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A. Polovkova, A. G. Martynov, Y. G. Gorbunova and M. Murugesu, Dalton Trans., 2016, DOI:

10.1039/C6DT00777E.

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This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

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a. Department of Chemistry and Biomolecular Sciences, and Centre for Catalysis

Research and Innovation, University of Ottawa, Ottawa, Ontario, K1N 6N5,

Canada. Email: [email protected] b. A. N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Leninsky

Prosp. 31, Moscow 119071, Russia c. N. S. Kurnakov Institute of General and Inorganic Chemistry RAS, Leninsky Prosp.

31, Moscow 119991, Russia

Electronic Supplementary Information (ESI) available: [UV-Vis, HR-ESI-MS, IR,

SQUID data]. See DOI: 10.1039/x0xx00000x

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

Impact of Coordination Environment on Magnetic Properties of

Single-Molecule Magnets Based on Homo- and Hetero-Dinuclear

Terbium(III) Heteroleptic Tris(Crownphthalocyaninate)

Rebecca J. Holmberg,a Marina A. Polovkova,

b Alexander G. Martynov,

b Yulia G. Gorbunova,*

,b,c

Muralee Murugesu*,a

A series of TbIII

triple-decker heteroleptic crownphthalocyaninate complexes consisting of a homodinuclear compound

[(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc) (1) and two novel heterodinuclear compounds [(15C5)4Pc]Tb[(15C5)4Pc]Y(Pc), (2), and

[(15C5)4Pc]Y[(15C5)4Pc]Tb(Pc) (3), have been synthesized. All compounds were characterised using UV-Vis spectroscopy,

HR-ESI-MS, MALDI-TOF-MS, and 1H NMR Spectroscopy, followed by exploration into the effects of lanthanide coupling and

ligand field symmetry on the magnetic properties of these complexes using SQUID magnetometry. Magnetic

measurements on the homonuclear TbIII

complex (1) displayed non-negligible ferromagnetic coupling between magnetic

ions, eliciting a high zero-field energetic barrier to magnetic relaxation of Ueff = 229.9(0) K, while the heteronuclear TbIII

/YIII

complexes displayed single-ion field-induced slow relaxation of the magnetization; yielding energetic barriers of Ueff =

129.8(0) K for 2, and 169.1(8) K for 3.

Introduction

Single-Molecule Magnets (SMMs) are unique molecular

entities capable of exhibiting slow relaxation of the

magnetisation below a certain blocking temperature. These

systems are continuously being improved through careful

synthetic design strategies in order to achieve higher energy

barriers to magnetic relaxation (Ueff), and higher magnetic

blocking temperatures (TB). Since SMMs exhibit clean quantum

behaviour1 they are appealing candidates for application in

molecular electronics.2,3

Thus, magnetochemists design and

promote SMMs with the goal of achieving breakthroughs in

information storage,4 quantum computing,

5-10 and molecular

spintronics applications,2,11-16

among others. There is extensive

interest in new synthetic approaches to further understand

these systems, and advance them toward a niche within

various high-density electronics applications.

Rare-earth-based systems are currently the benchmarks

within molecular magnetism due to their large magnetic

moments arising from unpaired f-electrons, and more

importantly their inherent magnetic anisotropy. Very recently

a new benchmark was set by a DyIII

-based SMM, which

displayed a TB = 20 K.17

This was an incredibly significant

discovery within the field of molecular magnetism, as the

previous record was set in 2011 by a homonuclear TbIII

dimer,

where the paramagnetic ions were coupled through a

dinitrogen radical bridging moiety (TB = 14 K).18

Interestingly,

the other benchmark within the SMM field was set in 2013 by

a heteroleptic TbIII

phthalocyanine (Pc) complex, with Ueff = 652

cm-1

(938 K, at Tm= 58 K).19

Thus, TbIII

systems have clearly

proven to be worthy of great interest to the magnetic

community. In particular, extensive work has been performed

on synthesizing and studying the magnetic properties of

homoleptic and heteroleptic TbIII

sandwich phthalocyanine

systems ever since they were first reported by Ishikawa in

2003.20

These systems possess a D4d 4-fold axial symmetry,

which promotes the substantial orbital contribution from the

single TbIII

ion to be axially aligned, thus creating a large

separation of states (large Ueff value). Besides their tendency

towards high energetic barriers to magnetic relaxation, they

have also been shown to be synthetically customizable for

surface adhesion.21

Thus, these systems are promising

candidates for the aforementioned electronics applications; as

SMMs will inevitably be necessitated to operate magnetically

upon surface adhesion in order to be viable candidates.21

Due to the coordinatively-induced axiality of the orbital

component within sandwich tetrapyrrolic compounds, interest

is now being directed toward the investigation of f–f

interactions. To this end, Ishikawa’s group has demonstrated

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2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

