Chapter 1

Introduction

1

1.1 Introduction

Design and construction of transition metal mediated, self-assembled molecular

entities have experienced extraordinary progress during the past decade, because of their

potential for use as sensors, probes, photonic devices and catalysts.14 Recently, there

has been extensive interest in transition-metal complexes containing chalcogen ligands

because of their relevance to biological processes and their interesting structural and

molecular recognition properties.5 The chalcogen atoms have been amongst the most

useful and stabilising ligands in numerous metal carbonyl compounds. Transition metal

complexes containing the disulfide ligand exhibit a range of interesting reactivity at the

sulphur atoms.6, 7 Early work in metal-chalcogenide complexes suggested that while

sulphur and selenium show similar behavior, the reactivity of tellurium is often unique.

During the past decade, compounds containing Re(I) centres have played an important

role as molecular components in generating well defined architectures.8, 9 Furthermore,

the structural and spectroscopic characteristics of Re(I) containing complexes employed

in the self-assembly process could permit functional properties such as Lewis acidity,

luminescence or redox activity to be introduced into the macrocyclic structure. These

properties could be tuned by changing the coordinated ligands.

1.2 Molecular and Supramolecular Chemistry

Molecular chemistry is the chemistry of compounds formed via interactions

between atoms. Supramolecular chemistry is the analogous chemistry of assemblies

formed via interactions between molecules.10 Molecular chemistry usually entails strong

particle-particle interactions, such as covalent bond formation, and takes place on an

angstrom scale. Supramolecular chemistry involves weaker interactions like hydrogen

bonding, dispersion interactions, electron-donor/electron-acceptor interactions and so on

and usually takes place on a nanometer scale. Because the organizing forces in

supramolecular chemistry are generally weak, advantage is often taken of: (a) favorable

symmetry and entropy effects (for example, to achieve macrocycle formation at the

expense of linear oligomer formation), (b) preprogramming of synthons toward a

particular reaction direction (for example, cis vs trans coordination of a reactive metal

centre), (c) molecular or ionic charge, size, and shape complementarity (for example, to

template the assembly of a desired higher-order structure), or (d) the reversibility of

2

supramolecular association (for example, to obtain thermodynamic rather than kinetic

products from assembly reactions).

1.3 Self-assembly

Self-assembly is a ubiquitous process and defined as the spontaneous formation

of highly ordered, complex, symmetrical molecular or supramolecular structures in pure

form, from many starting components (atoms, ions, ligands, etc.), in a single step.

Sometimes two or more of the above strategies can be combined to achieve self-

assembled products, as a results of tremendous variety of metal-coordination-based grid,

ladder and other semi-infinite structures,29 as well as discrete molecular structures

(triangles, squares, pentagons, hexagons, rectangles, cubes, cylinders, and more complex

structures),11 have been obtained. Recently, self-assembly has been shown to play an

important role in the development of molecular materials and in the bottom-up approach

to nanofabrication.12

1.4 Significance of Re(CO)3 fragment

The one-step production of the Re(CO)3 fragment is of great interest also for the

in situ preparation of stable compounds to be used in nuclear medicine (186/188Re

compounds for radiotherapy),13 for labelling biological molecules, such as amino-acids14

and peptides,15 either by direct binding or through bioconjugation mediated by

polyfunctional ligands.16 Unexpectedly, the cationic fac-[Re(CO)3(CH3CN)3](Y) (Y =

PF6, BF4, ClO4) complexes have been scarcely exploited for synthetic purposes. On the

other hand, (NEt4)2fac-[ReBr3(CO)3] is a convenient precursor for the preparation of fac-

Re(CO)3 compounds to be used in nuclear medicine, as it is water-soluble and can be

obtained also by low-pressure carbonylation of radioactive ReO4−. A similar synthetic

pathway also applies to the corresponding (NEt4)2[fac-99mTcBr3(CO)3] species.13

Mononuclear and dinuclear tricarbonylrhenium(I) complexes of the type

[ReBr(CO)3(Ph2PCH2)2NR] (R = Ph, CH2CH2OH, CH2COOCH2Ph,

CH2CONHCH2COOCH2Ph, CH2COOH) and [Re2(-OR)3(CO)6] have recently been

described to efficiently suppress the growth of solid and suspended tumor cell lines.17

Substitution of the alkoxide or hydroxide ligands and coordination to N7 in purine bases

in a fashion similar to cisplatin were anticipated to be a possible mode of action for some

3

of these complexes.16 Alberto and coworkers have studied the X-ray structures, kinetic

and thermodynamic data of two guanine bound to rhenium and technetium complexes.

