Modulating magnetic dynamics of Dy2 system through the coordination geometry and magnetic interaction

Article abstract of DOI:10.1021/ic400150f

Two new dinuclear dysprosium compounds, [Dy₂(HL ₁)₂(PhCOO)₂(CH₃OH)₂] (1) and [Dy₂(L₂)₂(NO₃) ₂(CH₃OH)₂]·2CH₃OH· 4H₂O (2), have been assembled through applying two ligands with different coordination pockets. The different features of ligands H ₃L₁ and H₂L₂ result in the distinct coordination geometry of the metal ions in their respective structures. The Dy ions of complexes 1 and 2 were linked by the alkoxide- and hydrazone-O, and display the hula hoop-like and the broken hula hoop-like coordination geometry, respectively. Consequently, these two compounds show distinct magnetic properties. Complex 1 behaves as a single molecule magnet (SMM) with rather slow quantum tunneling rate (τ > 274 ms), while no SMM behavior was observed for complex 2. In addition, the comparison of the structural parameters among the similar Dy₂ SMMs with hula hoop-like geometry reveals the significant role played by coordination geometry and magnetic interaction in modulating the relaxation dynamics of SMMs.

Full text of DOI:10.1021/ic400150f

Article
pubs.acs.org/IC
Modulating Magnetic Dynamics of Dy2 System through the
Coordination Geometry and Magnetic Interaction
Peng Zhang,†,‡ Li Zhang,†,§ Shuang-Yan Lin,†,‡ Shufang Xue,†,‡ and Jinkui Tang*,†
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People’s Republic of China
‡University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
§College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People’s Republic of China
S
* Supporting Information
ABSTRACT: Two new dinuclear dysprosium compounds, [Dy2(HL1)2(PhCOO)2(CH3OH)2] (1) and [Dy2(L2)2(NO3)2-
(CH3OH)2]·2CH3OH·4H2O (2), have been assembled through applying two ligands with different coordination pockets. The
different features of ligands H3L1 and H2L2 result in the distinct coordination geometry of the metal ions in their respective
structures. The Dy ions of complexes 1 and 2 were linked by the alkoxide- and hydrazone-O, and display the hula hoop-like and
the broken hula hoop-like coordination geometry, respectively. Consequently, these two compounds show distinct magnetic
properties. Complex 1 behaves as a single molecule magnet (SMM) with rather slow quantum tunneling rate (τ > 274 ms), while
no SMM behavior was observed for complex 2. In addition, the comparison of the structural parameters among the similar Dy2
SMMs with hula hoop-like geometry reveals the significant role played by coordination geometry and magnetic interaction in
modulating the relaxation dynamics of SMMs.
such as the approximate D4d symmetry in [LnPc2]14,15 and the
INTRODUCTION
■
16
axial hula hoop-like geometry in asymmetric Dy2 and Dy6
Since the seminal discovery of single molecule magnet (SMM)
triangular prism.17 In addition, the tune of magnetic
behavior in a Mn12 complex during the 1990s, the study of
interactions can also bring about some surprising results,
SMM has been the focus of chemistry, physics, and materials
although exchange coupling between lanthanide metal centers
science, where the quantum world of magnetization for single
is very weak.16,18 To date, extensive research attempts have
molecule clusters meets the bulk scale of classical physics.1−4
been performed to probe how the exchange coupling affects
SMM properties in lanthanide-based complexes, thus yielding a
The continued interest stems from their prospects of
applications in information storage and quantum computing.5
series of ground-breaking results, such as the high blocking
In particular, recent years have seen a flurry of results for
temperature discovered in N23−-bridged Tb2 complex and
lanthanide-based SMMs, including the highest relaxation
sulfur-bridged organometallic dysprosium complexes.7,12
Herein, to investigate the effects of magnetic interaction and
coordination geometry on the SMM behavior in Dy-based
compounds with hula hoop-like geometry, we employed two
ligands with different features, H3L119−21 and H2L2 (Scheme 1),
a n d o b t a i n e d t w o n o v e l D y 2 c o m p o u n d s ,
[Dy2(HL1)2(PhCOO)2(CH3OH)2] (1) and [Dy2(L2)2(NO3)2-
energy barriers for multinuclear clusters6 and the highest
blocking temperature,7 which mainly benefit from the
significant magnetic anisotropy of lanthanide ions arising
from the large, unquenched orbital angular momentum.8,9
Remarkably, the alteration of coordination geometry on local
metal sites and/or magnetic interaction between them turns
out to be a key factor in modulating the relaxation dynamics of
lanthanide-based SMMs.10−13 The high axial coordination
geometry around DyIII ions is an important feature enabling
lanthanide complexes functioning as SMMs with high barrier,
Received: January 21, 2013
Published: March 25, 2013
© 2013 American Chemical Society
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are corrected for the diamagnetism estimated from Pascal’s constants22
and sample holder calibration.