that TbIII

ions in a series of isomeric triple-decker

phthalocyaninato-porphyrinato complexes display distinctly

different magnetic behavior, depending on the symmetry of

the coordination polyhedron.22

The nature of peripheral

substituents on sandwich phthalocyanines can significantly

impact the structural properties, such as the tilt and twist

angles of decks, as well as interplanar and M-M distances.23

Thus, varying substituents on the ligand architecture, the

magnetic properties of the complex are undoubtedly

impacted. The impact of such substituents on the magnetic

behavior of TbIII

within sandwich phthalocyanines can be

uniquely studied through magnetic isolation of the

paramagnetic ion with a diamagnetic cation; for example YIII

.

This method involves the isolation of three dimeric complexes:

TbIII

/TbIII

. TbIII

/YIII

and YIII

/TbIII

; thus allowing for the magnetic

ion to be isolated in each pocket/deck and separately studied.

Interestingly, only two previous examples of heteroleptic

triple-decker TbIII

-based complexes have been synthesized and

magnetically studied in this manner.22,24

In each case, triple-

decker complexes were synthesized with two unsubstituted Pc

ligands, and one substituted Pc/Por ligand decorated with

electron donating groups. In the first study the electron

donating group was OBu, where both the heteroleptic and

homoleptic Ln coordination environments were square

antiprismatic (SAP).24

Conversely, in the more recent study the

homoleptic (unsubstituted Pc) pocket was SAP, whereas the

pocket with the OMe-substituted Por ligand was square

prismatic (SP).22

Interestingly, magneto-structural correlations

were thoroughly explored for each coordination environment

in this system; finding that the SAP coordination environment

elucidated slower magnetic relaxation in TbIII

.22

Furthermore,

the SP ion in the TbIII

/TbIII

complex was found to speed up the

overall relaxation of the SAP TbIII

ion. Thus, through selectively

isolating the magnetic TbIII

ion in each deck they were able to

elucidate the effects of their substituent and the resulting

coordination environment on the magnetic behavior of the

TbIII

ion in a heteroleptically substituted pocket.

We sought to build on the previous work performed on

Tb2Pc3 systems by employing new design strategies toward the

fundamental study of ligand substituent effects on the

promotion of stronger 4f coupling and thus better magnetic

properties. Since the record for the highest energy barrier is

held by a heteroleptic system, we endeavoured to

independently explore one homoleptic and one heteroleptic

TbIII

ion, both with SAP geometry, and their resulting f-f

coupling. However, instead of having the homoleptic pocket

be Pc ligand-based, as has been extensively studied for TbIII

-

based Pc systems, we chose to explore the effects of placing

substituents on two of the ligand decks in order to study both

a homoleptic SAP coordination environment with electron

donating substituents, and a heteroleptic SAP coordination

environment with an outside Pc ligand. The Pc ligands were

uniquely functionalized with an electron donating crown-ether

substituent. The choice of crown ether was due to its electron

donating properties, as well as its ability to be functionalized

as an integral part of the Pc ligand, thus providing rigidity to

the outer structure, as opposed previously employed

substituents which have been more flexible on the periphery

of the Pc rings.22,24

Furthermore, crown-ether substituents

provide potential for future study in these systems, as they

possess the ability to provide control over molecular

orientation through interaction with alkali metals.25,26

In order to facilitate these goals we synthesized one

homonuclear and two novel heteronuclear triple-decker

complexes with unsubstituted phthalocyanine and crown-

phthalocyanines as ligands (Fig. 1):

[(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc) (1),

[(15C5)4Pc]Tb[(15C5)4Pc]Y(Pc), (2), and

[(15C5)4Pc]Y[(15C5)4Pc]Tb(Pc) (3).

Fig. 1 Side-on view schematic structure of [(15C5)4Pc]M*[(15C5)4Pc]M(Pc) complexes 1,

2, and 3.

