Family of defect-dicubane Ni4Ln2 (Ln = Gd, Tb, Dy, Ho) and Ni4Y2 complexes: Rare Tb(III) and Ho(III) examples showing SMM behavior

Article abstract of DOI:10.1021/ic402973g

Reactions of Ln^III perchlorate (Ln = Gd, Tb, Dy, and Ho), NiCl₂·6H₂O, and a polydentate Schiff base resulted in the assembly of novel isostructural hexanuclear Ni₄Ln₂ complexes [Ln = Gd (1), Tb (2), D

Full text of DOI:10.1021/ic402973g

Article
pubs.acs.org/IC
Family of Defect-Dicubane Ni4Ln2 (Ln = Gd, Tb, Dy, Ho) and Ni4Y2
Complexes: Rare Tb(III) and Ho(III) Examples Showing SMM Behavior
Lang Zhao,†,‡ Jianfeng Wu,† Hongshan Ke,† and Jinkui Tang*,†
†State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, China
‡State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012,
China
S
* Supporting Information
ABSTRACT: Reactions of LnIII perchlorate (Ln = Gd, Tb,
Dy, and Ho), NiCl2·6H2O, and a polydentate Schiff base
resulted in the assembly of novel isostructural hexanuclear
Ni4Ln2 complexes [Ln = Gd (1), Tb (2), Dy (3), Ho (4)] with
an unprecedented 3d−4f metal topology consisting of two
defect-dicubane units. The corresponding Ni4Y2 (5) complex
containing diamagnetic YIII atoms was also isolated to assist the
magnetic studies. Interestingly, complexes 2 and 3 exhibit
SMM characteristics and 4 shows slow relaxation of the
magnetization. The absence of frequency-dependent in-phase
and out-of-phase signals for the Ni−Y species suggests that the
Ln ions’ contribution to the slow relaxation must be effectual
as previously observed in other Ni−Dy samples. However, the
observation of χ″ signals with zero dc field for the Ni−Tb and
Ni−Ho derivatives is notable. Indeed, this is the first time that such a behavior is observed in the Ni−Tb and Ni−Ho complexes.
benzylideneamino)phenol in Ni−Ln cluster chemistry, and a
family of Ni4Ln2 clusters consisting of two Ni2LnO4 defective
cubanes with unusual structural features and magnetic proper-
ties were synthesized, where slow magnetic relaxation was
observed for the Ni4Dy2 species.13 To assemble novel
heterometallic polynuclear complexes, we focused our research
efforts on a Schiff base ligand, N1,N3-bis(3-methoxy-
salicylidene) diethylenetriamine (H2L, Scheme S1, Supporting
Information). This ligand is a compartmental type ligand and
has two different coordination sites; the inner site (N3O2, two
amide, one imine, and two phenol functions) showing
preference for 3d metal ions and the outer site (O2O2, two
phenol groups and two oxygen atoms of the methoxy groups)
having preference for hard, oxophilic 4f metal ions. It is
noteworthy that Long and co-workers synthesized an exchange-
coupled Dy2 complex that behaves as an SMM with a very high
anisotropy barrier value of 76 K using this kind of Schiff base
ligand.17 Although some homometallic complexes and a
Co2Dy2 complex based on H2L have been previously reported
in the literature by our group18 and others,19 we recently
employ it in mixed Ni−Ln cluster chemistry, and a series of
Ni4Ln2 hexanuclear complexes were synthesized for the first
time by using this ligand. We herein report the synthesis,
characterization, and detailed magnetic properties of a series of
INTRODUCTION
■
Polymetallic complexes have attracted great attention because
of their wide promising applications in many significant areas
such as physics, chemistry, and materials science.1−3 For
instance, homo- and heteropolynuclear complexes are of high
interest by virtue of their importance as magnetic materials,
such as molecular nanomagnets,4 offering the possibility to test
the fundamental questions in physics, such as quantum
tunneling and quantum phase interference, as well as their
applications ranging from quantum computing5 and high-
density memory storage devices6 to magnetic refrigeration.7
3d−4f complexes are of interest because, in most cases, single
ion anisotropy can often be ensured. However, the zero-field
splitting of the MJ states in 4f SMMs usually results in complex
patterns with multiple maxima and minima instead of a simple
parabola, as seen for 3d-based SMMs, between the energetically
lowest MJ states. In this context, many 3d/4f systems,
particularly those containing Cu−Ln,8 Mn−Ln,9 Fe−Ln,10
and Co−Ln11 systems, are widely investigated. However, in
contrast, there are few examples known for Ni−Ln12−14 based
clusters exhibiting SMM behavior.15 The isolation of new Ni−
Ln complexes will help our understanding of these types of
complexes, in regard to their synthesis and magnetic analysis.
