Disorder-order transformation and significant dislocation motion cooperating with a surprisingly large hysteretic magnetic transition in a nickel-bisdithiolene spin system

Article abstract of DOI:10.1021/ic302571p

The compound [4′-CF₃bzPy][Ni(mnt)₂] (1) (where 4′-CF₃bzPy = 1-(4′-(trifluoromethyl)benzyl)pyridinium and mnt^2- = maleonitriledithiolate) was synthesized and displays a magnetic bistability with a surprisingly large thermal hysteresis loop (～49 K). X-ray crystallographic studies reveal that in the high-temperature (HT) phase the anions and cations form mixed stacks, with alternating anion dimers (AA) and cation dimers (CC) in an...AACCAACC... fashion along the crystallographic a + b direction, and disordered CF₃ groups in the cations are aligned into a molecular layer parallel to the crystallographic (001) plane. However, in the low-temperature (LT) phase, the c-axis length of the unit cell is roughly doubled, and the asymmetric unit switches from one [4′-CF₃bzPy][Ni(mnt)₂] pair in the HT phase to two [4′-CF₃bzPy][Ni(mnt)₂] pairs. Most interestingly, the CF₃ group in the cations becomes ordered, and the conformation of one of two crystallographically different cations changes significantly. A dislocation motion between the neighboring molecular layers emerges as well. The analyses of the magnetic susceptibilities and the density functional theory calculations suggest that the antiferromagnetic exchange interaction within one of two types of [Ni(mnt)₂]₂^2- dimers in the LT phase is much stronger than that within the [Ni(mnt)₂] ₂^2- dimer in the HT phase. The lattice reorganization during this phase transition is proposed to be responsible for the wide thermal hysteresis loop.

Full text of DOI:10.1021/ic302571p

Article
pubs.acs.org/IC
Disorder−Order Transformation and Significant Dislocation Motion
Cooperating with a Surprisingly Large Hysteretic Magnetic
Transition in a Nickel−Bisdithiolene Spin System
Hai-Bao Duan,† Xuan-Rong Chen,† Hao Yang,† Xiao-Ming Ren,*,†,‡ Fang Xuan,† and Shi-Ming Zhou*,§
†State Key Laboratory of Materials-Oriented Chemical Engineering and College of Science, Nanjing University of Technology,
Nanjing 210009, People’s Republic of China
‡State Key Laboratory & Coordination Chemistry Institute, Nanjing University, Nanjing 210093, People’s Republic of China
§Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026,
People’s Republic of China
S
* Supporting Information
ABSTRACT: The compound [4′-CF3bzPy][Ni(mnt)2] (1) (where 4′-CF3bzPy = 1-(4′-
(trifluoromethyl)benzyl)pyridinium and mnt2− = maleonitriledithiolate) was synthesized
and displays a magnetic bistability with a surprisingly large thermal hysteresis loop (∼49
K). X-ray crystallographic studies reveal that in the high-temperature (HT) phase the
anions and cations form mixed stacks, with alternating anion dimers (AA) and cation
dimers (CC) in an ...AACCAACC... fashion along the crystallographic a + b direction,
and disordered CF3 groups in the cations are aligned into a molecular layer parallel to the
crystallographic (001) plane. However, in the low-temperature (LT) phase, the c-axis
length of the unit cell is roughly doubled, and the asymmetric unit switches from one [4′-
CF3bzPy][Ni(mnt)2] pair in the HT phase to two [4′-CF3bzPy][Ni(mnt)2] pairs. Most
interestingly, the CF3 group in the cations becomes ordered, and the conformation of one
of two crystallographically different cations changes significantly. A dislocation motion
between the neighboring molecular layers emerges as well. The analyses of the magnetic susceptibilities and the density
2−
functional theory calculations suggest that the antiferromagnetic exchange interaction within one of two types of [Ni(mnt)2]2
dimers in the LT phase is much stronger than that within the [Ni(mnt)2]22− dimer in the HT phase. The lattice reorganization
during this phase transition is proposed to be responsible for the wide thermal hysteresis loop.
