Equatorially coordinated lanthanide single ion magnets

Article abstract of DOI:10.1021/ja500793x

The magnetic relaxation dynamics of low-coordinate Dy^III and Er^III complexes, namely three-coordinate ones with an equatorially coordinated triangle geometry and five-coordinate ones with a trigonal bipyramidal geometry, have been exploited for the first time. The three-coordinate Er-based complex is the first equatorially coordinated mononuclear Er-based single-molecule magnet (SMM) corroborating that simple models can effectively direct the design of target SMMs incorporating 4f-elements.

Full text of DOI:10.1021/ja500793x

Communication
pubs.acs.org/JACS
Equatorially Coordinated Lanthanide Single Ion Magnets
Peng Zhang,†,‡ Li Zhang,†,‡ Chao Wang,† Shufang Xue,†,‡ Shuang-Yan Lin,†,‡ and Jinkui Tang*,†
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P. R. China
‡University of Chinese Academy of Sciences, Beijing, 100049, P. R. China
S
* Supporting Information
and homoleptic [Er(COT)2]− have recently been proven to be
more efficient SMMs than their DyIII congeners.2,22,24 In spite
ABSTRACT: The magnetic relaxation dynamics of low-
coordinate DyIII and ErIII complexes, namely three-
of the seeming sandwich-type structure, the ab initio
calculations for [Dy(COT)2]− indicated that the equatorial
coordinate ones with an equatorially coordinated triangle
component of the ligand field is stronger than the axial one.25
geometry and five-coordinate ones with a trigonal
bipyramidal geometry, have been exploited for the first
time. The three-coordinate Er-based complex is the first
equatorially coordinated mononuclear Er-based single-
molecule magnet (SMM) corroborating that simple
models can effectively direct the design of target SMMs
incorporating 4f-elements.
Therefore, the interest in an equatorially coordinated molecular
species is driven greatly by the potential of developing SMMs
with high barriers and blocking temperature.
Given that low-coordinate complexes can provide more
opportunities to get the highly axial equatorially coordinate
geometry such as triangle and square, we determined to explore
low-coordinate lanthanide complexes in pursuit of a mono-
nuclear SMM model compound with equatorially coordinated
geometry. In fact, such low-coordinate lanthanide complexes
have been extensively investigated in organometallic chem-
istry,19,26 however their low-temperature magnetic dynamics is
substantially unexplored. Herein, two classes of lanthanide
compounds with low-coordination numbers were chosen by us
for their initial magnetic studies. One class is the three-
coordinate lanthanide complexes, Ln[N(SiMe3)2]3 (Ln = DyIII,
1a and ErIII, 1b), showing an equatorially coordinated triangle
geometry with a perfect C3 axis.27−30 Amazingly, the Er
complex, 1b, behaves as the first equatorially coordinated
mononuclear Er-based SMM (vide infra) that fulfills the simple
model developed by Long et al. In fact, the C3 symmetry
mononuclear Dy or Er SMMs with a high coordinate number,
such as six or seven-coordinate, have been explored before, but
all show the fast zero-field quantum tunneling of magnetization
(QTM) and thus a field-induced SMM behavior due to the
large mixing of different mJ states.31−33 Here on lowing the
coordinate number, the crystal field only in equatorial positions
around the Er ion extremely enhances the unaxial anisotropy
and thus efficiently suppresses the zero-field QTM in spite of
‘nonaxial’ parameters in C3 crystal field analysis. The other class
is the five-coordinate complexes, Ln(NHPhiPr2)3(THF)2 (Ln =
DyIII, 2a and ErIII, 2b), with a trigonal bipyramidal geometry.34
Both of them (2a and 2b) exhibit slow magnetic relaxation in a
zero/nonzero dc applied field, which can be elucidated through
the comparison of electronic structures between 1 and 2.