The construction of 3d−4f systems is usually facilitated by
the use of suitable compartmental ligands containing
appropriate pockets that can bind different metal ions.16
Recently, we have employed (E)-2-(2-hydroxy-3-methoxy-
Received: December 3, 2013
Published: March 24, 2014
© 2014 American Chemical Society
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isostructural hexanuclear Ni4Ln2 complexes [Ln = Gd (1), Tb
(2), Dy (3), Ho (4)] with an unprecedented 3d−4f metal
topology comprising two defect-dicubane units. The corre-
sponding Ni4Y2 (5) complex containing diamagnetic YIII atoms
was also isolated to assist the magnetic studies. Interestingly,
complexes 2 and 3 exhibit SMM characteristics and 4 shows
slow relaxation of the magnetization. The observation of χ″M
signals with zero dc field for the Ni−Tb and Ni−Ho derivatives
is notable while the absence of frequency-dependent signals for
the Ni−Y complex indicates that the contribution of 4f ions to
the anisotropy must be effectual. If such a behavior was
reported previously for other Ni−Dy samples,15 it is indeed
observed for the first time in the Ni−Tb and Ni−Ho
complexes.
(3)) and {[Ho2Ni4L2Cl2(OH)4(CH3OH)6]Cl2(ClO4)2-
(CH3OH)2·(H2O)2} (4) could be isolated as single crystalline
material. To probe the magnetic behavior of 3d−4f systems,
{[Y2Ni4L2Cl2(OH)4(CH3OH)6]Cl2(ClO4)2(CH3OH)2·
(H2O)2} (5) was synthesized to exclude the lanthanide
anisotropy and the 4f contribution, but all attempts to
substitute Ni with Zn were unsuccessful. Those reactions all
gave noncrystalline materials that could not be further
characterized. Complexes 1−3 crystallize in the triclinic space
group P1, whereas complexes 4−5 crystallize in the monoclinic
̅
space group P21/c. The complexes are similar to each other, but
having two bridging methanol molecules connecting the LnIII
and NiII metal ions for complexes 1−3 instead of two hydroxyl
groups for complexes 4−5. The structure of 3 will be described
as representative of the whole series.
Crystal Structures. The crystal structure consists of the
EXPERIMENTAL SECTION
■
entities [Dy2Ni4L2Cl2(OH)2(CH3O)2(CH3OH)6]4+, Cl−, and
Physical Measurements. All chemicals purchased were analytical
reagent grade and used as received. Elemental analysis (C, H, and N)
were performed on a PerkinElmer 2400 analyzer. IR spectra were
recorded with samples prepared as KBr disks in the 4000−300 cm−1
range on a PerkinElmer Fourier transform infrared spectrophotometer.
X-ray Crystal Structure Determinations. Single-crystal X-ray
data for complexes 1−5 were collected at 185(2) K on a Bruker Apex
II CCD diffractometer equipped with graphite-monochromatized
Mo−Kα radiation (λ = 0.71073 Å). The structures were solved by
direct methods and refined by full-matrix least-squares methods on F2
using SHELXTL-97. All non-hydrogen atoms determined from the
difference Fourier maps were refined anisotropically. Hydrogen atoms
were placed geometrically and refined using a riding model.
Crystallographic data are listed in Table S1 (Supporting Information).
CCDC 972379−972383 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
−
ClO4 as counteranions, CH3OH, and H2O as the solvent
molecule. A perspective view of the hexanuclear portion is
depicted in Figure 1. The core of the structure can be described
Magnetic Measurements. Variable-temperature magnetic sus-
ceptibility measurements were carried out using a Quantum Design
MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet in
the temperature range of 1.9−300 K with an external magnetic field of
1000 Oe. Diamagnetic corrections were made with the Pascal’s
constants for all the constituent atoms as well as the contributions of
the sample holder.
