Two spin-peierls-like compounds exhibiting divergent structural features, lattice compression, and expansion in the low-temperature phase

Article abstract of DOI:10.1021/jp8100333

Two quasi-one-dimensional (quasi-1D) compounds, [4′-CH ₃Bz-4-RPy][Ni(mnt)₂] (mnt² = maleonitriledithi-olate), where 4′-CH₃Bz-4-RPy⁺ = 1-(4′-methylbenzyl)pyridinium (denoted as compound 1) and 1-(4′-me

Full text of DOI:10.1021/jp8100333

8278
J. Phys. Chem. B 2009, 113, 8278–8283
Two Spin-Peierls-like Compounds Exhibiting Divergent Structural Features, Lattice
Compression, and Expansion in the Low- Temperature Phase
Zhengfang Tian,† Haibao Duan,‡ Xiaoming Ren,*,†,‡ Changsheng Lu,† Yizhi Li,† You Song,†
Huizhen Zhu,† and Qingjin Meng*,†
State Key Laboratory & Coordination Chemistry Institute, School of Chemistry and Chemical Engineering,
Nanjing UniVersity, Nanjing 210093, P.R. China, and College of Science, Nanjing UniVersity of Technology,
Nanjing 210009, P.R. China
ReceiVed: NoVember 11, 2008; ReVised Manuscript ReceiVed: April 25, 2009
Two quasi-one-dimensional (quasi-1D) compounds, [4′-CH3Bz-4-RPy][Ni(mnt)2] (mnt2- ) maleonitriledithi-
olate), where 4′-CH3Bz-4-RPy+ ) 1-(4′-methylbenzyl)pyridinium (denoted as compound 1) and 1-(4′-
methylbenzyl)-4-aminopyridinium (denoted as compound 2), show a spin-Peierls-like transition with TC ≈
182 K for 1 and TC ≈ 155 K for 2. The enthalpy changes for the transition are estimated to be ∆H ) 316.6
J · mol-1 for 1 and 1082.1 J · mol-1 for 2. From fits to the magnetic susceptibility, the magnetic exchange
constants in the gapless state are calculated to be J ) 166(2) K with g ) 2.020(23) for 1 Versus J ) 42(0)
K with g ) 2.056(5) for 2. In the high-temperature (HT) phase, 1 and 2 are isostructural and crystallize in
the monoclinic space group P21/c. The nonmagnetic cations and paramagnetic anions form segregated columns
with regular anionic and cationic stacks. In the low-temperature (LT) phase, the crystals of the two compounds
undergo a transformation to the triclinic space group P-1, and both anionic and cationic stacks dimerize. In
the transformation from the HT to LT phases, the two compounds exhibit divergent structural features, with
lattice compression for 1 but lattice expansion for 2, due to intermolecular slippage. Combined with our
previous studies, it is also noted that the transition temperature, TC, is qualitatively related to the cell volume
in the HT phase for the series of compounds [1-(4′-R-benzylpyridinium][Ni(mnt)2] (where R represents the
substituent). When there is a single substituent in the para position of benzene, giving a larger cell volume,
the transition temperature increases.
SCHEME 1
Introduction
Functional molecular materials with switching properties and
memory transduction are of considerable interest due to their
potential technological applications.1
As one of this group of functional molecular materials, spin
transition compounds display switchable stable/metastable spin
states in response to external stimuli such as temperature,
pressure, or light. In this rapidly emerging field, spin-crossover
compounds that exhibit crossover between high-spin (HS) and
low-spin (LS) states are promising examples of technologically
useful materials.2 In addition, spin transition phenomena can
probably occur in a charge/electron transfer compound, since
the spin transition is associated with the process of charge/
electron transfer between different magnetic centers.3-7 This also
applies to mixed-valence compounds (where the transition arises
from charge/valence ordering)8,9 and quasi-1D quantum mag-
netic chain compounds, where the strong spin-lattice interaction
leads to the spin system displaying a wide range of ground states
at low temperature, such as the spin-Peierls state,10 spin density
wave (SDW) state,11 and so on.
