less sensitive to moisture

compared to hydrogen bonds. For the application of metallo-‐supramolecular polymers in

self-‐healing materials, mostly nitrogen-‐based aromatic ligands were used (Figure 1). Metal-‐

ligand bond in these materials was either used for chain extension or as a cross-‐linker. 5,14–23

We have shown previously that bisimidazole-‐Pd complexes do not show any significant

ligand exchange between two different Pd centers at room temperature. However, excess

free imidazole, which is formed when Im-‐Pd bonds break under stress, initiates a ligand

exchange reaction and replaces one of the imidazoles coordinated to Pd via an associative

mechanism. We anticipated that polymer films, which possess such Im-‐metal coordination

as cross-‐linker, would be ideal candidates for autonomous self-‐healing materials. High

association affinity between imidazole and transition metals would make the material

strong while promoting a fast healing for a broken or partially deformed sample. In this

chapter, synthesis of polymethyl acrylate (PMA) with pendant imidazole (VIm) groups which

serve as the cross-‐linking sites, and preliminary results for self-‐healing property of p(MA-‐co-‐

VIm)-‐Metal films are reported.

Figure 1: Examples from recent literature for polymeric ligands and metals investigated in self-‐

healing metallosupramolecular polymers.5, 14-‐23

ON

NN

NN

O N

NN

NN

x yn

NH

NH

O O NH

NH

O NH

O O O O O

NH

O O NH

NH

O O O1 2 1 4 1 3 1

O4

1 OCNNCO

2N O NO₂

OHHO

3

4

NNN N N N

NHO

OH

HOO

H

NN N

OO ORn m

n m

/Zn²⁺/or/La³⁺ /Fe²⁺/or/Cd²⁺

/Zn²⁺/or/Eu³⁺

O O

OOO

O

/Fe²⁺

a) b) c)

d)

NR₂

NR₂R₂N

R₂N

NNN NO O

NH

R₂N

R₂N

NNO

O

HN

O

NR₂

NR₂

N NO

e)

/Pd²⁺/or/Pt²⁺

Supramolecular polymer networks

105

6


Scheme 1: Synthesis of copolymer poly(MA-‐co-‐VIm).

In order to obtain a cross-‐linked coordination network, methyl acrylate was copolymerized

with N-‐vinyl imidazole (Scheme 1) via Cu catalyzed single electron transfer living radical

polymerization (SET-‐LRP).24 The mechanism of SET-‐LRP is well established and it results in

low polydispersity polymers with predefined molecular weights. Copolymer was synthesized

with an initial monomer feed ratio in solution for VIm: MA of 5:95. The fraction of VIm in

the final polymer was determined with 1H NMR, using methyl peaks of initiator (ethyl 2-‐

bromo-‐2-‐methylpropanoate) as internal reference (Figure 2). The poly(MA-‐co-‐VIm) had ~4%

VIm units, indicating similar reactivity ratios for monomers.25 Molecular weight of the

copolymer was determined as Mn = 25 kDa by 1H NMR and confirmed by MALDI (Figure 3) to

give an average of n imidazole units per polymer chain. The molecular weight of the

polymer is also consistent with the results previously reported the homo-‐ polymerizations of

MA by SET-‐LRP. This shows that the presence of VIm does not inhibit or slow down the SET-‐

LRP.

O

OBr

O

O+ +O

N

NOO

Ox

Cu)wireDMSO

RTN

N

1 2 3

y n∗

Figure 2: 1H NMR spectrum of poly(MA-‐co-‐VIm) in CDCl3

Figure 3: MALDI-‐TOF spectrum of poly(MA-‐co-‐VIm)

A 3.7 mM solution of copolymer poly(MA-‐co-‐VIm) in MeCN was mixed with either

PdCl2(CH3CN)2 or Cu(CH3CN)4PF6 dissolved in CH2Cl2. Upon mixing, a gel was formed in both

cases, as depicted in Figure 4 for the Pd complex.

Chapter 6

106


Scheme 1: Synthesis of copolymer poly(MA-‐co-‐VIm).

In order to obtain a cross-‐linked coordination network, methyl acrylate was copolymerized

with N-‐vinyl imidazole (Scheme 1) via Cu catalyzed single electron transfer living radical

polymerization (SET-‐LRP).24 The mechanism of SET-‐LRP is well established and it results in

low polydispersity polymers with predefined molecular weights. Copolymer was synthesized

with an initial monomer feed ratio in solution for VIm: MA of 5:95. The fraction of VIm in

the final polymer was determined with 1H NMR, using methyl peaks of initiator (ethyl 2-‐

bromo-‐2-‐methylpropanoate) as internal reference (Figure 2). The poly(MA-‐co-‐VIm) had ~4%

VIm units, indicating similar reactivity ratios for monomers.25 Molecular weight of the

copolymer was determined as Mn = 25 kDa by 1H NMR and confirmed by MALDI (Figure 3) to

give an average of n imidazole units per polymer chain. The molecular weight of the

polymer is also consistent with the results previously reported the homo-‐ polymerizations of

MA by SET-‐LRP. This shows that the presence of VIm does not inhibit or slow down the SET-‐

LRP.

O

OBr

O

O+ +O

N

NOO

Ox

Cu)wireDMSO

RTN

N

1 2 3

y n∗

Figure 2: 1H NMR spectrum of poly(MA-‐co-‐VIm) in CDCl3

Figure 3: MALDI-‐TOF spectrum of poly(MA-‐co-‐VIm)

A 3.7 mM solution of copolymer poly(MA-‐co-‐VIm) in MeCN was mixed with either

PdCl2(CH3CN)2 or Cu(CH3CN)4PF6 dissolved in CH2Cl2. Upon mixing, a gel was formed in both

cases, as depicted in Figure 4 for the Pd complex.