The interaction data of this complexes with guanine compared to that of [Pt-

(NH3)2(H2O)2]2+ implies that these complexes were potential cytotoxic agent affecting

DNA like cisplatin. The complexes with 188Re or 99mTc could be used as novel

radiodiagnostic or therapeutic agents.17

1.5 Synthesis of dinuclear metallacycles

For more than a decade, the development of an effective strategy for the

preparation of discrete metallacycles in a controlled fashion has attracted a great deal of

attention.18 The coordination chemistry of the dinuclear tetracarboxylate complexes of

the transition metals are a focus of interest owing to their wide application in many

fields, such as material science, catalysis and as anticancer agents. Dirhodium

tetracarboxylate complexes were used as antitumor agents.19

Synthesis of sulphur bridged metallacycles

Deeming et al. have reported the reaction of Re2(CO)10 with pyridine-2-thione

(pySH) in xylene to afford the dinuclear complex [Re2(pyS)2(CO)6] which contains three

fused four-membered rings. The novel five-electron donating pyS bridges are easily

cleaved by ligand addition and in redistribution reactions with [Re2(MepyS)2(CO)6]

complex (Scheme 1.1).20

Scheme 1.1 Formation of [Re2(pyS)2(CO)6]

When trinuclear rhenium complex Re3(µ-H)3(CO)11(MeCN) was treated with

pyridine-2-thiol in toluene at reflux temperature, the sulphur bridged dinuclear rhenium

complex was obtained as white crystals (Scheme 1. 2).21

4

Scheme 1.2 Formation of [Re2(pyS)2(CO)6]

Hupp and coworkers reported the stepwise synthesis of rhenium dinuclear

metallacycle from Re(CO)5Br. Sulphur bridged dimeric complexes were obtained by

treatment of Re(CO)5OTf with alkyl and aromatic thiols at room temperature. The

rhenium thiolate dimer was used as a precursor for synthesising molecular rectangles

(Scheme 1.3).22

Scheme 1.3 Formation of Re2(-SPh)2(CO)8

Sulphur bridged manganese dinuclear complexes

Treatment of Mn2(CO)10 with 3,4-toluenedithiol and 1,2-ethanedithiol in the

presence of (CH3)3NO affords manganese based sulphur-bridged complexes

[Mn2(CO)6(µ-η4-SC6H3CH3SSC6H3CH3S)] and [Mn2(CO)6(µ-η4-

SCH2CH2SSCH2CH2S)] (Scheme 1.4).23

5

Mn2(CO)10 +

SH

SH

CH3

CH2Cl2, RTMn

S

S

OC

CO

SOCMn

S

CO

CO

CO

CH3

CH3

HS(CH2)2SH,

CH2Cl2

(CH3)3NO,

(CH3)3NO

Mn

SOC

CO

SOC

Mn

CO

S

CO

CO

S

RT

Scheme 1.4 Formation of [Mn2(CO)6(µ-η4-SC6H3CH3SSC6H3CH3S)] and

[Mn2(CO)6(µ-η4-SCH2CH2SSCH2CH2S)]

Dinuclear sulphur bridged cobalt complexes

Equimolar amount of Co2(CO)8 and C6F5SSC6F5 in hexane medium at room

temperature yields air stable black colour crystals of dinuclear cobalt complex (Scheme

1.5). The analogous compound was obtained with C6Cl5SSC6Cl5, as a greenish

coloured product.24

6

Scheme 1.5 Formation of [Co(CO)3(µ-SC6F5)]2

Dinuclear sulphur bridged platinum complexes

When trimethylplatinum(IV)chloride was treated with 1,3-bis(methylthio)propane

in benzene, afforded the air stable 1,3-bis(methylthio)propane]chlorotrimethyl-

platinum(IV) (Scheme 1.6).25

Scheme 1.6 Formation of [(PtXMe3)2(MeS(CH2)nSMe)]

