Assembling Dysprosium Dimer Units into a Novel Chain Featuring Slow Magnetic Relaxation via Formate Linker

Article abstract of DOI:10.1021/acs.inorgchem.6b02276

A dinuclear complex [DyLClCH₃OH)]₂ (1) and a one-dimensional compound [DyL(HCOO)(CH₃OH)]_n (2) have been synthesized using an organic ligand of N′-(2-hydroxybenzylidene)picolinohydrazide (H₂L). Complex

Full text of DOI:10.1021/acs.inorgchem.6b02276

Article
pubs.acs.org/IC
Assembling Dysprosium Dimer Units into a Novel Chain Featuring
Slow Magnetic Relaxation via Formate Linker
Qi Chen,† Fang Ma,† Yin-Shan Meng,§ Hao-Ling Sun,*,† Yi-Quan Zhang,*,‡ and Song Gao*,§
†Department of Chemistry and Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University,
Beijing 100875, P. R. China
‡Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R.
China
§Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
S
* Supporting Information
ABSTRACT: A dinuclear complex [DyLClCH3OH)]2 (1) and a one-
dimensional compound [DyL(HCOO)(CH3OH)]n (2) have been synthe-
sized using an organic ligand of N′-(2-hydroxybenzylidene)picolinohydrazide
(H2L). Complex 1 exhibits a symmetric dinuclear structure, in which the Dy3+
centers reside in a pentagonal-bipyramidal coordination environment. In 2,
the dinuclear units of 1 are strung into chains by formate anions, in which
Dy3+ ions are situated in an octa-coordinated, hula-hoop-like coordination
geometry. Magnetic studies reveal that ferromagnetic coupling is found
between Dy3+ ions in both compounds. Complexes 1 and 2 exhibit slow
magnetic relaxation under zero dc field with effective energy barriers of 88.4
and 175.8 K, respectively. Magnetic study combined with ab initio
calculations indicates that the better performance of 2 is related to the
unique molecular geometry and relatively stronger Dy3+−Dy3+ magnetic
interaction within and/or between the dimer units.
hydrazide based Schiff base ligands have become synthetic
chemists’ favorite because they can provide multichelating
groups that can easily coordinate with lanthanide ions. After the
first synthesis of a lanthanide compound containing an
isonicotinohydrazide based Schiff base ligand with slow
magnetic relaxation,6 a series of lanthanide molecular nano-
magnets with dinuclear,7 tetranuclear,8 hexanuclear,9 and
octanuclear9a,d structures have been constructed by employing
these Schiff base derivatives with additional coordination sites
or functional groups. In our previous work, we introduced the
pyridine-N-oxide group to this type of ligands in an effort to
enhance their coordination capabilities with lanthanide ions,
and we successfully constructed a one-dimensional (1D) and a
two-dimensional (2D) dysprosium system featuring slow
magnetic relaxation.10 More recently, we obtained a super-
paramagnetic dinuclear dysprosium compound [DyLCl(CH3-
OH)]2 (1) by reacting the ligand N′-(2-hydroxybenzylidene)-
picolinohydrazide (H2L) with DyCl3·6H2O. It was interesting
to us that one of the coordination sites of the Dy3+ ion in 1 was
occupied by a chloride ion. We contemplated whether we could
bridge this superparamagnetic dinuclear unit by replacing the
chloride ion with carboxylate linkers. We anticipated that this
would lead to lanthanide superparamangetic systems with 1D
INTRODUCTION
■
Molecular nanomagnet, exhibiting slow magnetic relaxation and
magnetic hysteresis at low temperature, is a revived subject in
magnetic materials and has attracted considerable attention in
the field of spin-based devices and high-density information
storage.1 Within the many characteristics required for chemical
design of high performance molecular nanomagnets, magnetic
anisotropy is the determinant factor.2 Owing to the strong
coupling between the spin and orbit angular moments and the
crystal-field splitting effects, lanthanide ions can provide large
magnetic moments and significant magnetic anisotropy, making
them ideal candidates for the construction of molecular
nanomagnets.3 To date, many lanthanide molecular nano-
magnets with diverse structural topologies have been reported,
including systems featuring the highest relaxation energy barrier
and the highest blocking temperature.4 However, the magnetic
behaviors of lanthanide molecular nanomagnets remain poorly
understood because of their complicated magnetic nature,
which can be affected by many factors, such as symmetry-
related single-ion anisotropy, spin−orbit coupling, magnetic
interactions, and crystal-field effects.5 Particularly, the ligand
can directly influence the coordination environments around
lanthanide ions and further affect the magnetic anisotropy of
lanthanide ions.
