Quasi-one-dimensional molecular magnets based on derivatives of (fluorobenzyl)pyridinium with the [M(mnt)2] monoanion (M = Ni, Pd or Pt; Mnt2- = maleonitriledithiolate): Syntheses, crystal structures and magnetic properties

Article abstract of DOI:10.1039/b514278d

The syntheses, structural characterizations and magnetic behaviors of three new complexes, 1-(3′,4′,5′-trifluorobenzyl)pyridinium [M(mnt)₂]^- [M = Ni (1), Pd (2) or Pt (3)], are reported. These complexes are isomorphous and their prominent structural character is that the [M(mnt)₂]^- anions form columnar stacks, in which the dimerization was observed. Complexes 2 and 3 are diamagnetic, while 1 possesses an energy gap of 2474 K. For crystal 4, 1-(4′-fluorobenzyl)pyridinium [Ni(mnt)₂] (its structure and magnetic susceptibility were briefly reported earlier), the magnetic behavior can be divided into two regimes, namely, weakly ferromagnetic coupling above 93 K and strongly antiferromagnetic coupling below 93 K. A transition occurs at 93 K which switches the magnetic exchange nature from ferromagnetic to antiferromagnetic. A sharp thermal abnormality with λ-shape, associated with the transition, appears from its heat capacity measurement to indicate that the transition is first order. The temperature dependences of the superlattice diffractions revealed the existence of the pretransitional phenomena up to at least 140 K. The unusual magnetic behavior of 4, such as the origin of the ferromagnetic interaction in the high temperature phase and what causes the spin transition, are discussed further. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b514278d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Quasi-one-dimensional molecular magnets based on derivatives of
(fluorobenzyl)pyridinium with the [M(mnt)2] monoanion (M = Ni,
Pd or Pt; mnt2− = maleonitriledithiolate): Syntheses, crystal structures and
magnetic properties†
X. M. Ren,*a,g S. Nishihara,b T. Akutagawa,a,g S. Noro,a,g T. Nakamura,*a,g W. Fujita,c K. Awaga,d Z. P. Ni,e
J. L. Xie,e Q. J. Menge and R. K. Kremer f
Received 7th October 2005, Accepted 5th January 2006
First published as an Advance Article on the web 23rd January 2006
DOI: 10.1039/b514278d
The syntheses, structural characterizations and magnetic behaviors of three new complexes,
1-(3ꢀ,4ꢀ,5ꢀ-trifluorobenzyl)pyridinium [M(mnt)2]− [M = Ni (1), Pd (2) or Pt (3)], are reported. These
complexes are isomorphous and their prominent structural character is that the [M(mnt)2]− anions
form columnar stacks, in which the dimerization was observed. Complexes 2 and 3 are diamagnetic,
while 1 possesses an energy gap of 2474 K. For crystal 4, 1-(4ꢀ-fluorobenzyl)pyridinium [Ni(mnt)2] (its
structure and magnetic susceptibility were briefly reported earlier), the magnetic behavior can be
divided into two regimes, namely, weakly ferromagnetic coupling above 93 K and strongly
antiferromagnetic coupling below 93 K. A transition occurs at 93 K which switches the magnetic
exchange nature from ferromagnetic to antiferromagnetic. A sharp thermal abnormality with k-shape,
associated with the transition, appears from its heat capacity measurement to indicate that the
transition is first order. The temperature dependences of the superlattice diffractions revealed the
existence of the pretransitional phenomena up to at least 140 K. The unusual magnetic behavior of 4,
such as the origin of the ferromagnetic interaction in the high temperature phase and what causes the
spin transition, are discussed further.
of columnar stacks is favored because such molecules possess
(1) a flat molecular and extended electronic structure, and (2)
Introduction
intermolecular interactions between [M(mnt)2]− anions, such as p–
p stacking, S · · · S or S · · · M interactions. The magnetic exchange
characteristic is highly sensitive to the intermolecular contacts and
overlap pattern between neighboring anions within an anionic
stack,4c,5 so that an external perturbation could trigger a spin
transition. Accordingly, a key issue in our research is the control of
the [M(mnt)2]− anions during the columnar arrangement. Because
the stacking pattern of the [M(mnt)2]− anion depends strongly
upon the molecular topology of the countercation, we proposed
that the formation of a columnar stack of [M(mnt)2]− was attain-
able by modifying the molecular structure of the countercation.