the countercation (Scheme 1).13,14 [Ni(mnt)2]- is a half spin
moiety (S ) 1/2) with planar geometry and an extended
electronic structure that favors the formation of stacks via
intermolecular π· · ·π interactions.12 Thus, it is often observed
that both anions and cations form segregated stacks in the crystal
structure of such compounds, with each magnetic stack of anions
surrounded by four nonmagnetic stacks of organic cations. From
the viewpoint of the crystal structure, the compound is an ideal
quasi-1D magnetic system with S ) 1/2. Theoretically, it is
possible for a spin-Peierls (SP) transition to occur in an
antiferromagnetically (AFM) coupled S ) 1/2 Heisenberg or
XY magnetic chain when magnetoelastic coupling induces a
progressive lattice dimerization.15 In fact, a spin-Peierls-like
transition is observed in these quasi-1D quantum magnetic
systems, with the following features: (1) the magnetic chain is
In our previous studies, a series of quasi-1D quantum
magnetic chain systems have been built using the bis(maleoni-
triledithiolato)nickelate monoanion ([Ni(mnt)2]-) as the basic
magnetic architecture and the benzylpyridinium derivative as
* To whom correspondence should be addressed. Fax: +86-25-83587438
(X.R.); +86-25-83314502 (Q.M.). Phone: +86-25-83587820 (X.R.); +86-
25-83597266 (Q.M.). E-mail: xmren@njut.edu.cn (X.R.); mengqj@nju.edu.cn
(Q.M.).
† Nanjing University.
‡ Nanjing University of Technology.
10.1021/jp8100333 CCC: $40.75  2009 American Chemical Society
Published on Web 05/27/2009

Two Spin-Peierls-like Compounds
J. Phys. Chem. B, Vol. 113, No. 24, 2009 8279
TABLE 1: Crystallographic Data for 1 and 2 at 298 and
123 Ka
uniform above the transition and dimerization occurs below the
transition; (2) applied pressure strongly promotes the transition,
while nonmagnetic doping suppresses the transition;16,17 (3) the
pretransitional structural fluctuation (which is a common phe-
nomenon existing in the quasi-1D conductors or magnets and
originates from electron-phonon or magnetoelastic coupling
interactions) appears in the temperature dependent oscillation
photographs.17,18 As a result, the transition is very similar to
the spin-Peierls transition. However, a sharp λ-shaped thermal
anomaly, a typical thermodynamic feature of a first-order
transition, is observed in the heat capacity measurements in some
cases.13c,17,18 In order to understand these issues further and extend
our research, this work reports the use of (4′-methylbenzyl)pyri-
dinium derivatives as the countercation of [Ni(mnt)2]- to prepare
two new compounds, which exhibit spin-Peierls-like transition
features.
compound
1
2
mol. formula
C21H14N5NiS4
C21H15N6NiS4
temperature
CCDC no.
mol. mass
space group
a/Å
293 K
612873
523.32
123 K
635758
523.32
P-1
7.239(3)
12.109(5)
26.347(11)
88.629(6)
86.360(6)
76.992(4)
293 K 123 K
612874
538.34
635759
538.34
P21/c
P21/c
P-1
12.123(4)
26.446(8)
7.389(2)
90.00
103.168(4)
90.00
12.137(3)
26.690(7)
7.526(2)
90.00
103.896(5)
90.00
7.311(3)
12.252(5)
28.680(11)
91.557(4)
92.682(4)
104.625(6)
b/Å
c/Å
R/deg
ꢀ/deg
γ/deg
V/Å3/Z
µ/mm-1
λ/Å
2306.7(12)/4 2245.6(16)/4 2366.6(11)/4 2481.0(17)/4
1.222
0.71073
1.507
0.0500
0.0980
1.255
0.71073
1.548
0.0520
0.1157
1.194
1.139
0.71073
1.511
0.71073
1.441
F/g · cm-3
R1
0.0561
0.1426
0.0717
0.1821
wR2
Experimental Section
a R1 ) Σ||Fo| - |Fc||/Σ|Fo| and wR2 ) {Σ[w(Fo - Fc2)2]/
2
Σ[w(Fo )2]}1/2
.