107

6

Figure 4: Supramolecular gel formed upon addition of PdCl2(MeCN)2 in MeCN

Figure 5: Polymer films prepared from the gels by evaporating solvent a) poly(MA-‐co-‐VIm)-‐PdCl2 and

b) poly(MA-‐co-‐VIm)-‐CuPF6

Polymer films (Figure 5) were prepared from the gels by slowly evaporating solvents on a

Teflon mold. Dumbbell shaped polymer films were subjected to tensile testing at a strain

rate of 10-‐3 s-‐1. The Pd containing polymer has a Young’s modulus of 2 MPa that is higher

than that of Cu containing polymer (0.25 MPa). This may be attributed to the higher binding

affinity of Pd for imidazole. The force induced bond scission has been already shown to take

place on the weakest bond.3 Therefore; the strength of the metal ligand coordination bond

determines the mechanical properties for the film.

Figure 6: Tensile tests for poly(MA-‐co-‐VIm)-‐PdCl2 and poly(MA-‐co-‐VIm)-‐CuPF6 before and after

breaking the polymer films.

Polymer films were tested for their self-‐healing properties at room temperature. Broken

pieces were brought in contact for 2 h right after rupture (<5 mins). In the time scale of the

experiment, Pd bearing polymer film did not heal significantly whereas Cu containing

polymer recovered approximately half of its initial tensile strength (Figure 6).

Conclusions

A copolymer of methyl acrylate (MA) with ~4% vinylimidazole (VIm) groups was synthesized.

The Imidazole pendant groups in the polymer serve as metal coordination sites and give rise

to gelation when Pd(II) or Cu(I) salts are added in solution. Freestanding films were prepared

with these metallo-‐supramolecular polymers. Metal-‐ligand bonds in these polymer films

break upon crack formation and yield free ligand and coordinatively unsaturated metal

center. These reactive groups may initiate the self-‐healing property once surfaces are

brought in contact. Healing efficiency of these polymers can be increased with fine-‐tuning of

the ligand metal ratios or healing time and temperatures.

Chapter 6

108

Figure 4: Supramolecular gel formed upon addition of PdCl2(MeCN)2 in MeCN

Figure 5: Polymer films prepared from the gels by evaporating solvent a) poly(MA-‐co-‐VIm)-‐PdCl2 and

b) poly(MA-‐co-‐VIm)-‐CuPF6

Polymer films (Figure 5) were prepared from the gels by slowly evaporating solvents on a

Teflon mold. Dumbbell shaped polymer films were subjected to tensile testing at a strain

rate of 10-‐3 s-‐1. The Pd containing polymer has a Young’s modulus of 2 MPa that is higher

than that of Cu containing polymer (0.25 MPa). This may be attributed to the higher binding

affinity of Pd for imidazole. The force induced bond scission has been already shown to take

place on the weakest bond.3 Therefore; the strength of the metal ligand coordination bond

determines the mechanical properties for the film.

Figure 6: Tensile tests for poly(MA-‐co-‐VIm)-‐PdCl2 and poly(MA-‐co-‐VIm)-‐CuPF6 before and after

breaking the polymer films.

Polymer films were tested for their self-‐healing properties at room temperature. Broken

pieces were brought in contact for 2 h right after rupture (<5 mins). In the time scale of the

experiment, Pd bearing polymer film did not heal significantly whereas Cu containing

polymer recovered approximately half of its initial tensile strength (Figure 6).

Conclusions

A copolymer of methyl acrylate (MA) with ~4% vinylimidazole (VIm) groups was synthesized.

The Imidazole pendant groups in the polymer serve as metal coordination sites and give rise

to gelation when Pd(II) or Cu(I) salts are added in solution. Freestanding films were prepared

with these metallo-‐supramolecular polymers. Metal-‐ligand bonds in these polymer films

break upon crack formation and yield free ligand and coordinatively unsaturated metal

center. These reactive groups may initiate the self-‐healing property once surfaces are

brought in contact. Healing efficiency of these polymers can be increased with fine-‐tuning of

the ligand metal ratios or healing time and temperatures.


109

6

Experimental

General

Dry tetrahydrofuran (THF, HPLC grade) was degassed with argon and purified by passage through

activated alumina solvent column. DMSO was dried over 4Å molecular sieves for at least 12h prior to

use. A Varian 400MR or a Varian Mercury 400 spectrometer was used to record 1H NMR (400 MHz).

Chemical shifts are reported in ppm and referenced to tetramethylsilane or solvent. Methyl acrylate

and N-‐vinyl imidazole were purchased from commercial sources and filtered through neutral

Alumina before polymerizations.

Synthesis of poly(MA-‐co-‐VIm):

Initiator (180 mg, 0.10 mmol), Me6TREN (46.5 mg, 0.20 mmol), methyl acrylate (5.0 mL) and N-‐vinyl

imidazole (0.7 ml) were dissolved in DMSO and degassed by three freeze-‐pump-‐thaw cycles, and

purged with Ar prior to polymerization. Me6TREN was weighed into aluminum weigh boat and added

to a Schlenk tube with the boat. Copper wire was wrapped around a magnetic stirring bar and used

as the source of Cu(0) catalyst. The Schlenk tube was stirred at room temperature in a water bath at

25oC for the period of polymerization (30 mins). Upon completion of the reaction, the tube was

opened to air and THF (10 mL) was added to the viscous solution. The reaction was filtered through

a plug of basic alumina using THF to remove Cu(0) particles and added onto stirring MeOH. The

polymer was centrifuged out from the turbid liquid mixture and left under vacuum overnight.

Molecular weight of polymer was determined by MALDI-‐TOF and 1H NMR as 25 kDa.

References

(1) Binder, W. H.; Zirbs, R. In Hydrogen Bonded Polymers; Binder, W., Ed.; Advances in Polymer

Science; Springer Berlin Heidelberg, 2006; pp 1–78.

(2) Aida, T.; Meijer, E. W.; Stupp, S. I. Science 2012, 335 (6070), 813–817.

(3) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F.

M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278 (5343), 1601–1604.

(4) Xu, D.; Craig, S. L. Macromolecules 2011, 44 (13), 5465–5472.


(6) Cordier, P.; Tournilhac, F.; Soulié-‐Ziakovic, C.; Leibler, L. Nature 2008, 451 (7181), 977–980.

(7) Murphy, E. B.; Wudl, F. Prog. Polym. Sci. 2010, 35 (1–2), 223–251.