Preparation of Ligand H2L. The Schiff base ligand H2L was
prepared according to the reported procedure.19a Diethylenetriamine
(2.575 g, 25 mmol) and o-vanillin (7.6 g, 50 mmol) were mixed in
ethanol (100 mL) and then heated at reflux for 5 h. The solvent was
evaporated under reduced pressure to give H2L as an orange oil. IR
(KBr, cm−1): 3438 (br), 1629 (S), 1469 (S), 1255 (m), 1080 (w), 964
(w), 734 (S). Elemental analysis (%) calcd: C, 64.85; H, 6.53; N,
11.34; found: C, 64.74; H, 6.87; N, 11.37.
Synthesis of Complexes 1−5. Ln(ClO4)3·6H2O (Ln = Gd, Tb,
Dy, Ho, Y) (0.15 mmol) was dissolved in CH3OH/CH2Cl2 (5 mL/10
mL), followed by the addition of H2L (0.2 mmol) and NiCl2·6H2O
(0.3 mmol). Then triethylamine (0.57 mmol, 0.08 mL) was added
after 0.5 h, and the resulting mixture was stirred for 3 h. Pale green
block-shaped single crystals, suitable for X-ray diffraction analysis, were
isolated after 6 days.
Figure 1. Molecular structure of 3 highlighting the [Ni2DyO3Cl]
heterometallic defective cubane subunits in bright green lines. The
noncoordinated solvent molecules and H atoms are omitted for clarity.
as two Ni2DyO3Cl defective cubane subunits held together by
two hydroxyl groups and two phenoxo bridging oxygen atoms
(Figure 2a). Each Ni2DyO3Cl subunit is made of two nickel
ions and one dysprosium ion arranged as a defective cubane
with one missing vertex where the three metal ions are linked
to each other by means of one phenoxo group, one hydroxyl
group, one methanol molecule, and one chloride anion. The
eight-coordinated environment of Dy1 is completed by two
phenolic bridging oxygen atoms and two methoxy oxygen
atoms from the ligand L, two μ3-OH and two alcoholic oxygen
atoms of the methanol molecules, close to a distorted square-
antiprismatic geometry with O8 coordination sites (Figure 2b).
Dy1 and Dy1a are double-bridged by two μ3-O atoms from two
μ3-OH to form a Dy2O2 rhombus with angles of 110.54(33)°
and 69.46(29)° for Dy−O−Dy and O−Dy−O, respectively.
The nickel ions in the N2O3Cl/NO4Cl surrounding are six
coordinated with the coordination polyhedron of distorted
octahedral geometry. The N2O2 donor atoms derived from two
imines, and one phenoxo group from one L ligand and one μ3-
OCH3 ligand reside in the equatorial positions of Ni1, while
one alcoholic oxygen atom of the methanol molecule and one
chloride are placed in the axial positions (Figure 2c). The
equatorial plane of Ni2 is composed of NO3 atoms derived
from one imine, one μ3-hydroxyl group, one μ3-OCH3 ligand,
and one alcoholic oxygen of the methanol as well as the axial
RESULTS AND DISCUSSION
■
Synthetic Aspects. N1,N3-bis(3-methoxysalicylidene) di-
ethylenetriamine ligand (H2L) was prepared by the Schiff base
condensation of o-vanilin and diethylenetriamine. Ln(ClO4)3·
6H2O (Ln = Gd, Tb, Dy, Ho) was reacted with H2L in a 3:4
ratio in the presence of NiCl2·6H2O. By using triethylamine as
the base in a MeOH/CH2Cl2 1:2 ratio mixture, the hexanuclear
3d−4f complexes {[Ln2Ni4L2Cl2(OH)2(CH3O)2(CH3OH)6]-
Cl2(ClO4)2(CH3OH)2·(H2O)2} (Ln = Gd (1), Tb (2), Dy
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Figure 2. (a) Coordination environment of nickel and dysprosium in [Dy2Ni4L2Cl2(OH)2(CH3O)2(CH3OH)6]4+ (3). (b) Distorted square
antiprismatic environment around the DyIII metal ion in 3. (c) Distorted octahedral environment around the Ni1 atom in 3. (d) Distorted octahedral
environment around the Ni2 atom in 3: Dy (yellow), Ni (light blue), Cl (green), N (blue), O (red).
