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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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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
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 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
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 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.
Notes and references
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39x26mm (300 x 300 DPI)
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