Among the numerous organic ligands employed for the
construction of lanthanide molecular nanomagnets, pyridyl-
Received: September 24, 2016
© XXXX American Chemical Society
A
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Figure 1. Dinuclear structure (a) and coordination polyhedral around Dy3+ ions in complex 1 (b).
or 2D structures not yet found in the known compounds
derived from the H2L ligand. Herein, we report the synthesis of
a 1D compound [DyL(HCOO)(CH3OH)]n (2) by a
coordination-driven self-assembly method using compound 1
as a building unit and formate group as a bridge. In compound
2, two formate groups adopting a syn−anti bridging mode
replace the coordinating chloride ion in 1 and link the dimer
units to form a 1D chain structure. Magnetic studies indicate
that there are ferromagnetic interactions between the Dy3+ ions
in complexes 1 and 2. As unambiguously indicated by the well-
defined temperature- and frequency-dependent ac signals under
zero dc field, they both show typical slow magnetic relaxation
behavior featuring a Ueff of 88.4 and 175.8 K, respectively.
Theoretical calculation suggests that the larger effective energy
barrier obtained for 2 than that observed for 1 is ascribed to its
larger Dy3+−Dy3+ magnetic coupling, which can largely
suppress the quantum tunneling magnetization at low temper-
atures.
RESULTS AND DISCUSSION
Crystal Structure of 1. Single-crystal X-ray analysis reveals
that complex [DyLCl(CH3OH)]2 (1) crystallizes in the
monoclinic space group P21/c and exhibits a symmetric
dinuclear structure (Figure 1a and Table 1). The asymmetric
■
Table 1. Crystallographic Data and Structure Refinement for
Complexes 1 and 2
1
2
formula
Mr
C14H13ClDyN3O3
469.22
C15H14DyN3O5
478.79
crystal system
space group
a (Å)
monoclinic
P21/c
monoclinic
P21/n
9.644(3)
7.117(2)
22.423(6)
90
9.321(2)
8.249(3)
20.125(4)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
97.503(4)
90
100.547(1)
90
EXPERIMENTAL SECTION
■
1525.9(7)
4
1521.2(7)
4
The pyridine-carbohydrazine based Schiff base ligand N′-(2-hydroxy-
benzylidene)picolinohydrazide (H2L) was synthesized by condensa-
tion of picolinoylhydrazide and salicylaldehyde in methanol using the
reported procedure.11 Other reagents and solvents employed were
commercially available and used as received without further
purification.
μ (mm−1
F(000)
GOF
)
5.086
4.944
900
924
1.106
1.065
data collected
unique
8694
7884
2988
2888
[DyLCl(CH3OH)]2 (1). H2L (0.2 mmol, 48.2 mg) was dissolved in 8
mL of methanol, to which a methanolic solution of Et3N (2.0 mL, 0.2
mol·L−1) was added. The reaction mixture was then stirred for 2 min,
after which solid DyCl3·6H2O (0.2 mmol, 75.4 mg) was added and the
yellow solution was continually stirred for 20 min. The mixture was
transferred to a 20 mL Teflon reactor and heated to 80 °C for 72 h
under autogenous pressure, before cooling to room temperature at a
rate of 5 °C·h−1. Yellowish block crystals of 1 were obtained. Yield:
57.8 mg (61.7% based on the metal salt). Elemental analysis (%) calcd
for C14H13ClDyN3O3: C, 35.84, N, 8.96, H, 2.79; found C, 35.85, N,
8.86, H, 2.90.IR (KBr, cm−1): 3238(br), 1610(s), 1560(m), 1541(m),
1477(m), 1438(w), 1334(m), 1197(m), 1153(m), 1004(m), 894(m),
686(m), 543(m).