Derivatives of benzylpyridinium (abbreviated as [RBzPy]+) can
serve as flexible cations which can be adjusted via modifying the
nature of the groups on the aromatic rings.6 Consequently, we have
recently prepared a series of [RBzPy][M(mnt)2] ion-pair complexes
that form a structure with segregated stacks of cation and anion,7,8
and in some of these a spin-Peierls-like transition was observed (the
substituent groups in the cation and the metal ions of the anion
for those reported complexes are listed in Table 1).8
Solids consisting of molecules with flat-shaped configurations and
extended electronic structures have been attracting considerable
interest in the field of molecule-based conductors and magnets
due to their unusual physical properties.1–4
Our studies have been focused on the design, preparation,
and investigation of novel spin transition molecular magnets
that are based on the molecular architecture of [M(mnt)2]−
(mnt2− = maleonitriledithiolate; M = Ni, Pd, or Pt). The formation
aResearch Institute for Electronic Science, Hokkaido University, Sapporo,
060-0812, Japan. E-mail: xmren@es.hokudai.ac.jp, tnaka@imd.es.hokudai.
ac.jp; Fax: +81 11-706-4972; Tel: +81 11-706-2849
bDepartment of Physical Science, Graduate School of Science, Osaka
Prefecture University, Sakai, Osaka, 599-8531, Japan
cResearch Center of Materials Science, Nagoya University, Chikusa-ku,
Nagoya, 464-8602, Japan
dDepartment of Chemistry, Graduate School of Science, Nagoya University,
Chikusa-ku, Nagoya, 464-8602, Japan
eState Key Laboratory & Coordination Chemistry Institute, Nanjing Univer-
sity, 210093, Nanjing, P. R. China
f
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, Post-
Recently, we briefly reported the room temperature crystal
structure of and the temperature dependence of the magnetic
susceptibility for [1-(4ꢀ-fluorobenzyl)pyridinium][Ni(mnt)2] (the
[FBzPy][Ni(mnt)2] complex is referred to in the current work as
compound 4), which undergoes a spin transition at ∼93 K.8k
The nature of the magnetic exchange is weakly ferromagnetic
above this transition (high temperature phase) and strongly
fach 800665, D-70569, Stuttgart, Germany
gCREST, Japan Science and Technology Corporation (JST), Kawaguchi,
332-0012, Japan
† Electronic supplementary information (ESI) available: Bond lengths for
the anion in compounds 1–4 (Table S1); spin density distributions in a
monomer of [Ni(mnt)2]− (Table S2); details of DFT calculations; ORTEP
and stacking pattern diagrams for 2–4 (Fig. S1–S5); and the EPR spectrum
of 4 (Fig. S6). See DOI: 10.1039/b514278d
1988 | Dalton Trans., 2006, 1988–1994
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Table 1 Reported complexes in the series of [R1R2R3BzPy][M(mnt)2] that
exhibit a magnetic transition
analysis: Calcd for C20H9N5S4F3Ni: C, 42.6; H, 1.60; N, 12.4%.
Found: C, 42.8; H, 1.71; N, 12.3%.
[F3BzPy][Pd(mnt)2] (2) and [F3BzPy][Pt(mnt)2] (3)
The procedure described for 1, except with the corresponding
derivatives of [fluorobenzyl)pyridinium]2[M(mnt)2] (M = Pd or Pt)
as the starting material, was followed to afford 2 and 3. Elemental
analysis: Calcd for C20H9N5S4F3Pd (2): C, 39.3; H, 1.48; N, 11.5%.