(8) Liu, Y.-‐L.; Chuo, T.-‐W. Polym. Chem. 2013, 4 (7), 2194–2205.

(9) Chen, Y.; Kushner, A. M.; Williams, G. A.; Guan, Z. Nat. Chem. 2012, 4 (6), 467–472.

(10) Chen, Y.; Guan, Z. Chem. Commun. 2014, 50 (74), 10868–10870.

(11) Jr, S. J. K.; Ward, T. C.; Oyetunji, Z. Mech. Adv. Mater. Struct. 2007, 14 (5), 391–397.

(12) Burattini, S.; Colquhoun, H. M.; Fox, J. D.; Friedmann, D.; Greenland, B. W.; Harris, P. J. F.;

Hayes, W.; Mackay, M. E.; Rowan, S. J. Chem. Commun. 2009, No. 44, 6717–6719.

(13) Yang, X.; Yu, H.; Wang, L.; Tong, R.; Akram, M.; Chen, Y.; Zhai, X. Soft Matter 2015, 11 (7),

1242–1252.

(14) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.;

Weder, C. Nature 2011, 472 (7343), 334–337.

(15) Yuan, J.; Fang, X.; Zhang, L.; Hong, G.; Lin, Y.; Zheng, Q.; Xu, Y.; Ruan, Y.; Weng, W.; Xia, H.;

Chen, G. J. Mater. Chem. 2012, 22 (23), 11515–11522.

(16) Hong, G.; Zhang, H.; Lin, Y.; Chen, Y.; Xu, Y.; Weng, W.; Xia, H. Macromolecules 2013, 46 (21),

8649–8656.

(17) Bode, S.; Zedler, L.; Schacher, F. H.; Dietzek, B.; Schmitt, M.; Popp, J.; Hager, M. D.; Schubert, U.

S. Adv. Mater. 2013, 25 (11), 1634–1638.

(18) Mozhdehi, D.; Ayala, S.; Cromwell, O. R.; Guan, Z. J. Am. Chem. Soc. 2014, 136 (46), 16128–

16131.

(19) Sandmann, B.; Happ, B.; Kupfer, S.; Schacher, F. H.; Hager, M. D.; Schubert, U. S. Macromol.

Rapid Commun. 2015, 36 (7), 604–609.

(20) Harrington, M. J.; Masic, A.; Holten-‐Andersen, N.; Waite, J. H.; Fratzl, P. Science 2010, 328

(5975), 216–220.

(21) Holten-‐Andersen, N.; Harrington, M. J.; Birkedal, H.; Lee, B. P.; Messersmith, P. B.; Lee, K. Y. C.;

Waite, J. H. Proc. Natl. Acad. Sci. 2011, 108 (7), 2651–2655.

Chapter 6

110

Experimental

General

Dry tetrahydrofuran (THF, HPLC grade) was degassed with argon and purified by passage through

activated alumina solvent column. DMSO was dried over 4Å molecular sieves for at least 12h prior to

use. A Varian 400MR or a Varian Mercury 400 spectrometer was used to record 1H NMR (400 MHz).

Chemical shifts are reported in ppm and referenced to tetramethylsilane or solvent. Methyl acrylate

and N-‐vinyl imidazole were purchased from commercial sources and filtered through neutral

Alumina before polymerizations.

Synthesis of poly(MA-‐co-‐VIm):

Initiator (180 mg, 0.10 mmol), Me6TREN (46.5 mg, 0.20 mmol), methyl acrylate (5.0 mL) and N-‐vinyl

imidazole (0.7 ml) were dissolved in DMSO and degassed by three freeze-‐pump-‐thaw cycles, and

purged with Ar prior to polymerization. Me6TREN was weighed into aluminum weigh boat and added

to a Schlenk tube with the boat. Copper wire was wrapped around a magnetic stirring bar and used

as the source of Cu(0) catalyst. The Schlenk tube was stirred at room temperature in a water bath at

25oC for the period of polymerization (30 mins). Upon completion of the reaction, the tube was

opened to air and THF (10 mL) was added to the viscous solution. The reaction was filtered through

a plug of basic alumina using THF to remove Cu(0) particles and added onto stirring MeOH. The

polymer was centrifuged out from the turbid liquid mixture and left under vacuum overnight.

Molecular weight of polymer was determined by MALDI-‐TOF and 1H NMR as 25 kDa.

References

(1) Binder, W. H.; Zirbs, R. In Hydrogen Bonded Polymers; Binder, W., Ed.; Advances in Polymer

Science; Springer Berlin Heidelberg, 2006; pp 1–78.

(2) Aida, T.; Meijer, E. W.; Stupp, S. I. Science 2012, 335 (6070), 813–817.

(3) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F.

M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278 (5343), 1601–1604.

(4) Xu, D.; Craig, S. L. Macromolecules 2011, 44 (13), 5465–5472.


(6) Cordier, P.; Tournilhac, F.; Soulié-‐Ziakovic, C.; Leibler, L. Nature 2008, 451 (7181), 977–980.

(7) Murphy, E. B.; Wudl, F. Prog. Polym. Sci. 2010, 35 (1–2), 223–251.

(8) Liu, Y.-‐L.; Chuo, T.-‐W. Polym. Chem. 2013, 4 (7), 2194–2205.

(9) Chen, Y.; Kushner, A. M.; Williams, G. A.; Guan, Z. Nat. Chem. 2012, 4 (6), 467–472.

(10) Chen, Y.; Guan, Z. Chem. Commun. 2014, 50 (74), 10868–10870.

(11) Jr, S. J. K.; Ward, T. C.; Oyetunji, Z. Mech. Adv. Mater. Struct. 2007, 14 (5), 391–397.

(12) Burattini, S.; Colquhoun, H. M.; Fox, J. D.; Friedmann, D.; Greenland, B. W.; Harris, P. J. F.;

Hayes, W.; Mackay, M. E.; Rowan, S. J. Chem. Commun. 2009, No. 44, 6717–6719.

