Assembled structures and magnetic properties of viologen-[M(mnt) 2] charge-transfer salts (mnt = maleonitriledithiolate; M = Cu, Ni, Pt)

Article abstract of DOI:10.1016/j.ica.2011.11.043

Ion-pair compounds of phenyl-substituted viologen dications with [M(mnt)₂] dianions were prepared and characterized. (Benzyl viologen)[M(mnt)₂] (M = Cu (1), Ni (2), Pt (3)) and (phenyl viologen)[Cu(mnt)₂] (4) exhibited mixed-stack crystal structures, whereas (dinitrophenyl viologen)[Cu(mnt)₂] (5) exhibited no π-π interactions. Magnetic susceptibility measurements revealed that 1 exhibits a ferromagnetic exchange interaction (Weiss constant θ = +0.9 K), possibly mediated by the diamagnetic cation. Antiferromagnetic interactions were observed for 4 (θ = -2.0 K) and 5 (θ = -2.9 K), whereas 2 and 3 were diamagnetic. Absorption bands assignable to charge transfer were observed in the mixed-stack salts in the visible to near-infrared region.

Full text of DOI:10.1016/j.ica.2011.11.043

Inorganica Chimica Acta 384 (2012) 111–116
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Assembled structures and magnetic properties of viologen–[M(mnt)2]
charge-transfer salts (mnt = maleonitriledithiolate; M = Cu, Ni, Pt)
a
b
b
Tomoyuki Mochida a,b, , Yusuke Funasako , Takanori Kishida , Chihiro Kachi-Terajima
⇑
a Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan
b Department of Chemistry, Faculty of Science, Toho University, Miyama, Funabashi, Chiba 274-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 March 2011
Received in revised form 11 November 2011
Accepted 14 November 2011
Available online 23 November 2011
Ion-pair compounds of phenyl-substituted viologen dications with [M(mnt)2] dianions were prepared
and characterized. (Benzyl viologen)[M(mnt)2] (M = Cu (1), Ni (2), Pt (3)) and (phenyl violo-
gen)[Cu(mnt)2] (4) exhibited mixed-stack crystal structures, whereas (dinitrophenyl viologen)[Cu(mnt)2]
(5) exhibited no p–p interactions. Magnetic susceptibility measurements revealed that 1 exhibits a fer-
romagnetic exchange interaction (Weiss constant h = +0.9 K), possibly mediated by the diamagnetic cat-
ion. Antiferromagnetic interactions were observed for 4 (h = ꢀ2.0 K) and 5 (h = ꢀ2.9 K), whereas 2 and 3
were diamagnetic. Absorption bands assignable to charge transfer were observed in the mixed-stack salts
in the visible to near-infrared region.
Keywords:
Magnetic properties
Charge transfer
Viologen
Ó 2011 Elsevier B.V. All rights reserved.
Metal dithiolenes
Copper
1. Introduction
interactions [10–12]. Among the anions, [Cu(mnt)2]2ꢀ is a para-
magnetic dianion that exhibits diverse coordination geometries
To date, a great deal of research has been carried out on charge-
transfer (CT) complexes consisting of electron donors (D) and elec-
tron acceptors (A). Organic CT complexes with mixed-stack crystal
structures usually become either neutral (D0A0) or ionic (D+Aꢀ),
depending on the balance between the redox potentials of the
components and the Madelung energy in the ionic state [1,2]. Sim-
ilarly, molecules that afford stable monocations and dications, such
as biferrocenes and viologens, produce monovalent ionic salts
(D+Anꢀ) or divalent ionic salts (D2+An2ꢀ). Indeed, biferrocenium
salts have been shown to exhibit transformation between the
monovalent and divalent states by external stimuli [3]. Concerning
viologens [4,5], many CT salts reported to date are predominantly
divalent ion-pair compounds (D2+An2ꢀ) [5–8], although a few
monovalent salts have also been reported [6b,9].
around the copper atom, although there are relatively few exam-
ples of its complexes [13–15].
Herein we report the preparation, crystal structures, and mag-
netic properties of (benzyl viologen)[M(mnt)2] (M = Cu (1), Ni (2),
Pt (3)), (phenyl viologen)[Cu(mnt)2] (4), and (dinitrophenyl violo-
gen)[Cu(mnt)2] (5) (Fig. 1). Complexes 1–4 exhibited mixed-stack
crystal structures, whereas 5 showed no
p–p stacking structure.
