New series of asymmetrically substituted bis(1,2-dithiolato)-nickel(III) complexes exhibiting near IR absorption and structural diversity

Article abstract of DOI:10.1021/ic701408m

The synthesis, structural characterization, and properties of a new series of asymmetrically substituted bis(dithiolene) nickel(III) compounds [Bu ₄N][Ni(Phdt)₂] (1) (Phdt = 2-Phenyl-1,2-dithiolate), [Bu₄N][Ni(NO₂Phdt)₂] (2) (NO₂Phdt = 2-(p-nitrophenyl)-1,2-dithiolate), [Bu₄N][Ni(FPhdt)₂] (3) (FPhdt = 2-(p-fluorophenyl)-1,2-dithiolate), [Bu₄N][Ni(ClPhdt) ₂] (4) (ClPhdt = 2-(p-chlorophenyl)-1,2-dithiolate), and [Bu ₄N][Ni(BrPhdt)₂] (5) (BrPhdt = 2-(p-bromophenyl)-1,2- dithiolate) have been described. All complexes 1-5 exhibit absorptions in the near-infrared region; the shift of these absorption bands can be tuned by the choice of the substituents on the relevant dithiolene moieties. The substituents on the dithiolene moiety are also responsible for their structural diversities. The nature of the substituents on the dithiolene moiety play an important role in tuning the redox potentials along this series. The nitro derivative (compound 2) exhibits several redox couples in its cyclic voltammogram in contrast to the other compounds in this series. The synthesis and characterization of two asymmetrically halogen substituted tetrathiafulvalene (TTF) derivatives 4,4′-bis(4-chlorophenyl)-tetrathiafulvalene ClPhTTF (6) and 4,4′-bis(4-bromophenyl)-tetrathiafulvalene (BrPhTTF) (7) have been described. One of these compounds has been structurally characterized. Iodine treatment of the monoanionic Ni(III) compound [Bu₄N][Ni(ClPhdt) ₂] (4) results in the formation of a neutral Ni(IV) complex [Ni(ClPhdt)₂] (8). All monoanionic compounds 1-5 are Ni(III) complexes, as evidenced by electron spin resonance spectroscopy. Interestingly, strong Cl...Cl interactions are observed in the solid state structures of the chlorinated compounds 6 and 8. Finally, the structural features of compound [Ni(ClPhdt)₂] (8) and the TTF derivative ClPhTTF (6) are compared based on their enormous structural similarities, and the neutral compound [Ni(ClPhdt)₂] (8) is classed as the an inorganic counterpart of TTF .

Full text of DOI:10.1021/ic701408m

Inorg. Chem. 2008, 47, 5055-5070
New Series of Asymmetrically Substituted Bis(1,2-dithiolato)-Nickel(III)
Complexes Exhibiting Near IR Absorption and Structural Diversity†
Vedichi Madhu and Samar K. Das*
School of Chemistry, UniVersity of Hyderabad, Hyderabad 500046, India
Received July 15, 2007
The synthesis, structural characterization, and properties of a new series of asymmetrically substituted bis(dithiolene)
nickel(III) compounds [Bu4N][Ni(Phdt)2] (1) (Phdt ) 2-Phenyl-1,2-dithiolate), [Bu4N][Ni(NO2Phdt)2] (2) (NO2Phdt )
2-(p-nitrophenyl)-1,2-dithiolate), [Bu4N][Ni(FPhdt)2] (3) (FPhdt ) 2-(p-fluorophenyl)-1,2-dithiolate), [Bu4N][Ni(ClPhdt)2]
(4) (ClPhdt ) 2-(p-chlorophenyl)-1,2-dithiolate), and [Bu4N][Ni(BrPhdt)2] (5) (BrPhdt ) 2-(p-bromophenyl)-1,2-dithiolate)
have been described. All complexes 1-5 exhibit absorptions in the near-infrared region; the shift of these absorption
bands can be tuned by the choice of the substituents on the relevant dithiolene moieties. The substituents on the
dithiolene moiety are also responsible for their structural diversities. The nature of the substituents on the dithiolene
moiety play an important role in tuning the redox potentials along this series. The nitro derivative (compound 2)
exhibits several redox couples in its cyclic voltammogram in contrast to the other compounds in this series. The
synthesis and characterization of two asymmetrically halogen substituted tetrathiafulvalene (TTF) derivatives 4,4′-
bis(4-chlorophenyl)-tetrathiafulvalene ClPhTTF (6) and 4,4′-bis(4-bromophenyl)-tetrathiafulvalene (BrPhTTF) (7) have
been described. One of these compounds has been structurally characterized. Iodine treatment of the monoanionic
Ni(III) compound [Bu4N][Ni(ClPhdt)2] (4) results in the formation of a neutral Ni(IV) complex [Ni(ClPhdt)2] (8). All
monoanionic compounds 1-5 are Ni(III) complexes, as evidenced by electron spin resonance spectroscopy.
Interestingly, strong Cl · · · Cl interactions are observed in the solid state structures of the chlorinated compounds
6 and 8. Finally, the structural features of compound [Ni(ClPhdt)2] (8) and the TTF derivative ClPhTTF (6) are
compared based on their enormous structural similarities, and the neutral compound [Ni(ClPhdt)2] (8) is classed as
the “an inorganic counterpart of TTF”.
Introduction
metal dithiolene complexes in the context of organometallic
compounds.6 To date, many of the symmetrically substituted
metal bis-dithiolene complexes7 have been investigated
extensively, but studies based on asymmetrical dithiolene
metal complexes are still limited. Mueller-Westerhoff and
co-workers reported nickel complexes of asymmetrically
substituted dithiolenes describing their electrochemical prop-
erties and NIR spectroscopy.8 Fourmigue´ and Bertran
described an interesting paramagnetic nickel-asymmetric-
The synthesis of asymmetrically substituted metal bis-
dithiolene complexes with more conjugated systems is
fascinating because of their unusual optical,1 electronic,2
near-infrared (NIR) absorption,3 magnetic,4 and electro-
chemical properties.5 These properties are often related to
the different substitutions or the modulation of the dithiolene
part, as well as the mode of the conjugations. The scope of
metal-dithiolene complexes has expanded to heteroleptic
(2) Cassoux, P.; Valade, L.; Kobayashi, H.; Kobayashi, A.; Clark, R. A.;
Underhill, A. E. Coord. Chem. ReV. 1991, 110, 115.
† Dedicated to Professor Sabyasachi Sarkar on the occasion of his 60th
birthday.
* To whom correspondence should be addressed. E-mail: skdsc@
uohyd.ernet.in.
(1) (a) Winter, C. S.; Oliver, S. N.; Ruth, J. D.; Hill, C. A. S.; Underhill,
A. E. J. Appl. Phys. 1992, 71, 512. (b) Winter, C. S.; Oliver, S. N.;
Manning, R. J.; Rush, J. D.; Hill, C. A. S.; Underhill, A. E. J. Mater.
Chem. 1992, 2, 443. (c) Aloukos, P.; Couris, S.; Koutselas, J. B.;
Anyfantis, G. C.; Papavassiliou, G. C. Chem. Phys. Lett. 2006, 428,
109.