Scheme 1. Schematic Diagram of Ligands Used in
Compounds 1, 2, 3, and 4
X-ray Crystallographic Analysis and Data Collection. Single-
crystal X-ray data of the title complexes were collected at 273(2) K on
a Bruker Apex II CCD diffractometer equipped with graphite-
monochromatized Mo Kα radiation (λ = 0.71073 Å). Data processing
was completed with the SAINT processing program. The structure
was solved by direct methods and refined by full matrix least-squares
methods on F2 using SHELXTL-97.23 The locations of the Dy atoms
were easily determined, and C, O, and N atoms were determined from
the difference Fourier maps. The nonhydrogen atoms were refined
anisotropically. All hydrogen atoms were introduced in calculated
positions and refined with fixed geometry with respect to their carrier
atoms.
Synthesis of [Dy2(HL1)2(PhCOO)2(CH3OH)2] (1). Ligand H3L1 (0.1
mmol) was dissolved in CH3OH/CH3CN (10 mL/5 mL) followed by
the addition of Dy(PhCOO)3·6H2O (0.1 mmol) and triethylamine
(0.2 mmol), which gave a clear pale-yellow solution after stirring for 2
h. Diethyl ether was allowed to diffuse slowly into this solution at
room temperature, and yellow single crystals were obtained in 1 week
in 52% yield (26 mg). Anal. Calcd for C36H40Dy2N2O12: C, 42.49; H,
3.96; N, 2.75. Found: C, 42.07; H, 3.65; N, 2.47. IR (KBr, cm−1): 3641
(m), 3196 (br), 2894 (m), 2839 (m), 2666 (w), 1630 (s), 1595 (s),
1537 (s), 1494 (m), 1470 (s), 1423 (s), 1344 (s), 1325 (m), 1248
(m), 1219 (w), 1192 (m), 1150 (m), 1121 (s), 1042 (m), 1030 (s),
936 (w), 886 (m), 864 (m), 824 (w), 757 (m), 721 (s), 688 (m), 627
(s), 595 (m), 527 (s), 504 (m).
Synthesis of [Dy2(L2)2(NO3)2(CH3OH)2]·2CH3OH·4H2O (2). Ligand
H2L2 (0.1 mmol) was dissolved in a mixed solvent of CH3OH/
CH3CN (5 mL/10 mL) followed by the addition of Dy(NO3)3·6H2O
(0.1 mmol) and triethylamine (0.2 mmol), which gave a clear red
solution after stirring for 3 h. This solution was left unperturbed to
allow the slow evaporation of the solvent. After 4 days, red block-
shaped crystals were formed in 38% yield (23 mg). Anal. Calcd for
C36H44Dy2N10O18: C, 35.15; H, 3.60; N, 11.38. Found: C, 34.71; H,
(CH3OH)2]·2CH3OH·4H2O (2), from such ligands. The two
compounds show the different structures concomitant with the
observation of distinct static and dynamic magnetism. Further,
the structure−property relationship can be revealed by the
comparisons of four Dy2 complexes containing two complexes
from vanillin picolinoylhydrazone ligands.