[DyL(HCOO)(CH3OH)]n (2). H2L (0.02 mmol, 4.8 mg) was
dissolved in 8 mL of methanol. A methanolic solution of Et3N (1.0
mL, 0.02 mol·L−1) was added to the mixture. After stirring for 2 min,
solid DyCl3·6H2O (0.02 mmol, 7.5 mg) and HCOONa·2H2O (0.06
mmol, 6.2 mg) were added and the yellow solution was continually
stirred for 20 min. The mixture was transferred to a 20 mL Teflon
reactor and heated to 80 °C for 72 h under autogenous pressure,
before cooling to room temperature at a rate of 5 °C·h−1. Yellowish
block crystals of 2 were obtained. Yield: 2.6 mg (26.5% based on the
metal salt). Elemental analysis (%) calcd for C15H14DyN3O5: C, 37.63,
N, 8.78, H, 2.95; found C, 37.56, N, 8.73, H, 2.88. IR (KBr, cm−1):
3425(br), 1606(s), 1560(s), 1475(s), 1440(m), 1336(s), 1195(m),
1014(m), 896(s), 798(m), 758(m), 686(m), 534(m).
Rint
0.0259
0.0573
R1, wR2 [I > 2σ(I)]
R1, wR2 [all data]
0.0226, 0.0488
0.0257, 0.0499
0.0415, 0.1210
0.0598, 0.1634
unit of 1 is composed of one crystallographically independent
Dy3+ ion, one L2− ligand, one chloride ion, and one methanol
molecule. As shown in Figure 1, each Dy3+ ion is hepta-
coordinated with an approximately pentagonal-bipyramidal
coordination geometry, in which the equatorial site is defined
by one pyridyl nitrogen atom (N1), two alkoxido oxygen atoms
(O1 and O1A), one hydrazide nitrogen atom (N3A), and one
phenolate oxygen atom (O2A). Additionally, one chloride ion
(Cl1) and one methanol molecule (O3) are situated at the axial
sites. The Dy−O distances are in the range of 2.177−2.366 Å
(Table S1), resembling those found in documented dysprosium
complexes constructed by related Schiff base ligands.9−12 It is
noteworthy that the Dy1−O2A bond length (2.177 Å) is
substantially shorter than those between Dy1 and the other
coordination atoms (2.312−2.627 Å), which is likely due to the
stronger coordination affinity of the phenolate oxygen atom
which has a relatively large negative charge. The shortest Dy1−
O2A bond distance may play an important role in mediating
the magnetic behavior of complex 1. The dysprosium centers in
B
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Figure 2. Dinuclear unit (a), coordination polyhedral around Dy3+ ion (b), and 1D chain in complex 2 (c).
the dinuclear core are bridged by two alkoxido groups (O1 and
O1A) from two antiparallel ligands adopting the conjugate
deprotonated enol form with a Dy···Dy distance of 3.900 Å and
a Dy−O−Dy angle of 113.7°. Two ligands bind to two Dy3+
ions with the tridentate (O1, N3, O2) and bidentate (N1, O1)
parts, as observed in most reported complexes containing a
pyridyl-carbohydrazide moiety.6−9 Adjacent dimer units are
connected via Cl1···H3A−O3 hydrogen bonds between
coordinated methanol molecules (O3) and neighboring
chloride ions (Cl1···O3 = 3.117 Å, ∠O3−H3A···Cl1 =
136.82°), forming a 1D supramolecular chain with the shortest
interdimer Dy···Dy distance at 7.117 Å (Figure S1a).
Neighboring chains along the a-axis are further linked to
produce a 2D layer through π−π interactions between
interdigitating pyridyl (phenyl) and phenyl with the shortest
interchain Dy···Dy distance of 8.272 Å (Figure S1b).
(O1 and O1A) with a Dy···Dy distance of 3.938 Å and a Dy−
O−Dy angle of 113.4°. The formate groups adopt the syn−anti
bridging mode and connect the adjacent dinuclear subunits to
produce a 1D chain structure along the b direction with the
shortest interdimer Dy···Dy distance at 5.752 Å (Figure 2c).
