Peculiar magnetic behavior in ion-pair complex [1-(4′-(fluorobenzyl)pyridinium][Ni(mnt)2] (mnt2- = maleonitriledithiolate)

Article abstract of DOI:10.1039/b205441h

An ion-pair complex [FBzPy][Ni(mnt)₂], where [FBzPy])⁺ = 1-(4′-fluorobenzyl)pyridinium and mnt^2- = maleonitriledithiolate, forms a discrete stacking column and shows a peculiar magnetic transition from paramagnetic to diam

Full text of DOI:10.1039/b205441h

Peculiar magnetic behavior in ion-pair complex
[1-(4A-fluorobenzyl)pyridinium][Ni(mnt)2] (mnt22
maleonitriledithiolate)†
=
Jingli Xie,a Xiaoming Ren,ab You Song,a Wenwei Zhang,a Wenlong Liu,a Cheng Hec and Qingjin
Meng*a
a State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute, Nanjing University,
210093, Nanjing, P.R. China. E-mail: njujlxie@yahoo.com
b Department of Electronic Science & Engineering, Nanjing University, 210093, Nanjing, P.R. China
c
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, 100871,
Beijing, P.R. China
Received (in Cambridge, UK) 6th June 2002, Accepted 28th August 2002
First published as an Advance Article on the web 17th September 2002
An ion-pair complex [FBzPy][Ni(mnt)2], where [FBzPy]+
=
column is considered as a one-dimensional (1-D) magnetic
chain from the crystal structure viewpoint (Fig. 1b). The
[FBzPy]+ cations orient themselves in the solid state. The
phenyl rings of the neighboring phenyls are parallel to each
other with identical center–center distances of 3.912 Å (Fig.
S2†).
1-(4A-fluorobenzyl)pyridinium and mnt22 = maleonitriledi-
thiolate, forms a discrete stacking column and shows a
peculiar magnetic transition from paramagnetic to diamag-
netic around 90 K.
Square-planar M(dithiolene)2 complexes have attracted ex-
tensive interest in the areas of conducting and magnetic
materials, dyes, non-linear optics, and catalysis.1,2 In this series
of complexes, previous works have shown that different
counterions could induce versatile stacking modes in the solid
state and, furthermore, have a great influence on molecular
interactions and the resultant magnetic diversity.3 Recently, we
The temperature dependence of cmT, measured under a field
of 1 T, is shown in Fig. 2. The observed cmT value at 350 K is
0.308 emu K mol21, slightly less than the value of 0.375
expected for one spin-only Ni(III) ion with S = 1/2, g = 2.0.
Upon cooling, cmT increases gradually and reaches a maximum
of 0.465 emu K mol21 at 93 K. At 93 K, the cmT value abruptly
drops to nearly zero, and below 90 K this complex is
fabricated and structurally characterized
a
series of
[BzPy]+[M(mnt)2]2 (M = Ni or Pt) complexes, where [BzPy]+
denotes a benzylpyridinium derivative, and found that there are
markedly different magnetic exchange properties although
these complexes have extremely similar molecular and stack
structures.4 Herein we report the synthesis and crystal structure
of an ion-pair complex (Scheme 1), [1-(4A-fluorobenzyl)pyr-
idinium][Ni(mnt)2], with a discrete stacking column. Its
magnetic properties show a peculiar magnetic transition from
paramagnetic to diamagnetic around 90 K.
[FBzPy]2[Ni(mnt)2] (1) was prepared by the direct combina-
tion of 1+2+2 mol equiv. of NiCl2·6H2O, Na2mnt and 1-(4A-
fluorobenzyl)pyridinium chloride in H2O. The title complex (2)
was prepared by I2 oxidation of 1.5 Fig. S1† depicts the
molecular structure of 2.‡ The coordination geometry of Ni(III
)
is square planar with four sulfur donor atoms, and Ni–S bond
lengths and S–Ni–S bond angles are in agreement with those
results reported.4 In the [FBzPy]+ cation, the dihedral angles
between the C(15)–C(14)–N(5) reference plane and aryl rings,
103.4° for the benzene ring, 97.9° for the pyridine ring, are
similar to previous results.4 It should be noted that 2 possesses
well-separated columns stacked along the c-axis direction (Fig.
1a). Within an anion column, there exists a slipped nickel-over-
sulfur configuration of the [Ni(mnt)2]2 anions and the Ni…Ni
distances between adjacent anions are identical (3.964 Å), and
the nearest S…S, S…Ni distances are 3.84 Å and 3.604 Å,
respectively. The closest Ni…Ni separation between anion
chains is 14.745 Å, and is significantly longer than the Ni…Ni
separation within a chain. Therefore, the [Ni(mnt)2]2 anion
Scheme 1 Structure of 2.
† Electronic supplementary information (ESI) available: experimental
details; crystal data, atomic coordinates, ORTEP view, side view of cation
chain, cM vs. T plot for 2. See http://www.rsc.org/suppdata/cc/b2/
b205441h/
Fig. 1 (a) Structure of the anions and cations of 2 viewed along the c-axis.
(b) Side view of the 1-D anion chain for 2 (symmetry codes: A = x, 0.5 2
y, 20.5 + z; B = C = x, y, z 2 1).
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This journal is © The Royal Society of Chemistry 2002