EXPERIMENTAL SECTION
■
General Information. All chemicals were used as received without
any further purification, and all manipulations were performed under
aerobic conditions. Elemental analysis (C, H, and N) were carried out
on a Perkin-Elmer 2400 analyzer. IR spectra were recorded with a
Perkin-Elmer Fourier transform infrared spectrophotometer with
samples prepared as KBr disks in the 4000−300 cm−1 range. Magnetic
susceptibility measurements were obtained in the temperature range
2−300 K, using a Quantum Design MPMS XL-7 SQUID magneto-
meter equipped with a 7 T magnet. The experimental magnetic data
Figure 1. The crystal structures of compounds from ligand H2L3 (3, 4, and Dy6). Adapted from refs 16, 13, and 17.
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3.15; N, 11.17. IR (KBr, cm−1): 3402 (br), 1633 (w), 1590 (s), 1548
(m), 1499 (s), 1475 (s), 1449 (s), 1433 (m), 1384 (s), 1340 (s), 1308
(s), 1255 (w), 1169 (w), 1106 (s), 1049 (m), 1012 (w), 973 (w), 944
(w), 922 (m), 904 (m), 839 (m), 768 (m), 753 (m), 742 (m), 716
(m), 692 (m), 653 (m), 489 (m).
Table 1. Crystallographic Data and Structure Refinement for
Complexes 1 and 2
1
2
formula
Mr
C36H40Dy2N2O12
1017.70
C36H44Dy2N10O18
1229.81
0.19 × 0.18 × 0.16
red
cryst size [mm]
color
0.20 × 0.18 × 0.16
pale yellow
RESULTS AND DISCUSSION
■
For ligand H2L3, two Dy2 compounds (Figure 1, asymmetric
Dy2 (3)16 and centrosymmetric Dy2 (4)13) with hula hoop-like
geometry have been reported by our group, which both show
the typical SMM behavior. In particular, the asymmetric Dy2
compound (3) shows the high axiality and strong Ising
exchange interaction, which efficiently suppresses quantum
tunneling of the magnetization. Furthermore, the assembly of
Dy2 unit leads to a novel Dy6 SMM with triangular prism
arrangement (Figure 1c), indicating that such Dy2 compounds
with hula hoop-like geometry are useful building blocks for
constructing new Dy-based SMMs.17 As compared to ligand
H2L3, ligand H3L1 provides similar coordination sites (two close
coordination pockets showing a linear arrangement), whereas a
clear difference was observed in the part of aldehyde for
hydrazone ligand H2L2, as shown in Scheme 1. Therefore,
compound 1 displays similar hula hoop-like coordination
geometry around each Dy ion, while such geometry is broken
in compound 2 due to the bent nature of H2L2 ligand. Detailed
magnetization dynamics studies reveal typical SMM behavior
for compound 1, but no SMM properties for compound 2
probably due to the changes of coordination geometry of
dysprosium ions and/or interactions between them. In
addition, compound 1 represents the rare alkoxide-O bridged
Dy2 complex that behaves as a SMM. Notably, until now the
Dy-SMMs based on alkoxide bridging ligand (R−O−) are still
rare for the lanthanide complexes with small-nuclearity (n <
5).24−27
cryst syst
space group
T [K]
triclinic
orthorhombic
Pnna
P1
̅
273(2)
273(2)
15.7438(10)
14.1723(9)
22.6961(14)
90
a [Å]
9.0109(4)
10.0056(5)
11.5680(6)
84.4320(10)
72.8300(10)
66.5430(10)
913.88(8)
1
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
V [Å3]
Z
Dcalcd [g cm−3
μ(Mo Kα) [mm−1
90
90
5064.09
4
]
1.849
1.613
]
0.71073
498
0.71073
2424.0
F(000)
reflns collected
unique reflns
Rint
5083
32 268
3555
7169
0.0130
0.0762
parameters/restraints
GOF
236/0
301/6
1.053
1.051
R1 [I > 2σ(I)]
wR2 (all data)
0.0249
0.0581
0.0560
0.2015
2.575(6) Å and Dy−O−Dy angle of 114.9(2)° (Figure 2b).
The two ligands bind two DyIII ions in a spiral twisted “head-to-
tail” fashion with the quadridentate (O1, O2, N3, and N4) and
bidentate (O1a, N1) unit (Figure 2b). A closer look at the
crystal structure of 2 reveals the broken hula hoop-like
coordination geometry on DyIII sites, as seen in Figure 3b,
which should arise from the different features of ligand H2L2
from H2L3. The bend of ligand H2L2 leads to the distortion of
the structure, thus breaking the cyclic ring from atoms N1, O1,
O1a, N3, N4, and O2 (Figure 3b). The different coordination
geometries around DyIII ion in these compounds are probably
responsible for the distinct magnetic behavior observed (see
below).