Adjacent chains along the a-axis are further linked to construct
a 2D layer structure through π−π interactions between
interdigitating pyridyl (phenyl) and phenyl groups with the
shortest interchain Dy···Dy distance of 8.794 Å (Figure S2). To
the best of our knowledge, complex 2 is a rare example of a 1D
chain derived from the assembly of neighboring dinuclear
subunits based on a pyridyl-carbohydrazine Schiff base ligand.
In order to confirm the phase purity, compounds 1 and 2
were analyzed by powder X-ray diffraction (XRD) at room
temperature using the same microcrystalline samples from the
magnetic measurements (Figure S3). The diffraction peaks of
the as-prepared samples are in good agreement with the
corresponding simulated patterns calculated from the single-
crystal X-ray diffraction data, demonstrating a high phase purity
of the experimental samples.
Crystal Structure of 2. It is interesting to note that, when
sodium formate was added to the reaction system of 1, the
formate group replaced the coordinated chloride ion and
connected the dinuclear units of 1, forming a distinct 1D chain
complex of [DyL(HCOO)(CH3OH)]n (2) (Figure 2).
Compound 2 crystallizes in the monoclinic space group P21/
n, the asymmetric unit of which consists of one Dy3+ ion, one
L2− ligand, one formate group, and one coordinated methanol
molecule. As shown in Figure 2, each Dy3+ ion in 2 is octa-
coordinated with an approximately hula-hoop-like coordination
geometry. The circle of the hula-hoop resembles that found in
1, which is defined by one pyridyl nitrogen atom (N1), two
alkoxido oxygen atoms (O1 and O1A), one hydrazide nitrogen
atom (N3A), and one phenolate oxygen atom (O2A); however,
the axial coordination groups are quite distinct, occupied by
two formate groups (O3 and O4) and one methanol molecule
(O5). The Dy−O distances are between 2.181 and 2.419 Å
(Table S1), slightly longer than that of 1 (2.179−2.366 Å).
Magnetic Behavior of 1. Temperature-dependent mag-
netic susceptibility measurement was performed on a micro-
crystalline sample of 1 in the temperature range of 2−300 K at
1 kOe. As shown in Figure 3, at room temperature, the
observed χMT value of 13.96 cm3 mol−1 K is slightly smaller
than the expected value (14.17 cm3 mol−1 K) for one Dy3+ ion
with a ground state H15/2 and g = 4/3.12 With decreasing
6
temperatures, the χMT value gradually decreases, reaching a
minimum value of 12.46 cm3 mol−1 K at 32 K, which is mainly
ascribed to the progressive depopulation of the excited Stark
components of Dy3+ ions. Upon further cooling, the χMT value
abruptly increases to a maximum of 14.33 cm3 mol−1 K at 3 K.
The increase of χMT in the low temperature range clearly
implies intramolecular ferromagnetic interactions between Dy3+
ions. The field dependence of the magnetization was also
investigated in the range of 0−50 kOe at 2, 3, 5, 8, and 10 K,
respectively (Figure S4a). A rapid increase of the magnetization
is observed at low magnetic fields, indicating well-separated
excited Kramer’s doublets. The nonsaturation and non-
superposition of the M vs H/T plots at higher fields suggest
−
Nevertheless, the bond length of Dy−OHCOO (2.373 and
2.404 Å) in 2 is obviously shorter than that of Dy−Cl (2.627 Å)
in 1, indicating a stronger coordination strength in the axial
direction, which might have a great impact on the magnetic
behavior of 2. Similar to 1, two neighboring Dy3+ ions in the
dinuclear subunits of 2 are also bridged by two alkoxido groups
C
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Figure 5. Magnetic relaxation dynamics for 1 under zero dc field.
Figure 3. Temperature dependence of χMT products for 1 and 2. The
black and red lines are the simulation from ab initio calculation.
coefficient of the Raman process) and the quantum tunnelling
process (τQTM−1). The fitting of the ln τ vs 1/T plots with the
consideration of all relaxation processes gives a good agreement
with the data over the whole temperature range, generating the
following parameters: Ueff = 88.4 K, τ0 = 2.65 × 10−7 s, C = 0.35
s−1 K−3.18, n = 3.18, and τQTM = 0.028 s. The values of τ0 are of
the correct order of magnitude expected for an Orbach
relaxation process, and the values of C and n are as expected for
Raman process. Furthermore, the Cole−Cole diagram is
created based on the frequency dependencies of the ac
susceptibility data, which is fitted to a generalized Debye
model, giving rise to a narrow distribution of coefficient α
parameters in the range of 0.015−0.26 (Tables S3 and S4).