Complexes 1, 4, and 5 were paramagnets. In particular, 1 exhibited
a small ferromagnetic interaction, despite its mixed-stack struc-
ture with a diamagnetic cation.
2. Results and discussion
2.1. Syntheses
In this context, given our interest in the magnetism and valence
states of viologen complexes, here we have prepared a series of CT
salts using phenyl-substituted viologens and [M(mnt)2] anions
(mnt = maleonitriledithiolate). Although many alkyl viologen com-
plexes with metal dithiolates have been synthesized to date [6],
their magnetic properties have not been thoroughly investigated.
CT salts with [M(mnt)2]nꢀ are interesting from the viewpoint of
magnetism, as they exhibit magnetic transitions and ferromagnetic
Single crystals of 1–5 were obtained by slow diffusion of solu-
tions of the viologen dications in a water–ethanol mixture and of
[M(mnt)2] dianions in acetone. These complexes were divalent sol-
ids with 1:1 stoichiometry (D2+A2ꢀ). In the infrared (IR) spectra, the
CN stretching vibrations of the anions were observed at around
2180–2200 cmꢀ1, indicating that the acceptors are dianions [16].
A characteristic signal of Cu(II) [17] was consistently observed in
the electron spin resonance (ESR) spectrum of 5. The intramolecu-
lar bond lengths of the viologens and [M(mnt)2] determined by
crystallographic analyses (vide infra) are typical of those of violo-
gen dications [6] and [M(mnt)2] dianions [18], respectively.
⇑
Corresponding author at: Department of Chemistry, Graduate School of Science,
Kobe University, Rokkodai, Nada, Hyogo 657-8501, Japan. Tel./fax: +81 78 803 5679.
E-mail address: tmochida@platinum.kobe-u.ac.jp (T. Mochida).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.11.043

112
T. Mochida et al. / Inorganica Chimica Acta 384 (2012) 111–116
Fig. 1. Chemical formulas of 1–5.
2.2. Structures of benzyl and phenyl viologen salts 1–4
Complexes 1–3 were isomorphous, although 1 was slightly lar-
ger in cell volume than the other two [V = 745.7 Å3 (1; M = Cu),
735.7 Å3 (2; M = Ni), and 736.5 Å3 (3; M = Pt) at 173 K; Table 1].
The packing diagram of 1 is shown in Fig. 2a. Both the viologens
and [M(mnt)2] are located on inversion centers, and half the mol-
ecules are crystallographically independent.
Fig. 2. (a) Packing diagram of 1 projected along the a-axis. Hydrogen atoms are
omitted for clarity. (b) Molecular arrangements viewed perpendicular to the
molecular planes. Dashed lines represent cation–anion contacts.
Complexes
1–3
exhibit
a
mixed-stack
structure
(ꢁ ꢁ ꢁD2+A2ꢀD2+
A
2ꢀꢁ ꢁ ꢁ) along the b-axis (Fig. 2a). The bipyridinium
moieties in the viologens are coplanar, and [M(mnt)2]2ꢀ dianions
exhibit planar geometries. This contrasts with [H2bpy][Cu(mnt)2],
in which non-planar cations and distorted anions are stacked alter-
nately [15]. As seen in Fig. 2b, the cations and anions exhibit side-
by-side co-planar arrangement along the [110] direction, forming
contacts between the pyridyl hydrogens and heteroatoms in
[M(mnt)2]. A similar cation–anion arrangement is commonly seen
in the structure of 4 (vide infra) as well as in (methyl violo-
gen)[M(mnt)2] (M = Ni, Pd, Pt [6a,d]). The benzyl groups are located
for molecules on the (001) plane. Within the stacking columns,
the molecular long axes of the viologen and [Cu(mnt)2] are aligned
in parallel, although their molecular planes are tilted with respect
to each other by 7°. In addition, the molecular planes of the cations
and anions are coplanar, arranged side-by-side as in 1–3.