Found: C, 39.3; H, 1.76; N, 11.4%. C20H9N5S4F3Pt (3): C, 34.3; H,
1.30; N, 10.0%. Found: C, 34.4; H, 1.32; N, 9.97%.
The preparation of crystal 4 was described previously.8k
R1
R2
R3
M
Reference
Magnetic susceptibility
I
H
H
H
H
H
H
NH2
NH2
NH2
H
H
H
H
H
H
F
H
H
H
H
Ni
Ni
Ni
Ni
Pt
Ni
Ni
Ni
Ni
Ni
[8a]
[8i]
NO2, Cl, Br
F
CN
NO2, Cl, Br
Br
Br
CN
Cl, H
Vinyl
Magnetic susceptibility measurements on polycrystalline samples
were carried out with a Quantum Design MPMS-XL super-
conducting quantum interference device (SQUID) magnetometer
between 2–350 K under a magnetic field of 1 T.
[8k]
[8h]
[8e]
[8f ]
[8g]
[8c]
[8b,8j]
[8d]
Heat capacity
The measurement of Cp for 4 was performed by the relaxation
method using a Quantum Design PPMS in the temperature range
2–140 K. A crystal with a mass of 0.52 mg was attached to the
sample platform using a small amount of grease.
antiferromagnetic below (low temperature phase). This magnetic
behavior is significantly different from other spin transition
complexes of [RBzPy][Ni(mnt)2] in which the natures of the
magnetic exchange are weakly and strongly antiferromagnetic in
the high and low temperature phases, respectively. It appears that
a delicate relationship exists between the nature of substituent
group (such as its electronic or steric nature) and the anionic
stacking pattern, as well as the magnetic coupling behavior in
the series [RBzPy][M(mnt)2] (M = Ni, Pd or Pt).7–9 In this
work, a new countercation, 1-(3ꢀ,4ꢀ,5ꢀ-trifluorobenzyl)pyridinium
(abbreviated as [F3BzPy]+) together with three new complexes,
[F3BzPy][M(mnt)2] (M = Ni, Pd or Pt), were designed and
prepared to gain more insight into the issues mentioned above
and to investigate the magnetostructural relationship. The crystal
structures and magnetic properties for these new complexes were
examined. Additionally, the nature of the magnetic transition in
[FBzPy][Ni(mnt)2] was further investigated by heat capacity as well
as temperature dependent superlattice diffraction measurements.
X-Ray structural analyses
Crystallographic data for 1–3 were collected using a Rigaku
˚
RAXIS RAPID diffractometer with Mo Ka (k = 0.71073 A)
radiation from a graphite monochromator. Structures were solved
by direct methods using the SHELXL-97 software package.11
The non-H atoms were refined anisotropically using the full-
matrix least-squares method on F2. All H atoms were placed
˚
at calculated positions (C–H = 0.930 A for benzene or pyridine
˚
rings and 0.970 A for methylene) and refined riding on the parent
atoms with U(H) = 1.2 × Ueq (bonded C atom). Details of the
crystal parameters, data collection, and refinement for 1–3 are
summarized in Table 2. The bond lengths in [M(mnt)2]− (M = Ni,
Pd or Pt) anions, along with their estimated standard deviations,
are listed in Table S1 of the ESI.† Some important intermolecular
distances are listed in Table 3.
CCDC reference numbers 182194 and 285872–285874.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b514278d
Experimental
Preparation of the complexes
Na2mnt, the derivatives of fluoro-substituted benzylpyridinium
bromide6 and their salts with the [M(mnt)2] dianion (M = Ni, Pd
or Pt) were prepared according to the published procedures.10
Temperature dependent oscillation photographs
The oscillation photographs at different temperature for 4 were
taken using a Rigaku RAXIS RAPID imaging plate area detector
˚
with graphite monochromated Mo Ka radiation (k = 0.71073 A).