positions are occupied by one phenoxo group from one L
ligand and one chloride anion (Figure 2d). The Ni−O and Ni−
N bond lengths cover ranges of 1.979(2)−2.036(2) and
1.960(3)−2.166(3) Å, which are similar to the values reported
in the literature. The Ni−Cl bond length is 2.465(2) Å,
significantly longer than the distances of Ni−O and Ni−N. The
value of the Ni1···Ni2 separation is 3.4221(16) Å. The Ni1−
O5−Ni2 angle is 103.33(5)°, which is larger than the Ni1−
Cl2−Ni2 angle (84.59(5)°). Ni−N, Ni−O, and Ni−Cl bond
distances and the coordination environment are in line with a
high-spin state of the NiII ion (S = 1). Two Ni ions are linked
through one μ3-OCH3 ligand and one chloride anion toward
the central Dy ion, resulting in a Ni2DyO3Cl defective cubane,
which are further connected by two hydroxyl groups and two
phenolic bridging oxygen atoms to adjacent cubane, resulting in
a Ni4Dy2O8Cl2 core with two edge-to-edge Ni2DyO3Cl
defective cubanes. Ni−O(i)−Dy angles are 106.2(1)° and
107.3(1)°, and the Ni−Dy distance is 3.4410(6) Å. The H2L
ligand wrapped around two Dy ions and two Ni ions with the
inner N3O2 donors coordinating two Ni ions, while the outer
O4 donors coordinate with two Dy ions, which consolidates this
Ni4Dy2 cluster.
Figure 3. Temperature dependence of the χMT products at 1000 Oe
for complexes 1 (red), 2 (green), 3 (black), and 4 (purple). The blue
circles correspond to the calculated behavior of complex 1 (see the
text for details).
It is also interesting to compare the series of Ni4Ln2
complexes previously reported by our group13 with the one
described herein. The reported complex13 contains four Ni ions
linked by two μ3-OH and two phenoxo groups that lie in the
central of the metal core, and the two Ln ions lie in the two
edges of the metal core. While all the Ni4Ln2 species described
in this work can be viewed that two Dy ions linked by two μ3-
OH are in the central of the metal core, and the two pairs of Ni
ions lie in the two edges of the metal core. Our Ni4Ln2 shows a
similar arrangement of metal cores as those reported in the
literature,12 but the two Ln ions are held together by two
bridged μ3-OH in our complexes instead of two bridged
carboxyl groups as observed elsewhere. Such differences will
affect the magnetic behavior of these complexes.
Magnetic Properties. The static magnetic susceptibilities
of the four Ni−Ln complexes 1−4 (Ln = Gd, Tb, Dy, Ho,
Figure 3) and 5 (Y2Ni4, Figure 4) were collected in the
temperature range of 2−300 K in an applied magnetic field of
1000 Oe. It is generally known that the lanthanide anisotropy
can be excluded by using isotropic Gd ions, and the 4f
contribution can be removed by using diamagnetic Y ions.
Hence, complexes 1 and 5, which contain Gd ions and Y ions,
respectively, have been prepared. In complex 5, the four Ni(II)
ions are linked by the diamagnetic Y(III) ion, resulting in a pair
of magnetically isolated dinuclear Ni2 units. The molar
magnetic susceptibility product (χMT) of 5 at room temper-
ature is 5.61 cm3 mol−1 K, which is slightly higher than the
Figure 4. Temperature dependence of the χMT product at 1000 Oe for
complex 5 (red). The black circles correspond to the calculated
behavior (see the text for details).