(13) Yang, X.; Yu, H.; Wang, L.; Tong, R.; Akram, M.; Chen, Y.; Zhai, X. Soft Matter 2015, 11 (7),

1242–1252.

(14) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.;

Weder, C. Nature 2011, 472 (7343), 334–337.

(15) Yuan, J.; Fang, X.; Zhang, L.; Hong, G.; Lin, Y.; Zheng, Q.; Xu, Y.; Ruan, Y.; Weng, W.; Xia, H.;

Chen, G. J. Mater. Chem. 2012, 22 (23), 11515–11522.

(16) Hong, G.; Zhang, H.; Lin, Y.; Chen, Y.; Xu, Y.; Weng, W.; Xia, H. Macromolecules 2013, 46 (21),

8649–8656.

(17) Bode, S.; Zedler, L.; Schacher, F. H.; Dietzek, B.; Schmitt, M.; Popp, J.; Hager, M. D.; Schubert, U.

S. Adv. Mater. 2013, 25 (11), 1634–1638.

(18) Mozhdehi, D.; Ayala, S.; Cromwell, O. R.; Guan, Z. J. Am. Chem. Soc. 2014, 136 (46), 16128–

16131.

(19) Sandmann, B.; Happ, B.; Kupfer, S.; Schacher, F. H.; Hager, M. D.; Schubert, U. S. Macromol.

Rapid Commun. 2015, 36 (7), 604–609.

(20) Harrington, M. J.; Masic, A.; Holten-‐Andersen, N.; Waite, J. H.; Fratzl, P. Science 2010, 328

(5975), 216–220.

(21) Holten-‐Andersen, N.; Harrington, M. J.; Birkedal, H.; Lee, B. P.; Messersmith, P. B.; Lee, K. Y. C.;

Waite, J. H. Proc. Natl. Acad. Sci. 2011, 108 (7), 2651–2655.


111

6

(22) Terech, P.; Yan, M.; Maréchal, M.; Royal, G.; Galvez, J.; Velu, S. K. P. Phys. Chem. Chem. Phys.

2013, 15 (19), 7338–7344.

(23) Kersey, F. R.; Loveless, D. M.; Craig, S. L. J. R. Soc. Interface 2007, 4 (13), 373–380.



(25) Pekel, N.; Rzaev, Z. M. O.; Güven, O. Macromol. Chem. Phys. 2004, 205 (8), 1088–1095.

Summary

Mechanochemical Scission of Transition Metal-‐Ligand Bonds in Coordination

Polymers

Mechanical activation of chemical bonds in polymers offers opportunities for a broad range

of uses such as mechanically activated catalytic activity, and the formation of block

copolymers. It has been shown that mechanical work done by an external force lowers the

energy barrier for bond dissociation to such an extent that thermal fluctuations can exceed

this barrier at room temperature. One of the most efficient ways to exert force on a

molecule in solution is sonication. Polymers with sufficient molecular weight undergo chain

scission when they are sonicated in solution due to elongational stresses experienced

around collapsing bubbles. As a consequence of the nature of the stress field, chain scission

occurs preferentially around the chain midpoint.



Therefore, the use of mechanical force to break coordination bonds between transition

metals and ligands was investigated.

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 and Pt(NHC-‐pTHF)2Cl2, coordination

complexes of PdII and PtII with polytetrahydrofuran functionalized N-‐heterocyclic carbene

(NHC) ligands, was investigated in Chapter 2. Application of force in solution by the use of

ultrasound resulted in selective chain scission at the metal–ligand coordination bond.

Scission in coordination bond is reversible; however, scission products were trapped and

monitored by NMR and GPC. Sonicating a series of polymer complexes with different

molecular weights (MW), it was found that the chain scission rate is directly proportional to

the MW of the polymers with above the limiting molecular weight. Comparing scission of

Palladium and Platinum containing polymers showed the influence of ligand dissociation

energy on mechanochemical response of the coordination polymers. Pd-‐NHC (195 kJ/mol)

and Pt-‐NHC (245 kJ/mol) have limiting MWs as 20 and 22 kDa respectively. The force

required to break the bond, Fmax , was calculated using COGEF method. Two important

Chapter 6

112

(22) Terech, P.; Yan, M.; Maréchal, M.; Royal, G.; Galvez, J.; Velu, S. K. P. Phys. Chem. Chem. Phys.

2013, 15 (19), 7338–7344.

(23) Kersey, F. R.; Loveless, D. M.; Craig, S. L. J. R. Soc. Interface 2007, 4 (13), 373–380.



(25) Pekel, N.; Rzaev, Z. M. O.; Güven, O. Macromol. Chem. Phys. 2004, 205 (8), 1088–1095.

Summary

Mechanochemical Scission of Transition Metal-‐Ligand Bonds in Coordination

Polymers

Mechanical activation of chemical bonds in polymers offers opportunities for a broad range

of uses such as mechanically activated catalytic activity, and the formation of block

copolymers. It has been shown that mechanical work done by an external force lowers the

energy barrier for bond dissociation to such an extent that thermal fluctuations can exceed

this barrier at room temperature. One of the most efficient ways to exert force on a

molecule in solution is sonication. Polymers with sufficient molecular weight undergo chain

scission when they are sonicated in solution due to elongational stresses experienced

around collapsing bubbles. As a consequence of the nature of the stress field, chain scission

occurs preferentially around the chain midpoint.



Therefore, the use of mechanical force to break coordination bonds between transition

metals and ligands was investigated.

Ultrasound induced chain scission in Pd(NHC-‐pTHF)2Cl2 and Pt(NHC-‐pTHF)2Cl2, coordination

complexes of PdII and PtII with polytetrahydrofuran functionalized N-‐heterocyclic carbene

(NHC) ligands, was investigated in Chapter 2. Application of force in solution by the use of

ultrasound resulted in selective chain scission at the metal–ligand coordination bond.