high- and low-temperature phase X-ray diffraction pattern, so
the magnetic transition may accompany the structural phase
transition, and the phenomenon is similar to cases where some
heterocyclic thiazyl radicals undergo an abrupt magnetic/
structural phase transition with thermal hysteresis, which are
related to distortions of the p-stack arrangement.10,11 Previous
studies have shown that the magnetic coupling between
[Ni(mnt)2]2 anions is very sensitive to not only the inter-
molecular separation,2a but also the manner of overlap between
neighboring [Ni(mnt)2]2 anions.7,12 Several shorter contact
distances, such as Ni…Ni, Ni…S, S…S, S…N, S…C and
C…N, play important roles in the superexchange pathway due
to extensive electron delocalization in the [Ni(mnt)2]2 unit.13
Therefore, changes of the contact distance between neighboring
anions, which can arise from variation of the external pressure
or temperature, may lead to the sign of the magnetic coupling
constant (J) changing.
Fig. 2 Temperature dependence of cmT for 2 (inset: d(cmT)/dT versus T).
The solid line represents the best fit in the high temperature range according
to the Baker equation.
Financial support from the National Natural Science Founda-
tion (No. 29771017, No. 29831010), the State Education
Commission of China and the State Key Laboratory of
Structural Chemistry are gratefully acknowledged.
diamagnetic. When the temperature is increased from 2 K to 350
K, an identical curve was observed without hysteresis. Ob-
viously, a phase transition takes place around 93 K. The
transition temperature, 90 K, may be estimated from the d(cmT)/
dT vs. T plot (inset of Fig. 2). In the temperature range 93–350
K, an estimation was made by fitting cm data to the Baker
equation6 (applicable for a chain of s = 1/2 spin) derived from
a high-temperature series expansion. A fit of the data to the
equation6c gives g = 2.006 (fixed), J/kB = 14.61 K, TIP =
23.5 3 1024 emu with a final agreement factor R = 2.3 3 1024
[R = S(cmTobs 2 cmTcalc)2/S(cmTobs)2]. The fitting results in
the high temperature range are in agreement with those of
similar Ni(III) complexes.2c,7
It is possible that there exist spin-Peierls-like dimeric lattic
distortions8–10 in the low-temperature phase for 2. To measure
the crystal structural data in the low-temperature phase are
closely linked to explore the phase transition feature, however,
this attempt up to now has been unsuccessful. As an accessorial
measure, the variable-temperature X-ray diffraction (XRD)
experiments of 2 were carried out and are displayed in Fig. 3,
and the results show that there exist differences between the
Notes and references
‡ Crystal data: C20H11FN5NiS4, M = 527.29, monoclinic, space group P21/
c, a = 12.1500(4), b = 25.9523(6), c = 7.3397(3) Å, b = 101.74°, V =
2265.95(13) Å3, Z = 4, Dc = 1.546 g cm23, m(Mo-Ka) = 1.251 mm21, l
= 0.71073 Å. A crystal of dimensions 0.15 3 0.10 3 0.10 mm was selected
for indexing and intensity data collection at 298 K. w-Scans covering
reciprocal space up to qmax 25.05° with 99.8% completeness, total of 7854
reflections (4013 unique) with Rint = 0.0436. Structure solution SHELX-
97, full matrix least-squares based on F2 using SHELXL-97, final R =
0.062, wR
= 0.154. CCDC reference number 182194. See http://
www.rsc.org/suppdata/cc/b2/b205441h/ for crystallographic data in CIF or
other electronic format.
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C = 1.0 + 5.7979916y + 16.902653y2 + 29.376885y3 + 29.832959y4
+ 14.036918y5
D
=
1.0 + 2.7979916y + + +
7.0086780y2 8.653644y3
4.5743114y4
y = J/2kT.
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