(3) (a) Mueller-Westerhoff, U. T.; Vance, B.; Yoon, D. I. Tetrahedron
1991, 47, 909. (b) Bigoli, F.; Deplano, P.; Devillanova, F. A.; Lippolis,
U.; Lukes, P. J.; Mercuri, M. L.; Pellinghelli, M. A.; Trogu, E. F.
J. Chem. Soc., Chem. Commun. 1995, 371. (c) Bigoli, F.; Deplano,
P.; Devillanova, F. A.; Frraro, J. R.; Lippolis, U.; Lukes, P. J.; Mercuri,
M. L.; Pellinghelli, M. A.; Trogu, E. F.; Williams, J. M. Inorg. Chem.
1997, 37, 1218. (d) Marshall, K. L.; Painter, G.; Lotito, K.; Noto,
A. G.; Chang, P. Mol. Cryst. Liq. Cryst. 2006, 454, 449.
(4) Jeannin, O.; Cle´rac, R.; Fourmigue, M. J. Am. Chem. Soc. 2006, 128,
14649.
10.1021/ic701408m CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 12, 2008 5055
Published on Web 05/15/2008

Madhu and Das
Scheme 1
Scheme 2. Synthesis of [Bu4N][Ni(Phdt)2] (1) (Phdt ) 2-Phenyl-1,2-dithiolate)
dithiolene complex analog [Ni(edt-CN)2]2- (edt ) 1,2-
ethanedithiolate) of Ni(mnt)2- (mnt ) maleonitrile dithiolate)
with two cyano groups only for both oxidation states,
together with the singlet-triplet behavior.9The asymmetrically
substituted dithiolene complex bis(2-cyano-1,2-ethanedithiolato),
nickelate(III) has been stabilized with (meso-tetraphenylpor-
phinato)manganese(III) resulting in an interesting mag-
netic material [MnTPP]{Ni[S2C2H(CN)]2}.10 The unsym-
metrical character of the dithiolate ligand 2-(trifluoromethy-
l)acrylonitrile-1,2-dithiolate (tfadt) makes it possible for the
first time to evaluate experimentally the inversion barrier for
cis-trans isomerization of the complex [Ni(tfadt)2].11 Syn-
thesis and structural characterization of gold(III) complex-
es[Au(dithiolene)2]-, each involving two asymmetrically
substituted dithiolene ligands (SC(H)C(R)S; R ) phenyl,
pyridin-2-yl, pyridin-3-yl, or pyridin-4-yl) have been reported
with interesting electrochemical properties.12 The rational
synthesis of transition metal (Ni, Pd, and Pt)-asymmetrically
substituted dithiolene complexes has been described with
mesomorphic properties.13 The effect of bulky substituents
adjacent to the dithiolene ring has been addressed by study-
ing Ni(II)-asymmetrically substituted dithiolenes [Ni(SCR′
n
) CR′′S)2] (R′ ) Ph; R′′ ) Bu, cyclopentylmethyl, or
4-pentylcyclohexyl).14 The addition of bulky groups to the
phenyl ring may affect the ability of the ring to rotate and
thus limits the conjugation between the ring and the
dithiolene core. This can result in a decrease in the width of
the absorption band.14 Laguna and co-workers have chosen
an asymmetrically substituted phenyl group on dithiolene-
transition metal complexes, in which the phenyl group
contains the -CN group as substituent, and studied the
resulting optoelectronic properties.15 Drexhage and co-
workers discovered that transition metal dithiolene com-
plexes, especially those of Nickel, are particularly suitable
for Q-switching lasers-dyes for Nd:YAG lasers operating at
1064 nm.8c,d The NIR absorption maxima, observed in nickel
dithiolenes system, is a function of both extensive electron
delocalization within the dithiolene ring system, and the
interaction of this delocalized system with available d-orbitals
on the central metal.16 Thus, the rational design and
development of NIR absorbing dyes are a challenging task
because of their technological significances. Indeed, some
liquid crystalline (LC) display systems are known to use a
(5) (a) Lowe, N. D.; Garner, C. D. J. Chem. Soc., Dalton Trans. 1993,
2197. (b) Nomura, M.; Takayama, C.; Kajitani, M. Inorg. Chem. 2003,
42, 6441.
(6) (a) Nomura, M.; Okuyama, R.; Fujita-Takayama, C.; Kajitani, M.
Organometallics 2005, 24, 5110. (b) Avarvari, N.; Fourmigue´, M.
Organometallics 2003, 22, 2042.
(7) (a) Progress in Inorganic Chemistry; Stiefel, E. I., Karlin, K., Eds.;
John Wiley & Sons, Inc.: New York, 2004; Vol. 52. (b) Stiefel, E. I.
In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard,
R. D., McCleverty, J. A., Eds.; Pergamon Press: New York, 1987. (c)
Stiefel, E. I Prog. Inorg. Chem. 1977, 22, 34.
(8) (a) Mueller-Westerhoff, U. T.; Vance, B. In ComprehensiVe Coordina-
tion Chemistry, Vol.2; Wilkinson, G., Gillard, R. D., McClverty, J. A.,
Eds.; Pergamon Press: Oxford, 1987; p 595. (b) Mueller-Westerhoff,
U. T.; Yoon, D. I.; Plourde, K. Mol.Cryst. Liq. Cryst. 1990, 183, 291.
(c) Drexhage, K. H.; Muller-Westerhoff, U. T. IEEE J. Quantum
Electron. 1972, QE-8, 759. (d) Drexhage, K. H.; Mueller-Westerhoff,
U.T. U.S. Patent 3,743, 964, 1973.
(13) (a) Giroud, A. M.; Nazzal, A.; Mueller-Westerhoff, U. T. Mol. Cryst.
Liq. Cryst. 1980, 56, 225. (b) Mueller-Westerhoff, U. T.; Nazzal, A.;
Cox, R. J.; Giroud, A. M. Mol. Cryst. Liq. Cryst. 1980, 56, 249.
(14) Shi, S. In Optoelectronic Properties of Inorganic Compounds;
Roundhill, D. M., Fackler, J. P., Jr., Eds.; Plenum: New York, 1999;
pp 52-153.
(9) Fourmigue´, M.; Bertran, J. N. Chem. Commun. 2000, 2111.
(10) Dawe, L. N.; Miglioi, J.; Turnbow, L.; Taliaferro, M. L.; Shum, W. W.;
Bagnato, J. D.; Zakharov, L. N.; Rheingold, A. L.; Arif, A. M.;
Fourmigue, M.; Miller, J. S. Inorg. Chem. 2005, 44, 7530.
(11) Jeannin, O.; Delaunay, J.; Barrie´re, F.; Fourmigue´, M. Inorg. Chem.
2005, 44, 9763.
(15) Cerrada, E.; Diaz, C.; Laguna, M.; Romeo, I.; Teruel, H.; Urzay, N.
Synth. Met. 2001, 120, 817.
(12) Tunney, J. M.; Blake, A. J.; Davies, E. S.; McMaster, J.; Wilson, C.;
Garner, C. D. Polyhedron 2006, 25, 591.