2
The starting materials Na2mnt,19 [4′-CH3Bz-4-RPy]Br and [4′-
CH3Bz-4-RPy]2[Ni(mnt)2]13 were synthesized according to lit-
erature procedures. Please refer to Scheme 1, R1 ) CH3, R2 )
R ) H; R1 ) CH3, R2 ) R ) NH2 for compounds 1 and 2,
respectively.
method on F2. All H atoms were placed at calculated positions
(C-H ) 0.930 Å for benzene, 0.970 Å for methylene, and 0.860
Å for -NH2) and refined riding on the parent atoms with U(H)
) 1.2Ueq (bonded C or N atoms). Details of the crystal
parameters, data collection, and refinements for crystals 1 and
2 at 293 and 123 K are summarized in Table 1. It is noteworthy
that the diffuse scattering around the phase transition that is
probably driven by magnetoelastic interactions13c,18 influences
the inaccuracy of the diffraction data of 1 and 2, and leads to
the parameters, wR2 and R1, being somewhat higher than those
in the high-temperature phase.
Preparation of [4′-CH3BzPy][Ni(mnt)2] (1). An acetonitrile
(MeCN) solution (10 cm3) of I2 (150 mg, 0.59 mmol) was slowly
added to a MeCN solution (20 cm3) of [4′-CH3BzPy]2[Ni(mnt)2]
(710 mg, 1.0 mmol). Then, the mixture was stirred for 15 min
and then allowed to stand overnight after methanol (MeOH)
(90 cm3) was added. Brown microcrystals (400 mg) were
precipitated and filtered off, washed with MeOH, and dried in
a vacuum. Yield: 76.5%. Anal. Calcd for C21H14N5NiS4: C,
48.20; H, 2.70; N, 13.38%. Found: C, 48.15; H, 2.65; N,
13.34%. IR spectrum (cm-1): ν(CN), 2205s; ν(CdC) of mnt2-
1458m.
,
Results and Discussion
Preparation of [4′-CH3Bz-4-NH2Py][Ni(mnt)2] (2). The
procedure for preparing compound 2 is similar to that for 1.
Yield: 78.0%. Anal. Calcd for C21H15N6NiS4: C, 46.85; H, 2.81;
N, 15.61%. Found: C, 46.80; H, 2.79; N, 15.60%. IR spectrum
(cm-1): ν(CN), 2205s; ν(CdC) of mnt2-, 1456m.
Single crystals of 1 and 2 suitable for X-ray analysis were
obtained by evaporating solutions of the corresponding com-
pounds in MeCN.
Crystal Structures of 1 and 2. In the HT phase, the crystals
of 1 and 2 belong to the monoclinic system with space group
P21/c. These are isostructural with the reported [4′-RBzPy][N-
i(mnt)2] compounds (in which the single substituent is located
at the para position of benzene in the cation). The asymmetric
units in the unit cell of 1 (2) comprise one [Ni(mnt)2]- together
with one 4′-CH3BzPy+ (4′-CH3Bz-4-NH2Py+), as shown in
Figure 1a (Figure 1b). The NiS4 core in the [Ni(mnt)2]- anion
exhibits a slightly distorted square planar geometry. The Ni-S
bond lengths, ranging from 2.1338(9) to 2.1436(9) Å in 1
(2.1347(11) to 2.1477(10) Å in 2), and the S-Ni-S bite angles
of 92.21(4) and 92.58(4)° in 1 (92.27(4) and 92.41(4)° in 2)
are in agreement with the values reported for other [Ni(mnt)2]-
compounds.13
As expected, the anions and cations in the crystals of 1 and
2 form completely segregated stacks along the c-axis direction
(as displayed in Figure 2 for 1 and Figure S1 in the Supporting
Information for 2). Both cationic and anionic stacks are regular,
similar to other reported compounds in the series [4′-RBzPy][N-
i(mnt)2]. The nearest Ni · · ·Ni distance is 3.865 Å for 1 versus
3.947 Å for 2 within an anionic stack, and the centroid-to-
centroid separation is 4.488 Å for 1 versus 4.296 Å for 2 within
a cationic stack. Other significant interatomic contacts within
an anionic or a cationic stack are illustrated in Figure 3, Figure
4, and Figure S3 in the Supporting Information, and summarized
in Table 2. It is worth noting that only weak van de Waals forces
exist between the adjacent anionic and cationic stacks in 1, while
strong H-bonding interactions from the NH2 groups of the cation
to the CN groups of the mnt2- ligands as well as CN · · ·π