[F3BzPy][Ni(mnt)2] (1)
To a solution of [F3BzPy]2[Ni(mnt)2] (844 mg, 1.0 mmol) in
acetonitrile (5 mL) was added I2 (158 mg, 0.62 mmol) and
methanol (30 mL). The mixture was stirred for 10 min, and then
placed in a refrigerator at 4 ◦C for 24 h. The resulting brown
microcrystalline product was collected by filtration, washed with
methanol, and dried under vacuum (yield: 470 mg, 83%). Single
crystals suitable for X-ray structural analysis were obtained by
diffusing diethyl ether into an acetonitrile solution of 1. Elemental
Results and discussion
Crystal structures of 1, 2 and 3
The complexes 1, 2 and 3 are isomorphous, so only the detailed
structure of 1 is presented here. The asymmetric unit within
the unit cell of 1, comprised of one [Ni(mnt)2]− together with
one F3PyBz+, is shown in Fig. 1(a). The anion possesses a
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Table 2 Crystallographic data for 1–4
Complex
1
2
3
4
Mol. formula
Temp./K
C20H9F3N5NiS4
100
CCDC-285872
563.27
C20H9F3N5PdS4
293
CCDC-285873
610.96
C20H9F3N5PtS4
293
CCDC-285874
699.65
C20H11FN5NiS4
293
CCDC-182194
527.29
CCDC no.
Mol. mass
Space group
P21/c
P21/c
P21/c
P21/c
˚
a/A
7.445(2)
22.234(6)
13.400(5)
90
98.400(10)
90
2194.3(12)
4
1.310
7.4749(15)
22.786(5)
13.566(3)
90
98.55(3)
90
2284.8(8)
4
1.221
7.5498(15)
22.745(5)
13.546(3)
90
98.70(3)
90
2299.3(8)
4
6.509
12.112(2)
25.875(5)
7.3230(15)
90
101.72(3)
90
2247.2(7)
4
1.261
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
l/mm−1
˚
k/A
0.71070
1.705
0.0288
0.71070
1.776
0.0525
0.71070
2.021
0.0387
0.71070
1.559
0.0353
q/g cm−3
a
R1
b
wR2
0.0548
0.1045
0.0838
0.0781
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = (ꢁFo| − |Fcꢁ)/ |Fo|. b wR2 = w(|Fo|2 − |Fc|2)2/ w(|Fo|2) 2]1/2
.
Fig. 1 (a) ORTEP11 diagram of 1 (with displacement ellipsoids at a 30% probability level) and H atoms omitted for clarity. (b) Packing diagram of 1
showing the anionic stacks along the direction of a-axis.
roughly planar geometry, and most of the bond lengths and
angles are in good agreement with various [Ni(mnt)2]− complexes.8
Similar to other members of the [RBzPy][Ni(mnt)2] series, the
anions form columnar stacks which lie parallel to the a-axis
(Fig. 1(b)). Within an anionic stack, two kinds of overlapping
patterns between neighbors were observed, as shown in Fig. 2.
Face-to-face overlapping (eclipse overlapping) involves an anionic
pair containing Ni(1) and Ni(1)i (symmetry code i = 1 − x,
the pyridine ring and reference plane, and between the benzene
and pyridine rings. For convenience, these dihedral angles are
represented by h1, h2 and h3, respectively, and are summarized in
Table 4. In 1, h1, h2 and h3 are 102.3, 116.3 and 108.2◦, respectively.
The angles h1 and h3 are of comparable value within the series, but
the value of h2 for 4 differs markedly from those measured in 1–3.8k
As demonstrated in Fig. 2(c) and (d), the benzene rings between
the adjacent cations keep away from each other which is probably
due to the steric repulsion between the F atoms (the steric repulsion
should become stronger with an increase in the number F atoms
on the benzene ring). As a result, the cations in 1–3 do not form the
columnar stacks (cf. Fig. 1(b)) that are a common feature of the
stacking patterns in the series of the benzylpyridinium derivatives
with [M(mnt)2] (M = Ni, Pd or Pt).