(16) Fang, Q.; Sun, Y.-M.; You, X.-Z. Chin. J. Chem. Phys. 1992, 1, 129.
5056 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
Scheme 3. Synthesis of [Bu4N][Ni(NO2Phdt)2] (2) (NO2Phdt ) 2-(p-nitrophenyl)-1,2-dithiolate)
Scheme 4. Synthesis of [Bu4N][Ni(ClPhdt)2] (4) (ClPhdt ) 2-(p-chlorophenyl)-1,2-dithiolate)
Scheme 5. Synthesis of [Bu4N][Ni(BrPhdt)2] (5) (BrPhdt ) 2-(p-bromophenyl)-1,2-dithiolate)
Scheme 6. Synthesis of 4,4′-Bis(4-chlorophenyl)-tetrathiafulvalene
nickel dithiolene complex as the NIR absorbing dye.17 This
is because the guest nickel bis(dithiolene) dyes possess a
number of important criteria such as high solubility in the
liquid crystal host to maximize contrast, excellent chemical
and thermal stability, and low impact on the long-range
molecular ordering in the LC host, besides its strong
absorbance maximum at or near the wavelength of incident
laser radiation. Most of the commercially available NIR dyes
of organic compounds are ionic or highly polar and accord-
ingly show poor solubility in hydrocarbon-like liquid crystal
hosts.17 Thus, the inorganic nickel dithiolenes are of special
interest for their high solubilities (up to 10 wt %) in LC
hosts. It is important to note that nickel-dithiolene dyes
themselves can possess LC properties18 if appropriate
terminal functional groups are selected on the relevant
dithiolene moiety.
We wish to report herein the synthesis, structural charac-
terization, and properties of a new series of asymmetrically
substituted Ni(III)-dithiolene complexes 1-5 as shown in
Scheme 1. We have demonstrated how the nature of
substituents on the dithiolene moiety in 1-5 influences the
position of the NIR band maxima in their electronic
absorption spectra and the overall packing of the molecules
in the solid state. While working with this system (com-
pounds 1-5), two “sulfur” containing organic compounds
6 and 7, known as tetrathiafulvalenes (TTFs) as shown in
(ClPhTTF) (6)
Scheme 1, have been prepared based on a structural analogy
of 6 and 7 from the anions of compounds 1-5. They are
analogues in the sense that if the nickel ion (in the anions of
compounds 1-5) is replaced by a CdC bond, an organic
TTF will be obtained. We have also reported a neutral Ni(IV)
compound 8 (Scheme 1), which can be described as the
inorganic counterpart to an organic TTF molecule as far as
the structural analogies between compounds 8 and 6/7
(Scheme 1) are concerned.
Experimental Details
General Consideration. All chemicals were purchased from
commercial sources and used without further purification. Mi-
croanalytical (C, H, N, S) data were obtained with a FLASH EA
1112 Series CHNS Analyzer. The IR spectra (with KBr pellets)
were recorded in the range of 400-4000 cm-1 on a JASCO FT/
IR-5300 spectrometer. UV-vis-NIR spectra were recorded using
a 3101 PC/UV/vis-NIR Philips spectrometer equipped with a
diffuse reflectance accessory. The electron spin resonance (ESR)
spectra were recorded on a (JEOL) JESFA200 ESR spectrometer.
1H NMR and 13C NMR spectra were recorded on Bruker DRX-
400 spectrometer using Si(CH3)4 (TMS) as an internal standard.
Solution mass spectra (LCMS) were obtained on a LCMS-2010A
Shimadzu spectrometer. A Cypress model CS-1090/CS-1087 elec-
(17) (a) Kenneth, L. M.; Benjamin, S.; Irene, A. L. Proc. SPIE 2004, 5213,
201; Liquid Crystals VII. Edited by Iam-Choon, K. (b) Marshall, K. L.;
Guardalben, M. J.; Corsello, S. M.; Moore, M. S.; Lippa, I. A.; Brecker,
R. P. Presented at the 16th Interdisciplinary Laser Science Conference
(ILS-XVI), Providence, RI, 22-26 October, 2000.
(18) Marshall, K. L.; Jacobs, S. D. Mol. Cryst. Liq. Cryst. 1988, 159, 181.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5057

Madhu and Das
Scheme 7. Synthesis of Neutral [Ni(ClPhdt)2] (8) (ClPhdt ) 2-(p-chlorophenyl)-1,2-dithiolate)
Scheme 8. Synthetic Route for Preparation of Asymmetrically Substituted Anionic Ni(III) Bis-(dithiolene) Complexes and Asymmetrically Substituted
Tetrathiafulvalenes
Table 1. Cyclic Voltammetry Dataa
troanalytical system was used for cyclic voltammetric experiments.
reduction wave
oxidation wave
The electrochemical experiments were measured in acetonitrile
solvent containing Bu4NClO4 as a supporting electrolyte, using a
conventional cell consisting of two platinum wires as working and
counter electrodes and a saturated calomel electrode(SCE) as
reference. The potentials reported here are uncorrected for junction
contributions.
compound
E1/2, V (∆Ep, mV)
E1/2, V (∆Ep, mV)
[Bu4N][Ni(Phdt)2] (1)
-0.63
0.26 (70)
0.42 (130)
[Bu4N][Ni(NO2Phdt)2] (2) -0.31 (90), -0.56 (170),
-0.98 (170)
[Bu4N][Ni(ClPhdt)2] (4) -0.44 (60)
[Bu4N][Ni(BrPhdt)2] (5) -0.43 (60)
a Cyclic voltammogram of compounds 1,2, 4, and 5 (10-3 M) in CH3CN.
0.36 (120)
0.37 (130)
Synthetic Procedures19 for the 1,3-Dithiol-2-one Derivatives.
(4-Phenyl-1,3-dithiol-2-one, 4-(p-nitrophenyl)-1,3-dithiol-2-one, 4-(p-
fluorophenyl)-1,3-dithiol-2-one, 4-(p-chlorophenyl)-1,3-dithiol-2-
one, 4-(p-bromophenyl)-1,3-dithiol-2-one), which have been used
as starting precursors for the preparation of compounds 1 through
5 respectively, are presented in the Supporting Information, section
S1.
Synthesis of [Bu4N][Ni(Phdt)2]. A 0.2 g (1 mmol) quantity of
4-phenyl-1,3-dithiol-2-one was suspended in 10 mL of absolute
EtOH; to this reaction mixture was added sodium (0.06 g, 2.6 mmol)
under a dry N2 atmosphere, and the color of the reaction mixture
turned from yellow to orange. Then, the solution of NiCl2 ·6H2O
(0.12 g, 0.5 mmol) in absolute EtOH (10 mL) was added dropwise
to the orange solution. After complete addition of the NiCl2 ·6H2O
solution, it was continuously stirred for 20-25 min followed by
the addition of absolute EtOH solution of [Bu4N]Br (0.322 g, 1
mmol). This immediately resulted in the precipitation of 1 as a red
solid. This product was then recrystallized from a 1:1 mixture of
acetone and 2-propanol. Yield 0.165 g (52%). Anal. Calcd for
C32H48NNiS4: C, 60.65; H, 7.63; N, 2.21; S, 20.237. Found: C,
60.37; H, 7.11; N, 2.39; S, 19.81. IR (KBr pellet): (ν/cm-1) ) 2959,
2866, 1588, 1477, 1452, 1429, 1373, 1206, 1024, 931, 877, 835,
750, 690, 613, 449. LCMS mass spectrum (neg ion mode; CH3CN
solution): m/z ) 390 [Ni(Phdt)2]-. UV-vis (CH3CN): λmax, nm (ε,
M-1 cm-1) ) 383 (2898), 514 (1920), 623 (455), 837 (2987). CV (in
CH3CN vs SCE): E1 ) 0.26, E2 ) -0.63 V (irrev.) (Scheme 2).