Experimental

Materials and methods

Commercial 1-chloronaphthalene (1-ClN) (Acros Organics) and

1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, Aldrich) were used

as received. Reagent grade chloroform was dried and distilled

from CaH2. Methanol (Merck) was dried over 4 A molecular

sieves. Diphthalocyaninatolanthanum La(Pc)2,27,28

di-tetra-15-

crown-5-phthalocyaninates M[(15C5)4Pc]2,29

H2[(15C5)4Pc]30

and acetylacetonates M(acac)3⋅nH2O,31

M = Tb,Y were

prepared by previously published procedures. Column

chromatography was performed on neutral Al2O3 (Merck, II-III

Brockman activity).

UV-Vis absorption spectra were recorded on a Cary-100

Varian spectrophotometer in 10-mm quartz rectangular cells.

The 1H NMR spectra were recorded on a Bruker Avance 300

spectrometer operating at 300 MHz with internal deuterium

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lock at room temperature. The proton chemical shifts (δ, ppm)

were referenced to the residual proton signals (δ=7.26 ppm) in

CDCl3, which was employed as solvent. High-resolution mass

spectra (HRMS) were recorded on an Orbitrap ESI-TOF mass

spectrometer. Positive-ion MALDI TOF mass spectra were run

on Bruker Daltonics Ultraflex spectrometers using a reflector

mode with a 20 mV voltage at the target. 2,5-

Dihydroxybenzoic acid was used as the matrix.

Magnetic measurements were performed using a Quantum

Design SQUID magnetometer MPMS-XL7, operating between

1.8 and 300 K for dc-applied fields ranging from -7 to 5 T.

Susceptibility measurements were performed on powder

samples of the following amounts: 9.5 mg 1, 18.9 mg 2, and

33.7 mg 3, each wrapped within a polyethylene membrane.

Direct current (dc) susceptibility measurements were

performed at 1000 Oe, and alternating current (ac)

susceptibility measurements were performed under an

oscillating ac field of 3.78 Oe, with ac frequencies ranging from

0.1-1500 Hz. Samples 2 and 3 did not exhibit a strong zero-field

ac signal, and thus were measured under an optimized applied

dc field of 1500 Oe. The magnetization data was initially

collected at 100 K to check for ferromagnetic impurities, found

to be absent in all samples.

Synthetic procedures

[(15C5)4Pc]Tb[(15C5)4Pc]Tb(Pc) [Tb*,Tb] (1). A mixture of La(Pc)0•

2

(55.0 mg, 47 µmol), H2[(15C5)4Pc] (60 mg, 47 µmol) and

Tb(acac)3⋅H2O (67 mg, 141 µmol) was dissolved under sonication in

20 ml of 1-chloronaphtalene and refluxed under a slow steam of Ar

for 20 min until no further changes in the UV-Vis spectrum of the

reaction mixture were observed. After cooling to room

temperature, the mixture was transferred to a chromatographic

column packed with neutral alumina in CHCl3. Multiple gradient

elution with CHCl3+1.25-1.5 vol.% MeOH, and evaporation of

solvents afforded the target complex as a dark-blue solid (27 mg,

34%).

UV-Vis (λmax, nm (r.u.)): 698(0,17), 642(1), 343(0,61), 291(0,5).

HR-ESI: found 1148.333 - [M+3Na]3+

, calculated for

[C160H160N24O40Tb2+3Na]3+

- 1148.315 1H-NMR, CDCl3, δ ppm: -154.91 (br s, 8H, HPc

in), -68.91 and -66.34

(2s, 2x8H, 1,1′-CH2in

), -55.45 and -54.38 (2s, 2x8H, HPcout

+α-HPcout

), -

36.86 and -36.03 (2s, 2x8H, 2,2′-CH2in

), -34.66 (s, 8H, β-HPcout

), -

33.15 (s, 8H, 1-CH2out

), -22.32 and -21.98 (2s, 2x8H, 3,3′-CH2in

), -

18.76 and -18.47 (2s, 2x8H, 4,4′-CH2in

), -17.87 (s, 8H, 1′-CH2out

), -

15.35, -11.46, -10.30, -7.40, -6.60, -4.60 (6s, 6x8H, 2,2′,3,3′,4,4′-

CH2out

).