This is consistent with the presence of a unique coordination
sphere of Dy3+ ion in 1. Since the external field can suppress the
QTM process, we applied an optimized dc field of 2 kOe. As a
consequence, the out-of-phase signal is decreased below 5 K
compared to the zero field case (Figure S6). In the high
temperature region, the magnetic relaxation is similar to that of
the zero field case, since both of them originate from the single
lanthanide center. There is no observable evidence of the
existence of the direct process.
the presence of significant magnetic anisotropy caused by the
crystal-field effect in 1.
To further probe the dynamics of magnetization, alternating
current (ac) susceptibility measurements were performed for 1
in the frequency range of 100−10 000 Hz under zero dc field.
As shown in Figure 4, both the in-phase (χM′) and out-of-phase
Magnetic Behavior of 2. The variable-temperature
magnetic susceptibility measurement for polycrystals 2 was
also investigated in the temperature range of 2−300 K under 1
kOe dc field (Figure 3). The experimental χMT value of 13.79
cm3 mol−1 K at room temperature is slightly smaller than the
expected value of 14.17 cm3 mol−1 K for one uncoupled
dysprosium ion with a ground state 6H15/2 and g = 4/3. Similar
to 1, the χMT value of 2 gradually decreases in a wide
temperature range from 300 to 24 K, reaching a minimum value
of 12.64 cm3 mol−1 K at 24 K, which is mainly ascribed to the
depopulation of the excited Stark sublevels. Below 24 K, the
χMT value goes up to a maximum value of 15.37 cm3 mol−1 K at
2 K, similarly denoting the presence of dominant ferromagnetic
interactions between the dysprosium ions within and/or
between the dinuclear subunits. The field dependence of the
magnetization of 2 was investigated in the range of 0−50 kOe
at 2, 3, 5, 8, and 10 K, respectively (Figure S4b). A rapid
increase of the magnetization is observed at low magnetic fields,
followed by a linear increase up to 5.5 Nβ at 2 K. The lack of
saturation of magnetization at 50 kOe or superposition of the
M vs H/T plots suggests the presence of a significant magnetic
anisotropy and/or low-lying excited states in 2.
Figure 4. Temperature dependence of the in-phase χM′ (top) and out-
of-phase χM″ (bottom) ac signals under zero dc field for 1.
(χM″) signals of ac susceptibilities are strongly temperature-
dependent below 28 K, which is a positive proof for the slow
magnetic relaxation and superparamagnetic behavior in 1. In
the χac−T plots, good peak shapes are clearly observed, which
shift to high temperatures with increasing frequencies. Upon
decreasing the temperature, the upturns of χM′ and χM″ are
observed below 6 K, implying that the quantum tunneling
mechanism, which occurs between the degenerated ground
states, may play a dominant role at low temperatures.13 The
relaxation time can be deduced from the frequency-dependent
ac data at different temperatures using the generalized Debye
model and is plotted as ln τ vs 1/T as shown in Figure 5 (also
see Figure S5; Tables S3 and S4). Generally speaking, the
relaxation process of a lanthanide molecular nanomagnet is a
combination of the Orbach process, Raman process, and
quantum tunneling process.14 However, different relaxation
processes dominate in different temperature ranges. For
example, at high temperatures, the relaxation is dominated by
the Orbach process (τOrbach−1 ∼ τ0−1 exp(−Ueff/kBT)), whereas,
at low temperatures, gradual transitions are observed as a result
Ac susceptibility measurements were also conducted for 2
under zero dc field to further explore the dynamics of
magnetization. Similar to 1, both the real (χM′) and imaginary
of the Raman process (τRaman ∼ CTn, where C is the
−1
D
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(χM″) part of the ac susceptibility for 2 show strong
temperature dependence, with a clear evidence of the peaks
covering the range of 100−10 000 Hz (Figure 6), suggesting
atures. As shown in Figures S9 and S10, magnetic memory
effects are clearly visible below 5 K at a field sweep rate of 500
Oe/s for both complexes, which is characteristic of lanthanide
molecular nanomagnets. When the temperature drops down to
2 K, the butterfly-shaped hysteresis loops for both complexes
become much larger and wider. As expected for a molecular
nanomagnet, upon decreasing the sweeping rate to 300 or 100
Oe/s, the loops become narrower, also indicating the dynamic
behavior of 1 and 2. The strong dependence of hysteresis loops
on the temperature and field sweep rates is clearly in agreement
with the slow magnetic relaxation suggested by the ac
susceptibility data.