The synthesis of Ln[N(SiMe3)2]3 (1) was originally
pioneered in 1972 by D. C. Bradley.27 Subsequently, the
complexes with different lanthanide ions attracted considerable
attention as special examples for the low coordination number
of lanthanide compounds and as starting compounds for
n area of particular concern in the research of single
A
molecule magnets (SMMs)1 in recent years is the
investigation of systems with only one spin carrier within a
molecule.2−5 Remarkably, the record value of effective barrier in
SMM field is still kept to date by a heteroleptic bis-
(phthalocyaninate) SMM, [TbIII(Pc)(Pc′)] (Ueff = 652 cm−1/
939 K).6 Here the high barriers and blocking temperatures in
bis(phthalocyaninate) lanthanide complexes can be directly
related to the crystal field with highly axial symmetry (C4, even
S8 axis) created by the surrounding ligands.7−9 Therefore, how
to enhance the axial anisotropy around lanthanide ion has been
an important subject for the improvement of SMM proper-
ties.10−15 In 2011, Long et al. put forward a simple but
amenable model for predicting the ligand architectures that will
generate strong magnetic anisotropy for a variety of 4f-element
ions based on their basic overall shape of free-ion electron
density.16,17 Two representative examples are the Kramers ions
DyIII (6H15/2) and ErIII (4I15/2) with oblate and prolate-shaped
electron densities, respectively.18 Herein, to give a highly
anisotropic ground state with a large mJ, DyIII ion should be
located in sandwich-type ligand geometry, maximizing the
anisotropy of an oblate ion. Inversely, an equatorially
coordinated geometry is predicted to be preferable for ErIII
ion. However, such a coordination geometry is difficult to be
constructed because of the high coordination numbers of
lanthanide ions,19 and thus mononuclear Er-based SMM with a
perfect equatorially coordinate geometry has been lacking so
far. Actually Er-containing SMMs are very rare, and a few
examples exhibiting only equatorially coordinated ligands or
equatorial field have been reported.2,20−23 Nevertheless, in an
intermetallic compound SmCo5, the SmIII ions with prolate
electron densities are coupled with six equatorially coordinated
cobalt atoms with the delocalized electrons, providing the high
magnetic anisotropy and thus generating one of the strongest
magnets known.16 Indeed, both the heteroleptic ErCp*(COT)
Received: January 28, 2014
© XXXX American Chemical Society
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preparing the tetrahydrofuran-free molecular compounds of
lanthanides ions (amide route).26,28 Single-crystal X-ray studies
revealed that compound 1 crystallizes in the trigonal space
group P-31c with high symmetry (Table S1 and Figure S1).
The molecular structure in Figure 1a demonstrates that the
Figure 2. Energy and mJ values of the sublevels of the ground
multiplets of Ln[N(SiMe3)2]3 (Ln = DyIII and ErIII).28,29
states mJ = 15/2 and the order of mJ states from highest to
lowest. The completely opposite results for Dy and Er ions are
consistent with the prediction of the model from Long et al.
For Er ion, such an equatorially coordinating geometry
minimizes charge contact with the axially located f-element
electron density, which stabilizes high magnitude mJ as ground
state, as shown in Figure 1c.16
The variable-temperature dc magnetic susceptibility data for
1 and 2 collected under a 1 kOe applied field reveal the
reasonable room-temperature χMT values of 13.95 (1a), 11.16
(1b), 14.56 (2a), and 11.39 (2b) cm3 K mol−1 (Figure 3). As
Figure 1. Molecular structures of compounds 1 (a) and 2 (b). The H
atoms have been omitted for clarity. (c) Depiction of low- and high-
energy configuration of the f-orbital electron density with respect to an
equatorial crystal field for a 4f ion with prolate electron density.
Reproduced from ref 16 with permission of The Royal Society of
Chemistry.
central ion is coordinated by three N atoms in the shape of a
flat trigonal pyramid and disordered over two positions above
and below the N3 plane, leading to an effective ligand field of
C3v symmetry. In terms of most mononuclear lanthanide
SMMs, the axial symmetry is only limited to the coordination
geometry of central ion, but not for the whole molecule. It is
noteworthy that such a high axial symmetry in the whole
molecule provides us a great opportunity for exploring its
effects on QTM and SMM behavior. Compound 2 was
synthesized by adopting the procedure given by W. J. Evans in
1996.34 As shown in Figure 1b, the metal center is
approximately trigonal bipyramid (Table S2) with the three
large electronegative amido ligands in the equatorial positions
and two axially coordinating THF molecules. Thus the negative
charges are still concentrated in the equatorial positions of
metal center, and compound 2b shows an average ErIII−N
distance of 2.1871 Å close to 2.1914 Å in 1b. In contrast to 1,
the coordination of two THF molecules breaks the equatorially
coordinating geometry, enabling us to study its effects on SMM
behavior of DyIII and ErIII complexes, respectively. In a packing
diagram for compounds 1 and 2, the shortest intermolecular
Dy···Dy distances are more than 9 Å, suggesting the negligible
intermolecular magnetic interactions.
Before discussing the magnetic properties, the electronic
structures of compounds 1a and 1b were provided because the
crystal field splitting pattern has been stimulated through
applying the crystal field Hamiltonians for C3v symmetry by H.-
D. Amberger in 1998 and 2008.28,29 As shown in Figure 2, the
doublet ground states are distinct for Dy (1a) and Er (1b)
compounds due to their opposite shape of free-ion electron
density. Clearly, the Dy compound (1a) shows ground states
with the smallest mJ = 1/2 component, and the mJ states are
ordered from lowest to highest, behaving as the hard-axis or
easy-plane properties. On the contrary, the easy-axis property is
realized in the Er compound (1b) showing the highest ground
Figure 3. Susceptibility temperature product χT as a function of
temperature recorded on samples of 1 and 2 in an applied field of 1
kOe.
the temperature is lowered, χMT product displays a slight
decrease for compounds 2a and 2b as a result of the
depopulation of the Stark sublevels and/or significant magnetic
anisotropy present in lanthanide systems. In contrast,
compound 1a demonstrates an obvious decrease to 6.35 cm3
K mol−1 at 2 K, which can be ascribed to the ground states with
smallest mJ component ( 1/2).