[(15C5)4Pc]Tb[(15C5)4Pc]Y(Pc) [Tb*,Y] (2). A mixture of

Tb[(15C5)4Pc]2 (60 mg, 22 µmol), La(Pc)2 (25,6 mg, 22 µmol) and

Y(acac)3⋅H2O (27 mg, 66 µmol) was dissolved under sonication in 20

ml of 1-chloronaphtalene and refluxed under a slow steam of Ar for

15 min, until no further changes in the UV-Vis spectrum of the




elution with CHCl3+1.25 vol.% MeOH and evaporation of solvents

afforded target complex as a dark-blue solid (59 mg, 82%).

UV-Vis (λmax, nm (r.u.)): 702(0,18), 641(1), 349(0,62), 292(0,5).

HR-ESI: found 1124.981 – [M+3Na]3+

, calculated for

[C160H160N24O40TbY+3Na]3+

- 1124.975 1H-NMR, CDCl3, δ ppm: -70.02 and -69.27 (2s, 2x8H, HPc

in+HPc

out), -

33.71 (s, 8H, 1-CH2in

), -29.27 (s, 8H, 1-CH2out

), -22.11 (s, 8H, 1`-

CH2out

), -19.22 (s, 8H, 1`-CH2in

), -14.72 (s, 8H, 2-CH2in

), -13.25 (s, 8H,

2-CH2out

), -12.04 (s, 8H, 2`-CH2in

), -10.37 (s, 8H, 2`-CH2out

), -7.77 – -

3.25 (8s, 8x8H, 3,3`,4,4`-CH2in,out

), 9.22 (s, 8H, β-HArout

), 24.66 (s, 8H,

α-HArout

)

[(15C5)4Pc]Y[(15C5)4Pc]Tb(Pc) [Y*,Tb] (3). A mixture of

Y[(15C5)4Pc]2 (60 mg, 22,8 µmol), La(Pc)2 (26,4 mg, 22,8 µmol) and

Tb(acac)3⋅H2O (28 mg, 68,4 µmol) was dissolved under sonication in

20 ml of 1-chloronaphtalene and refluxed under slow steam of Ar

for 20 min until no further changes in the UV-Vis spectrum of the




elution with CHCl3+1.25 vol.% MeOH and evaporation of solvents

afforded target complex as a dark-blue solid (58 mg, 78%).

UV-Vis (λmax, nm (r.u.)): 702(0,21), 641(1), 346(0,69), 291(0,53).

HR-ESI: found 1124.974 – [M+3Na]3+

, calculated for

[C160H160N24O40TbY+3Na]3+

- 1124.975 1H-NMR, CDCl3, δ ppm: -74.13 and -69.04 (2s, 2xH, HPc

in+α-HPc

outut), -

41.68 (s, 8H, 1`-CH2in

), -33.64 (s, 8H, β-HArout

), -27.78 (s, 8H, 1-CH2in

),

-19.99 (s, 8H, 2`-CH2in

), -16.57 (s, 8H, 2-CH2in

), -11.50, -9.77, -9.50, -

8.01 (4s, 4x8H, 3,3`,4,`4-CH2in

), 0.49 – 3.46 (signals overlapped with

N2H4 – 1,2,2`,3,3`,4,4`-CH2outut

), 8.97 (s, 8H, 1`-CH2outut

), 24.39 (s, 8H,

HPcin

).

Results and Discussion

Synthesis and Structural Characterization

The synthetic approach toward complexes 1-3 was motivated by a

desire to obtain isostructural homonuclear and heteronuclear

complexes containing paramagnetic TbIII

and diamagnetic YIII

atoms

in an SAP coordination environment. The homonuclear complex (1)

was prepared as previously described.28

Reported methods of

preparation of heteronuclear trisphthalocyaninates utilize a “raise-

by-one-story” strategy, where a monophthalocyaninate is added to

a double-decker phthalocyaninate complex containing a different

metal. Conversely, herein we used another approach to synthesize

heteronuclear trisphthalocyaninates, which was previously

developed for the preparation of heteronuclear Yb/Y triple-decker

phthalocyanines.32

This approach includes the use of La(Pc)2 as an

efficient and convenient source of Pc2-

dianion.27,28

Indeed, the

reaction between Tb[(15C5)4Pc]2 and La(Pc)2, in the presence of

Y(acac)3, under reflux in 1-chloronaphtalene, afforded the

heteronuclear complex [(15C5)4Pc]Tb[(15C5)4Pc]Y(Pc) (2), which

was isolated in 82% yield. Remarkably, the reaction rate was

exceptionally high; only 15 min were required for complete

conversion of the starting double-decker into the target

heteronuclear compound. The isomeric complex,

[(15C5)4Pc]Y[(15C5)4Pc]Tb(Pc) (3), was prepared in the same


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manner, and was isolated in 78% yield. After column

chromatographic purification on alumina, bulk purity of all

complexes was investigated and structural characterization were

performed.