Sulphur bridged rhodium complexes

Connelly and Johnson have been reported the synthesis of dinuclear rhodium

complexes of the type [Rh(µ-SR)(η5-C5H5)]2 from [Rh(η5-C5H5)(CO)2] and disulphides

RSSR (R = Ph, p-C6H4Me, Ph, But, CH2Ph) in toluene or cyclohexane medium (Scheme

1.7).26

7

Scheme 1.7 Formation of [{Rh(µ-SR)(η5-C5H5)}]2

1.6 Synthesis of Selenium and Tellurium bridged Metallacycles

Selenium bridged ruthenium complex

The reaction of [Ru3(CO)10(-dppm)] with PhSeSePh furnished the binuclear

compound [Ru2(CO)4(-SePh)2(-dppm)]. The metal core of the compound contains a

Se2Ru2 butterfly geometry with the wingtips of Se atoms linked with two phenyl groups.

The ruthenium-ruthenium backbone is ligated by four terminal carbonyl ligands and

bridged by a dppm ligand (Scheme 1.8).27

Scheme 1.8 Formation of [Ru2(CO)4(-SePh)2(dppm)]

Selenium bridged palladium complex

Liaw and coworkers have prepared thermally stable dinuclear selenium bridged

palladium complexes [PPN][Cl2Pd(-SeR)2PdCl2] from PdCl2 and cis-[Mn(CO)4(SeR)2].

The core geometry of the compound was described as a Pd2Se2 distorted square planar

complex and each PdII atom makes four bonds, two to selenium atoms of bridging

phenylselenolates and two with terminal chloride atoms. cis-[Mn(CO)4(SeR)2] serve as

8

a chelating metalloligand for heterotrinuclear complexes and act as a selenolate ligand-

transfer reagent (Equation 1.9).28

Selenium bridged iron complex

An equimolar amount of [-C5H5(CO)2MnCC6H5]BBr4 was reacted with

[Et3NH][Fe2(-CO)(-SeR)(CO)6] in THF at low temperature to yield selenolate bridged

FeFe bonded dinuclear complex [Fe2(-SeR)2(CO)6] (Scheme 1.10).29

Scheme 1.10 Formation of [Fe2(-SeR)2(CO)6]

Selenium bridged chromiun complex

Goh and coworkers have accounted for the preparation of cyclopentadienyl

chromium chalcogenide complex [CpCr(SePh)]2Se from the reaction of [CpCr(CO)3]2

with Ph2Se2. [CpCr(SePh)]2Se readily reacts with Cr(CO)5THF to form the black colour

crystals of [CpCr(SePh)]2Se[Cr(CO)5] (Scheme 1.11).30

9

Scheme 1.11 Formation of [CpCr(SePh)]2SeCr(CO)5]

Selenium bridged platinum complexes

The selenium bridged platinum complexes were synthesised by the reaction of

trimethylplatinum(IV)chloride with 1,3-bis(methylthio)propane in benzene, to afford the

air stable dinuclear platinum complexes [1,3-bis(methylthio)propane]chlorotrimethyl

platinum(IV) (Scheme 1.12).31

Scheme 1.12 Formation of [(PtBrMe3)2(MeSe(CH2)nSeMe)]

Tellurium bridged osmium complexes

The reaction of Ph2Te2 with unsaturated cluster Os3(-H)2(CO)10 affords the final

product [OsH(CO)3(TePh)]2 via a trinuclear osmium complex Os3H2(CO)10(-TePh)2.