Scission in coordination bond is reversible; however, scission products were trapped and

monitored by NMR and GPC. Sonicating a series of polymer complexes with different

molecular weights (MW), it was found that the chain scission rate is directly proportional to

the MW of the polymers with above the limiting molecular weight. Comparing scission of

Palladium and Platinum containing polymers showed the influence of ligand dissociation

energy on mechanochemical response of the coordination polymers. Pd-‐NHC (195 kJ/mol)

and Pt-‐NHC (245 kJ/mol) have limiting MWs as 20 and 22 kDa respectively. The force

required to break the bond, Fmax , was calculated using COGEF method. Two important

113

conclusions can be drawn from the results of these calculations: (i) Mlim scales with Fmax and

(ii) direction of pulling and the choice of attachment points are important when determining

the value of Fmax.

Free NHC released during sonication was used to induce chemiluminescence via proton

abstraction from a 2-‐coumaranone derivative, which decomposes via a chemiluminescent

pathway in the presence of oxygen. Using the chemiluminescent response, the rate of


chain scission were determined in Chapter 3. The rate constants were also simulated using

GEPASI, which showed that the rate-‐determining step is the decomposition of

coumaranone.

The MW dependence of chain scission rates proves that the bond scission observed during

sonication is mechanically induced. However, it is still arguable that trapping agents

promote the mechanochemical chain scission by decreasing the mechanical stability of the

ligand-‐metal coordination bond. In Chapter 4 the rates of mechanically induced ligand

exchange reactions were determined. It was established by NMR that the ligands of Pd-‐

Imidazole complexes exchange via an associative pathway, and stoichiometric complexes of

Pd-‐Imidazole do not show ligand exchange because free ligands are absent. However, free

ligands released by ultrasound initiate ligand exchange in a mixture of polymeric and low

molecular weight Pd(Im)2 complex. The rate of consumption of polymer complex, Pd(Im-‐

pTHF)2Cl2, was monitored by GPC and kinetic simulations showed that the ligand association

is the rate determining step.

In Chapter 5, mechanochemically induced ligand exchange of Pd(II) complexes was used to

direct the formation of heterocomplexes. Symmetric complexes with high and low

molecular weight polymer-‐attached ligands were mixed in solution and sonicated. When

one of the complexes has a molecular weight higher than Mlim for mechanochemical chain

scission, while the other is smaller, sonication leads to the directed formation of a

heterocomplex with two different ligands. Two Pd(Im)2Cl2 complexes with different

molecular weights were mixed in toluene and the solution was subjected to ultrasound. GPC

traces showed that the main polymer fraction after sonication was the heterocomplex.

Furthermore, pTHF attached NHC-‐Pd complexes, Pd(NHC-‐pTHF)2Cl2, and

poly(methylacrylate) attached pyridine-‐Pd complexes, Pd(PyPMA)2Cl2, were also mixed with

Pd(Im-‐pTHF)2Cl2 and sonicated. In all cases, ultrasound-‐induced formation of heterocomplex

was observed.

In Chapter 6, preliminary results for the self-‐healing properties of polymer films of

poly(methyl acrylate)-‐vinyl imidazole copolymers, cross-‐linked by ligand metal coordination

are described.

Summary

114

conclusions can be drawn from the results of these calculations: (i) Mlim scales with Fmax and

(ii) direction of pulling and the choice of attachment points are important when determining

the value of Fmax.

Free NHC released during sonication was used to induce chemiluminescence via proton

abstraction from a 2-‐coumaranone derivative, which decomposes via a chemiluminescent

pathway in the presence of oxygen. Using the chemiluminescent response, the rate of


chain scission were determined in Chapter 3. The rate constants were also simulated using

GEPASI, which showed that the rate-‐determining step is the decomposition of

coumaranone.

The MW dependence of chain scission rates proves that the bond scission observed during

sonication is mechanically induced. However, it is still arguable that trapping agents

promote the mechanochemical chain scission by decreasing the mechanical stability of the

ligand-‐metal coordination bond. In Chapter 4 the rates of mechanically induced ligand

exchange reactions were determined. It was established by NMR that the ligands of Pd-‐

Imidazole complexes exchange via an associative pathway, and stoichiometric complexes of

Pd-‐Imidazole do not show ligand exchange because free ligands are absent. However, free

ligands released by ultrasound initiate ligand exchange in a mixture of polymeric and low

molecular weight Pd(Im)2 complex. The rate of consumption of polymer complex, Pd(Im-‐

pTHF)2Cl2, was monitored by GPC and kinetic simulations showed that the ligand association

is the rate determining step.

In Chapter 5, mechanochemically induced ligand exchange of Pd(II) complexes was used to

direct the formation of heterocomplexes. Symmetric complexes with high and low

molecular weight polymer-‐attached ligands were mixed in solution and sonicated. When

one of the complexes has a molecular weight higher than Mlim for mechanochemical chain

scission, while the other is smaller, sonication leads to the directed formation of a

heterocomplex with two different ligands. Two Pd(Im)2Cl2 complexes with different

molecular weights were mixed in toluene and the solution was subjected to ultrasound. GPC

traces showed that the main polymer fraction after sonication was the heterocomplex.

Furthermore, pTHF attached NHC-‐Pd complexes, Pd(NHC-‐pTHF)2Cl2, and

poly(methylacrylate) attached pyridine-‐Pd complexes, Pd(PyPMA)2Cl2, were also mixed with

Pd(Im-‐pTHF)2Cl2 and sonicated. In all cases, ultrasound-‐induced formation of heterocomplex

was observed.

In Chapter 6, preliminary results for the self-‐healing properties of polymer films of

poly(methyl acrylate)-‐vinyl imidazole copolymers, cross-‐linked by ligand metal coordination

are described.

Summary

115

Curriculum Vitae

Abidin Balan was born on August 27th, 1983 in Malatya, Turkey. He received his BSc in chemistry from Bilkent University in 2007. He then joined the group of Prof. Levent Toppare at Middle East Technical University (METU) to pursue an MSc in chemistry. In his MSc studies, he focused on the synthesis of the Donor–Acceptor type conjugated polymers, mainly benzotriazole derivatives for the application in electrochromic devices and organic solar cells. His master thesis was awarded as the thesis of the year in 2009 (METU). In 2010, he started his PhD in Macromolecular and Organic Chemistry Group at Eindhoven University of Technology, under supervision of Prof. Rint Sijbesma. The most important results of this research are described in this thesis.