Crystal Structures of 1 and 2. The reaction of
Dy(PhCOO)3·6H2O with H3L1 in 2:1 CH3OH/CH3CN in
the presence of triethylamine followed by Et2O diffusion leads
to the formation of pale-yellow crystals of 1, while compound 2
was obtained by the reaction of Dy(NO3)3·6H2O with H2L2 in
1:2 CH3OH/CH3CN in the presence of triethylamine. Crystal
data and structure refinement details for 1 and 2 are
summarized in Table 1. Their crystal structures have been
depicted in Figure 2. Compounds 1 and 2 crystallize in the
triclinic P1 and orthorhombic Pnna space group, respectively.
̅
In compound 1, two DyIII ions are bridged by the μ-Oalkoxide
atoms from two ligands, with Dy−O bond lengths of 2.295(2)
and 2.261(0) Å, Dy···Dy distance of 3.769(9) Å, as well as Dy−
O−Dy angle of 111.66(9)° (Figure 2a). Here, eight-coordinate
Dy1 center demonstrates a hula hoop-like coordination
geometry with the cyclic ring defined by the atoms N1, O1,
O1a, O3, and O2 from two ligands (Figure 3a), which
resembles that in compounds 3 and 4. In general, the “hula
hoop” configuration may favor persistent axiality of Dy ions,
and the orientations of easy axes will further affect the dipolar−
dipolar magnetic interactions within the molecule. In addition,
one PhCOO− and one CH3OH are coordinated to each
dysprosium ion, completing the coordination sphere around it.
Furthermore, in the crystal of compound 1, the strong intra-
and intermolecular hydrogen-bonding interactions result in a
one-dimensional supramolecular chain similar to that in
complex 3 (Figure S1).
MAGNETIC PROPERTIES
■
Direct Current (dc) Magnetism. Direct current (dc)
magnetic susceptibility studies of poly crystalline samples
(Figure 4) reveal a room-temperature χMT value of 27.1 cm3 K
mol−1 and 29.5 cm3 K mol−1 for 1 and 2, respectively, which is
in agreement with the expected value of 28.34 cm3 K mol−1 for
two uncoupled DyIII ions (6H15/2, g = 4/3). With decreasing the
temperature, χMT product only displays a slight decrease to
26.0 cm3 K mol−1 at 30 K for compound 1, but a clear decrease
to 22.7 cm3 K mol−1 at 9 K in compound 2, which may result
from the depopulation of the Stark sublevels and/or significant
magnetic anisotropy present in DyIII systems. Furthermore, for
compound 2, χMT product shows a slight increase to 25.6 cm3
K mol−1 at 2 K (Figure 4). In contrast, the product increases
sharply to a maximum value of 36.2 cm3 K mol−1 at 2 K in
compound 1 (Figure 4), suggesting stronger intramolecular
ferromagnetic interactions in 1 than those in 2. As seen from
Figure 4, compound 1 demonstrates stronger ferromagnetic
In contrast, compound 2 displays a μ-Ohydrazone bridged Dy2
metal core, with Dy−O/N distances in the range of 2.290(6)−
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value of 40 μB (4 × 10 μB), most likely due to the crystal field
effect at the dysprosium(III) ion.13 The nonsuperposition on a
single mastercurve of the M versus H/T data (Figure 5)
suggests the presence of a significant magnetic anisotropy and/
or low-lying excited states in these systems.28 In addition, it is
worth mentioning that the M versus H data (Figure 4) give rise
to a butterfly shaped hysteresis cycle at 1.65 K for compound 1.
Alternating Current (ac) Magnetism. The temperature-
(Figure 6, Figures S3 and Figure S4) and frequency-dependent
(Figure S5) ac susceptibilities of compounds 1 and 2 were
measured under zero dc field. At high temperature region (>5
K), out-of-phase (χ″) signals of compound 1 are observed with
maxima at ∼15 K for 1488 Hz (Figure 6, left), revealing a slow
relaxation of the magnetization that is typical for SMM
behavior, which should arise from the strong axiality of DyIII
ion due to the hula hoop-like coordination geometry around it.