1/2
Synthesis of [Bu4N][Ni(NO2Phdt)2] (2). A 0.13 g (0.54 mmol)
(19) Bhattacharya, A. K.; Hortmann, A. G. J. Org. Chem. 1974, 39, 95.
5058 Inorganic Chemistry, Vol. 47, No. 12, 2008
quantity of 4-(p-nitrophenyl)-1,3-dithiol-2-one was suspended in

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
an orange solution. To this mixture was added 0.03 g (1.3 mmol)
of sodium metal, and the reaction mixture color became dark It
was then stirred for 10 min at room temperature under a dry N2
atmosphere. Subsequently, 5 mL of a MeOH solution of
NiCl2 ·6H2O (0.070 g, 0.3 mmol) was added to above reaction
mixture, and it was additionally stirred for 10 min followed by the
addition of 5 mL of a MeOH solution of [Bu4N]Br (0.2 g, 0.62
mmol). The red precipitate was separated out by adding of 7 mL
of H2O. Compound 3 was then washed with MeOH and dried in
vacuum. Single crystals, suitable for X-ray structure analysis, were
grown from a (1:1) mixture of acetone and 2-propanol mixed
solvent. Yield 0.122 g (60%). Anal. Calcd for C32H46F2NNiS4: C,
57.39; H, 6.92; N, 2.092; S, 19.15. Found: C, 57.12; H, 6.11; N,
2.39; S, 20.01. IR (KBr pellet): (ν/ cm-1) ) 2955, 2868, 1583,
1476, 1443, 1378, 1204, 1070, 1009, 879, 813, 784, 698, 473, 421.
UV-vis (CH3CN): λmax, nm (ε, M-1 cm-1) ) 390 (2190), 515
(1790), 628 (130), 1020 (2850).
Figure 1. Electronic absorption spectra of NO2-substituted Ni(III) bis-
(dithiolene) complex 2 (black line), F-substituted Ni(III) bis(dithiolene)
compound 3 (red line), and Cl-substituted Ni(III) bis(dithiolene) compound
4 (blue line; 1 × 10-3 M, CH3CN).
Synthesis of [Bu4N][Ni(ClPhdt)2] (4). A 0.38 g (1.66 mmol)
quantity of 4-(p-chlorophenyl)-1,3-dithiol-2-one was dissolved in
15 mL of MeOH. This reaction mixture was stirred until its
complete dissolution resulting in an orange solution. To this mixture
was added 0.07 g (3 mmol) of sodium metal. The ensuing dark
reaction mixture was then stirred for 10 min at room temperature
under dry N2 atmosphere. A 5 mL MeOH solution of NiCl2 ·6H2O
(0.17 g, 0.71 mmol) was added and the resulting reaction mixture
was further stirred for 10 min followed by the addition of a 5 mL
MeOH solution of [Bu4N]Br (0.5 g, 1.55 mmol). The red precipitate,
thus obtained, was washed with MeOH and dried in vacuum. Single
crystals, suitable for X-ray structure analysis, were grown from
CH2Cl2-hexane (1:1) mixed solvents. Yield 0.22 g (44%). Anal.
Calcd for C32H46Cl2NNiS4: C, 54.70; H, 6.59; N, 1.994; S, 18.25.
Found: C, 54.43; H, 6.10; N, 1.69; S, 19.02. IR (KBr pellet): (ν/
cm-1) ) 2957, 2866, 1585, 1481, 1444, 1377, 1207, 1087, 1008,
879, 815, 779, 709, 474. LCMS mass spectrum (neg ion mode;
CH3CN solution): m/z ) 460 [Ni(ClPhdt)2]-. UV-vis (CH3CN):
λ
max, nm (ε, M-1cm-1) ) 401 (2000), 510 (2180), 651 (230), 1033
(2760). CV (in CH3CN vs SCE): E1 ) 0.36, E2 ) -0.44 V
1/2
1/2
Figure 2. ESR spectrum of the powder form of compound [Bu4N]-
[Ni(Phdt)2] (1) at liquid nitrogen temperature.
(Scheme 4).
Synthesis of [Bu4N][Ni(BrPhdt)2] (5). A 0.2 g (0.73 mmol)
quantity of 4-(p-bromophenyl)-1,3-dithiol-2-one was suspended in
15 mL of MeOH. This reaction mixture was then stirred until
complete dissolution, resulting in an orange solution. To this mixture
was added 0.04 g (1.73 mmol) of sodium metal. The dark reaction
mixture, thus obtained, was stirred for 10 min at room temperature
under a dry N2 atmosphere. Subsequently, a 5 mL MeOH solution
of NiCl2 ·6H2O (0.1 g, 0.42 mmol) was added to the above reaction
mixture, and it was then stirred for 10 min followed by the addition
of 5 mL MeOH solution of [Bu4N]Br (0.23 g, 0.71 mmol). The
red precipitate, thus obtained, was washed with MeOH and dried
in vacuum. Single crystals, good enough for X-ray structure
analysis, were grown from an acetone-isopropyl alcohol solvent
mixture. Yield 0.13 g (40%). Anal. Calcd for C32H46Br2NNiS4: C,
48.56; H, 5.86; N, 1.77; S, 16.20. Found: C, 47.97; H, 5.58; N,
1.63; S, 16.32. IR (KBr pellet): (ν/cm-1) ) 2957, 2870, 1580, 1478,
1379, 1206, 1068, 1005, 927, 879, 814, 783, 692, 466, 412. LCMS
mass spectrum (neg ion mode; CH3CN solution): m/z ) 547
[Ni(BrPhdt)2]-. UV-vis (CH3CN): λmax, nm (ε, M-1 cm-1) ) 400
(1990), 510 (2100), 648 (220), 1037 (2800). CV (in CH3CN vs
SCE): E1 ) 0.37, E2 ) -0.43 V (Scheme 5).