expected value for the presence of four noninteracting Ni(II)
ions (S = 1, if g = 2). On cooling, a gradual χMT increase to 5.65
cm3 mol−1 K (150 K), followed by an abrupt increase, up to
7.10 cm3 mol−1 K at 15 K, and then an abrupt decrease to 5.54
cm3 mol−1 K at 2 K were observed. The curve indicates that the
exchanges within complex 5 are dominated by ferromagnetic
interactions between the Ni(II) ions. The decrease at low
temperature is probably due to intermolecular antiferromag-
netic interaction, to the presence of zero-field splitting (ZFS),
or to a combined effect of the above two factors. Thus, to
simulate the magnetic data, we can treat the model as two
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isolated dimers of Ni(II) ions with the Hamiltonian of H =
−2J1(SNi1·SNi2) − 2[D(Sz2 − 1/3S(S + 1)) + gμBHSz] using the
MAGPACK20 program, where J1 represents the exchange
parameter between the Ni(1)···Ni(2) and Ni(1)#1···Ni(2)#1
ions and D accounts for axial single-ion zero-field splitting
(ZFS) of the Ni(II) ions. A fit to the experimental data gives J1
= 2.78 cm−1, D = 5.16 cm−1, and g = 2.28 with R = 3.79 × 10−3
that further indicated ferromagnetic coupling between nickel-
(II) ions. The magnetization measurement for complex 5 at 1.9
K (Figure S1, Supporting Information) shows a rapid increase
of the magnetization and eventually reaches 8.4 μB at 1.9 K and
7 T, in line with the presence of ferromagnetic interactions in
this system.
At room temperature, the χMT values of complexes 2, 3, and
4 are 30.24, 34.28, and 34.19 cm3 mol−1 K, respectively. These
values are almost in good agreement with the expected
theoretical values (2: 27.64; 3: 32.34; 4: 32.16 cm3 mol−1 K)
for two noninteracting lanthanide ions and four uncoupled NiII
ions. Upon cooling, the χMT value increases gradually before 50
K and then increases abruptly to the maximum value of 56.53
cm3 mol−1 K for 2, 54.72 cm3 mol−1 K for 3, and 46.52 cm3
mol−1 K for 4 at 7 K, and finally decreases to 20.28, 19.85, and
19.02 cm3 mol−1 K for 2, 3, and 4, respectively, at 2 K. The
curve suggests that the couplings within complexes 2, 3, and 4
are dominated by ferromagnetic interactions between the
paramagnetic centers in the defect-dicubane units. The decrease
of the χMT values at low temperature is ascribed to the
antiferromagnetic interaction between the spin carriers and the
thermal depopulation of the Stark levels of the TbIII, DyIII, and
HoIII centers. Compared with the reported Ln2Ni4 com-
plexes,12,13 the direct-current (dc) magnetic measurements
indicate that ferromagnetic interaction is dominated in
complexes 1−5, while antiferromagnetic coupling for the
reported Ln2Ni4. The field-dependent magnetization at low
temperatures reveals a steady increase approaching the value of
18.79 μB for 2, 22.46 μB for 3, and 20.73 μB for 4 at 70 kOe
without saturation (Figures S2−S4, Supporting Information).
This behavior suggests the presence of magnetic anisotropy
and/or the population of low-lying excited states.21
For the analogous gadolinium complex 1, the two NiII ions
are linked through the paramagnetic GdIII ion that can mediate
the magnetic interactions between the two types of metal ions.
The χMT vs T plot shows similar thermal behavior to that of 5.