The stacking structures of 1 and 4 projected nearly along the
molecular long axes are shown in Fig. 4a and b, respectively. In
both structures, the intermolecular Sꢁ ꢁ ꢁS distances (>4.41 Å) are
much longer than the vdW distance of 3.8 Å, and there are no
direct contacts between the anions. Significant intermolecular
interactions exist only between the cation and anion through
between the columns, having mutual p–p contacts. The centroid–
centroid distance between the phenyl rings are 3.912 Å (1), 3.921 Å
(2), and 3.981 Å (3).
The packing diagram of 4 is shown in Fig. 3. Both viologen and
[M(mnt)2] are located on inversion centers. The bipyridinium moi-
ety in the cation is planar, and the terminal phenyl groups are
tilted with respect to the bipyridinium moiety by 33°. This salt
exhibits a mixed-stack structure, in which the stacking extends
along [110] for molecules on the (002) plane, and along [1–10]
p–p stacking and side-by-side contacts. Hence, direct magnetic
exchange between the anions should be negligible.
2.3. Structure of a dinitrophenyl viologen salt 5
The structure of 5 is much more complicated than those of
1–4. In addition, it is exceptional in that it lacks a mixed-stack
Table 1
Crystallographic parameters.
1
2
3
4
5
Formula
C
32H22N6CuS4
C30H22N6NiS4
677.51
C32H22N6PtS4
813.92
C30H18N6CuS4
654.28
0.61 ꢂ 0.14 ꢂ 0.02
monoclinic
13.391(4)
C30H14N10CuO8S4
834.48
Formula weight
Crystal size (mm)
Crystal system
a (Å)
b (Å)
c (Å)
682.34
0.38 ꢂ 0.15 ꢂ 0.03
0.15 ꢂ 0.08 ꢂ 0.05
0.30 ꢂ 0.25 ꢂ 0.03
0.5 ꢂ 0.13 ꢂ 0.03
triclinic
triclinic
triclinic
monoclinic
6.6279(6)
8.9564(8)
13.5994(12)
95.555(2)
90.727(2)
111.6750(10)
745.66(12)
P1
6.5384(11)
8.9767(19)
13.536(2)
83.376(5)
88.017(3)
68.794(3)
735.7(2)
6.4844(6)
9.0975(9)
13.4236(13)
94.424(2)
93.233(2)
9.3198(7)
22.5924(16)
23.7992(17)
7.542(2)
27.630(7)
a
(°)
b (°)
90.634(4)
90.6920(10)
c
(°)
110.517(2)
736.48(12)
V (Å3)
2790.4(13)
C2/c
5010.7(6)
P21/c
ꢀ
ꢀ
ꢀ
Space group
P1
P1
Z value
1
1
1
4
6
Dcalc
1.52
1.529
1.835
1.557
1.643
F(000)
349
348
398
1332
2105
Number of reflections
Number of observations
Parameters
4753
3412
197
4687
3328
196
5012
3591
196
7464
3101
187
31439
11464
718
T (K)
173
173
173
173
173
b
R1a, wR2 (I > 2
r(I))
0.0470, 0.1266
0.0695, 0.1473
1.026
0.0767, 0.1283
0.1359, 0.1464
1.046
0.0353, 0.0747
0.0363, 0.0752
0.975
0.0356, 0.0835
0.0467, 0.0893
1.028
0.0476, 0.0832
0.1211, 0.1043
0.953
R1a, wR2 (all data)
b
Goodness-of-fit (GOF) on F2
a
R1
wR2 = [
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
b
2
R
w(Fo ꢀ Fc2)2/
R .
w(Fo2)2]1/2

T. Mochida et al. / Inorganica Chimica Acta 384 (2012) 111–116
113
Fig. 3. Packing diagram of 4 viewed along the a-axis. Hydrogen atoms are omitted for clarity.
Fig. 4. Molecular arrangements in (a) 1 and (b) 4, viewed nearly along the molecular long axes. The p–p stacking direction is shown by an arrow. Hydrogen atoms are omitted
for clarity.
structure. The packing diagram is shown in Fig. 5. There are two
pairs of crystallographically independent molecules in the crystal,
denoted as D1, D2, A1, and A2. D1 and A1 are located on inver-
sion centers, whereas D2 and A2 have no center of symmetry.