˚
2 − y, 1 − z) in which the nearest Ni · · · Ni distance (3.420 A)
is comparable to the corresponding interplane distance (3.433
˚
A) (the mean molecular plane is defined by four coordinated S
atoms). Longitudinal and transverse offsets involve an anionic
pair containing Ni(1) and Ni(1)ii (symmetry code ii = 2 − x, 2 −
y, 1 − z) in which the Ni · · · Ni and corresponding interplane
˚
distances are 4.465 and 3.484 A, respectively. In other words, the
anionic stack is irregular and dimeric which is different from that
found in 4.
Magnetic susceptibility
In the complexes with substituted benzylpyridinium derivatives,
the molecular configuration was completely determined by three
dihedral angles, which were the angles between the benzene
ring and reference plane (defined by CAr–CH2–NPy), between
The magnetic behaviors of 1–3 have been determined using
polycrystalline samples between 2–350 K. The results are given
in Fig. 3 as vm = f (T). As shown in Fig. 3, they all exhibited a
very weak paramagnetism. For 2 and 3, vm = f (T) follows the
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Table 4 Dihedral angles (in ◦) in the cation moieties of complexes 1–4
1
2
3
4
h1
h2
h3
102.3
116.3
108.2
100.6
116.2
108.4
100.1
115.5
108.7
103.0
98.0
112.4
Fig. 2 The anionic arrangement patterns for 1 as viewed from (a) the side
and (b) above (symmetry codes: 1A = − x, −2 − y, 1 − z; 1B = 1 − x,
−2 − y, 1 − z). The cationic arrays for 1 as viewed from (c) the side and
(d) above .
Curie–Weiss law, whereas in the case of 1 the typical magnetic
behavior of a spin gap system was observed. Therefore, eqn
(1) was used to fit the temperature dependence of the magnetic
susceptibilities of 2 and 3
C
T − h
vm
=
+ v0
(1)
while eqn (2) was used for 1
a
C
T
vm
=
exp(−D/T) + + v0
(2)
T
where a is a constant value corresponding to the dispersion
of excitation energy, D is the magnitude of the spin gap, v0
contributes from the core diamagnetism and the possible Van
Vleck paramagnetism.12 The best fitting yielded the parameters
of a = 6.8(7) emu K mol−1, D/kB = 2474(23) K, C = 3.6(1) ×
10−4 emu K mol−1, and v0 = − 2.4(1) × 10−4 emu mol−1 for 1;
C = 1.39(1) × 10−3 emu K mol−1, h = − 0.58(2) K and v0 = −
1.7(1) × 10−4 emu mol−1 for 2, C = 3.6(3) × 10−3 emu K mol−1, h
= − 0.49(1) K and v0 = − 1.59(2) × 10−4 emu mol−1 for 3. The v0
values in 1–3 range from −1.6 × 10−4 to −2.4 × 10−4 emu mol−1,
which are comparable to their diamagnetic corrections calculated
from tabulating Pascal constants. For 1, the spin gap leads to
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Fig. 3 Temperature dependence of the magnetic susceptibilities of 1–4 (open circles: experimental data; solid lines: fitted data. The inset in the graph of
4 shows the hysteresis loop).
the magnetic susceptibility decreasing exponentially upon cooling
at higher temperature region. For 2 and 3, the very small Curie
constant indicated that the weak paramagnetism originates from
magnetic impurities, and hence the inherent magnetic behaviors
of both 2 and 3 should be diamagnetic due to the existence
of very strong antiferromagnetic coupling interactions in the
eclipsed [M(mnt)2]− (M = Pd or Pt) pair in the anionic stack.
The distinction between the magnetic behaviors of [Ni(mnt)2]−
and [M(mnt)2]− (M = Pd or Pt) series (the former possesses a
finite energy gap whereas the latter are diamagnetic), indicates that
the antiferromagnetic exchange interactions are much stronger in
[M(mnt)2]− (M = Pd or Pt) than in [Ni(mnt)2]−, which is associated
with the fact that a more efficient molecular orbital overlap takes
place in [M(mnt)2]− (M = Pd or Pt) complexes due to the bigger
ionic radii of Pd and Pt [cf. Table 3, the plane-to-plane distances
in the eclipsed overlapping pair of anions for 1–3 are comparable
to each other].