Theoretical Calculations. Ab initio calculation has proved
to be an efficient method for probing the insight of magnetic
anisotropy in lanthanide-containing compounds. In particular,
MOLCAS 7.815 and SINGLE_ANISO16 programs with the
complete-active-space self-consistent field (CASSCF) method
have been used to probe individual Dy3+ fragments of
complexes 1 and 2 on the basis of geometries determined by
X-ray diffraction analysis (see the Supporting Information for
details). The results provide the lowest spin−orbit energies and
the corresponding g tensors of one dysprosium fragment
extracted from 1 and 2. Table S7 shows that the energy
separations between the two lowest KDs for the Dy3+ fragments
of 1 and 2 are 184.6 and 152.0 cm−1, respectively, and the gz
values of both fragments are 19.487 and 19.474, close to 20.
Thus, the Dy3+−Dy3+ magnetic interactions in both complexes
can be approximately regarded as the Ising type. The main
magnetic axes on dysprosium ions of two complexes are
indicated in Figure 8, where the magnetic axes on magnetic
centers for each complex are parallel to each other and pointing
Figure 6. Temperature dependence of the in-phase χM′ (top) and out-
of-phase χM″ (bottom) ac signals under zero dc field for 2.
the existence of slow magnetic relaxation expected for
molecular nanomagnets.1−10 The imaginary frequency peaks
gradually shift to the high temperature region with increasing ac
frequency, and the peak maxima fall in the range of 8.99−19.02
K, more concentrated compared with that of 1 (8.49−20.02 K).
The relaxation time can be extracted from the frequency-
dependent ac data at different temperatures (Figure S7; Tables
S5 and S6). The plot of ln τ vs T−1 was also fitted with the
consideration of all relaxation processes, yielding Ueff = 175.8 K,
τ0 = 2.1 × 10−9 s, C = 0.0153 s−1 K−4.62, n = 4.62, τQTM = 0.079
s. The obtained τQTM value for 2 at zero dc field is notably
larger than that found for 1, indicating the slowing down of the
zero-field quantum tunneling relaxation rate in 2, which is also
supported by the smaller tail value in the out-of-phase signal of
the ac magnetic data (Figure 7). Similar to the case of 1, when
to the direction of Dy−Ophenoxide
.
Figure 8. Orientations of the local main magnetic axes of the ground
doublets on magnetic centers of 1 (a) and 2 (b).
The magnetic susceptibilities of complexes 1 and 2 were
simulated with the program POLY_ANISO16 (see Figure 3)
using the exchange parameters from Table S8. We note that all
Figure 7. Magnetic relaxation dynamics for 2 under zero dc field.
of the fitted Dy3+−Dy3+ coupling constants in Table S8 were
3+
̃
an optimized dc field of 1.5 kOe was applied, the QTM process
was suppressed (Figure S8). Compared with compound 1,
which has a dimer structure, the 1D complex 2 shows higher
Ueff and τQTM values at zero dc field, probably due to the
distinct coordination environment around Dy3+ ions and/or the
different Dy3+−Dy3+ magnetic coupling existing in 2.