the other hand, it is worthy of noting that the disorder-to-order
transformation can provide an intrinsic impulse for the
initiation of the spin transition. By coupling the magnetic
transition to the disorder−order transformation rather than
strengthening intermolecular interactions, magnetic bistability
with large hysteresis loops was observed in the unsymmetrical
dithiolene complexes [Cp2M][Ni(tfadt)2] (Cp = cyclopenta-
diene; M = Fe or Co; tfadt = (trifluoromethyl)acrylonitrile-1,2-
dithiolate), in which the disorder−order transformations of the
spin-inactive components, CF3 and cyclopentadienyl groups,
are coupled with the magnetic bistability.11
In this paper, we present a novel example that a π-type dimer
compound, [4′-CF3BzPy][Ni(mnt)2] (1) (4′-CF3BzPy = 1-(4′-
(trifluoromethyl)benzyl)pyridinium; mnt2− = maleonitriledi-
thiolate), exhibits magnetic bistability with a ca. 49 K thermal
hysteresis loop. It was confirmed that the magnetic bistability in
such a dimer system is coupled with the disorder−order
transformations of the CF3 group in the spin-inactive cation
and the dislocation motion of the neighboring molecular layers.
INTRODUCTION
■
Functional materials with bistable (or hysteresis) behavior on
the molecular scale are in large demand in electronic and
photonic technologies.1 Generally, magnetic bistability is
observed in spin-crossover (SCO) complexes,2−5 valence
tautomeric compounds,6 and one-dimensional (1-D) S = 1/2
spin-Peierls-type transition compounds.7 The transition tem-
perature TC and the hysteresis loop width (ΔTC = TC↑ − TC↓)
for a magnetically bistable compound are technologically
important parameters for practical applications, and the 50 K
and more width of the loop and it being centered at near room
temperature are thought to be necessary for high-density
storage media.1a
For a magnetic bistable compound with a wide hysteresis
loop, it is necessary to strengthen cooperative interactions
between the spin-active units in the lattice. It has been observed
that an absence of strong intermolecular interactions between
spin-active centers (polymeric units) results in a low
cooperativity in the SCO complex.10 Therefore, a general
strategy for this purpose is to enhance the lattice rigidity by
using covalent linkers to8 or promoting the noncovalent
interaction (such as H-bonding, π stacking, guest inclusion, van
der Waals forces, and so on) between9 the spin-active units. On
Received: November 24, 2012
Published: March 21, 2013
© 2013 American Chemical Society
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Table 1. Crystal Data and Structural Refinement of 1
293 K
190 K
577.30
160 K
131 K
577.30
chemical formula
C21H11F3N5NiS4
577.30
fw
577.30
wavelength (Å)
CCDC number
cryst syst
space group
a (Å)
0.71073
911452
triclinic
0.71073
911453
triclinic
0.71073
911454
triclinic
0.71073
911455
triclinic
P1
̅
P1
̅
P1
̅
P1
̅
8.3497(13)
11.6251(19)
13.813(2)
71.189(3)
75.117(3)
77.007(3)
1211.7(3)/2
1.582
8.2623(6)
11.5851(6)
13.8576(9)
71.043(5)
74.970(6)
77.509(5)
1198.87(13)/2
1.599
8.2247(4)
11.5827(4)
13.8825(7)
70.995(4)
74.753(4)
77.577(4)
1194.18(9)/2
1.605
8.5682(8)
11.4127(9)
25.008(2)
96.294(7)
93.068(7)
103.850(7)
2352.0(4)/4
1.599
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)/Z
density (g·cm−3
)
abs coeff (mm−1
F(000)
)
1.188
1.201
1.206
1.223
582.0
582.0
582.0
1120.0