As is illustrated in Fig. 3, the prominent magnetic characteristics
of 4 are a peculiar spin transition that occurs at ∼93 K, the
ferromagnetic coupling that dominates above this transition (high
temperature phase, abbreviated as HT phase) and a spin gap that
opens below the transition (low temperature phase, abbreviated
as LT phase).
In the HT phase, the Baker equation (eqn (3)),13 derived from
a high temperature series expansion for a ferromagnetic chain, is
used to analyze the magnetic susceptibility, and the contribution
(v0) originating from the closed shells (diamagnetism) and possible
Van Vleck paramagnetism was further taken into account (eqn (4))
where C = 1.0 + 5.7979916y + 16.902653y2 + 29.376885y3 +
29.832959y4 + 14.036918y5, D = 1.0 + 2.7979916y + 7.008678y2 +
8.653644y3 + 4.5743114y4 and y = J/kBT. The best fit for the
magnetic susceptibility above 93 K yielded the parameters, J/kB =
15.2(3) K and v0 = − 3.7(8) × 10−4 emu mol−1 with a fixed g-factor
of 2.035 (which was determined from the room temperature EPR
spectrum, see Fig. S6 of the ESI†).
Below the transition,
a spin-Peierls-like dimeric lattice
distortion5,8,14,15 happens which is supported by the fact that the
XRD pattern is more complicated in the LT than the HT phase,8k
and several new diffractions were observed in LT phase to indicate
the symmetry of the lattice is lowered upon moving from HT to LT
phase.16 Eqn (2) is accordingly used to evaluate the temperature
dependence of the magnetic susceptibility. A reasonable fit of the
magnetic susceptibility data in the temperature range of 4–88 K
gave the parameters a = 9.04(43), D/kB = 581(12) K, v0 = −
2.8(1) × 10−4 emu mol−1, C = 8.8(6) × 10−4 emu K mol−1.
For the magnetic behavior of 4, our initial questions regarding
the origin of the ferromagnetic interaction in the HT phase and
the cause of the switch in the nature of the magnetic exchange
in the HT and LT phases, are interesting. Coomber et al.4c
calculated the spin density distributions in a single [Ni(mnt)2]−
anion at the UHF/INDO level, and the results indicated that
the Ni ion and S atoms possess large positive and small negative
spin densities, respectively. Based on these calculations, they
thought that the spin-polarization effect plays an important
role in the magnetic exchange process and gave the following
physical picture about the nature of magnetic exchange, namely,
for the [Ni(mnt)2]− neighbors overlapping in a slipped Ni-over-S
configuration, the spin polarization effect (McConnell’s theory17)
between large positive spin densities on Ni ions and small negative
spin densities on S atoms of adjacent anions of [Ni(mnt)2]− leads
ꢁ
ꢂ
2/3
Ng2l2B
4kBT
C
D
vchain
=
(3)
(4)
vm = vchain + v0
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to a ferromagnetic coupling interaction (illustrated in Fig. 4(a)),
while the direct overlapping of Ni · · · Ni or S · · · S gives rise to
an antiferromagnetic coupling interaction. In fact, a slipped Ni-
over-S stacking pattern was observed in the anionic stack of 4 as
depicted in Fig. 4(b). In our most recent study,18 we recalculated
the spin distributions in a dimer of [Ni(mnt)2]− utilizing a series of
functionals and basis sets in the framework of density functional
theory (DFT) and found that the calculated spin populations
depended upon the method used. Among them, the values of spin
density distributions at the UBPW91/LANL2DZ level matched
the EPR experimental results well, which revealed that the S atoms
possessed large spin densities, but the centre Ni ion did not due to
the strong delocalization effect from the Ni ion to mnt2− ligands,
and that the sum of the spin population in the core of NiS4 was
0.887 from our calculation vs 0.85 from experiment.19 However,
our results do not match the report by Coomber et al.4c In addition,
we examined the variation of the magnetic exchange constant (J) in