10

The dinuclear compound was characterized by single crystal X-ray crystallographic

studies. [OsH(CO)3(TePh)]2 compound is centrosymmetric and hence Os2Te2 ring is

constrained to be planar. The two phenyl rings in this product are trans to each other

(Scheme 1.13).32

Os3( -H)2(CO)10 + Ph2Te2 Os

Te

Te

Os

CO

HOs

CO

COOC

H

CO

COPh

Ph

silica

CO

CO

OC

OC

Os

Te

Te

Os

CO

H CO

CO

HPh

Ph

CO

OC

OC

Scheme 1.13 Formation of [OsH(CO)3(µ-TePh)]2

Tellurium bridged manganese complexes

Liaw and coworkers have reported the synthesis of tellurium bridged manganese

metallacycles, from [Mn(CO)5] fragment by the reaction with (PhTe)2 in a three step

process (Scheme 1.14).33

Scheme 1.14 Formation of [(CO)4Mn(-TePh)2Mn(CO)4]

11

1.7 Synthesis of tetranuclear molecular rectangles

Bipyrimidine bridged Re(I) molecular rectangles

Rhenium (I) based molecular rectangles were synthesised from dimetallic edges

containing chelating bridge, 2,2-bipyrimidine (bpym) and Re(CO)5Cl. Bidentate pyridyl

ligands were reacted with dinuclear complexes to furnish tetranuclear ionic molecular

rectangles (Scheme 1.15).34

N NN

N ,

Re

Re

OC

OC CO

N

OC

OC CO

N

Re

Re

N

OC CO

CO

N

OC CO

COL

L

N

N

N

N

N

N

N

N

NN

NNRe

CO

COOC

OC CO

Toluene

Re

Re

OC

OC CO

Cl

OC

OC CO

Cl

ClN

N

N

N

(OTf)4

(1) 2AgOTf/acetone, N2,

(2) N-L-N/THF, N2,

N-L-N

Scheme 1.15 Formation of [{Re2(CO)6(-bpym)}2(-N-L-N)2]4+.

12

2,2-bisbenzimidazolate bridged Re (I) rectangles

Hupp and coworkers have accounted for the synthesis of molecular rectangles

featuring rhenium(I) sites organized in pairs by doubly chelating planar ligands such as

2,2-bisbenzimidazolate (BiBzIm) and then linked by rigid bidentate azine ligands such

as 4,4-bipyridine (bpy) (Scheme 1.16). The pyridyl ligands in the neutral molecular

rectangles are stabilized by van der Waals contact of the centres of these ligands. The

pyridyl ligands remain relatively planar and in good spatial overlap to facilitate direct

electronic interaction.35

Scheme 1.16 Formation of [Re2(CO)6(μ-BiBzIm)]2(μ-bpy)2]

Synthesis of luminescent, pyridyl ligands bridged Re (I) molecular rectangles

Lu and coworkers have achieved neutral rhenium rectangles from Re(CO)5Br

with rigid and bidentate pyridyl ligands as edges. The first step involves synthesis of

bimetallic edges obtained from rhenium precursor and a variety of bidentate ligands.

Further treatment of bidentate ligands with bimetallic edges with larger bidentate pyridyl

ligands produced the tetrametallic rhenium (I) molecular rectangles. These Re (I)

molecular rectangles are neutral in nature with larger cavities and exhibited interesting

luminescence and molecular recognition properties (Scheme 1.17).9a

13

Scheme 1.17 Formation of [{(CO)3Re(μ-L1)Br} {Re(CO)3(μ-L2)Br}]2

To increase the cavity dimensions and molecular recognition properties of the

metallasupramolecules, novel Re (I) molecular rectangles were synthesised using

acetylene, butadiyne and 1,4-bis(ethynyl)-benzene bridges between two pyridine moieties

in the azine ligands (L2) on one side and 4,4’-bipyridine ligand was keeping on the other

side.9b

Synthesis of oxamide bridged Ru (II) molecular rectangles

Jin and coworkers have reported a new series of bi- and tetranuclear half

sandwich ruthenium complexes bearing oxamidato bridges and pyridyl ligands. The

binuclear complexes were obtained from oxamide and [(p-cymene)RuCl2]2 as starting

materials. The binuclear oxamide bridged ruthenium complexes were treated with

bipyridyl linkers such as bpy and bpe ligands to furnish Ru (II) molecular rectangles