10 mL of MeOH. This mixture was then stirred until complete
dissolution, resulting in an orange solution. To this mixture was
added 0.03 g (1.3 mmol) of sodium metal, and the color of the
reaction mixture became dark. It was then stirred for 10 min at
room temperature under a dry N2 atmosphere. Subsequently, 5 mL
MeOH solution of NiCl2 ·6H2O (0.070 g, 0.295 mmol) was added
to the above reaction mixture, and the resulting reaction mixture
was additionally stirred for 10 min followed by the addition MeOH
solution (5 mL) of [Bu4N]Br (0.2 g, 0.62 mmol). The resulting red
precipitate was separated out by adding of 7 mL of H2O. This red
precipitate was then crystallized from a (1:1) mixture of acetone/
2-propanol. The thin-plate shaped crystals of 2 were washed with
MeOH and dried in vacuum. Yield 0.110 g (51%). Anal. Calcd for
C32H46N3O4NiS4: C, 53.11; H, 6.41; N, 5.80; S, 17.72. Found: C,
53.76; H, 5.98; N, 5.59; S, 17.32. IR (KBr pellet): (ν/cm-1) ) 2955,
2874, 1643, 1584, 1504, 1334, 1209, 1103, 850, 748, 690. LCMS
mass spectrum (neg ion mode; CH3CN solution): m/z ) 479
[Ni(NO2Phdt)2]-. UV-vis (CH3CN): λmax, nm (ε, M-1 cm-1) ) 483
(2480), 616 (1980), 970 (2910). CV (in CH3CN vs SCE): E11/2 ) 0.42,
E2 ) -0.31 V, E3 ) -0.56, E4 ) -0.98 V (Scheme 3).
1/2
1/2
1/2
1/2
1/2
Synthesis of [Bu4N][Ni(FPhdt)2] (3) (FPhdt ) 2-(p-fluorophe-
nyl)-1,2-dithiolate). A 0.15 g (0.71 mmol) quantity of 4-(p-
fluorophenyl)-1,3-dithiol-2-one was suspended in 10 mL of MeOH.
This mixture was stirred until its complete dissolution, resulting in
Synthesis of 4,4′-Bis(4-chlorophenyl)-tetrathiafulvalene
(ClPhTTF) (6). A 0.33 g (1.44 mmol) quantity of 4-(p-chlorophe-
nyl)-1,3-dithiol-2-one was suspended in a 20 mL round-bottom flask
under a dry N2 atmosphere. To this mixture was added 5 mL of
Inorganic Chemistry, Vol. 47, No. 12, 2008 5059

Madhu and Das
Figure 3. Cyclic voltammograms of unsymmetrical dithiolene complexes in acetonitrile. (a) [Bu4N][Ni(Phdt)2] (1), (b) [Bu4N][Ni(NO2Phdt)2] (2), (c)
[Bu4N][Ni(ClPhdt)2] (4), and (d) [Bu4N][Ni(BrPhdt)2] (5). All compounds peak potential were recorded vs SCE at 298 K.
Table 2. Crystal Data and Structural Refinement for Compounds 3
freshly distilled P(OEt)3, it was continuously heated for 2 h at
140-150 °C, and the color of the reaction mixture became dark. It
was then cooled to room temperature, after which time 10 mL of
MeOH was added resulting in the formation of an orange
precipitate. Single crystals, suitable for X-ray analysis, were grown
by slow evaporation of CHCl3 solution. Yield 0.52 g (85%). Anal.
Calcd for C18H10Cl2S4: C, 50.82; H, 2.37; S, 30.15. Found: C, 50.43;
and 4
3
4
empirical formula
formula weight
T [K]
C32H46F2NNiS4
669.65
298(2)
C32H46Cl2NNiS4
702.55
298(2)
λ [Å]
0.71073
monoclinic
C2/c
20.410(5)
8.949(2)
19.574(4)
90
109.772(3)
90
3364.2(13)
4
0.71073 A
monoclinic
P2(1)/c
9.6059(16)
40.727(7)
9.0719(15)
90
91.483(3)
90
3547.9(10)
4
crystal system
space group
a [Å]
1
H, 2.41; S, 29.86. H NMR (d6-DMSO): (δ/ppm) ) 6.67 (s, 2H),
7.30 (d, 4H), 7.50 (d, 4H). IR (KBr pellet): (ν/cm-1) ) 1651, 1541,
b [Å]
c [Å]
1473, 1394, 1072, 1005, 918, 817, 761, 457 (Scheme 6).
R [deg]
ꢀ [deg]
γ [ deg]
V [Å3]
Z
Dcalc [Mg m-3
]
1.322
0.858
1420
1.315
0.955
1484
µ [mm-1
F [000]
]
crystal size [mm3]
θ range for data
0.35 × 0.34 × 0.12 0.20 × 0.14 × 0.06
2.12 to 25.00
2.00 to 25.00
collection [deg]
reflections collected/unique
R (int)
15593/2964
0.0522
33998/6255
0.0666
refinement method
data/restraints/parameters
goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
full-matrix least-squares on F2
2964/0/185
1.100
0.0356/0.1027
0.0407/0.1057
6255/0/365
0.918
0.0419/0.0855
0.0658/0.0921
0.455/-0.214
largest diff. peak/hole [e Å3] 0.473/-0.291
Synthesis of 4,4′-Bis(4-bromophenyl)-tetrathiafulvalene (Br-
PhTTF) (7). Ten milliliters of freshly distilled P(OEt)3 was added
to 0.5 g (1.83 mmol) of 4-(p-bromophenyl)-1,3-dithiol-2-one, taken
Figure 4. Thermal ellipsoidal plot of the complex [Bu4N][Ni(FPhdt)2] (3)
(30% probability).
5060 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
Table 3. Selected Bond Lengths [Å] and Angles [°] for Compound 3a
SAINTPLUS software,20 absorption correction using an empirical
method SADABS,21 structure solution using the SHELXS-97
program,22 and refined using the SHELXL-97 program.23 Hydrogen
atoms on the aromatic rings were introduced on the calculated
positions and included in the refinement riding on their respective
parent atoms.
C(1)-C(2)
C(1)-S(1)
1.349(3) C(1)-C(3)
1.739(2) C(2)-S(2)#1
1.352(3) C(9)-N(1)
1.518(3) C(13)-N(1)
1.515(2) N(1)-C(9)#2
2.1384(6) Ni(1)-S(2)#1
2.1478(7)
1.477(3)
1.713(2)
1.516(2)
1.515(2)
1.516(2)
2.1384(6)
C(7)-F(1)
C(10)-C(11)
N(1)-C(13)#2
Ni(1)-S(2)
Ni(1)-S(1)
Results and Discussion
C(2)-C(1)-S(1)
C(1)-C(2)-S(2)#1
F(1)-C(7)-C(8)
C(10)-C(9)-N(1)
117.09(17) C(3)-C(1)-S(1)
122.57(17) S(2)#1-C(2)-H(2)
119.03(15)
118.7
118.4(3)
Synthesis. We have synthesized the asymmetrically sub-
stituted anionic nickel bis-1,2-dithiolene complexes 1-5
(Scheme 1) from the corresponding 1,3-dithiol-2-one deriva-
tives; the 1,3-dithiol-2-one derivatives were synthesized by
the reaction of the respective R-haloketones and alkyl-
xanthates following previously reported procedures,13,19,24
Scheme 8.