Accordingly, the bipyridinium moiety is planar in D1, whereas
the moiety is twisted by 32° in D2. The dinitrophenyl groups
are twisted with respect to the pyridinium rings by 64° in D1
and by 80° and 83° in D2.
2.4. Magnetic susceptibilities
Magnetic susceptibility measurements revealed that 1, 4, and 5
were paramagnets, whereas 2 and 3 were diamagnetic. This is con-
sistent with the valence states of the salts; [M(mnt)2]2ꢀ is para-
magnetic for M = Cu and diamagnetic for M = Ni and Pt. The
temperature dependence of the magnetic susceptibilities of the
paramagnetic salts are shown in Fig. 6 in the form of
plots. The T values at room temperature correspond to one spin
per formula unit (
T = 0.375 emu K molꢀ1), which resides on the
[Cu(mnt)2]2ꢀ anion. The temperature dependence of the
T value
obeyed the Curie–Weiss law,
= C/(T ꢀ h). With decreasing tem-
vT versus T
The cations and anions are arranged in a zigzag fashion with their
long axes aligned in parallel (Fig. 5). The molecular planes of the
bipyridinium moiety and the anion are highly twisted with respect
v
v
v
to each other, and there is no direct
p–p
overlap between them.
v
Although the nearest cation–anion pair is D1 and A2, their molecular
planes are twisted relative to one another by 56°. Hence, CT interac-
tions between D1 and A2 are expected to be small. The anions are
surrounded by the dinitrophenyl groups and bipyridinium
hydrogens of the cations. A2 forms a side-by-side centrosymmetric
dimer-like arrangement, which has a long Sꢁ ꢁ ꢁS intermolecular
distance of 4.44 Å.
perature, the values for 1 increased at low temperatures, indicating
the presence of a ferromagnetic interaction (h = +0.89 K). On the
other hand, the values for 4 and 5 decreased, indicating antiferro-
magnetic interactions. The Weiss constants for 4 and 5 were
h = ꢀ2.04 and ꢀ2.92 K, respectively.
Considering the mixed-stack structure with the diamagnetic
cation, the ferromagnetic interaction in 1 is noteworthy despite

114
T. Mochida et al. / Inorganica Chimica Acta 384 (2012) 111–116
Fig. 5. Packing diagram of 5 viewed along the b-axis. Molecules on the ac planes are
omitted. Hydrogen atoms are omitted for clarity.
Fig. 7. Solid-state electronic spectra of 1–5.
in the visible to near-IR region [6,15]. The solid-state absorption
spectra for 1–5 are shown in Fig. 7. Comparison of the spectra for
1–3 suggests that the intense broad bands at around 1.8 eV
(ꢃ700 nm) as a shoulder in 1 and 1.5 eV (ꢃ850 nm) in 2 and 3,
not observed in their precursors, are assignable to the CT transition
from [M(mnt)2]2ꢀ to [benzyl viologen]2+. These are consistent with
the redox potentials (E2ꢀ/1ꢀ) of the anions [17b], 330, 226, and
210 mV (versus SCE), for M = Cu, Ni, and Pt, respectively. The very
weak absorption bands observable at around 1.1 eV (ꢃ1100 nm) in
the copper complexes 1, 4 and 5 may be d–d transitions in
[Cu(mnt)2]2ꢀ [15], although the absorption in 5 is very slight. In
4, a broad peak is observed at around 1.8 eV (ꢃ750 nm), which
likely reflects the intramolecular transitions in phenyl viologen
overlapped by a CT transition. No clear CT band was observed in
5, consistent with the absence of
p–p interactions between the
cation and anion, as crystallographically demonstrated.
Fig. 6. Temperature dependence of magnetic susceptibilities of 1 (s), 4 (}), and 5
3. Conclusion
(+), represented in the form
vmT vs. T.