data collection θ range (deg)
1.59−26.00
−10 ≤ h ≤ +9
−12 ≤ k ≤ +14
−12 ≤ l ≤ +17
6916
2.82−29.25
−10 ≤ h ≤ +11
−14 ≤ k ≤ +15
−17 ≤ l ≤ +17
5618
2.98−28.99
−10 ≤ h ≤ +10
−14 ≤ k ≤ +14
−18 ≤ l ≤ +15
5456
2.02−29.20
index range
−8 ≤ h ≤ +11
−8 ≤ k ≤ +15
−31 ≤ l ≤ +31
7427
no. of reflns collected
no. of independent reflns
no. of data/restraints/params
goodness-of-fit on F2
4716
3658
3559
3817
4716/0/335
0.918
5618/0/335
0.928
5456/0/335
0.905
7424/0/613
1.080
a
final R indices [I > 2σ(I)]
R1 = 0.0583
wR2 = 0.1387
R1 = 0.0552
wR2 = 0. 1378
R1 = 0.0359
wR2 = 0.0865
R1 = 0.1171
wR2 = 0.3060
a
R1 = ∑(∥Fo| − |Fc∥)/∑|Fo|. wR2 = ∑w(|Fo|2 − |Fc|2)2/∑w(|Fo|2)2]1/2
.
Figure 1. (a) χm vs T, where the solid squares and lines represent the experimental data and theoretically reproduced magnetic susceptibility data,
respectively. (b) dχm/dT vs T in the cooling (green) and warming (blue) modes for 1.
MPMS-5 superconducting quantum interference device (SQUID)
EXPERIMENTAL SECTION
■
magnetometer over the temperature range of 2−300 K.
Chemicals and Materials. The starting materials Na2mnt12 and
Preparation of a Sample of [4′-CF3bzPy][Ni(mnt)2]. A MeOH
[4′-CF3bzPy]I13 were synthesized using the methods described in the
solution (10 cm3) of I2 (205 mg, 0.80 mmol) was slowly added to a
MeCN solution (25 cm3) of [4′-CF3bzPy]2[Ni(mnt)2] (573 mg, 1.0
mmol), and the mixture was allowed to stand overnight after being
stirred for 25 min. The dark powder formed was filtered off, washed
with MeOH, and dried in a vacuum. Yield: ∼69%. Anal. Calcd for
C21H11F3N5NiS4: C, 43.69; H, 1.92; N, 12.13. Found: C, 43.76; H,
1.78; N, 12.25.
literature, and [4′-CF3bzPy]2[Ni(mnt)2] was prepared employing a
procedure similar to that reported in the literature.13
Physical Measurements. Elemental analyses for C, H, and N
were performed with an Elementar Vario EL III analytic instrument.
Powder X-ray diffraction (PXRD) data were collected on a Bruker D8
diffractometer with Cu Kα radiation (λ = 1.54018 Å). FT-IR spectra
were recorded on an IF66 V FT-IR (4000−400 cm−1) spectropho-
tometer with KBr pellets. Differential scanning calorimetry (DSC) was
carried out on a Pyris 1 power-compensation differential scanning
calorimeter in the range of 98−298 K (from −175 to +25 °C) for 1,
and the warming/cooling rate was 10 K·min−1 during the thermal
cycles. Magnetic susceptibilities were measured on a Quantum Design
Crystals of 1 suitable for single-crystal X-ray diffraction can be
obtained by slow evaporation of the saturated acetonitrile or acetone
solution of the above powdered sample at ambient temperature for 5−
7 days.
X-ray Crystallography. Selected crystals of 1 were centered on an
Oxford Diffraction Xcalibur diffractometer equipped with a Sapphire 3
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charge-coupled device (CCD) detector and graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). Measurements were performed at
293, 190, 160, and 131 K for 1 (cooling mode). Note that it is really
difficult to obtain good single-crystal X-ray diffraction data in the low-
temperature phase since the crystal is often broken below ∼160 K
upon cooling. Single-crystal X-ray diffraction data at 131 K were finally
successfully collected by slow cooling of the selected crystal at a rate of
2 K/h; even so, the crystal was broken into parts in the subsequent
heating process.