Fig. 2

1H NMR

spectra of synthesized compounds 1, 2, and 3.

The UV-Vis spectra for 1-3 (Fig. S1-S3) are typical for triple-

decker complexes; displaying the N-band (~292 nm) and the

bathochromically shifted broad Soret band (343-349 nm),

which is characteristic of the positioning and number of

electron donating crown-ether substituents.33,34

Furthermore,

we also observe the lanthanide-dependent Q-band at 642 nm;

which experiences a hypsochromic shift and splitting upon

decreasing ionic radii of the metals (the Q2-band is located at

698 nm for 1 and is shifted to 702 nm for heteronuclear 2 and

3).33,34,35,36

Further characterization employed the use of HR-ESI-MS

and MALDI-TOF MS, which were able to provide unambiguous

evidence of the absence of Lanthanum ions in the final

complexes, as well as the distinct composition and purity of

the desired products 1, 2 and 3 (Fig. S4-S9).

Finally, 1H NMR Spectra were employed for structural

confirmation, particularly as regards the positioning of the

crown-substituted ligand (Fig. 2). 1H NMR spectra of the triple-

decker complexes are expected to consist of 20 resonance

signals: aromatic α- and β-protons of the unsubstituted

phthalocyanine ring (α/β-HPc), as well as evidence of the

crown-ether substitutions through aromatic protons on the

crown-substituted ligand (HPcin/out

), and CH2 protons from the

crown-ether substituents themselves. The assignment of 1H-

NMR spectra was made through the application of previously

developed approaches for characterizing paramagnetic metal-

containing trisphthalocyaninates; through the shifting of their

resonance signals in comparison with that of their diamagnetic

counterparts.37,38,39

Thus, through this method we were able

to assign the shifts shown in Fig. 2, where the positioning of

various signals is altered by the position of the corresponding

diamagnetic and paramagnetic ions. This is due to the fact that

the lanthanide-induced shift (LIS) phenomenon in NMR signals

is dependent both on the nature and arrangement of the

diamagnetic nuclei and paramagnetic lanthanide ion.

Magnetic Characterization

In order to probe our goal of inducing stronger 4f interactions

through uniquely substituted heteroleptic ligands, we

performed static magnetic susceptibility studies. Direct current

(dc) magnetic measurements were performed on all samples

between 1.8 and 300 K, with an applied dc field of 1000 Oe.

The temperature dependence of the χT product for all samples

can be observed in Fig. 3.


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Fig. 3 Temperature dependence of the χT product at 1000 Oe for: 1 (violet), 2 (green), and 3 (blue) samples.

At room temperature the observed χT values are as

follows: 23.62 cm3Kmol

-1 (1), 11.81 cm

3Kmol

-1 (2), and 11.79

cm3Kmol

-1 (3). The values are very consistent with the

theoretical values of 11.82 cm3Kmol

-1, and 23.64 cm

3Kmol

-1for

one and two non-interacting TbIII

ions (7F6, S = 3, L = 3, g = 3/2),

respectively. The χT product remains fairly stable upon

decrease in temperature for all samples, until ~30 K for 1;

where a gradual increase can be observed prior to a more

dramatic increase to 33.46 cm3Kmol

-1 at 1.8 K. This behaviour

is indicative of low temperature ferromagnetic interactions

between TbIII

ions. This ferromagnetic interaction between the

magnetic dipoles of TbIII

ions in triple-decker Tb/Tb

phthalocyanine complexes has been attributed to the

stabilization and interaction between the ground, Jz = ± 6, state

spins.22,24,40-44

The low temperature behaviour for 2 and 3 is

similar, displaying a plateau to values of 9.94 cm3Kmol

-1 and

9.22 cm3Kmol

-1, respectively. The consistently observed slight

negative deviation of the χT product from 300 K is most likely

attributed to a combination of possible factors, such as:

inherent magnetic anisotropy present in TbIII

and/or

depopulation of excited states. These results are consistent

with previously studied triple-decker Tb/Y phthalocyanine

complexes.22,24,40,42

Confirmation of the presence of magnetic anisotropy was

achieved through the collection of isotherm magnetisation

data between 1.8 and 7 K (Fig. S10, S11, and S12). In each

complex the M vs. H data below 7 K demonstrates a rapid

increase in the magnetization at low magnetic fields. A

shoulder develops below 1 T, followed by a gradual increase of

M at 1.8 K to reach near magnetic saturation: 7.90 µB (1), 4.38

µB (2), 4.23 µB (3) at 5 T. The M vs. H/T data also displays

similar behaviour for all three complexes, where at high fields

isotemperature lines do not completely overlay onto a single

master curve. These properties indicate the presence of non-

negligible magnetic anisotropy and/or low-lying excited states.

Given that TbIII

sandwich complexes with Pc ligands have

become some of the highest-performing Single-Molecule

Magnets to date, we chose to investigate the magnetisation

dynamics of 1-3 through ac susceptibility studies. Under zero

applied static dc field and a small 3.78 Oe oscillating field there

was a significant temperature dependent ac signal observed

for 1. The dynamic behaviour was therefore investigated at

various frequencies (0.1-1500 Hz) between 28 and 1.9 K.

Frequency dependent in-phase (χ’) and out-of-phase (χ”)

magnetic susceptibility data was fit using the corresponding

generalized Debye models for a single relaxation process, with

a distribution (α) of relaxation times (τ).45,46

The corresponding

plots of fitted data can be observed in Fig. 4 and S13. The

effective energy barrier and relaxation time were obtained

through fitting the data using the Arrhenius equation (τ =

τ0exp(Ueff/kT), which elicited a value of Ueff = 229.9(0) K (τ0 =

7.1(4)x10-9

s), as can be seen in Fig. 5 and S13. This thermal

relaxation barrier is attributed to the energy gap between

ground and first excited Stark sublevels, and is consistent with

previously reported barriers of 149 cm-1

,44

and 230 cm-1

for

homoleptically substituted Tb2Pc3 complexes.

41,43,47

The speed of relaxation is related to both the f-f interaction

between TbIII

ions, and to the symmetry of the crystal field

around the TbIII

ion. In the mononuclear TbPc2 molecule, the

SAP D4d symmetry inherently has no mixing amongst |Jz} states

due to the diagonal matrix elements.22,24

However, if that SAP

symmetry is lowered, the introduction of off-diagonal matrix

elements allows admixing of states, thus leading to faster

relaxation processes. In this case we clearly observe a

reduction in the SAP symmetry, as compared with previously

reported TbIII

triple-decker phthalocyanine complexes

discussed above, thus leading to a lower energetic barrier to

magnetic relaxation. Interestingly, 1 does not display the same

distinct relaxation processes for each TbIII

ion that was

observed in the Tb/Tb version of the aforementioned

systems.22,24

This could be due to a more closely related SAP

coordination environment between Ln ions, or a stronger f-f

coupling interaction between TbIII

ions.

For 2 and 3 no significant temperature dependent signal

was observed under zero applied dc field, however, under an

applied dc field of 1000 Oe a signal was observed for both

complexes. Such behaviour is indicative of faster relaxation

processes, likely due to the aforementioned admixing of states

in lower symmetry environments. In comparison to 1, these

complexes do not have close dipolar contacts due to the

diamagnetic YIII

ion, and therefore they cannot experience the

same coupling interaction to aid in hindering the admixing of

states. Thus, through the application of a static magnetic field

we are able to lift the degeneracy of these states, and thus

clearly observe the faster relaxation processes.


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Fig. 4 Frequency dependence of the in-phase, χ’, (top) and out-of-phase, χ’’, (bottom) magnetic susceptibility at 0 Oe applied dc field for 1, and 1500 Oe applied dc field for 2 and 3. Solid lines are the best fit to the generalized Debye model.

The dynamic behaviour for these complexes was

investigated through ac susceptibility measurements at various

frequencies (0.1-1500 Hz) between 25 and 1.9 K and under an

optimized applied dc field of 1500 Oe (Fig. 4). Frequency

dependent in-phase (χ’) and out-of-phase (χ”) magnetic

susceptibility data was fit using the corresponding generalized

Debye models for a single relaxation process, with a

distribution (α) of relaxation times (τ),45,46

and can be observed

in Fig. 4, as well as S15 (2) and S17 (3). Effective energy

barriers and relaxation times were obtained by fitting data

using the Arrhenius equation (τ = τ0exp(Ueff/kT), which elicited

values of Ueff = 129.8(0) K (τ0 = 6.7(1)x10-7

s) for 2 (Fig. 5 and

S14) and Ueff = 168.1(8) K (τ0 = 2.7(4)x10-7

s) for 3 (Fig. 5 and

S16).