interactions between the CN group and the pyridine ring are
Physical Measurements. Elemental analyses for C, H, and
N were performed with a Perkin-Elmer 240 analytical instru-
ment. IR spectra (KBr pellets) were obtained in the 4000-400
cm-1 region using a IFS66 V FT-IR spectrophotometer. Dif-
ferential scanning calorimetry (DSC) experiments were per-
formed with a Perkin-Elmer calorimeter; polycrystalline samples
were placed in an aluminum crucible and measurements carried
out during warming (at a rate of 20 K min-1) over the
temperature range -170 to 20 °C (103-293 K). Magnetic
susceptibility measurements were carried out on a Quantum
Design MPMS-XL superconducting quantum interference device
(SQUID) magnetometer over the range 2-300 K, and diamag-
netic correction for each compound was performed utilizing
Pascal’s constants 2.35 × 10-4 and 2.42 × 10-4 emu·mol-1
for 1 and 2, respectively.20
X-ray Single Crystallography. Crystallographic data for 1
and 2 were collected using a Smart CCD area detection
diffractometer with Mo KR (λ ) 0.71073 Å) radiation from a
graphite monochromator at 293 K (HT) and 123 K (LT),
respectively. Structures were solved by the direct method using
the SHELXL-97 software package.21 The non-hydrogen atoms
were refined anisotropically using the full-matrix least-squares

8280 J. Phys. Chem. B, Vol. 113, No. 24, 2009
Tian et al.
Figure 1. Molecular structures with atomic labels for (a) 1 and (b) 2 at 293 K (H atoms omitted for clarity).
Figure 4. Strong H-bonding and CN · · · π interactions between the
pyridine moiety of the cation and the CN groups of the mnt2- ligands
in 2 (H atoms omitted for clarity).
TABLE 2: Significant Interatomic Separations in the
Crystals of 1 and 2
distance/Å
1 (293 K)
1 (123 K)
2 (293 K)
2 (123 K)
b1a
b1′
b2
b2′
b3
b3′
a1
a1′
a2
a2′
a3
a3′
a4
a4′
c1
c1′
3.865
3.865
3.620
3.620
3.710
3.710
4.488
4.488
3.742
3.742
3.698
3.698
3.879
3.879
3.795
3.656
3.584
3.589
3.675
3.507
4.304
4.516
3.612
3.718
3.680
3.565
4.017
3.801
3.947
3.947
3.700
3.700
3.794
3.794
4.296
4.296
3.805
3.805
3.912
3.912
3.641
3.641
3.416
3.416
3.964
3.896
3.702
3.548
3.799
3.708
4.353
4.332
3.524
3.922
3.813
3.623
3.756
4.034
3.477
4.192
Figure 2. Packing diagram of 1 showing the segregated cationic and
anionic stacks at 293 K.
a b1, b2, b3, a1, a2, a3, a4, and c1 represent the distances
between Ni · · ·Ni, Ni · · ·S, S · · ·S, the centroid-to-centroid distance
between the benzene rings, the C atom-to-centroid of benzene ring,
the
C atom-to-centroid of the pyridine ring, and the N
Figure 3. Illustrations showing the shortest distances for (a) Ni · · ·Ni,
Ni· · ·S, and S · · ·S within an anionic stack and (b) the C atom of the
methyl group to the centroid of aromatic rings within a cationic stack
in the crystal of 1 (H atoms omitted for clarity).
atom-to-centroid of the pyridine ring, respectively (see Figures 4
and 5 and Figure S3 in the Supporting Information).
stacks and the intermolecular slippage. The features of the
transition are the following: (1) the cell volume reduces from
2306.7(12) Å3 at 293 K to 2245.6(16) Å3 at 123 K; (2) the
adjacent anions shrink unevenly along the longer molecular axis
direction giving alternating Ni · · ·Ni distances in an anionic stack
(3.656 and 3.795 Å); (3) the distortion within a cationic stack
is reflected in the centroid-to-centroid separations between
adjacent benzene rings (from 4.488 Å in the HT phase to 4.304
and 4.516 Å in the LT phase). Other typical distances which
reflect the intermolecular slippage in the HT and LT phases are
listed in Table 2.
present in 2 (as shown in Figure 4). The corresponding geo-
metric parameters are listed in Tables 2 and 3.
