Observation of an unusual hysteretic magnetic transition in a heteroleptic nickel-bis-1,2-dithiolene compound

Article abstract of DOI:10.1016/j.inoche.2011.09.023

A heteroleptic nickel-bis-1,2-dithiolene ion-pair complex, [BzQl][Ni(dmit)(mnt)] (where BzQl⁺ = 1-(benzyl)quinolinium; dmit ^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate, mnt^2- = maleonitriledithiolate), was synthesized and characterized structurally, which exhibited novel magnetic bistability. The compound crystallized in triclinic system with space group P-1. The anions and cations form alternating layered alignments, and the anionic layer is built by the irregularly heteroleptic [Ni(dmit)(mnt)]^- chains, where the neighboring anions are connected via lateral-to-lateral S...S contacts of dmit^2- ligands. The temperature dependences of magnetic susceptibility follow the S = 1/2 Heisenberg alternating linear-chain model in high-temperature phase and Curie-Weiss law in low-temperature phase.

Full text of DOI:10.1016/j.inoche.2011.09.023

Inorganic Chemistry Communications 14 (2011) 1971–1974
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Observation of an unusual hysteretic magnetic transition in a heteroleptic
nickel-bis-1,2-dithiolene compound
a
a,b,
Bin Cai a, Jian-Lan Liu a, , Xiao-Li Sheng , Xiao-Ming Ren
⁎
⁎
a
State Key Laboratory of Materials-Oriented Chemical Engineering and College of Science, Nanjing University of Technology, Nanjing 210009, PR China
State Key Lab & Coordination Chemistry Institute, Nanjing University, Nanjing 210093, PR China
b
a r t i c l e i n f o
a b s t r a c t
A heteroleptic nickel-bis-1,2-dithiolene ion–pair complex, [BzQl][Ni(dmit)(mnt)] (where BzQl+ =1-(ben-
zyl)quinolinium; dmit2− =2-thioxo-1,3-dithiole-4,5-dithiolate, mnt2− =maleonitriledithiolate), was syn-
thesized and characterized structurally, which exhibited novel magnetic bistability. The compound
crystallized in triclinic system with space group P-1. The anions and cations form alternating layered align-
ments, and the anionic layer is built by the irregularly heteroleptic [Ni(dmit)(mnt)]− chains, where the
neighboring anions are connected via lateral-to-lateral S…S contacts of dmit2− ligands. The temperature de-
pendences of magnetic susceptibility follow the S=½ Heisenberg alternating linear-chain model in high-
temperature phase and Curie–Weiss law in low-temperature phase.
Article history:
Received 31 July 2011
Accepted 15 September 2011
Available online 23 September 2011
Keywords:
Heteroleptic metal–dithiolene complex
Alternating linear spin chain
Magnetic bistability
© 2011 Elsevier B.V. All rights reserved.
Molecular materials with magnetic bistability have aroused a
great deal of attention owing to their potential applications in magne-
to-thermal switching, information storage, and other molecular elec-
tronic devices [1–3]. Up to date, various molecules with magnetic
bistability, in which the most well-known example is transition
metal spin–crossover (SC) complexes [4–9], including charge transfer
(CT) complexes [10,11], valence ordering compounds [12,13], and
pure organic radical compounds [14–16], have been developed.
As conducting and magnetic materials as well nonlinear optical
materials, metal-bis-1, 2-dithiolene ion–pair complexes have been
actively studied for a long time [17–21]. Experimental and theoretical
investigations disclosed that the unusual physical properties in such
ion–pair complexes result from the intermolecular interactions of
metal-bis-1,2-dithiolate anions, which are strongly affected by the
structural features of both dithiolene ligand and countercation [22–
28]. It is favorable for metal-bis-1,2-dithiolene anions to form the
cofacial π…π stacking between the anions, owing to the planar struc-
ture and delocalized negative charge. Especially, for the sulfur-rich
dithiolene, such as 2-thioxo-1,3-dithiole-4,5-dithiolate (dmit2−),
the lateral-to-lateral and head to tail S…S contacts as well cofacial
stacking between metal-bis-1,2-dithiolene anions generally align
the anions in the form of ladders or layers.
spin-Peierls-type ion–pair complexes [BzPy][Ni(mnt)2] were synthe-
sized, in which the spin-Peierls-type magnetic transition tempera-
ture can be finely tuned via systematically modifying the nature of
substituents in the benzylpyridinium derivative [29]. Furthermore,
replacement of the anion mnt2− in [BzPy][Ni(mnt)2] with sulfur-
rich dmit2− produced one-dimensional (1-D) laddered spin-Peierls
compounds [27] and two-dimensional (2-D) bi-layered magnetic
bistability compounds [28].
To investigate the effect of the structural feature of dithiolene li-
gand on the stacking structure of metal-bis-1,2-dithiolene anions
and magnetic property of the corresponding ion–pair complex, we
are focusing on the study of correlation of the structure and magnetic
behavior for the heteroleptic metal-bis-1,2-dithiolene spin systems
of [BzPy][Ni(dmit)(mnt)]. Herein we report the crystal structure
and magnetic property of a heteroleptic nickel-bis-1,2-dithiolene
complex, [BzQl][Ni(dmit)(mnt)] (1), which exhibits an unusual
thermal hysteresis loop in the temperature dependent magnetic
susceptibility.
Compound 1-benzylquinolinium bromide ([BzQl]Br) was pre-
pared using a similar process for preparation of [BzPy]Br [30] and
(n-Bu4N)[Ni(dmit)(mnt)] were synthesized following the published
procedure [31]. Complex 1 was prepared by means of a metathetical
reaction [32].
Heteroleptic nickel-bis-1,2-dithiolene complex 1 crystallizes in
the triclinic space group P-1 [33]. As shown in Fig. 1a, its asymmetric
unit contains one [Ni(dmit)(mnt)]− anion and one BzQl+ cation. The
[Ni(dmit)(mnt)]− anion adopts a planar geometry with a dihedral
angle of 6.4° between the mean molecular planes of dmit2− and
mnt2− ligands. The Ni−S bond distances range from 2.152 to
2.174 Å, and the S−Ni−S bite angles are 92.74(5)° and 92.94(5)°,
In our previous studies, the flexible Λ-shaped benzylpyridinium
derivatives (BzPy+) were chosen as the countercation of S=½
[Ni(mnt)2]− anion (mnt2− =maleonitriledithiolate), and a series of
⁎
Corresponding author at: State Key Laboratory of Materials-Oriented Chemical
Engineering and College of Science, Nanjing University of Technology, Nanjing
210009, PR China. Tel./fax: +86 25 83587820.
E-mail address: xmren@njut.edu.cn (X.-M. Ren).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.09.023