Curriculum Vitae

Abidin Balan was born on August 27th, 1983 in Malatya, Turkey. He received his BSc in chemistry from Bilkent University in 2007. He then joined the group of Prof. Levent Toppare at Middle East Technical University (METU) to pursue an MSc in chemistry. In his MSc studies, he focused on the synthesis of the Donor–Acceptor type conjugated polymers, mainly benzotriazole derivatives for the application in electrochromic devices and organic solar cells. His master thesis was awarded as the thesis of the year in 2009 (METU). In 2010, he started his PhD in Macromolecular and Organic Chemistry Group at Eindhoven University of Technology, under supervision of Prof. Rint Sijbesma. The most important results of this research are described in this thesis.

117

Acknowledgements

First and foremost, I would like to express my deepest gratitude, profound respect and sincere thanks to my supervisor prof. dr. Rint Sijbesma for giving me the opportunity to perform my PhD study in his group. It was a great pleasure and privilege for me to be a member of your group. Your scientific guidance always helped me to find my way out during my PhD.

I would like to thank my co-‐supervisor prof. dr. Bert Meijer and members of my promotion committee prof. dr. Giancarlo Cravotto, prof. dr. Joost Reek, prof. dr. ir. Emiel Hensen and prof. dr. Albert Schenning for their valuable comments and suggestions on my thesis.

I would like to thank dr. Bartek Szyja for theoretical calculations reported in Chapter 2 and Serge Söntjens for helping me with the 500 MHz NMR used for the experiments reported in Chapter 4.

Ralph Bovee and Xianwen Lou, I would like to thank both of you for the help and support you provided in the analytical lab. I appreciate the great discussions we had over scientific and non-‐scientific topics.

Ramon and Sascha, thank you very much for being by my side as my paranymphs. Ramon, I always enjoyed your companionship in and outside the lab. Having thought provoking discussions with you was inspiring for me. Sascha, thank you for being such a good friend and sharing the most memorable moments of my life. It was a lot of fun to enjoy Turkish cuisine with you and have you in the big Turkish community in Eindhoven.

Dear Bob, thank you for being the one who provided me with the most generous support during my PhD. Having you in the next office made me feel more confident as I always knew that you would be ready to help whenever I needed. I think we should keep our quarterly meetings going together with Ramon. Being friend with you guys means a lot to me.

My former office mate, Yulan Chen; you are a very good friend and an excellent scientist. Sometimes I miss our conversations in the office over many different topics. Thank you for introducing me different aspects of Chinese culture. I hope to visit you in China soon. Jody Lugger; you have taken Yulan’s seat after she left and I should say I enjoyed a lot being in the same office with you. You are very enthusiastic and always eager to learn person and I wish you a lot of success for the rest of your PhD.

Acknowledgements

First and foremost, I would like to express my deepest gratitude, profound respect and sincere thanks to my supervisor prof. dr. Rint Sijbesma for giving me the opportunity to perform my PhD study in his group. It was a great pleasure and privilege for me to be a member of your group. Your scientific guidance always helped me to find my way out during my PhD.

I would like to thank my co-‐supervisor prof. dr. Bert Meijer and members of my promotion committee prof. dr. Giancarlo Cravotto, prof. dr. Joost Reek, prof. dr. ir. Emiel Hensen and prof. dr. Albert Schenning for their valuable comments and suggestions on my thesis.

I would like to thank dr. Bartek Szyja for theoretical calculations reported in Chapter 2 and Serge Söntjens for helping me with the 500 MHz NMR used for the experiments reported in Chapter 4.

Ralph Bovee and Xianwen Lou, I would like to thank both of you for the help and support you provided in the analytical lab. I appreciate the great discussions we had over scientific and non-‐scientific topics.

Ramon and Sascha, thank you very much for being by my side as my paranymphs. Ramon, I always enjoyed your companionship in and outside the lab. Having thought provoking discussions with you was inspiring for me. Sascha, thank you for being such a good friend and sharing the most memorable moments of my life. It was a lot of fun to enjoy Turkish cuisine with you and have you in the big Turkish community in Eindhoven.

Dear Bob, thank you for being the one who provided me with the most generous support during my PhD. Having you in the next office made me feel more confident as I always knew that you would be ready to help whenever I needed. I think we should keep our quarterly meetings going together with Ramon. Being friend with you guys means a lot to me.

My former office mate, Yulan Chen; you are a very good friend and an excellent scientist. Sometimes I miss our conversations in the office over many different topics. Thank you for introducing me different aspects of Chinese culture. I hope to visit you in China soon. Jody Lugger; you have taken Yulan’s seat after she left and I should say I enjoyed a lot being in the same office with you. You are very enthusiastic and always eager to learn person and I wish you a lot of success for the rest of your PhD.

119

Pauline, Lech, Katja (and Alexandra Sophie J), Samanneh, Erik, Marta, Veronique, Marcel K., Dana, Berry, Jessica, Xiao, Remco, Marcel S., Jurgen, Olga, Ralph, Thuur, Asish, Abhijit, Gajanan; I would like to thank all of you for being so friendly. Having such great scientists like you around me made my life in MST perfectly pleasant.

I would like to thank Joke Rediker, Marjo van Hoof, Jolanda Spiering, Bas de Waal, Hans Damen and Henk Eding for their great help during my PhD. My life would be much more difficult without them.

Now comes the Turkish part.

Tezin en zor bölümü teşekkür bölümüymüş. Ne çok anı, ne güzel dostluklar biriktirmişiz. Düşündükçe daha da yoğunlaşıyor duygular. Keşke şu anda hissettiklerim kendiliğinden dökülse bu sayfaya.