From the HT to LT phases, the space group of the crystal
changes from P21/c to P-1 for both 1 and 2, with the asymmetric
unit containing either one pair of anion and cation or two pairs
of anions and cations. Although the molecular structures in the
LT and HT phases for each compound are almost identical,
distinct differences were observed between their stacking
structures.
For 1, the phase transition results in a nonuniform lattice
compression due to dimerization of both cationic and anionic
In the transition from the HT to LT phase of compound 2,
the lattice is compressed along the direction of the anionic and

Two Spin-Peierls-like Compounds
J. Phys. Chem. B, Vol. 113, No. 24, 2009 8281
TABLE 3: H-Bond Lengths (Å) and Angles (deg) in the
Crystal of 2 at 293 and 123 Ka
structural viewpoint, the magnetic chain is regular. On the other
hand, it was found that the single crystal magnetic susceptibili-
ties are almost independent of the magnetic field direction for
other members in this series of spin-Peierls-like compounds.13c
Therefore, we have further analyzed their magnetic behavior
in the HT phase using the regular antiferromagnetic Heisenberg
model for a linear chain with S ) 1/2.23 The temperature
dependences of the magnetic susceptibility in the range 187-300
K for 1 and 156-300 K for 2 are simulated using eq 1:
D-H· · ·A
d(D-H) d(H· · ·A) angle(D-H· · ·A) d(D· · ·A)
293 K
2.47
2.48
N(6)-H(6A)· · ·N(1)1b
N(6)-H(6B)· · ·N(3)1c
0.86
0.86
129.6
126.5
3.086(6)
3.073(6)
123 K
2.57
2.52
N(12)-H(12B)· · ·N(3)2b
N(12)-H(12A)· · ·N(4)2c
N(10)-H(10B)· · ·N(6)2d
0.86
0.86
0.86
124.8
141.3
143.7
3.140(6)
3.233(6)
3.361(7)
2.63
a Symmetry codes: b ) -x + 1, y + 1/2, -z + 1/2; c ) -x +
1, -y + 1, -z. Symmetry codes: c ) -x, -y + 1, -z + 2; c ) x
- 1, y - 1, z; d ) -x, -y + 1, -z + 1.
5
1 +
1 +
N X-i
∑
i)1
6
2
i
Ng2µB
4kBT
ꢁm )
·
(1)
cationic stacks due to the dimerization of the stacks (to give
rise to the alternating adjacent Ni · · ·Ni distances of 3.896 and
3.964 Å), which is similar to that which occurs in 1, with
expansion along the longer and shorter molecular axes of the
planar anion. Such lattice expansions give rise to the cell volume
increase from 2366.6(11) Å3 at 293 K to 2481.0(17) Å3 at 123
K. To the best of our knowledge, it is a rare case that the lattice
increases upon cooling.22 As illustrated in Figures 3 and 4 and
Tables 2 and 3, it has been found that the separations of the C
atom in the methyl group to the centroid of pyridine, the
centroid-to-centroid separations between the adjacent benzenes
in the cationic stack, as well as the separations of the N atom
in the CN group to the centroid of pyridine between the anionic
and cationic stacks become longer in the LT phase as a result
of the slippages of the anions and cations along the longer and
shorter molecular axes of the planar anion.
D X-i
∑
i
i)1
where X ) kBT/||J||, where J is the magnetic exchange constant
of the neighboring spins in a magnetic chain, and the coefficients
of Ni and Di in the power series. The best fit yielded J/kB )
166(2) K with a g-factor of 2.020(23) for 1 and J/kB ) 42 (0)
K with a g-factor of 2.056(5) for 2 (Figure 5). The fitted
g-factors are close to the experimental values from EPR
measurements.24
In the low-temperature phase, an opening of the spin gap
was observed. Accordingly, we simulated the magnetic suscep-
tibilities of both 1 (temperature range 2-123 K) and 2 (2-141
K) using eq 223,25
Magnetic Susceptibilities of 1 and 2. The variable temper-
ature molar magnetic susceptibilities of 1 and 2 were investi-
gated over the temperature range 2-300 K. Upon cooling from
300 to 2 K, a spin-Peierls-like transition occurred at around
182 K for 1 and 155 K for 2. On raising the temperature back
to room temperature, no sizable hysteresis was detected for
either compound. In the HT phase, the overall magnetic
behaviors of 1 and 2 correspond to an antiferromagnetically
coupled system. The value of ꢁmT at 300 K is 0.275
emu·K·mol-1 for 1 versus 0.333 emu ·K·mol-1 for 2, less than
the 0.375 emu ·K·mol-1 for a spin-only value with S ) 1/2 per
formula unit. Below the transition, the magnetic susceptibility
drops steeply and a small paramagnetic-like tail is observed in
the low-temperature region for the ꢁm values of 1 and 2.