1972
B. Cai et al. / Inorganic Chemistry Communications 14 (2011) 1971–1974
The purity of the polycrystalline sample was checked by powder
X-ray diffraction technique besides elemental analysis, the simulated
and experimental diffraction patterns are displayed in Fig. 2 and indi-
cated that the sample of 1 has high phase purity.
The plots of magnetic susceptibility versus temperature measured
in the temperature range of 2–300 K under an applied field of
1000 Oe is shown in Fig. 3, where χm represents the mole magnetic
susceptibility with one [Ni(dmit)(mnt)]− ion per molecular formula
and the diamagnetism correction for 1 was not made. The magnetic
behavior of 1 features in: (1) the temperature dependent magnetic
susceptibility shows typical characteristics of low-dimensional anti-
ferromagnetic spin system in temperature range of 40–300 K. It is
worthy of note that two maximums of magnetic susceptibility occur
around 85 and 65 K whereas the minimum of magnetic susceptibility
between two maximums is localized at ~70 K upon cooling; the mag-
netic susceptibility plots upon cooling and heating do not coincide
with each other in the temperature 60–140 K, which gives rise to an
unusual shaped thermal hysteresis loop (the inset of Fig. 3a) and in-
dicates the occurrence of a magnetic transition below 70 K. (2) The
Curie–Weiss type magnetic susceptibility nature appears in the low
temperature below 40 K. Even if the homoleptic nickel-bis-1,2-dithiolene
[Ni(mnt)2]− and [Ni(dmit)2]− ion–pair complexes have been compre-
hensively studied so far, the heteroleptic [Ni(dmit)(mnt)]− complexes
have very rarely been investigated in the field of magnetochemistry. To
the best of our knowledge, this is the first report of [Ni(dmit)(mnt)]−
complex with magnetic bistability.
In the nickel-bis-1,2-dithiolene crystals, the magnetic exchange
interaction may be transmitted through long nonbonded contacts
involving sulfur atoms. As noted in the crystal description, the
magnetic [Ni(dmit)(mnt)]− anions form inequivalent distanced
1-D chains via short lateral-to-lateral S…S contacts between the
neighboring dmit2− ligands, therefore, it is reasonable that an
alternating-exchange S=½ Heisenberg linear-chain model was cho-
sen for the fits of the magnetic susceptibility in high-temperature
phase. The spin Hamiltonian for the Heisenberg alternating linear
chain may be written as
Fig. 1. (a) ORTEP drawing with 30% thermal ellipsoids probability and (b) packing
diagram showing the alternating layered structures of anions and cations as well
as the anionic layer built from the anionic chains for 1.
ꢀ
ꢁ
n=2
^
^
^
^ ^
H ¼ −2J
S 2j−1 S 2j þ αS 2j S 2jþ1
ð1Þ
∑
i¼1
both of which are comparable to those in reported (n-Bu4N)[Ni(dmit)
(mnt)] [31]. The BzQl+ cation exhibits a non-planar conformation
with normal bond lengths and bond angles. The characteristic
dihedral angles are 46.9° and 67.7° between the referred plane N3–
C17–C18 and phenyl ring as well as quinolyl ring; 83.9° between
the quinolyl and the phenyl fragments. Two mean molecule planes
of dithiolene ligands in the anion moiety are approximately parallel
to quinolyl ring in the cation moiety with a dihedral angle of 7.9° be-
tween dmit2− ligand and quinolyl ring versus 2.6° between mnt2−
ligand and quinolyl ring.
where J is the exchange constant between a spin and its left neighbor
and αJ is the exchange constant between a spin and its right neighbor.
For an antiferromagnetic exchange system (Jb0 and 0≤α≤1), ex-
tremely, when α=0 the alternating linear-chain model is simplified
to the dimer model with pairwise interactions and when α=1 the
alternating linear-chain model to the regular linear-chain model [34].
As displayed in Fig. 1b, the anions and cations form an alternating
anion and cation layered arrangements and the molecular layers are
parallel to the crystallographic (101) plane. It is noted that the
magnetic anion layer is consisted of the anionic anti-parallel chains,
which are connected via the lateral-to-lateral S…S contacts of dmit2−
ligands between neighboring anions with dS3…S3#1=3.666 Å and
dS4…S4#2=3.429 Å (symmetric codes #1=1−x, −y, 2−z and
#2=1−x, −y, 1−z) anions along the crystallographic c-axis direc-
tion. This situation is similar to the homoleptic [Ni(dmit)2]− complexes
[27,28]. On the other hand, it is noticeable that there exist nonclassical
C–H…S and C–H…N hydrogen-bonding interactions between the
anions and cations, for instance, the shorter S…H and N…H distances,
d
d
C17–H17A…S7#3=2.936, dC22–H22…S7#4=2.870, dC20–H20…S1#5=2.996,
C17–H17B…N1#6=2.467, dC8–H8…N1#6=2.736, dC9–H9…N2#6=2.704
and dC10–H10…N2#7=2.598 Å, and the symmetric codes #3=−x, 1−
y, 1−z; #4=x, y, z; #5=x, 1+y, z; #6=1+x, y, −1+z and #7=
−x, 2−y, 1−z, are observed in the crystal of 1, respectively.
Fig. 2. Powder X-ray diffraction patterns of 1 (upper line: simulation and bottom: exper-
imental patterns).