The observed χMT value at 300 K of 22.20 cm3 mol−1 K for 1 is
slightly higher than the calculated value of 19.75 cm3 mol−1
K
for two noninteracting GdIII (S = 7/2, C = 7.875 cm3 mol−1 K)
and four high-spin NiII (S = 1, if g = 2) ions. On cooling, the
χMT value increases gradually before 50 K, then increases
abruptly to a maximum value of 43.35 cm3 mol−1 K at 6 K, and
finally decreases to 23.35 cm3 mol−1 K at 2 K, also showing the
predominantly ferromagnetic character. The ferromagnetic
interaction in 1 is in contrast to the antiferromagnetic
interactions observed in the reported Gd2Ni4 cluster with the
same arrangement of the metal motif.12 The experimental data
were fitted on the basis of the following simplified spin
Temperature- and frequency-dependent ac susceptibility
measurements were carried out under zero dc fields. Strikingly,
the results of the ac magnetic susceptibility observed for both
Tb-containing and Dy-containing complexes 2 (Figure 5, left)
and 3 (Figure 5, right) show that both in-phase and out-of-
phase susceptibilities are strongly frequency- and temperature-
dependent with a series of frequency-dependent peaks for the
out-of phase ac signals (Figure S5, Supporting Information),
typical for an SMM. However, only a weak frequency-
dependent ac signal below 6 K without peaks in the χ′ and
χ″ vs T plots, showing the onset of slow relaxation of the
Hamiltonian using the three-J models: = −2J1(SNi1·SNi2A
+
SNi1A·SNi2) − 2J2(SNi2·SGd1 + SNi2·SGd1A + SNi2A·SGd1A + SNi1·SGd1
+ SNi2A·SGd1 + SNi2A·SGd1A) − 2J3(SGd1·SGd1A), in which J is the
exchange coupling constant between the two ions, as shown in
Scheme 1. We used the MAGPACK20 program, including
Scheme 1. Three-J Model Magnetic Exchange Interactions
Employed To Simulate the Susceptibilities of Complex 1
magnetization, was observed for the HoIII2NiII complex 4
4
(Figure S6, Supporting Information). The out-of-phase (χ″)
component of the ac susceptibility of 2 and 3 clearly shows a
frequency-dependent peak, respectively. In contrast to what is
commonly observed, namely, a decrease of maximum peaks of
the χ″ signal upon increasing the temperatures, these peaks
increase at the temperature range of the test. These results are
striking since most Ni−Ln complexes reported so far do not
show SMM behavior under zero dc field.12−14 Complex 4 does
not show similar out-of-phase susceptibility signals with 2 and 3
above 1.8 K under zero dc field. However, the situation is
unaltered by the presence of a dc field of 2 kOe for 4. Such
behavior is observed for the first time in the Ni−Tb and Ni−
Ho complexes, even though it was previously reported for Ni−
Dy systems.15 No out-of-phase signals were observed for 1 and
5 above 1.8 K.
The blocking temperatures (Tb, Dy maximum of χ″ vs T
plot) at 1500 Hz for 2 and 3 are observed at 2.9 and 3.2 K,
respectively. The frequency-dependent behavior reveals that the
relaxation follows a thermally activated mechanism above 1.9 K,
and the plots of ln(τ) vs 1/T are linear (Figure 6). Fitting the
data to the Arrhenius law [τ = τ0 exp(Ueff/kBT)] afforded an
energy barrier of 30 and 32 K for 2 and 3, respectively, with a
pre-exponential factor (τ0) of τ0 = 2.09 × 10−9 and 1.41 × 10−8
s, in line with the expected τ0 of 10−6 −10−12 s for an SMM.
From these data, Cole−Cole plots of χ″ vs χ′ (Figure 6, bottom
intermolecular interaction (and/or zero-field splitting) by θ
value, to model the drop of χMT data at low temperatures. The
best-fit parameters for the data were J1 = 1.95 cm−1, J2 = −1.46
cm−1, J3 = −0.04 cm−1, θ = −0.06 cm−1, and g = 2.22. The
results indicate ferromagnetic interaction between Ni(II) ions,
antiferromagnetic interaction between NiII ions and GdIII ions,
and weakly antiferromagnetic interaction within the two GdIII
ions. The Gd−Gd and the Ni−Gd coupling are very weak as
expected. This simulation study is helpful to understand the
magnetic behavior of the species containing anisotropic
lanthanide ions, which is much more complicated. The field
dependence of the magnetization at 1.9 K appears to be
saturated at 7 T to 23.17 μB, which is close to the value
expected if all the metal ions were in the same direction (22
μB).
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Figure 5. Frequency dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibilities of 2 (left) and 3 (right) at different temperatures in a
zero dc field and a 3 Oe ac field. Solid lines are guides for the eyes.