Like the preparation of other substituted nickel bis(1,2-
dithiolene) complexes,13,25 1,3-dithiol-2-one is needed as the
starting precursor for the preparation of both metal-asym-
metric-dithiolene complexes and tetrathiafulvalene deriva-
tives. Even though the Ni(II) salt has been used during the
preparation of compounds 1-5, we obtained monoanionic
Ni(III)-dithiolene complexes as evidenced by electron para-
magnetic resonance (EPR) spectroscopy (vide infra). Mo-
lecular oxygen from air is the possible oxidant in this
oxidation from Ni(II) to Ni(III). The neutral nickel(IV)
complex [Ni(ClPhdt)2] (8) was synthesized by the chemical
oxidation of the anionic nickel(III) dithiolene complex
[Bu4N][Ni(ClPhdt)2] (4) using molecular iodine (oxidant) in
acetonitrile as shown in Scheme 7.
119.6(3)
F(1)-C(7)-C(6)
117.47(18) C(14)-C(13)-N(1) 116.72(17)
C(13)#2-N(1)-C(13) 106.1(2)
C(13)#2-N(1)-C(9)#2 110.92(11) C(13)-N(1)-C(9)#2 111.68(11)
110.92(11)
C(14)-C(13)-H(13A) 108.1
C(13)#2-N(1)-C(9) 111.68(11) C(13)-N(1)-C(9)
C(9)#2-N(1)-C(9)
S(2)-Ni(1)-S(1)
105.6(2)
88.70(2) S(2)#1-Ni(1)-S(1)
S(2)-Ni(1)-S(2)#1 180.0
91.30(2)
S(2)#1-Ni(1)-S(1)#1 88.70(2) S(1)-Ni(1)-S(1)#1 179.999(1)
C(1)-S(1)-Ni(1)
105.16(7) C(2)#1-S(2)-Ni(1) 103.83(8)
a Symmetry transformations used to generate equivalent atoms: #1, -x
+ 2, -y, -z; #2, -x + 2, y, -z + 1/2.
Figure 5. C-H· · ·F hydrogen bonding interactions (purple dotted lines)
between [Bu4N]+cation and [Ni(FPhdt)2]- anion in the compound 3. Color
code: F, green; C, gray; S, yellow; Ni, cyan; N, blue.
As shown in Scheme 8, we have succeeded in preparing
a number of dithiolene derivatives with a range of substit-
uents, X ) H(1), NO2(2), F(3), Cl(4), and Br(5), leading to
compounds 1-5, respectively. We also intended to use -CN
(an electron withdrawing group) as one of the substituents
because of their importance of the derivatives as optoelec-
tronic materials.15 The significance of cyano-substituted
dithiolene complexes are evidenced by the fact that a
platinum dithiolene complex with cyano substituents gives
a ꢁ(3) () third order non linearity) value of 4.2 × 10-12 esu
compared to 1.4 × 10-13 esu exhibited by a CF3 analogue.14
In this context, the synthesis of heteroleptic Ni(II) and Pd(II)-
dithiolenes with asymmetrically substituted cyanide group
has recently been reported.26 However, in our synthesis
(Scheme 8), we could not isolate the cyano-substituted
in a 20 mL round-bottom flask, under a dry N2 atmosphere. This
mixture was continuously heated for 1 h at 140-150 °C after which
time the reaction mixture became red. The reaction mixture was
then cooled to room temperature affording an orange crystalline
product over a period of 24 h. Single crystals, suitable for X-ray
analysis, were grown by slow evaporation of a CHCl3 solution.
Yield 0.76 g (81%). Anal. Calcd for C18H10Br2S4: C, 42.03; H,
1
1.96; S, 24.93. Found: C, 41.86; H, 2.01; S, 24.48. H NMR (d6-
DMSO): (δ/ppm) ) 6.52 (s, 2H), 7.28 (d, 4H), 7.47 (d, 4H). IR
(KBr pellet): (ν/cm-1) ) 1651, 1541, 1473, 1394, 1072, 1005, 918,
817, 761, 457. LCMS mass spectrum (pos ion mode; CH3CN
solution): m/z ) 514 (Scheme 6).
Synthesis of [Ni(ClPhdt)2] (8). A 0.14 g (0.2 mmol) quantity
of [Bu4N][Ni(ClPhdt)2](4) was dissolved in 10 mL of dry aceto-
nitrile. To this solution was added 0.05 g (0.197 mmol) of I2 in
dry acetonitrile (5 mL). The reaction mixture was then continuously
stirred for 30 min resulting in the formation of a bluish green
precipitate. The single crystals, suitable for X-ray analysis, were
grown via vapor diffusion of diethyl ether into a THF solution of
the precipitate. Yield 0.08 g (84%). Anal. Calcd for C16H10Cl2NiS4:
C, 41.77; H, 2.19; S, 27.88. Found: C, 41.78; H, 2.32; S, 27.15. IR
(KBr pellet): (ν/cm-1) ) 1478, 1440, 1334, 993, 815, 785, 693,
461 (Scheme 7).
Single Crystal Structure Determination. Data were measured
at room temperature for compounds 3, 4, 6, and 7 on a Bruker
SMART APEX CCD area detector system [λ (Mo KR) ) 0.7103
Å], graphite monochromator; 2400 frames were recorded with an
ω scan width of 0.3°, each for 8 s, crystal-detector distance 60
mm, collimator 0.5 mm. Data reduction was performed with the
(20) Software for the CCD Detector System; Bruker Analytical X-Ray
Systems, Inc.: Madison, WI, 1998.
(21) Sheldrick, G. M., SADABS, Program for Absorption Correction with
the Siemens SMART Area-Detector System; University of Go¨ttingen:
Go¨ttingen, Germany, 1996.
(22) Sheldrick, G. M. SHELXS-97, Program for Solution of Crystal
Structures. University of Go¨ttingen: Go¨ttingen, Germany. 1997.
(23) Sheldrick, G. M. SHELXS-97, Program for Refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(24) (a) Svenstrup, N.; Becher, J. Synthesis 1995, 215. (b) Clausen, R. P.;
Becher, J. Tetrahedron 1996, 52, 3171. (c) Cerrada, E.; Garrido, J.;
Laguna, M.; Lardies, N.; Romeo, I. Synth. Met. 1999, 102, 1709. (d)
Gorgues, A.; Hudhomme, P.; Salle, M Chem. ReV. 2004, 104, 5151.
(e) Fabre, J. M Chem. ReV. 2004, 104, 5133. (f) Kobayashi, A.;
Fujiwara, E.; Kobayashi, H. Chem. ReV. 2004, 104, 5243.
(25) Keefer, C. E.; Purrington, S. T.; Bereman, R. D.; Boyle, P. D. Inorg.
Chem. 1999, 38, 5437.
(26) Lardies, N.; Cerrada, E.; Laguna, M. Polyhedron 2006, 25, 2785.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5061

Madhu and Das
Figure 6. View (wire-frame representation) of the crystal packing of 3 in (2 × 2) cells looking down to the crystallographic b axis: (a) packing diagram
of [Bu4N][Ni(FPhdt)2] and (b) packing diagram of the same without [Bu4N]+ cation. Color code: F, green; C, gray; S, yellow; Ni, cyan; N, blue.