a [Ni(mnt)2]− dimer as a function of the interplane distance as well
as the rotation angle (h)18 [the angle made by the long molecular
axis of each monomer] by the symmetry-broken method in the
DFT framework and the X-ray structures. The results indicated
that the nature of the magnetic exchange in a [Ni(mnt)2]− dimer is
sensitive to these two factors, and the switching of the magnetic
exchange nature between ferromagnetic and antiferromagnetic is
probably achieved by changes in the interplane distance or rotation
angle (h). The magnetic orbital interaction is the main contributor
to the spin transition and magnetic exchange characteristics of
a spin system even if weak spin-polarization effect exists in the
system. Therefore, the peculiar magnetic behavior of 4 can be
interpreted as follows. The [Ni(mnt)2]− array within a stack in
the HT phase accidentally satisfy the conditions required for
a ferromagnetic interaction, but, upon cooling, the anisotropic
contraction of the crystal leads to a non-uniform compression and
slippage of the [Ni(mnt)2]− stack, which trigger the spin transition.
Fig. 5 Temperature dependence of molar heat capacity of crystal 4, the
inset shows the plot of Cp/T vs T.
In the first order approximation, a normal heat capacity is
considered a smooth curve.20 Upon subtracting this contribution
from the normal heat capacity, the entropy change of this
transition is estimated as 1.02 J K−1 mol−1, and this value is less
than the theoretical maximum of spin entropy for a mole of S =
1
2
ions, Rln2 ≈ 5.76 J K−1 mol−1,3a suggesting that the short range
order persists over the temperature of the transition in 4. The short
range ordering is an indication of the lowered dimensionality of
the magnetic system, and is consistent with the chain structure of
[Ni(mnt)2]− in crystal 4.21
Superlattice diffractions
Local structural fluctuation is a common phenomenon existing in
the quasi-one-dimensional conductor or magnet which originates
from electron–phonon or magnetoelastic coupling interactions.
Fig. 6 displays the temperature dependent diffraction patterns,
from which the superlattice diffractions were observed below
140 K and became stronger upon cooling. This observation pro-
vided the evidence that the pretransitional structural fluctuation
exists above the transition temperature.
Conclusion
The crystal structures and magnetic properties were measured for
three new ion-pair complexes which consist of derivatives of multi-
fluorine substituted benzylpyridinium with [M(mnt)2]− (M = Ni,
Pd or Pt). The dependence of the stacking effect on the nature of
the substituent groups was observed, and the common stacking
pattern of the cations seen for the series of [M(mnt)2]− with
mono-substituted benzylpyridinium derivates, in which adjacent
benzene rings overlap in a face-to-face fashion, was not seen in 1–3.
Instead, the neighboring cations keep away from each other due to
steric and/or electrostatic repulsion between the multi-substituted
benzene rings. As a result, the cations do not form columnar stacks
in the solid state. The anionic stacking columns dimerize in 1–3.
The complexes [Pd(mnt)2]− and [Pt(mnt)2]− are diamagnetic, while
the complex [Ni(mnt)2]− possesses an energy gap of 2274 K. This
difference originates from the greater molecular orbital overlap in
Fig. 4 (a) Schematic representation of spin density distributions in [Ni-
(mnt)2]−. The dotted line shows the ‘local’ antiferromagnetic interactions.
(b) Ni-over-S stacking pattern observed in an anionic stack of 4.
Heat capacity
Fig. 5 shows the results of the heat capacity measurement for 4. A
sharp thermal abnormality with k-shape, which is a typical feature
of first order transitions, was observed, and the peak temperature,
at 93.4 K, coincides fairly well with the temperature where the
abrupt drop of susceptibility takes place.