Figure 6. Fitting of the relaxation time (τ) from frequency dependence of the out-of-phase (χ″) parts of the ac susceptibility using Arrhenius law for
complexes 2 (left) and 3 (right). Inset: Cole−Cole plots under zero dc field. The solid lines indicate the fits using a generalized Debye model.
inset) can be constructed and fitted to a generalized Debye
model22 to determine α values and relaxation times (τ) in the
temperature ranges 1.9−2.8 K for 2 and 1.9−3.1 K for 3. The
relatively symmetrical plots suggest a single relaxation process.
The α values, ranging from 0.13−0.16 for 2 and 0.07−0.16 for
3, indicate a narrow distribution of relaxation times for the
single relaxation. All of these magnetic parameters clearly
indicate that the remarkable DyIII and TbIII complexes possess
SMM nature. This indicates that the d−f polynuclear complex
provides a useful design for SMMs due to the high-spin state
generated by the frequently observed ferromagnetic interaction
between d and f elements and the inherent magnetic anisotropy
of the 4f component. Noteworthy, the dynamic magnetic
behavior of 2 and 3 is different from that of previously reported
Ln2Ni4 complexes.13 Indeed, the literature reported complexes
are antiferromagnetically coupled and show only slow magnetic
relaxation of magnetization, whereas complexes 2 and 3 with
ferromagnetic interaction are found to be SMMs. It is well-
established that the magnetic anisotropy of an exchange-
coupled system was affected by not only the single-ion
anisotropies but also the relative orientation of the local
axes.23 The different coordination environments (eight-
coordinated in 2 and 3 and nine-coordinated in the reported
Ln2Ni4) around the DyIII ion and/or the different relative
orientation of the local axes within 2 and 3 and the reported
Ln2Ni4 are probably responsible for the different relaxation
dynamics observed. However, detailed theoretical calculations
are required to elucidate the mechanisms operating in
polynuclear 3d−4f complexes.
CONCLUSION
■
In summary, we have successfully obtained five heterometallic
Ln−Ni clusters based on the Schiff base ligand H2L. The core
of each structure consists of two distorted [Ni2LnO3Cl]
defective cubane-like moieties that are further bridged by two
hydroxyl groups and two phenolic bridging oxygen atoms to
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N.; Brechin, E. K.; Alonso, J. J. Angew. Chem., Int. Ed. 2011, 50, 6606−
6609. (d) Langley, S. K.; Chilton, N. F.; Moubaraki, B.; Hooper, T.;
Brechin, E. K.; Evangelisti, M.; Murray, K. S. Chem. Sci. 2011, 2,
1166−1169. (e) Zheng, Y.-Z.; Evangelisti, M.; Winpenny, R. E. P.
Chem. Sci. 2011, 2, 99−102. (f) Sessoli, R. Angew. Chem., Int. Ed. 2012,
51, 43−45.
adjacent cubane, generating a Ni4Dy2O8Cl2 core with two edge-
to-edge Ni2DyO3Cl defective cubanes. Investigation of their
magnetic properties shows ferromagnetic interactions in 1−5.
Interestingly, complexes 2 and 3 exhibit SMM characteristics
and 4 shows slow relaxation of the magnetization. The absence
of frequency-dependent in-phase and out-of-phase signals for
the Ni−Y species suggests that the contribution of Ln ions to
the anisotropy must be effectual as previously observed in other
Ni−Dy samples. However, such a behavior has never been
reported in Ni−Tb and Ni−Ho complexes. Efforts to generate
new interesting molecules using this ligand and other 3d/4f
ions are underway. This synthetic approach represents a
promising route toward the assembly of novel 3d−4f clusters
and new magnetic materials.
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ASSOCIATED CONTENT
* Supporting Information
■
(9) (a) Zaleski, C. M.; Depperman, E. C.; Kampf, J. W.; Kirk, M. L.;
Pecoraro, V. L. Angew. Chem., Int. Ed. 2004, 43, 3912−3914.
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S
Details of the structure solution and refinement (Table S1) and
magnetic measurements (Figures S1−S6) for complexes 1−5.
This material is available free of charge via the Internet at
http://pubs.acs.org.
́
10476−10485. (d) Ako, A. M.; Mereacre, V.; Clerac, R.; Wernsdorfer,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tang@ciac.ac.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grants 21201160, 21371166, 21241006, and 21331003) for
financial support.
́
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