Gönüllerin muhtarı Barış Yağcı ve sevgili başkan Seda Cantekin’le başlamalıyım sanırım. Eindhoven’a ilk geldiğim zamanlar hissettiğim sudan çıkmış balık şaşkınlığını, o zamandan bugüne hiç eksilmeyen muhteşem dostluğunuz sayesinde aştım. Vefalı arkadaş tabiri sanırım en çok ikinize uyar. İkinizi de çok seviyorum. Barış hocam, hem Hollanda’daki arkadaşlığın hem de İstanbul’daki misafirperverliğin için ayrı ayrı teşekkür ederim. Şu an sana sarılmak istiyorum J. Sedacım, seninle vakit geçirmek benim için hep çok keyifli oldu. Nereye gittiğimizi bilmediğimiz araba yolculukları, bilimsel tartışmalar veya zaman zaman senden sopa yemek… Samimiyetin için çok teşekkürler.

Başar efendi… Tartışmalar, sohbetler, geziler, belgeseller, filmler, şarkılar, türküler, çaylar kahveler, çeşitli içmeler… Sana bu paylaşımlarımızın her biri için tek tek teşekkür ederim. Dostluğun benim için çok değerli. Şimdi bu teşekkürden sonra benden kurtulacağını düşünüyorsan yanılıyorsun J. Umuyorum ki çok uzun yıllar daha görüşmeye devam edeceğiz sevgili dostum.

Ali Can kardeşim, hayatı sen yaşıyorsun J. Sohbetlerimiz ve tartışmalarımız bana çok şey öğretti. Doktoramın en sıkıntılı zamanlarını aydınlatıcı belgesel seanslarımız ve içtiğimiz kırmızı şaraplar renklendirdi. Bütün bunların yeri ayrı bende ama sana en çok kalbinde taşıdığın özgürlük ateşi ve her türlü otoriteyi sorgulama kararlılığı için teşekkür ederim.

Gökhan hocam, etrafına yaydığın neşe için, ne olursa olsun pozitif düşünüp umut dolu olabildiğin için çok teşekkür ederim. Umarım ileride sahip olmayı düşündüğüm çocuk(lar) senin gibi bir akademisyenden eğitim alma şansını yakalar(lar). Bu satırlar yazıya dökülürken hayatını birleştirmek üzere olduğun eşine ve sana çok mutlu bir hayat dilerim.

Kamiiiil… Hocam seninle ilgili anılarımızı düşünürken, çok yağmur yağdığı bir gün evde atomların yapısı hakkında yaptığımız uzun ve çok keyifli bir sohbete takıldı aklım. En son bir su hortumunun iki ucundan karşılıklı tutup sallayarak elektronların hareketlerini ve orbitalleri anlamaya çalıştığımızı hatırlıyorum J. Pek çok ufuk açan, algı zorlayan tartışmalarımız oldu seninle. Bunların yanında hayattan keyif almak için de neredeyse hiçbir fırsatı kaçırmadık. Tüm bu anılar için sana çok teşekkür ederim.

Teyzemin evladı Can Nemlioğlu. Gerçek bir sincan delikanlısı J. Büyük projelerin insanı. Her ne kadar politik tartışmalarda ayrı görüşleri savunsak da seninle tartışmaktan her zaman büyük keyif aldım. Seni tanıdığım için çok mutluyum.

Egelstraat 11a ve bu adresin hayatıma kattığı güzel insanlar. Başar, Kamil, Can, Barış, güzel sohbetiyle, titizliğiyle, çektiği birbirinden güzel fotoğraflarıyla İlkin, bütün bir günü tek cümleyle geçirebilecek kadar az ve öz konuşan Cengiz, heyecanı cümlelerinde devrilen Gözde, hikayeleriyle hepimizi gözyaşlarına boğan, acıların çocuğu Alios, tanıdığım en sakin insan olan Öncü, kavgaların eşiğinde, direnişçi Kıvılcım... Hepinizi çok seviyorum.

Ceylan, Sinan, Banu, Oktay, Demet, Ümit, Roos, Atilla, Sena, Bahadır, Seda L., Kubilay, Tekin Hocam, Tayfun abi ve ETÜD sakinleri, hepinize Hollanda’daki hayatıma anlam kattığınız için teşekkür ederim. Benim için çok kıymetli insanlarsınız. Daha pek çok partilerde, kamplarda, gezilerde, toplantılarda, gösteri-‐protesto ve forumlarda buluşmak dileğiyle.

Sevgili Sema, hayatımın ciddi bir şekilde değişmeye başladığı, pek çok önemli adımın atıldığı bir dönemde tam da ihtiyacım olan arkadaşlığı sunduğun ve en unutulmaz anlarda tüm mesafelere rağmen yanımda olmayı tercih ettiğin için teşekkür ederim.

Hollanda Gezi Dayanışması üyeleri, Osman Hocam, Kıvılcım, Hande, Maral, Leyla, Yaşar, Burhan, Çiğdem, Hakan, Nil, Levent, Elif, Eylem, Derya ve adını yazmadığım herkese sonsuz teşekkürler. Umut dolu, samimi, sıcacık insanlarsınız ve iyi ki varsınız.

Annem, babam ve Ceren... Sizin gibi bir ailem olduğu için çok şanslıyım. Sevginizi, desteğinizi ve güveninizi benden hiçbir zaman esirgemediğiniz için size ne kadar teşekkür etsem azdır.

ve Gizem... En iyi arkadaşım, diğer yarım... Bu tezin içindeki her deney hasretinle, sana kavuşmanın hayaliyle yapıldı. Çok bekledik, zorlandık, kızdık, küstük ama demek ki en çok da sevdik. Yoksa aşılır mıydı onca sınır? Ceylan’ın da dediği gibi beklediğime değdi J.

Abidin

Acknowledgements

120

Pauline, Lech, Katja (and Alexandra Sophie J), Samanneh, Erik, Marta, Veronique, Marcel K., Dana, Berry, Jessica, Xiao, Remco, Marcel S., Jurgen, Olga, Ralph, Thuur, Asish, Abhijit, Gajanan; I would like to thank all of you for being so friendly. Having such great scientists like you around me made my life in MST perfectly pleasant.

I would like to thank Joke Rediker, Marjo van Hoof, Jolanda Spiering, Bas de Waal, Hans Damen and Henk Eding for their great help during my PhD. My life would be much more difficult without them.

Now comes the Turkish part.