At low temperatures (below 5 K), the temperature-dependent
ac susceptibility shows almost negligible increase, suggesting
the “freezing” of the spins by the anisotropy barriers and the
effective suppression of zero-field tunneling of magnetization.16
In contrast, no SMM behavior was observed under zero-field
for compound 2, as indicated by the temperature-dependent ac
susceptibility data (Figure S4).
For compound 1, the magnetization relaxation time (τ) is
extracted from the frequency dependence measurements and is
plotted as a function of 1/T (between 1.8 and 18 K) in Figure
7a. Remarkably, although the ac susceptibility was measured
below the frequency of 1 Hz within the investigated
temperature domain (Figure 7, inset), the frequency-
independent peaks signaling the quantum tunneling region
were not observed, indicating a slow quantum tunneling
relaxation in compound 1. At 1.9 K, the position of frequency-
dependence peak is about 0.58 Hz, indicating a relaxation time
of 274 ms (τ = 1/(2πν)). Thus, the quantum tunneling time
should be more than 274 ms, which is rather long in contrast to
many reported Dy2 SMMs.29−31 At high temperature, the
relaxation follows an Arrhenius-like behavior, affording a high
spin reversal barrier of Ueff = 94 K with τ0 = 2.1 × 10−7 s. The
Cole−Cole plots from the zero field measurements (Figure 7b)
show a near symmetrical shape and can be fitted to the
generalized Debye model, with α parameters below 0.19
between 1.9 and 15 K, indicating a very narrow distribution of
relaxation times.
Figure 2. The crystal structures of compounds 1 (a) and 2 (b).
Structure−Property Relationship. To probe the struc-
ture−property relationship in compounds 1, 2, 3, and 4, some
crucial parameters of structure, including bond lengths of Dy−
O in Dy2O2 core (dDy−O(core)), the average bond lengths of
Dy−O/N at the cyclic ring of hula hoop (daverage), Dy···Dy
distances, and Dy−O−Dy angles have been listed in Table 2. It
is noteworthy that all compounds have similar Dy−O−Dy
angles (>110°), but compound 1 displays the shortest Dy−O
bonds in Dy2O2 cores of four compounds, thus leading to the
shortest Dy···Dy distance, further confirming the occurrence of
stronger intramolecular interactions between metal ions, as
reflected by Figure 4. Here, the strong interactions of
compound 1 should be as a result of the more strongly
negative charge of alkoxide group of ligand H3L1 as compared
to that of the hydrazone-O. Further, the shortest average bond
length of Dy−O/N at the position of cyclic ring was observed
in compound 1, suggesting the formation of a strong ligand
field on the local DyIII sites of compound 1. In addition, the
weakest ferromagnetic interactions were observed in compound
Figure 3. Left: Hula hoop-like geometry with the cyclic ring defined
by the atoms N1, O1, O1a, O3, and O2 for compound 1. Right:
Broken hula hoop-like geometry for compound 2.
interactions between metal centers, while weaker interactions
are observed in compound 2.
Magnetization (M) data for 1 and 2 were collected in the 0−
70 kOe field range below 5 K. The M versus H data below 5 K
(Figure S2) show a rapid increase in the magnetization at low
magnetic fields, which is consistent with the presence of
intramolecular ferromagnetic interactions. The magnetization
eventually reaches the value of 12.0 μB for 1 (13.1 μB for 2) at 2
K and 70 kOe. This value is lower than theoretical saturation
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Figure 4. Left: Plots of χMT versus T for compounds 1, 2, 3, and 4. Right: M versus H data of 1 at 1.65 K emphasizing the butterfly shaped
hysteresis.
Figure 5. M versus H/T plots between 1.9, 3.0, and 5 K for compounds 1 (left) and 2 (right). The solid lines are a guide for the eyes.