Ac susceptibility measurements reveal that complex 1a does
not exhibit any out-of phase (χ″) ac signals typical of an SMM
under zero applied dc field, and the application of a dc field
resulted in very weak ac signals with no frequency-dependent
peaks in plots of χ″ vs ν (Figure S3). In contrast, SMM
behavior is observed for the Er analogue 1b as evidenced by
well resolved out-of-phase ac susceptibility maxima that vary
with frequency (Figure 4 top and S4). Here the maxima in χ″ vs
ν plots indicates the “freezing” of the spins by the anisotropy
barriers at low-temperature region. The distinct relaxation
B
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S10). For Er-based compound 2b, the SMM behavior was also
observed through the application of a static field, as indicated
by the appearance of peaks in the out-of-phase (χ″) component
in χ″ vs ν plots (Figures S11 and S12). The results can be
rationalized in terms of the crystal structure of compound 2b,
where the negative charges are still concentrated in the
equatorial positions of Er center. However, the increasing
transverse components of anisotropy as a consequence of the
coordination of THF molecules lead to the fast quantum
tunneling and the subsequent disappearance of SMM behavior
under zero applied field.
The magnetization relaxation time (τ) is extracted from the
frequency dependence measurements and an Arrhenius fit to
the data gives the effective relaxation barriers of compounds 1b,
2a, and 2b. For 1b, τ has not reached the pure quantum regime
yet but nevertheless shows a weak deviation from the expected
thermally activated behavior (Figure 5 top), which should be
Figure 4. Temperature dependence of χ″ for samples 1b (top) and 2a
(bottom) under a zero applied dc field, with an ac field of 3 Oe.
behavior between 1a and 1b is a consequence of the completely
different electronic structures, as evidenced in Figure 2, of Dy
and Er analogue under the special equatorially coordinating
crystal field. The ground states with very small quantum
numbers, mJ = 1/2, of Dy ions is unfavorable for strong
single-molecule magnetism, well in agreement with the results
of ac susceptibility measurements (only a weak field-induced
relaxation behavior). Fitting the Cole−Cole plots to the
generalized Debye functions to determine relaxation times
gave small distributions (0.01 < α < 0.17 at temperature
between 6 and 14 K, Figure S6) for 1b.
By contrast, ac magnetic susceptibility measurements were
also performed on microcrystalline samples of compounds 2a
and 2b. For Dy-based compound 2a, the slow relaxation
behavior of magnetization can be observed at temperature as
high as 40 K, but the rapidly increasing out-of-phase (χ″) ac
magnetic susceptibility below 10 K in χ″ vs T plots is indicative
of a strong tunneling relaxation between the ground Kramers
doublet for this compound (Figures 4 bottom and S7 and S9).
In contrast to the absence of SMM behavior in compound 1a,
compound 2a shows clear zero-field SMM behavior in spite of
the fast tunneling relaxation at low temperature. Indeed, the
coordination of two THF molecules above and below
equatorial plane breaks the equatorially coordinating crystal
field around Dy ions, which shifts one state with a high mJ
quantum number to its ground state and thus leads to the large
magnetic moment of ground states. It seems that for Dy-based
compounds low-symmetry crystal field components could
promote the presence of SMM behavior for some specific
symmetries or ligand surroundings. This also provides a
reasonable explanation why some mononuclear lanthanide
compounds with low-symmetry behave as effective SMMs.35−37
The Cole−Cole plots can be fitted to the generalized Debye
model with α parameters below 0.25 between 1.9 and 20 K,
indicating a very narrow distribution of relaxation times (Figure
Figure 5. Magnetization relaxation time constant as ln(τ) vs T−1 for
1b (top) and 2a (bottom) in a zero static field from best fit to the
Arrhenius law of the thermally activated regime (solid line). Inset:
Molar magnetization at 1.9 K.
due to the weak quantum tunneling of magnetization induced
by the presence of rhombic anisotropy component in C3v
symmetry or other relaxation processes (Raman and direct).
At a high-temperature regime, the effective barrier (Ueff) is 122
K (85 cm−1, τ0 = 9.33 × 10−9 s), which is consistent with the
energy separation (82 cm−1) between the first excited states
and the ground states determined by the simulation of crystal
field splitting pattern (Figure 2), suggesting a thermally
activated tunneling mechanism or an Orbach process via the
first excited states.38 For Dy-based compound 2a, a crossover
from a thermally activated to a temperature-independent
regime in relaxation is clearly observed (Figure 5 bottom). At
high temperature (T > 11 K), the relaxation follows an
Arrhenius-like behavior, affording a barrier Ueff = 34 K with τ0 =
2.07 × 10−5 s. In addition, an effective barrier of 25 K (Figure
S13) is given through an Arrhenius fit with τ0 = 6.44 × 10−8
s
C
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In conclusion, we have provided the first equatorially
coordinated mononuclear Er-based molecular species behaving
as an effective SMM following the useful model developed by
Long et al. The three-coordinate lanthanide compounds
demonstrate an equatorially coordinating crystal field with a
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedure; physical measurements; crystallog-
raphy; structural and magnetic tables and figures. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
tang@ciac.ac.cn
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21371166, 21221061 and 21331003).
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