The data collection routine, unit cell refinement, and data
processing were carried out with the program CrysAlis.14 Structures
were solved by the direct method and refined by the full-matrix least-
squares procedure on F2 using the SHELXL-97 program.15 The non-
hydrogen atoms were anisotropically refined using the full-matrix least-
squares method on F2. Disorder models for the CF3 group were used
in the structure refinements of 1 at 293, 190, and 160 K. Each CF3
group was assumed to have two possible sites, and the occupied factors
for each site were refined. The crystallographic details on the data
collection and structural refinement are summarized in Table 1.
Density Functional Theory (DFT) Calculation Details. All DFT
calculations were carried out utilizing the Gaussan98 program16 on an
SGI 3800 workstation. The calculations for the single-point energy of
the broken-symmetric (BS) singlet states of [Ni(mnt)2]2− spin dimers
at the ubpw91/lanl2dz level17−22 were performed on the non-
modelized molecular geometry from the single-crystal X-ray analyses,
and the SCF convergence criterion was 10−8.
Figure 2. ORTEP view with non-hydrogen atom labeling and thermal
ellipsoids drawn at the 20% probability level for 1 at 293 K. The
structurally disordered CF3 group has two possible sites.
shaped 4′-CF3BzPy+ cation are 67.8° between the phenyl and
pyridyl rings and 87.8° and 88.5° between the reference plane
N(5)C(14)C(15) and the phenyl ring and the pyridyl ring,
respectively. The CF3 group is structurally disordered with two
possible sites for each fluorine atom.
As shown in Figure 3a,b, the anions and cations form mixed
stacks, with alternating anionic dimers (AA) and cationic
dimers (CC) in an ...AACCAACC... fashion, along the
crystallographic a + b direction. The mixed stacks are aligned
into a molecular layer, which is parallel to the crystallographic
(001) plane. Two cofacial [Ni(mnt)2]− ions in an anionic
dimer are related to each other via an inversion center, and
their long molecular axes are parallel to each other. The
slippage of two [Ni(mnt)2]− anions along the long molecular
axes leads to the existence of slightly longer interatomic
separations of 4.119 Å for Ni(1)···Ni(1)#1, 4.053 Å for
S(2)···S(3)#1, and 4.019 Å for Ni(1)···S(3)#1, with symmetric
code #1 being −x, 1 − y, −z. Two 4-CF3bzPy+ ions in a
cationic dimer are arranged in a chair conformation alignment.
Two (trifluoromethyl)benzene moieties exhibit an antiparallel
arrangement, and the phenyl rings are parallel to each other
with a longer centroid-to-centroid separation of 4.823 Å. The
phenyl ring in the cation moiety is superimposed over one of
two chelate rings in the anion moiety with a dihedral angle of
5.9°.
RESULTS AND DISCUSSION
■
Variable-Temperature Magnetic Susceptibility. The
temperature dependence of the magnetic susceptibility of 1
measured in both cooling and warming modes in the
temperature range of 2−300 K is displayed in Figure 1, in
which χm represents the molar magnetic susceptibility for one
[Ni(mnt)2]− unit of [4′-CF3bzPy][Ni(mnt)2]. The magnetic
behavior of 1 is two magnetic susceptibility maxima and a
surprisingly large thermal hysteresis loop in the χm versus T
plot. Upon cooling, the χm value increases gradually, reaches the
first maximum at around 169 K, and then drops abruptly. The
second magnetic susceptibility maximum appears at around 50
K, and the magnetic susceptibility decreases exponentially
below this temperature, the typical characteristic of a spin-gap
system. During the warming process, the history dependence in
the magnetic susceptibility occurs in the temperature range of
110−225 K, indicating thermally induced magnetic bistability.