A comparison of the magnetic features for two compounds
reveals the following two significant differences: (1) the
transition temperature of 2 is lower than that of 1; (2) below
the transition temperature, the magnetic susceptibility of 2
changes much more rapidly and attains a nonmagnetic state,
whereas the magnetic susceptibility of 1 decreases progressively,
which is the characteristic of a spin-Peierls transition due to
the dimerized state possessing a continuum of excited states.15
To estimate the magnetic coupling interaction in the HT
phase, we fitted the temperature dependent magnetic suscepti-
bilities of 1 (in the range 187-300 K) and 2 (in the range
156-300 K) utilizing a simple Curie-Weiss equation, to obtain
the Curie-Weiss parameters C ) 0.575(26) emu ·mol-1 with
θ ) -290(23) K and C ) 0.371(4) emu ·mol-1 with θ ) -10(2)
K for 1 and 2, respectively. The theoretical plot of the
temperature dependence of the magnetic susceptibility well
matches the experimental data for 2 (please refer to Figure S4
in the Supporting Information), while the Curie constant for 1
is not reasonable. On the other hand, the phenomenology of
the magnetic transitions in 1 and 2 closely resembles that of a
spin-Peierls transition in a quasi-1D spin system and, from the
R
C
T
ꢁm )
exp(-∆/kBT) +
(2)
Tγ
0
where R is a constant corresponding to the dispersion of the
excitation energy, ∆/kB is the magnitude of the spin gap, γ0 is
a constant (γ0 ) 0.5), and the C/T term represents the con-
tribution from magnetic impurities. Simulation gave the results
R ) 1.68(18), ∆/kB ) 933(17) K, and C ) 0.52(1) × 10-3
emu·mol-1 for 1 and R ) 54(166), ∆/kB ) 1474(442) K, and
C ) 1.36(2) × 10-3 emu·mol-1 for 2. Obviously, the fitted
results for 2 appear to be unreasonable.
Thermodynamic Properties. DSC measurements on 1 and
2 were performed, and the thermogramms for a warming rate
of 20 K ·min-1 are depicted in Figure 6, in which 2 shows a
more abrupt endothermic peak around its transition temperature.
The peak temperatures Tmax (∼182 K for 1 and 154 K for 2)
are close to the results from the magnetic susceptibility
measurements. The endothermic enthalpy changes (∆H), which
were calculated from the integrated peak areas, are estimated
to be 316.6 J·mol-1 for 1 and 1082.1 J·mol-1 for 2, confirming
that the transitions are first-order.26-29 Compared with 1,
compound 2 shows a larger endothermic enthalpy change. This
observation is probably related to the strong H-bonding interac-
tions in 2, and the larger enthalpy change is essential contribu-
tion of lattice degrees of freedom in the transition of 2.
Combined with our previous studies, the qualitative relationship
between the transition enthalpy and the magnetic susceptibility
below the transition temperature has been noted; namely, a larger
enthalpy change corresponds to the sharp drop of magnetic
susceptibility, while a smaller one matches the progressive
decrease of magnetic susceptibility in the LT phase. The latter
case is similar to a spin-Peierls transition (where the transition
enthalpy is zero). On the basis of the above analysis, it is further
assumed that the magnetic gap in the dimerized state follows

8282 J. Phys. Chem. B, Vol. 113, No. 24, 2009
Tian et al.
Figure 5. Plots of ꢁm(T) vs T for compounds 1 and 2 (open circles, experimental data; solid lines, fits; for details, see the text).