(Scheme 1.18).36

14

RHN

NHR THFRu

Cl

Cl

Ru

Cl

Cl

+

O

O

RN

NR

Ru

Cl

Ru

Cl

O

O

RN

NR

Ru

Cl

Ru

Cl

O

O

RN

NR

Ru

Cl

Ru

Cl

L

L

R Ph, C6H5-p-Me, R C6H5-p-Cl

AgOTf / CH3OH

L

N NN

NL ,

O

O

4+

Scheme 1.18 Formation of [(p-cymene)4Ru4(µ-4-oxa-R)2(L)2] (OTf)4

Synthesis of oxalato bridged Ru (II) molecular rectangles

Tetranuclear cationic metallosupramolecules assemblies were generated by

Therrien and coworkers from dinuclear ruthenium complex, AgCF3SO3 and 1,2-bis(4-

pyridyl)ethylene ligand. The product was isolated and characterized as triflate salt.37

15

Scheme 1.19 Formation of [Ru4(6-p-cymene) 4(µ-oxalate)2(µ-be)2][OTf]4

1.8 Photophysical properties of Re(I) complexes

A variety of Re(I)-containing complexes have been synthesized because of their

photophysical and photochemical properties.22, 38 The photoluminescent molecules have

the advantage of allowing researchers to study the electronic excited-state reactivity and

to identify guest inclusion.8c, 38b, 39 Several strategies have been developed to improve

the luminescence quantum yield, the most prominent being the suitable design of

ligands.39 The photophysical properties of rhenium(I) tricarbonyl complexes of the type

fac-[Re(CO)3(NN)L] (NN = polypyridyl ligand, L nitrogen donor atom) continue to

attract much attention due to their interesting excited state properties.40 The low energy

excited state of these rhenium(I) polypyridyl systems is mixed in character involving low

lying metal-to-ligand charge transfer (MLCT) states and intraligand (IL) 3–* excited

states. The spin-orbit coupling enhanced the singlet–triplet mixing of the rhenium

complex. As a result, rhenium complexes exhibit strong phosphorescent emitters

possessing long-lived excited states.41 Such systems are useful for the design of

luminescent sensors and materials for supramolecular devices. Hence, it is necessary to

understand their electronic structure and correctly assign their low-lying electronic

transitions. The excited states property of the complexes have been explained the

potential to act as active catalytic species in the photo and electrocatalytic reduction of

CO2 to CO,42 and as labeling reagents for biomolecules.43, 44 The photophysical and

16

photochemical properties of these systems can be tuned by varying the ligand

coordinated to the rhenium(I) tricarbonyl moiety. Complexes that undergo multi-photon

process have potential applications as probes in multi-photon microscopy or in tissues.45

1.9 Supramolecular interactions

The non-covalent interaction is the dominant type of weak interaction between

macromolecules.46 In general, non-covalent bonding refers to a variety of interactions

that are not covalent in nature between molecules or parts of molecules that provide force

to hold the molecules, usually in a specific orientation or conformation. These non-

covalent interactions include: ionic bonds, hydrophobic interactions, hydrogen bonds,

van der Waals forces, and dipole-dipole bonds. The non-covalent interactions or van der

Waals forces involved in supramolecular entities may be a combination of several

interactions, e.g. ion-pairing, hydrogen bonding, cation−π, π−π interactions etc.47

1.10 Interactions

Non-covalent interactions form the backbone of supramolecular chemistry and

include hydrogen bonds (H-bonds), stacking, electrostatic, hydrophobic, and charge-

transfer interactions as well as metal ion coordination.48 Extensive efforts have been

made to design components that mimic natural systems by undergoing molecular self-

organisation through selective non-covalent interactions such as H-bonding, electrostatic,

and - stacking interactions. Aromatic–aromatic or π–π interactions are important non-

covalent intermolecular forces similar to hydrogen bonding. They can contribute to self-

assembly or molecular recognition processes when extended structures are formed from

building blocks with aromatic moieties.49 As such, π–π interactions range from large

biological systems to relatively small molecules.50, 51 Non-covalent interactions between

aromatic groups play a role in the binding and conformations all the way from nucleic

acids and proteins to benzene.52ac The understanding and utilization of non-covalent

interactions including π–π stacking is of fundamental importance for the further

development of supramolecular chemistry and the tuning and prediction of crystal

structures.52dg

“π–π Interaction” is commonly used for stacks of aromatic groups with

approximately parallel molecular planes separated by interplanar distances of about 3.3–

17

3.8 Å. Stacking does not necessarily have to be a perfect face-to-face alignment of the

atoms but can also be an offset or slipped packing. The T-shaped conformation is a C–

H… π interaction.53 Both face-to-face and T-shaped conformations are limiting forms in

aromatic interactions. Among these, the stacked (facial) arrangements are of particular

interest as π–π interactions.