Table 2. Some Crucial Structural Parameters for
Compounds 1, 2, 3, and 4
compound
1
2
3
4
dDy−O (Å)
2.295(2)
2.261(0)
2.333(4)
3.769(9)
111.67°
2.463(5)
2.370(5)
2.334(8)
2.333(3)
2.370(8)
3.864(4)
112.20°
2.348(1)
2.318(6)
2.355(1)
3.825(8)
110.12°
daverage (Å)
Dy···Dy (Å)
Dy−O−Dy
4.074(3)
114.88°
Figure 6. Temperature dependence of the out-of-phase (χ″) ac
susceptibility of compounds 1 (left) and 4 (right). Solid lines are
guides for the eyes.
Among the four compounds, both 1 and 4 show similar
crystal structure and ac magnetic properties, and some
surprising results can be grasped through the comparison of
them. At high temperature (>5 K), ac susceptibility curves
demonstrate the higher blocking temperature in compound 1
than that in compound 4 (Figure 6), which should arise from
2 (Figure 4), possibly resulting from the large Dy···Dy
separation induced by the bent of ligand H2L2.
Figure 7. (a) Fitting of the relaxation time (τ) from frequency dependence of the out-of-phase (χ″) parts of the ac susceptibility using Arrhenius law
for compound 1 (inset: frequency dependence of χ″ of the ac susceptibility below 100 Hz). (b) Cole−Cole plots under zero-dc field for compound
1. The solid lines indicate the fits using a generalized Debye model.
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CONCLUSION
■
Two novel Dy2 compounds (1 and 2) have been assembled
from different types of ligands (H3L1 and H2L2). Compound 1
with H3L1 represents the rare alkoxide-O bridged Dy2 complex
and displays the hula hoop-like coordination geometry around
each DyIII ion, thus leading to the typical SMM behavior in
combination of the stronger ferromagnetic interactions
between DyIII ions. The distorted coordination geometry
around DyIII ion and much weaker interactions observed in
compound 2 due to the introduction of bent H2L2 result in the
disappearance of SMM behavior. Our simple comparative
investigations may shed light on the structure−property
relationship of lanthanide-based SMMs, which is crucial to
the advancement of single-molecule data storage and
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A. K.; Wernsdorfer, W.; Ungur, L.; Chibotaru, L. F.; Shen, M.; Cheng,
P. Chem. Sci. 2012, 3, 3366.
31,33
34
Dy2
and Dy2CoIII
.
2
In this regard, further investigations
including ab initio calculations and magnetic dilution are
required to elucidate the mechanisms operating in our
compounds.
ASSOCIATED CONTENT
* Supporting Information
Crystal structures and magnetic measurement (Figures S1−S7).
X-ray crystallographic data in CIF format (CCDC 894094 (1)
and 894095 (2)). This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
(28) Lin, P.-H.; Sun, W.-B.; Yu, M.-F.; Li, G.-M.; Yan, P.-F.;
Murugesu, M. Chem. Commun. 2011, 47, 10993.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tang@ciac.jl.cn.
Notes
■
(29) Lin, P.-H.; Burchell, T. J.; Cler
Chem., Int. Ed. 2008, 47, 8848.
(30) Xu, G. F.; Wang, Q. L.; Gamez, P.; Ma, Y.; Cler
́
ac, R.; Murugesu, M. Angew.
́
ac, R.; Tang, J.
K.; Yan, S. P.; Cheng, P.; Liao, D. Z. Chem. Commun. 2010, 46, 1506.
(31) Long, J.; Habib, F.; Lin, P.-H.; Korobkov, I.; Enright, G.; Ungur,
L.; Wernsdorfer, W.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc.
2011, 133, 5319.
(32) Sulway, S. A.; Layfield, R. A.; Tuna, F.; Wernsdorfer, W.;
Winpenny, R. E. P. Chem. Commun. 2012, 48, 1508.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grants 91022009, 21241006, and 21221061) for financial
support.
(33) Habib, F.; Lin, P.-H.; Long, J.; Korobkov, I.; Wernsdorfer, W.;
Murugesu, M. J. Am. Chem. Soc. 2011, 133, 8830.
(34) Langley, S. K.; Chilton, N. F.; Ungur, L.; Moubaraki, B.;
Chibotaru, L. F.; Murray, K. S. Inorg. Chem. 2012, 51, 11873.
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