Figure 6. DSC curves for compounds 1 and 2 showing Tpeak and ∆H of the phase transition.
NO2 and CH3 show different effects of electron donating and
withdrawing, but the two corresponding compounds show a
similar transition temperature. As shown in Figure 7, the
variation of TC looks to be dependent on the size of the
substituent group, since the spin-Peierls-like transition temper-
ature rises with the cell volume of the crystal, which increases
with the atomic size as well as atomic numbers of the substituent
located in the para position of the benzene ring. The effect of
the size of the substituent group on TC is probably related to
the so-called “chemical pressure” effect.30
Conclusion and Remarks
Figure 7. The linear relationship between TC and cell volume for two
series of spin-Peierls-like compounds.
In conclusion, we have shown that two molecular compounds
(1 and 2) assemble into novel spin transition systems, via π-π
stacking between the planar molecular groups [Ni(mnt)2]-. For
1 and 2, the spin transition is associated with a structural
transition. However, both the structural and magnetic aspects
of the transition are distinct from each other. From the HT phase
to the LT phase, a lattice compression is observed for 1 but a
lattice expansion for 2. Below the transition temperature and
upon further cooling, the magnetic susceptibility progressively
decreases for 1 (the progressive decrease of the magnetic
susceptibility is probably related to the dimerized state pos-
sessing a temperature dependence of the BCS energy gap),
whereas it drops sharply for 2. Combined with our previous
studies, we find that the variation of the transition temperature,
TC, increases with the cell volume in the HT phase for two series
of compounds with a single substituent in the para position of
benzene [4′-RBzPy][Ni(mnt)2] (see Figure 7).
the usual temperature dependence of the BCS energy gap in
the case of the transition with smaller enthalpy change.15
Relationship between the Substituent Group and the
Transition Temperature. Over the last half decade, we have
investigated the crystal structure and magnetic properties of a
series of [Ni(mnt)2]- compounds using different benzylpyri-
dinium derivatives as the counter cations, and observed the
presence of a spin-Peierls-like transition in many cases. It is
interesting that the transition temperature TC can be tuned by
modifying the substituents on the aromatic rings. For two series
of compounds with a single substituent in the para position of
benzene, the variation of TC seems to be independent of the
electronic effect of the substituent. The compounds in question,
abbreviated as [4′-RBzPy][Ni(mnt)2], are isostructural and
crystallize in monoclinic space group P21/c in the HT phase
(see inset of Figure 7).13 For example, the electron affinity of
the substituents should follow the order NO2, CN > F > Cl >
Br > I, whereas the variation of TC in the corresponding
compounds does not show this simple order. On the other hand,
Theoretically, the relationship between the transition tem-
perature TC and applied pressure P, at thermal equilibrium, is
given by the Clapeyron equation, dP/dTC ) ∆S/∆V ) ∆H/
(TC ·∆V), where ∆H, ∆S, and ∆V represent the enthalpy,
entropy, and volume changes between the HT and LT phases,

Two Spin-Peierls-like Compounds
J. Phys. Chem. B, Vol. 113, No. 24, 2009 8283
(10) Fujita, W.; Awaga, K.; Kondo, R.; Kagoshima, S. J. Am. Chem.
Soc. 2006, 128, 6016.
respectively. In the case of lattice compression in the LT phase,
TC is actually observed to shift to higher temperature under
pressure.16 For 2, the cell volume increases in the LT phase,
and as a consequence, a new issue is opening, which is whether
TC does or does not decrease with P. We are exploring
collaboration for the study of the magnetic susceptibility under
pressure.
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Supporting Information Available: Crystallographic data
(excluding structure factors) for the structures of 1 and 2 at 293
and 123 K and figures showing the packing diagram of 2, the
interactions between the pyridine moieties of the cation in 2,
the nearest distances in anionic and cationic stacks, magnetic
susceptibility vs T plots for 1 and 2, and a ꢁm vs T plot for 2.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. We thank the National Nature Science
Foundation of China, Jiangsu Science & Technology Depart-
ment, and the Center of Analysis and Determining of Nanjing
University for financial support (Grant Nos. BK2007184,
20871068, 20771056, and 20490218).
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