The width of the hysteresis loop in this bistable magnetic
system was calculated as ∼49 K on the basis of the peak
temperatures in the dχm/dT versus T plot (see Figure 1b).
A large thermal hysteresis loop is commonly observed in the
transition metal SCO complexes23 and the cyano-bridged
metal-to-metal charge-transfer complexes,24 but located at
organic radicals7d,25 and metal−bisdithiolene complexes. To
the best of our knowledge, only three other examples in metal−
bisdithiolene complexes have been reported to show a more
than 30 K hysteretic loop so far, namely, the 40 K hysteretic
loop reported in [Cp2Co][Ni(tfadt)2]11b and the ∼37 and ∼60
K hysteretic loops observed in two polymorphs of [NO2bzql]-
[Ni(dmit)2] (where NO2bzql+ represents 1-(4-nitrobenzyl)-
quinolinium).26
The crystal structures of 1 at 190 and 160 K (not shown
here) are quite analogous to that at 293 K. The characteristic
bond parameters in the crystal structures at both temperatures
are summarized in the Supporting Information.
Crystal Structure in the Low-Temperature (LT) Phase
at 131 K. Upon cooling from 293 to 131 K, the space group of
the crystal remains the same and the lengths of the a- and b-
axes do not change remarkably, but the length of the c-axis is
roughly doubled compared to that of the structure at 293 K,
indicating that the asymmetric unit switches from one [4′-
CF3bzPy][Ni(mnt)2] pair in the HT phase to two [4′-
CF3bzPy][Ni(mnt)2] pairs in the LT phase (Figure 4). Even
though the bond lengths and angles (Table S1, Supporting
Information) in two planar anions are comparable to those at
293 K, the relative orientations between two crystallo-
graphically different anions/cations change significantly in the
LT phase (Figure S5, Supporting Information). The mean
molecular planes (defined by four S atoms) as well as the long
molecular axes (defined by a straight line passing through the
center of two CC bonds of mnt2− ligands) of two
crystallographically independent anions make angles of 8.9°
and 22.6°, respectively. For two crystallographically inequiva-
lent cations, the characteristic dihedral angles are 71.1° (82.4°)
between the phenyl and pyridyl rings and 83.7° (76.1°) and
59.2° (55.0°) between the reference plane and the phenyl ring
Crystal Structure in the High-Temperature (HT) Phase
at 293, 190, and 160 K. Crystal 1 crystallizes in the triclinic
space group P1 at 293 K; its asymmetric unit comprises one
̅
[Ni(mnt)2]− together with one 4′-CF3bzPy+ (Figure 2). The
bond lengths and angles in the planar anion are in good
agreement with the values in the reported [Ni(mnt)2]−
compounds.13,27 The characteristic dihedral angles in the Λ-
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Figure 3. (a) Packing structure of 1 at 293 K projected along the crystallographic a + b direction. (b) A monolayer, being parallel to the
crystallographic (001) plane, shows the alternating stacks of dimeric anions and dimeric cations along the crystallographic a + b direction. (c)
Anionic dimer. (d) Cationic dimer.
layer, and this slippage leads to a shift of the phenyl ring
superimposed over one of two chelate rings in the L-1 layer
away from the flat anion. (2) A rotation of the whole molecule,
being relative to the anion plane, takes place for the cation in
the L-2 layer, which gives rise to the phenyl ring tilting against
the mean molecular plane of the anion (defined by four S
atoms) with a dihedral angle of 36.5° in the L-2 layer versus
11.2° in the L-1 layer, while the corresponding dihedral angle is
5.9° in the crystal structure at 293 K. (3) The relative molecular
orientation between the anions as well as between the cations
in the neighboring molecular layers shows a significant change.
As analyzed above, the structural changes from the HT to the
LT phase can be concluded to be the disorder-to-order
transformation of the CF3 groups and the dislocation motion
between the L-1 and L-2 molecular layers.