π–π stacking interactions C-H… interaction

Enlargement or polarization of the π system changes the picture and conclusions

drawn for benzene quite drastically. For example, stacked structures become

increasingly favorable with increasing arene size.54 Larger systems such as pyrene or

coronene show an offset π stacking with the hydrogens roughly over ring centres.55 As

such, non-polarized benzene is not necessarily a good model to arrive at an understanding

of π–π interactions. Experimental investigations showed that electron withdrawing

substituents or hetero atoms lead to the strongest π–π interaction. The face-to-face π

stacking of aromatic moieties shows increased stability when both partners are electron-

poor, whereas electron donating substituents disfavored a π–π interaction. The π-electron

density within the rings is increased and so is the π-electron repulsion.51c, 56

Pyridine, bipyridine and other aromatic nitrogen heterocycles are known as

electron poor ring systems (A) (relatively inert to electrophilic attack, enhanced reaction

with nucleophiles).57 A metal which is co-ordinated to a nitrogen heteroatom will further

enhance the electron-withdrawing effect through its positive charge (B). Hence, aromatic

nitrogen heterocycles should in principle well suited for π–π interactions because of their

low π-electron density. The introduction of heterocyclic N increases the tendency to

stack.

18

Multidentate ligands with pyridine groups or nitrogen heterocycles in general

feature prominently as building blocks in the design of metal–ligand networks.58 π–π

Stacking is an increasingly noted feature in the structural description of metal complexes

with these ligands. Furthermore, it is not just a structural phenomenon but also correlates

with the solid state luminescence properties of some metal complexes, notably with

platinum,59 gold60 and other metals.61 At the same time, few details on the π interaction

aside from the interplanar distance are usually given. A closer look at the so-called π–π

interactions reveals that the term appears to be used for all degrees of slipped stacking. It

could even involve a marked degree of deviation from coplanarity, i.e. an angular

orientation of the ring planes towards a C–H…π interaction.53a, 62 The π–π Interactions

can play an important role in controlling the packing or assembly of compounds. In

many structural descriptions of metal–ligand complexes π–π stacking is invoked as a

vital role for structural motif. Complexes with aromatic nitrogen heterocycles as ligands

reveals that such a π–π stacking is usually an offset or slipped facial arrangement of the

rings. The following examples showing π–π and C–H…π interaction, (i) 2,2-

bis(benzimidazole) bridged rhenium metallacycle, (ii) 2,5-bis(4-pyridyl)-1,3,5-

oxadiazole bridged tetranuclear [(Cp*Ir)4(-bpo)2-(-2-2-C2O4](OTf)4 complex cation.

(iii) macrocycle of salicylidene copper complex.63

19

1.11 Synthetic strategy for the design and synthesis of metallacycles

and metallacyclophanes

When rhenium carbonyl Re2(CO)10 was treated with diaryl dichalcogenides

REER (E = S, Se and Te) in presence of monodendate pyridine ligands (pyridine,

picoline, phenylpyridine), the oxidative addition of aryl disulphides to rhenium carbonyl

led to the formation of dinuclear metallacycles. The dinuclear metallacyclic compounds

having two rhenium and two chalcogens with substitution of monodentate azine ligands

were isolated under facile reaction conditions. Moreover, in case of oxidative addition of

bidendate pyridine ligand to Re2(CO)10 are afforded the tetranuclear metallacyclophanes.

The same synthetic strategy was also utilized for the synthesis of selenium-bridged

tetranuclear metallacyclophanes using bidentate ligands like 4,4’-bipyridine, 1,2-

bis(pyridylethylene) and 1,4-bis(4-pyridylethenyl)benzene.

Dinuclear Metallacycles:

Tetranuclear Metallacyclophanes:

20

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