8), a 1,3-dithiol-2-one derivative has been shown to react
with triethylphosphite (P(OEt)3) at 140-150 °C. This meth-
odology is well-established in literature.27
Compounds 1-8 were characterized by elemental analyses
and routine spectral studies including cyclic voltammetry
studies for compounds 1, 2, 4, and 5. Whereas compounds
3, 4, 6 and 8 were unambiguously characterized by single
crystal X-ray structure determinations, compounds 1, 2, 5,
and 7 could not be studied by single crystal X-ray structural
studies because of their poor crystal quality.
Spectroscopic and Electronic Characterization. Nickel
based transition metal unsymmetrical dithiolene complexes
[Bu4N][Ni(XPhdt)2] (X ) H, F, Cl, Br, and NO2) exhibit
strong absorbance bands in the region of 850-1000 nm.
Interestingly, these compounds are extremely soluble in
Figure 7. Thermal ellipsoidal plot of the compound [Bu4N][Ni(ClPhdt)2]
nonpolar organic solvents. The functional groups attached
to the nickel dithiolene core have a large effect on both the
position of the electronic absorbance maxima and the
solubility of the relevant dye. We found that the NO2-
substituted unsymmetrical metal dithiolene complex of Ni2+
(compound 2) shows the absorption band at 970 nm in
CH3CN solution (ε ) 2910 M-1 cm-1), whereas for the F
and Cl substituted Ni2+ dithiolene complexes, the absorption
maxima are red-shifted by 50 and 60 nm, respectively. This
low energy NIR broad feature in the region of 800 to 1000
nm (for compounds 1-5) as shown in the electronic
absorption spectra (Figure 1) is a characteristic of bis(dithi-
olene)nickel complexes and is generally assigned to πfπ*
transition between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO). It usually appears as a broad but intense band with
ε ) 15000-40000 M-1 cm-1 which is important in con-
nection with its use as NIR dyes3a,28 and in turn its
(4) (30% probability).
Table 4. Selected Bond Lengths [Å] and Angles [°] for Compound 4
C(1)-C(2)
C(9)-S(3)
C(17)-C(18)
Ni(1)-S(1)
Ni(1)-S(3
C(8)-Cl(2)
1.337(4)
1.712(3)
1.516(4)
2.1374(9) Ni(1)-S(2)
2.1433(8) Ni(1)-S(4)
1.745(3)
C(9)-C(10)
C(17)-N(2)
C(29)-N(2)
1.338(4)
1.514(3)
1.520(3)
2.1411(9)
2.1515(9)
C(2)-C(1)-S(1)
C(3)-C(2)-S(2)
N(2)-C(17)-C(18) 116.1(2)
C(17)-N(2)-C(25) 110.6(2)
C(25)-N(2)-C(21) 106.59(19)
S(2)-Ni(1)-S(4)
C(1)-S(1)-Ni(1)
C(9)-S(3)-Ni(1)
122.6(2)
118.8(2)
C(1)-C(2)-S(2)
17.2(2)
C(14)-C(16)-Cl(1) 119.8(3)
C(30)-C(29)-N(2) 116.0(2)
C(17)-N(2)-C(21) 110.5(2)
S(1)-Ni(1)-S(4)
S(3)-Ni(1)-S(4)
C(2)-S(2)-Ni(1)
89.49(3)
91.58(3)
105.49(11)
178.48(4)
103.74(11)
103.48(10)
derivative and obtained an insoluble product in the second
step (by the use of 80% H2SO4, cyclization purpose). We
believe that, this insoluble product is most likely the result
of acid hydrolysis of the -CN group to the corresponding
carboxylic acid. This is consistent with the IR data. The
relevant IR spectrum does not show the characteristic IR
bands for the -CN group. The insolubility of this product
precludes us from synthesizing and characterizing further
-CN substituted asymmetrical 1,2-dithiolene complexes
following our reaction Scheme 8. In the synthesis of
asymmetrically substituted tetrathiafulvalenes 6 and 7 (Scheme
(27) (a) Svenstrup, N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J.
Synthesis 1994, 809. (b) Becher, J.; Lau, J.; Leriche, P.; Mørk, P.;
Svenstrup, N J. Chem. Soc., Chem. Commun. 1994, 2715. (c) Lau, J.;
Simonsen, O.; Becher, J. Synthesis 1995, 521. (d) Simonsen, K. B.;
Svenstrup, N.; Lau, J.; Simonsen, O.; Mørk, P.; Kristensen, G. J.;
Becher, J. Synthesis 1996, 407.
(28) Bigoli, F.; Chen, C.-T.; Wu, W.-C.; Deplano, P.; Mercuri, M. L.;
Pellinghelli, M. A.; Pilia, L.; Pintus, G.; Serpe, A.; Trogu, E. F. Chem.
Commun. 2001, 2246–2247.
5062 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
Figure 8. View (wire-frame representation) of the crystal packing of 4 in (2 × 2) cells viewed down to the crystallographic c axis: (a) packing diagram of
[Bu4N][Ni(ClPhdt)2] and (b) packing diagram of the same without [Bu4N]+ cation.
Table 5. Crystal Data and Structural Refinement for Compounds
8 and 6
8
6
empirical formula
formula weight
T [K]
C16H10Cl2NiS4
474.10
298(2)
0.71073
monoclinic
P2(1)/c
10.790(3)
9.143(2)
8.849(2)
90
90.965(4)
90
872.9(4)
2
1.804
1.893
C18H10Cl2S4
425.40
298(2)
0.71073
monoclinic
P2(1)/c
10.5339(6)
9.0891(5)
8.8956(5)
90
92.8190(10)
90
850.67(8)
2
1.661
0.869
λ [Å]
crystal system
space group
a [Å]
Figure 9. Thermal ellipsoidal plot of the molecule ClPhTTF (6) (30%
probability).
b [Å]
c [Å]
application in Q-switching infrared lasers.8c,d However, in
the present system, even though the πfπ* transition appears
in the NIR region (Figure 1), it is not as intense a transition
(ε values are around 3000 M-1 cm-1) in comparison with
that of bis(dithiolene)nickel complexes that are used as NIR
dyes.28 One of the major conditions in obtaining high ε
values is coplanarity of the ligand moieties around the nickel
ion. In the present study, the outer two phenyl rings
(substituents on dithiolenes) are found to be twisted out of
the plane defined by the dithiolene moiety (vide infra). This
probably affects the intensity of the πfπ* transition in the
NIR region by perturbing the symmetry of the frontier
orbitals.