Figure 4. ORTEP view with non-hydrogen atom labeling and thermal
ellipsoids drawn at the 20% probability level for 1 at 131 K.
and the pyridyl ring for the cation containing F4 (F1).
Obviously, the dihedral angle between the reference plane and
the pyridyl ring changes significantly in the conformation of the
cations upon cooling from the HT to the LT phase.
Additionally, the structurally disordered CF3 group in the HT
phase becomes ordered in the LT phase.
Analysis of the Relationship between Structure and
Magnetic Behavior. From a structural viewpoint, the dimer
magnetic exchange models with one J constant and two
different J constants are respectively chosen for the fit of the
temperature-dependent magnetic susceptibility in the HT and
LT phases, since there exist one and two types of crystallo-
2−
The packing structure of 1 in the LT phase is displayed in
Figure 5. The phase transition results in a switch from one type
of molecular layer to two types of crystallographically different
molecular layers, comprised of, respectively, the Ni1 anions
together with F4 cations (this layer is labeled as L-1) and the
Ni2 anions together with F1 cations (this layer is labeled as L-
2). The L-1 and L-2 molecular layers alternate in an ...L-1/L-2/
L-1/L-2... manner along the c-axis direction, which is
responsible for the lengthening of the c-axis at low temperature.
The alignment of anions and cations is quite similar to that at
293 K in the L-1 layer (see Figures 3b and 5c), while distinct
from that at 293 K in the L-2 layer (see Figure 5d). The
molecular arrangement differences between the L-1 and L-2
layers are illustrated in Figure 5e,f and are as follows: (1) The
significant slippage along the short molecular axis of the anion
occurs between the neighboring anion and cation in the L-2
graphically different π-type [Ni(mnt)2]2 dimers in the HT
and LT phases:
2
Ng2μB
1
C
χm
χm
=
=
+
+ χ0
kBT 3 + exp(−2J/kBT)
T − Θ
(1)
2
Ng2μB
2kBT
⎧
⎨
⎩
1
3 + exp(−2J /k T)
B
1
⎫
⎬
⎭
1
C
+
+
+ χ0
3 + exp(−2J /k T)
T − Θ
B
(2)
2
where the C/(T − Θ) term represents the uncoupled magnetic
impurity and χ0 is the temperature-independent magnetic
susceptibility, which includes the diamagnetism originated from
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Figure 5. (a) Packing diagram of 1 at 131 K. (b) Two crystallographically independent layers, which are marked in red (containing Ni1-type anions
and F4-type cations) and blue (containing Ni2-type anions and F1-type cations). Arrangements of anions and cations in the monolayer containing
(c) Ni1-type anions and (d) Ni2-type anions. Relative alignment of the anion and cation with (e) Ni1-type anions and (f) Ni2-type anions.
the closed atom shells and possible van Vleck paramagnetism
contributed by the coupling of the ground and excited states
through a magnetic field. In the LT phase, the magnetic
susceptibility in the temperature range of 1.8−130 K was fitted
to eq 2 to yield the parameters g = 1.991(8), J1/kB = −43.4(1)
K, J2/kB = −268(9) K, C = 7.9(3) × 10−3 emu·K·mol−1, Θ =
−1.8(1) K, and χ0 = −5.12(1) × 10−4 emu·mol−1 with a ratio of
J2 to J1 of ∼6.2. On the basis of the fitted Curie constant, the
amount of S = 1/2 magnetic impurities, arising from the
uncoupled spins, is estimated to be ca. 2%. In the HT phase,
the temperature-dependent magnetic susceptibility in the
temperature range of 167−300 K was fitted to eq 1, and the
C, Θ, and χ0 parameters remained the same as obtained from
the fitting in the LT phase. The best fit gave g = 2.02(2) and J/
kB = −50.3(7) K. These results indicated that the
antiferromagnetic (AFM) coupling interaction in the LT
phase is approximately unchanged within one of two types of
dimers whereas it is much stronger in the other one compared
to that in the HT phase.