Ion-pair compounds of phenyl-substituted viologens with
[M(mnt)2] dianions were prepared. Salts with benzyl viologen
and phenyl viologen exhibited mixed-stack crystal structures,
its small magnitude. In general, pyridinium salts with [Cu(mnt)2]2ꢀ
exhibit antiferromagnetic interactions smaller than few
a
K
whereas
p–p interactions were absent in the salt with dinitro-
[14a–c], although there is a salt that exhibits a slight ferromagnetic
interaction (h = +0.38 K) [14d]. In 1, the ferromagnetic exchange
between [Cu(mnt)2]2ꢀ dianions may be communicated through
the cation, giving rise to an intermolecular superexchange. Indeed,
magnetic exchange mediated through non-magnetic counterions is
suggested for [BDTA]2[Cu(mnt)2] (J = ꢀ16 K) [13] and [BDTA]-
phenyl viologen. CT absorption bands were observed for the
mixed-stack salts. Notably, a small but clear ferromagnetic interac-
tion was found in (benzyl viologen)[Cu(mnt)2]. This is an interest-
ing example of a ferromagnetic interaction that is mediated
through diamagnetic molecules, as has previously been suggested
only in a few other salts. Although many salts with viologens have
been reported to date, magnetic investigations have been scarce.
Our results suggest that ferromagnetic interactions may also be
found in other viologen salts.
[Ni(mnt)2] (J = +2.20 K) [19], as well as for
a ferromagnet
[NH4][Ni(mnt)2]ꢁH2O [10a,b,19] (TC = 4.5 K). In 1, however, a CT
interaction is likely to exist between the cation and anion (vide
infra), which might lead to an antiferromagnetic superexchange.
Further studies, mainly theoretical, are needed to elucidate the
interaction mechanism.
4. Experimental
2.5. Solid-state absorption spectra
4.1. Materials and physical measurements
Ion-pair compounds between bipyridinium dications and
metal-bis(dithiolene) dianions often exhibit CT absorption bands
Benzyl viologen dichloride (=1,10-dibenzyl-4,40-bipyridinium
dichloride), phenyl viologen dichloride (=1,10-diphenyl-4,40-

T. Mochida et al. / Inorganica Chimica Acta 384 (2012) 111–116
115
bipyridinium dichloride), dinitrophenyl viologen dichloride (=1,10-
Acknowledgments
bis (2,4-dinitrophenyl)-4,40-bipyridinium dichloride), and (NBu4)2
[Ni(mnt)2] were purchased from TCI. (NBu4)2[M(mnt)2] (M = Cu,
Pt) were prepared according to methods in the literature [20]. IR
spectra were recorded as KBr pellets on a Thermo Nicolet Avatar
360 spectrometer. NIR–UV–Vis absorption spectra of powder sam-
ples were recorded as KBr pellets on a JASCO V-570 UV–Vis/NIR
spectrophotometer in a range of 250–2000 nm. Magnetic suscepti-
bilities were measured under a magnetic field of 1 T in the temper-
ature range of 2–300 K using a Quantum Design MPMS-XL SQUID
magnetometer.
We thank Dr. K. Mitsunaga (Toho University) for elemental
analysis and T. Akasaka (Toho University) for his help with X-ray
crystallography. This work was financially supported by a Grant-
in-Aid for Scientific Research from Japan Society for the Promotion
of Science (23110719) and was carried out as part of the ‘‘High-
Tech Research Center’’ Project (Toho University) 2005–2009 from
the Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan. We thank M. Nakama (WarpStream Tokyo, Co., Ltd.)
for providing a Web-based database system.
4.2. Preparation of salts
Appendix A. Supplementary material
Ion-pair compounds were prepared by diffusion methods. Typ-
ically, viologen dichloride (1.1 ꢂ 10ꢀ5 mol) and (NBu4)2[M(mnt)2]
(1.1 ꢂ 10ꢀ6 mol) were dissolved in 3 mL of a water–ethanol mix-
ture (2:1 v/v) and acetone, respectively. These solutions, which
were layered in a test tube and allowed to diffuse for a week, pro-
duced crystalline ion-pair compounds of 1–5. Crystalline salts of
(dinitrophenyl viologen)[Pt(mnt)2] and (benzyl viologen)[Ni(ben-
zene-1,2-dithiolate)2]2 were also obtained but not structurally
characterized because of their poor crystal quality.
CCDC 793659, 793658, 793657, 795496, and 793660 contain
the supplementary crystallographic data for (1), (2), (3), (4), and
(5). These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2011.11.043.
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47.22; H, 2.72; N, 10.33. Found: C, 46.98; H, 2.88; N, 9.94%.
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54.61; H, 2.96; N, 12.55%.
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