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Fig. 6 Temperature dependent oscillation photographs of 4 which show superlattice diffractions below 140 K.
dimers of [M(mnt)2]− (M = Pd or Pt) than [Ni(mnt)2]− due to the
ionic radii of Pd and Pt being much larger than Ni.
7 (a) X. M. Ren, T. Akutagawa, S. Nishihara and T. Nakamura, Synth.
Met., 2005, 150, 57; (b) J. L. Xie, X. M. Ren, Y. Song, Y. Zou and Q. J.
Meng, J. Chem. Soc., Dalton Trans., 2002, 2868.
The sharp thermal anomaly with k-shape seen for 4 associated
with the spin transition indicated that the transition was of first
order and the superlattice diffraction provided evidence for a
pretransitional phenomena up to at least 140 K. The theoretical
study suggested the magnetic exchange nature in a [Ni(mnt)2]−
complex depends highly on parameters relating to the stacking
pattern, such as the distance between intermolecular planes and
the rotation angle between the long molecular axes. Therefore, the
anisotropic contraction of the crystal 4 upon cooling led to the
non-uniform compression and slippage of the [Ni(mnt)2]− stack,
which could be responsible for the variation of the nature of the
magnetic exchange and triggering the spin transition.
8 (a) X. M. Ren, T. Akutagawa, S. Nishihara, T. Nakamura, W. Fujita
and K. Awaga, J. Phys. Chem. B, 2005, 109, 16610; (b) C. L. Ni, D. B.
Dang, Y. Z. Li, S. Gao, Z. P. Ni, Z. F. Tian and Q. J. Meng, J. Solid
State Chem., 2005, 178, 100; (c) C. L. Ni, Y. Z. Li, D. B. Dang, Z. P. Ni,
Z. F. Tian, Z. R. Yuan and Q. J. Meng, Inorg. Chem. Commun., 2005,
8, 105; (d) D. B. Dang, C. L. Ni, Y. Bai, Z. F. Tian, Z. P. Ni, L. L. Wen,
Q. J. Meng and S. Gao, Chem. Lett., 2005, 34, 680; (e) X. M. Ren, H.
Okudera, R. K. Kremer, Y. Song, C. He, Q. J. Meng and P. H. Wu,
Inorg. Chem., 2004, 43, 2569; (f) C. L. Ni, D. B. Dang, Y. Song, S. Gao,
Y. Z. Li, Z. P. Ni, Z. F. Tian, L. L. Wen and Q. J. Meng, Chem. Phys.
Lett., 2004, 396, 353; (g) C. L. Ni, D. B. Dang, Y. Z. Li, Z. R. Yuan,
Z. P. Ni, Z. F. Tian and Q. J. Meng, Inorg. Chem. Commun., 2004, 7,
1034; (h) J. L. Xie, X. M. Ren, C. He, Z. M. Gao, Y. Song, Q. J. Meng
and R. K. Kremer, Chem. Phys. Lett., 2003, 369, 41; (i) X. M. Ren,
Q. J. Meng, Y. Song, C. S. Lu, C. J. Hu and X. Y. Chen, Inorg. Chem.,
2002, 41, 5686; (j) X. M. Ren, Q. J. Meng, Y. Song, C. J. Hu, C. S. Lu,
X. Y. Chen and Z. L. Xue, Inorg. Chem., 2002, 41, 5931; (k) J. L. Xie,
X. M. Ren, Y. Song, W. W. Zhang, W. L. Liu, C. He and Q. J. Meng,
Chem. Commun., 2002, 2346.
Acknowledgements
This work was partly supported by a Grant-in-Aid for Science
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. Authors thank Prof. I. Nomura and Dr
Ichimura for use of the SQUID magnetometer. Ren (JSPS fellow
ID No. P03271) thanks the Japan Society for the Promotion of
Science for financial support.
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1994 | Dalton Trans., 2006, 1988–1994
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The Royal Society of Chemistry 2006
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