B. Cai et al. / Inorganic Chemistry Communications 14 (2011) 1971–1974
1973
low-temperature region (see next section). Since 1 exhibits the his-
torical dependence of the magnetic susceptibility around magnetic
transition temperature, the magnetic susceptibility data in the range
of 70–300 K (in the elevating temperature mode) were also fitted
using Eqs. (2) and (3) with the same values for the parameters C, P,
α, θ and χ0 obtained in the cooling model, to give J/kB =70.9(1)K
with the g-factor value of 2.087(2) that are quite similar to those
gained from the fits in the cooling model.
In the low-temperature region below 40 K, the temperature de-
pendence of magnetic susceptibility of 1 was fitted to Eq. (4),
C
T−θ
χm
¼
þ χ0
ð4Þ
where the symbols of C, θ and χ0 have normal meanings. The best fits
for the magnetic susceptibility of 1 in the temperature range of 2–
35 K gave rise to C=3.78(3)×10−2 emu Kmol−1, θ=−1.20(3)
K and χ0 =3.5(2)×10−4 emu mol−1, respectively.
In summary, a heteroleptic nickel-bis-1,2-dithiolene ion–pair complex
[BzQl][Ni(dmit)(mnt)] was synthesized and characterized structurally,
which exhibited novel magnetic bistability. In comparison with the
homoleptic [Ni(dmit)2]− and [Ni(mnt)2]− compounds, the stacking pat-
tern of the heteroleptic [Ni(dmit)(mnt)]− ions is similar to the homolep-
tic [Ni(dmit)2]−, namely, the neighboring anions are connected via
lateral-to-lateral S…S contacts of dmit2− ligands. This work shed a light
on the design and preparation of new magnetic bistability materials.
The further and systematical exploration in this context is in progress.
Acknowledgements
Fig. 3. (a) Temperature dependent magnetic susceptibility and inset showing the thermal
hysteresis loop (b) fits of magnetic susceptibility in low- and high-temperature regimes
(solid squares: experimental data and lines: fits).
Authors thank the Science and Technology Department of Jiangsu
Province and the National Nature Science Foundation of China for fi-
nancial support (grant nos. BK2010551, 20871068 and 21071080);
Ren thanks Prof. C. J. Fang for reading this manuscript.
Appendix A. Supplementary data
Based on the above-mentioned spin Hamiltonian, the molar magnetic
susceptibility as a function of temperature for an alternating-exchange
S=½ Heisenberg linear chain which is deduced from the cluster
approach [34] can be expressed as
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.09.023.
Ng2μB2
kBT
A þ Bx þ cx2
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