Hysteresis Behaviors of Complexes 1 and 2. To further
study the dynamic magnetic behavior of complexes 1 and 2, the
hysteresis loops measurement was done on microcrystalline
samples using a SQUID-VSM magnetometer at low temper-
calculated with respect to the pseudospin S = 1/2 of the Dy
ions. For complex 1, there is only one type of J constant, which
is mediated by two alkoxido oxygen atoms within the dinuclear
core. However, for complex 2, two possible exchange pathways
could be considered: one pathway involves the double alkoxido
bridges within the dinuclear core that connect the Dy3+ ions at
a distance of 3.938 Å, while the other, a much longer pathway,
concerns with the formate groups in a syn−anti bridging mode
that links the Dy3+ ions at a distance of 5.752 Å. Therefore, for
complex 2, we consider two types of J values (Figure S11). The
E
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total coupling parameters J (dipolar and exchange) were
included in the fitting of the magnetic susceptibilities for both
compounds. Plots of calculated and experimental χMT versus T
for both complexes are shown in Figure 3, where the calculated
data are reasonably consistent with the corresponding
experimental data in the entire temperature range. Details of
the fitting results are shown in Table S8, which reveals that the
Dy3+ ions in the dinuclear core of both complexes are all
ferromagnetically coupled but the overall interdimer magnetic
interactions in 2 are weakly antiferromagnetic. The exchange
states splitting for the lowest KD of both complexes are shown
in Table S9, where the high excited states are much smaller
than their energy barriers. Thus, the relaxations of two
complexes are all through excited KDs of the magnetic centers.
The theoretical calculation of the anisotropy of individual
Dy3+ ions and of the magnetic coupling between them provide
us with a good platform to understand the different magnetic
behaviors of the two complexes. It is interesting to note, as
revealed by Table S7, that the energy difference between the
two lowest KDs of individual Dy3+ fragments in 2 is smaller
than that in 1 (Table S7), but the corresponding experimental
energy barrier associated with the slow relaxation is larger than
that of 1. The larger separation between the two lowest KDs in
1 is due to the stronger interaction between the Dy3+ ion and
phenolate and alkoxido oxygen atoms along the magnetic axis
(as evidenced by the shorter Dy−O bond lengths) and weaker
interaction perpendicular to the magnetic axis. The higher
energy barrier found in 2 might arise from the difference within
the core dimer structure and the connection between them,
since these structural factors can affect the relaxation process of
individual Dy3+ ions. Within the dimer core, a smaller Dy−O−
Dy angle and a longer Dy···Dy distance result in stronger
intradimer Dy3+−Dy3+ magnetic interaction, as indicated by the
significantly larger coupling constant (6.31 cm−1) than that
found for 1 (3.82 cm−1). Additionally, while the overall
interdimer magnetic coupling appears small (J = −0.04 cm−1),
its exchange component J2 (0.75 cm−1) is considerably larger,
indicating the existence of substantial magnetic interaction
between the dimer cores through syn−anti formate bridges,
which likely imposes an effect on the slow magnetic relaxation.
The larger intra- and interdimer magnetic interactions can
effectively quench the quantum tunneling at low temperatures,
thus affording a larger energy barrier in 2. Another reason for
the energy barriers differences may be attributed to the different
molecular geometries and molecular rigidity. The latter can
affect the spin−phonon coupling, which has been recently
evidenced in the spin−phonon bottleneck mechanism study.17
Our finding thus provides a promising strategy for the rational
design of novel lanthanide-based magnetic materials.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02276.
Some experimental and computational details; selected
bond lengths and angles for 1 and 2; relaxation fitting
parameters from least-squares fitting of χ(f) data; crystal
structure of 1 and 2; XRD spectra of 1 and 2; and
magnetic data for 1 and 2 (PDF)
X-ray crystallographic data of compound 1 (TXT)
X-ray crystallographic data of compound 2 (TXT)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: haolingsun@bnu.edu.cn (H.-L.S.).
*E-mail: zhangyiquan@njnu.edu.cn (Y.-Q.Z.).
*E-mail: gaosong@pku.edu.cn (S.G.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 21171023), the National Key Basic
Research Program of China (2013CB933402), the Natural
Science Foundation of Jiangsu Province of China
(BK20151542), the Beijing Higher Education Young Elite
Teacher Project, and the Priority Academic Program Develop-
ment of Jiangsu Higher Education Institutions. We thank Prof.
Zhenqiang Wang of the University of South Dakota for helpful
comments.
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CONCLUSION
■
In summary, using a coordination-driven self-assembly method,
we have synthesized a novel 1D dysprosium chain compound,
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F
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