Fig.5 Relaxation times of the magnetization ln(τ) vs. T

-1 for 1 (Arrhenius plot using

χ’ and χ’’ ac data) under 0 Oe applied dc field for 1 (violet), and 1500 Oe applied

dc field for 2 (green)and 3 (blue). The solid lines correspond to the fits of the data.

As was the case for 1, these results are relatively

consistent with previously reported Tb/Y phthalocyanine triple

decker complexes.22,24

Since the previous Tb/Y studies did not

report energetic barriers it is difficult to make a quantitative

comparison. However, quantitatively, as discussed, SAP TbIII

exhibits slower relaxation than SP TbIII

. Interestingly, another

trend from the previous studies was that the SAP pocket with

unsubstituted homoleptic Pc ligands exhibited slower

relaxation than the heteroleptically substituted pocket.24

In the

case of 2 and 3 we observe similar field-induced behaviour was

observed in both isolated TbIII

systems, however, interestingly

the heteroleptically substituted SAP TbIII

displayed slower

relaxation of the magnetization than the homoleptically

substituted site. This was shown in the differing energy

barriers for 2 and 3 under the same optimized applied dc field,

which display the effect of the substituent-induced change in

the coordination environment of the paramagnetic TbIII

ion.

The heteroleptically substituted coordination environment

around the TbIII

ion in 3 elicits a higher barrier to magnetic

relaxation than the homoleptically substituted coordination

environment of 2. This is likely due to the change in symmetry

between the decks, as has been discussed above, as well as

the electron donating substituents on the peripheral deck,

where the paramagnetic metal ion in 3 is pushed closer to the

outer Pc ligand, thus causing enhanced ligand field effects.19

Cole-Cole plots (χ’’ vs. χ’) for all complexes can be observed in

Fig. 6, S15 and S17. These plots were employed in order to confirm

the presence of single relaxation processes occurring within each

complex. Furthermore, through fitting with a generalized Debye


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model45,46

the extracted values for α and τ can be observed in

Tables S1-S3.

Fig. 6 Cole-Cole plot for ac susceptibility data of 1 under 0 Oe applied dc field. Solid

lines are the best fit to the generalized Debye model.

Conclusions

A detailed magnetic study of one homonuclear TbIII

and two

novel heteronuclear TbIII

/YIII

phthalocyanine structures was

performed. These complexes were synthesized in order to

perform a magnetic study on a SAP Tb2Pc3 complex with

electron donating crown-ether substituents on two of the

three ligands in the system. Thus, through isolating the

paramagnetic ions separately in each pocket using diamagnetic

YIII

, we were able to study TbIII

in a heteroleptic and

homoleptic environment of electron donating substituents.

We found that the f-f coupling between lanthanide ions within

a heteroleptic homodinuclear crown-phthalocyanine complex

was indeed of a non-negligible ferromagnetic nature. This

magnetic dipolar interaction was able to lift the degeneracy of

states, thus prohibiting the faster relaxation processes

observed in the heterodinuclear complexes. Thus, a high

energy barrier to magnetic relaxation was extracted. The two

heterodinuclear complexes with TbIII

and YIII

displayed that the

static behavior of the single paramagnetic ion was consistent

between both structures, however, the dynamic

measurements illustrated the significant effect of the altered

environment, eliciting different energy barriers under the

same optimized applied dc field. The complex with

paramagnetic TbIII

in a heteroleptically substituted

environment exhibited a higher energy barrier, likely due to

increased symmetry in the square antiprismatic coordination

environment, thus reducing faster relaxation processes due to

admixing of states, as well as the presence of electron

donating substituents on the central ring pushing the

paramagnetic ion towards an enhanced ligand field from the

unsubstituted Pc ligand. Thus, we can strive towards higher

energy barriers in these complexes using the design principles

of incorporating: f-f coupling, idealized SAP symmetry, and

electron donating substituents on a heteroleptic Pc ligand.

Through these design strategies, and others learned through

fundamental studies such as these, we may design better

magnetic materials.

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39x26mm (300 x 300 DPI)


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