dimer in the LT phase. It is noted that (1) the unpaired
electrons are predominantly delocalized throughout the anion
skeleton, in which approximately 85% of the unpaired electron
spins are localized on the NiS4 core and the spin polarization
with an alternation in sign of the spin density appears in the C−
CN moieties of the mnt2− ligands for each [Ni(mnt)2]− anion
within the dimer in the HT phase, (2) the α and β spins almost
localize in two individual [Ni(mnt)2]− anions in the dimer,
indicating that the unpaired electrons between two individual
[Ni(mnt)2]− anions are magnetically coupled via a spin
polarization mechanism in the HT phase, and (3) the
calculated Mulliken spin population of the Ni1 dimer in the
LT phase is similar to that of the dimer in the HT phase (see
Figure 6a,b), while that of the Ni2 dimer in the LT phase is
quite different from that of the dimer in the HT phase. The
spin density in all the atoms is zero, suggesting a closed-shell
configuration for the Ni2 dimer and clearly reflecting the
strongly delocalized character of the two singly occupied π
orbitals of the [Ni(mnt)2]− anions; therefore, the unpaired
electrons between two individual [Ni(mnt)2]− anions are
magnetically coupled via a spin delocalization mechanism. This
situation is further identified from the frontier molecular
orbitals of the BS singlet states of three spin dimers in the HT
and LT phases. Figure 7 shows a stronger overlap between the
Mulliken spin population analyses based on the DFT
calculations were performed for the dimers to identify the
relationship between the structure and the magnetic behavior.
Figure 6 presents the calculated Mulliken spin densities of the
BS singlet state for the dimer in the HT phase and the Ni1
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Figure 6. Spin density distribution in the BS singlet state of (a) the
spin dimer in the HT phase and (b) the Ni1-type spin dimer in the LT
phase.
Figure 7. Kohn−Sham-type orbitals (HOMO) of the spin dimer (a)
in the HT phase and (b, c) corresponding to the spin dimers with Ni1-
type and Ni2-type anions in the LT phase, respectively.
orbitals of two [Ni(mnt)2]− anions within the Ni2 dimer in the
LT phase, but no significant orbital overlap for the dimer in the
HT phase as well as for the Ni1 dimer in the LT phase,
indicating the presence of a much stronger AFM exchange
interaction within the Ni2 dimer and a relatively weaker AFM
exchange interaction within the Ni1 dimer in the LT phase as
well as a weaker AFM exchange interaction within the dimer in
the HT phase, since the AFM exchange constant is propor-
tional to the overlap integral of the orbitals.
Differential Scanning Calorimetry. The DSC plot of 1 is
depicted in Figure 8, which shows a gradual endothermic peak
around the transition temperature in the heating process, with
an onset temperature of 200.2 K, a peak temperature Tmax
≈
211.4 K, and an endset temperature of 226.2 K. The peak
temperature Tmax ≈ 211.4 K is close to the TC obtained from
the magnetic susceptibility measurement. The enthalpy change
(ΔH), calculated from the integrated peak area, is estimated to
be only 282.6 J·mol−1 for 1. Such a small enthalpy change
means that the difference in lattice energy between the HT and
LT phases is very small.
Cooperativity of the Magnetic Phase Transition. The
width of thermal hysteresis loop is a technologically crucial
parameter for practical applications of the magnetic bistability.
In general, the width of the thermal hysteresis loop is related to
the cooperative effects and becomes larger as the cooperativity
of the magnetic phase transition increases. A variety of
cooperative effects are widespread in the different magnetic
bistability systems; for example, the bond distance changes in
the coordination sphere of a spin-active metal ion usually
cooperate with the spin-crossover process in an SCO complex.