Tezin en zor bölümü teşekkür bölümüymüş. Ne çok anı, ne güzel dostluklar biriktirmişiz. Düşündükçe daha da yoğunlaşıyor duygular. Keşke şu anda hissettiklerim kendiliğinden dökülse bu sayfaya.

Gönüllerin muhtarı Barış Yağcı ve sevgili başkan Seda Cantekin’le başlamalıyım sanırım. Eindhoven’a ilk geldiğim zamanlar hissettiğim sudan çıkmış balık şaşkınlığını, o zamandan bugüne hiç eksilmeyen muhteşem dostluğunuz sayesinde aştım. Vefalı arkadaş tabiri sanırım en çok ikinize uyar. İkinizi de çok seviyorum. Barış hocam, hem Hollanda’daki arkadaşlığın hem de İstanbul’daki misafirperverliğin için ayrı ayrı teşekkür ederim. Şu an sana sarılmak istiyorum J. Sedacım, seninle vakit geçirmek benim için hep çok keyifli oldu. Nereye gittiğimizi bilmediğimiz araba yolculukları, bilimsel tartışmalar veya zaman zaman senden sopa yemek… Samimiyetin için çok teşekkürler.

Başar efendi… Tartışmalar, sohbetler, geziler, belgeseller, filmler, şarkılar, türküler, çaylar kahveler, çeşitli içmeler… Sana bu paylaşımlarımızın her biri için tek tek teşekkür ederim. Dostluğun benim için çok değerli. Şimdi bu teşekkürden sonra benden kurtulacağını düşünüyorsan yanılıyorsun J. Umuyorum ki çok uzun yıllar daha görüşmeye devam edeceğiz sevgili dostum.

Ali Can kardeşim, hayatı sen yaşıyorsun J. Sohbetlerimiz ve tartışmalarımız bana çok şey öğretti. Doktoramın en sıkıntılı zamanlarını aydınlatıcı belgesel seanslarımız ve içtiğimiz kırmızı şaraplar renklendirdi. Bütün bunların yeri ayrı bende ama sana en çok kalbinde taşıdığın özgürlük ateşi ve her türlü otoriteyi sorgulama kararlılığı için teşekkür ederim.

Gökhan hocam, etrafına yaydığın neşe için, ne olursa olsun pozitif düşünüp umut dolu olabildiğin için çok teşekkür ederim. Umarım ileride sahip olmayı düşündüğüm çocuk(lar) senin gibi bir akademisyenden eğitim alma şansını yakalar(lar). Bu satırlar yazıya dökülürken hayatını birleştirmek üzere olduğun eşine ve sana çok mutlu bir hayat dilerim.

Kamiiiil… Hocam seninle ilgili anılarımızı düşünürken, çok yağmur yağdığı bir gün evde atomların yapısı hakkında yaptığımız uzun ve çok keyifli bir sohbete takıldı aklım. En son bir su hortumunun iki ucundan karşılıklı tutup sallayarak elektronların hareketlerini ve orbitalleri anlamaya çalıştığımızı hatırlıyorum J. Pek çok ufuk açan, algı zorlayan tartışmalarımız oldu seninle. Bunların yanında hayattan keyif almak için de neredeyse hiçbir fırsatı kaçırmadık. Tüm bu anılar için sana çok teşekkür ederim.

Teyzemin evladı Can Nemlioğlu. Gerçek bir sincan delikanlısı J. Büyük projelerin insanı. Her ne kadar politik tartışmalarda ayrı görüşleri savunsak da seninle tartışmaktan her zaman büyük keyif aldım. Seni tanıdığım için çok mutluyum.

Egelstraat 11a ve bu adresin hayatıma kattığı güzel insanlar. Başar, Kamil, Can, Barış, güzel sohbetiyle, titizliğiyle, çektiği birbirinden güzel fotoğraflarıyla İlkin, bütün bir günü tek cümleyle geçirebilecek kadar az ve öz konuşan Cengiz, heyecanı cümlelerinde devrilen Gözde, hikayeleriyle hepimizi gözyaşlarına boğan, acıların çocuğu Alios, tanıdığım en sakin insan olan Öncü, kavgaların eşiğinde, direnişçi Kıvılcım... Hepinizi çok seviyorum.

Ceylan, Sinan, Banu, Oktay, Demet, Ümit, Roos, Atilla, Sena, Bahadır, Seda L., Kubilay, Tekin Hocam, Tayfun abi ve ETÜD sakinleri, hepinize Hollanda’daki hayatıma anlam kattığınız için teşekkür ederim. Benim için çok kıymetli insanlarsınız. Daha pek çok partilerde, kamplarda, gezilerde, toplantılarda, gösteri-‐protesto ve forumlarda buluşmak dileğiyle.

Sevgili Sema, hayatımın ciddi bir şekilde değişmeye başladığı, pek çok önemli adımın atıldığı bir dönemde tam da ihtiyacım olan arkadaşlığı sunduğun ve en unutulmaz anlarda tüm mesafelere rağmen yanımda olmayı tercih ettiğin için teşekkür ederim.

Hollanda Gezi Dayanışması üyeleri, Osman Hocam, Kıvılcım, Hande, Maral, Leyla, Yaşar, Burhan, Çiğdem, Hakan, Nil, Levent, Elif, Eylem, Derya ve adını yazmadığım herkese sonsuz teşekkürler. Umut dolu, samimi, sıcacık insanlarsınız ve iyi ki varsınız.

Annem, babam ve Ceren... Sizin gibi bir ailem olduğu için çok şanslıyım. Sevginizi, desteğinizi ve güveninizi benden hiçbir zaman esirgemediğiniz için size ne kadar teşekkür etsem azdır.

ve Gizem... En iyi arkadaşım, diğer yarım... Bu tezin içindeki her deney hasretinle, sana kavuşmanın hayaliyle yapıldı. Çok bekledik, zorlandık, kızdık, küstük ama demek ki en çok da sevdik. Yoksa aşılır mıydı onca sınır? Ceylan’ın da dediği gibi beklediğime değdi J.

Abidin

Acknowledgements

121

mechanochemical scission of transition metal-ligand bonds ... · 11/11/2015 · useful chemical...

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