It was also observed that the coordination bond formation/
dissociation cooperates with the hysteresis magnetic phase
Figure 8. DSC curve for 1 showing the Tonset, Tmax, Tendset, and latent
heat ΔH of the phase transition.
transition in (BDTA)2[Co(mnt)2]28a and Na[Ni-
(pdt)2]·2H2O.28b The disorder-to-order transformation of the
spin-inactive components, CF3 and cyclopentadienyl groups,
couples with the magnetic bistability in [Cp2M][Ni(tfadt)2] (M
= Fe or Co) complexes.11 Furthermore, the rupture of the C−
H···π interactions between the neighboring dimers and the
large-amplitude motion of the phenalenyl rings assist in the
magnetic bistability in the charge delocalization spirobiphena-
lenyl neutral radical conductors.29 The disorder-to-order
transformation of the spin-inactive component CF3 and the
dislocation motion of the neighboring molecular layers couple
with the magnetic bistability in 1.
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Article
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CONCLUDING REMARKS
■
In summary, we presented a novel magnetic bistable example
which shows a surprisingly large thermal hysteresis loop but no
strong intermolecular interaction in the lattice. To the best of
our knowledge, this is the first example of an isolated metal−
bisdithiolene dimer system that displays a spin transition and,
moreover, with a wide hysteresis loop. In this study, it was
proposed that the lattice reorganization stemming from the
change of the bond parameters or motion of groups/molecules
during a magnetic phase transition cooperates kinetically with
the thermal hysteresis effect, and the higher energy barrier for
the lattice reorganization in this example is responsible for the
wider thermal hysteresis loop. This finding demonstrated that a
larger hysteresis loop magnetic bistability system is only found
in a compound with a greater lattice reorganization energy
between the thermodynamically stable phases and the
transition state.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables giving the characteristic bond lengths and angles in the
[Ni(mnt)2]− moiety of 1 at different temperatures, significant
interatomic separations of dimer anions in the crystals of 1 at
different temperatures, and characteristic dihedral angles in the
cation moiety of 1, figures showing ORTEP views with non-
hydrogen atom labeling and thermal ellipsoids drawn at the
20% probability level for 1 at 160 and 190 K, packing diagrams
of 1 at 160 and 190 K viewed along the a + b direction, an
illustration of the relative orientations between two crystallo-
graphic inequivalent anions and cations for 1 at 131 K, two
types of anion dimers in the crystal structure at 131 K, and
variable-temperature EPR spectra upon cooling and heating,
and CIF data for 1. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86 25 58139476. Fax: +86 25 58139481. E-mail:
xmren@njut.edu.cn (X.-M.R.), zhousm@ustc.edu.cn (S.-M.Z.).
(9) (a) Lara, F. J. M.; Gaspar, A. B.; Aravena, D.; Ruiz, E.; Munoz, M.
̃
■
C.; Ohba, M.; Ohtani, R.; Kitagawa, S.; Real, J. A. Chem. Commun.
2012, 48, 4686−4688. (b) Kepenekian, M.; Guennic, B. L.; Robert, V.
J. Am. Chem. Soc. 2009, 131, 11498−11502. (c) Hayami, S.; Gu, Z. Z.;
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Notes
The authors declare no competing financial interest.
(10) (a) Murray, K. S. Eur. J. Inorg. Chem. 2008, 3101−3121.
(b) Bauer, W.; Dirtu, M. M.; Garcia, Y.; Weber, B. CrystEngComm
2012, 14, 1223−1231.
ACKNOWLEDGMENTS
■
(11) (a) Jeannin, O.; Cler
19, 5946−5964. (b) Jeannin, O.; Cler
Chem. Soc. 2006, 128, 14649−14656. (c) Jeannin, O.; Cler
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ac, R.; Fourmigue,
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M. Chem. Mater. 2007,
M. J. Am.
ac, R.;
We thank the Priority Academic Program Development of
Jiangsu Higher Education Institutions and National Natural
Science Foundation of China (Grants 91122011 and
21071080) for financial support. X.-M.R. thanks Prof. C. J.
Fang for reading this manuscript.
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