The absorption maxima shift toward the NIR region in
the order of the nature of the substituents NO2 < F < Cl
(Figure 1). Thus, tuning of the NIR absorption bands is
attributed to the substituents on dithiolene ligands. In this
series, the -NO2 group, being the most electron withdrawing
substituent, decreases the electron density of the dithiolene
moiety and thereby increases the gap of HOMO and LUMO
(the πf π* transition). This results in the blue-shift of the
NIR absorption maximum of the nitro-substituted compound
2 compared to the absorption maxima of fluoro-, chloro- and
bromo-substituted compounds 3, 4, and 5, respectively. The
R [deg]
ꢀ [deg]
γ [ deg]
V [Å3]
Z
Dcalc [Mg m-3
]
µ [mm-1
F[000]
]
478
432
crystal size [mm3]
θ range for data
collection [deg]
0.36 × 0.32 × 0.10 0.20 × 0.10 × 0.06
1.89 to 24.99
1.94 to 25.00
reflections collected/unique
R (int)
7926/1539
0.0826
7900/1497
0.0323
refinement method
data/restraints/parameters
goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
R1/wR2 (all data)
full-matrix least-squares on F2
1539/0/107
1.115
0.0295/0.0724
0.0309/0.0736
1497/0/109
1.085
0.0242/0.0635
0.0269/0.0647
0.286/-0.239
largest diff. peak/hole [e Å3] 0.281/-0.432
present system (unsymmetrical nickel(III) dithiolene com-
plexes, [Bu4N][Ni(XPhdt)2] (X ) H, F, Cl, Br, and NO2)) is
of great attraction in the sense that further work can be put
forward by the substitution of more electron donating
substituents in the place of halogens or by the reduction of
the electron withdrawing -NO2 group to an electron donating
-NR2(R ) H, alkyl) group.
The ESR spectra of solid samples for compounds 1-5
were recorded at room temperature and at liquid nitrogen
Inorganic Chemistry, Vol. 47, No. 12, 2008 5063

Madhu and Das
Table 6. Selected bond Lengths [Å] and Angles [°] for Compound 6a
in our present system from crystal structure analysis (vide
infra). Thus, ESR studies are consistent with electronic
absorption studies, where we have argued that this type of
twist or distortion (with square planar geometry) might
diminish the intensity of the πfπ* NIR transition as
described in the preceding section. A similar Ni(III) rhombic
type ESR signal has been shown earlier for the dithiolene
complex, [Ni(DDDT)2]- (DDDT ) 5,6-dihydro-1,4-dithin-
2,3-dithiolate).31 Considering all the EPR features and
comparing these data with already known mononuclear
square-planar Ni(III) complexes,30–32 it is logical to describe
the present compounds 1-5 as a formal Ni(III) system.
C(1)-C(1)#1
C(1)-S(2)
C(2)-S(1)
C(9)-Cl(1)
1.346(4)
C(2)-C(3)
1.340(3)
1.7649(18)
1.7342(18)
1.7591(18) C(1)-S(1)
1.7707(18) C(3)-S(2)
1.7404(18)
C(1)#1-C(1)-S(2) 122.12(19)
C(1)#1-C(1)-S(1) 123.30(19)
S(2)-C(1)-S(1)
C(4)-C(2)-S(1)
C(7)-C(9)-Cl(1)
C(1)-S(1)-C(2)
114.55(10)
119.81(13)
119.76(14)
95.06(8)
C(3)-C(2)-S(1)
C(2)-C(3)-S(2)
C(8)-C(9)-Cl(1)
C(3)-S(2)-C(1)
115.63(14)
119.63(14)
119.03(14)
94.64(8)
a Symmetry transformations used to generate equivalent atoms: #1, -x
+ 2, -y + 1, -z.
1
The H NMR spectra for the Ni(III) monoanionic com-
plexes were measured in DMSO-d6. We have already
confirmed that the compounds 1-5 are paramagnetic (vide
supra). The presence of unpaired electron in the same
molecule gives rise to large isotropic shifts (up to several
hundred ppm) and severely broadened resonances, which
usually make integration of signals difficult.33 For a para-
magnetic chromium complex, protons attached to carbon
atoms directly bonded to Cr(III) (e.g., methyl groups) are
typically unobservable.33 Any report/discussion on 1H NMR
spectrum of Ni(III) coordination complex is hardly known.34
In the 1H NMR spectra for compounds 1-5, only the signals
corresponding to the [n-Bu4N]+ cations were observed; very
weak broad features are detected in the shifted aromatic
region (see the Supporting Information, section S3 for the
1
relevant H NMR spectra). The severely low solubility of
1
the neutral compound 8 precluded us from studying its H
NMR spectra. The organic compounds 6 and 7 exhibited
the expected 1H NMR signals (see Supporting Information,
section S3 for the NMR spectrum of compound 7).
The LCMS for the monoanionic Ni(III) complexes show
the parent complex anion in all cases. The organic com-
pounds (6 and 7) also exhibited the expected molecular mass
(see Supporting Information, section S4 for relevant mass
spectra).
The redox behavior of the unsymmetrical substituted nickel
bis(dithiolene) complexes [Bu4N][Ni(Phdt)2] (1), [Bu4N][Ni-
(NO2Phdt)2] (2), [Bu4N][Ni(ClPhdt)2] (4), and [Bu4N][Ni-
(BrPhdt)2] (5) in acetonitrile solutions, each containing 0.10
M [Bu4N]ClO4 as supporting electrolyte, was investigated
by cyclic voltammetry versus SCE. The results are sum-
marized in Table 1. As shown in Figure 3, the cyclic vol-
tammograms of compounds 1, 4, and 5 exhibit an oxidative
response and a reductive response (irreversible for compound
1 and quasi-reversible for compounds 4 and 5). Initial
developments on electrochemistry of transition metal dithi-
Figure 10. Packing diagram (wire frame representation) of compound
ClPhTTF (6) viewed down to the crystallographic b axis. Color code: C,
gray; S, yellow; Cl, green.
temperature. All exhibit a rhombic type signal (gx ) 2.095,
gy ) 2.053, and gz ) 2.026 for compound 1; gx ) 2.096, gy
) 2.062, and gz ) 2.004 for compound 4; gx ) 2.089, gy )
2.061, and gz ) 2.008 for compound 5 at liquid nitrogen
temperature) indicating that compounds 1-5 are formally
Ni(III) complexes. A representative ESR spectrum (com-
pound 1) is shown in Figure 2 (see Supporting Information,
section S2 for ESR spectra of compounds 4 and 5). The
anisotropy in the ESR spectra implies some contribution of
the d orbital of nickel in the total spin density, which is
usually, in this class of materials,29 distributed 40% on nickel
and 60% on the ligands. The similarity of the ESR parameters
of our systems and similar compounds, reported earlier,29
suggests this spin distribution for our compounds too.
Such rhombic feature is not unusual for square-planar
Ni(III) complexes that arises because of a distortion from
square-planar geometry.30 This distortion is indeed realized
(30) (a) Pal, S.; Mukhopadhyay, A. Polyhedron 2004, 23, 1997. (b) Collins,
T. J.; Nichols, T. R.; Uffelman, E. S. J. Am. Chem. Soc. 1991, 113,
4708. (c) Shimazaki, Y.; Tani, F.; Fukui, K.; Naruta, Y.; Yamauchi,
O. J. Am. Chem. Soc. 2003, 125, 10512.
(31) Vance, C. T.; Bereman, R. D.; Bordner, J.; Hatfield, W. E.; Helms,
J. H. Inorg. Chem. 1985, 24, 2905.
(32) Maki, A. H.; Edelstein, N.; Davison, A.; Holm, R. H. J. Am. Chem.
Soc. 1964, 86, 4580.
(29) (a) Ray, K.; Weyhermuller, T.; Neese, F.; Weighardt, K. Inorg. Chem.
2005, 44, 5345. (b) Kokatam, S.; Ray, K.; Pap, J.; Bill, E.; Geiger,
W. E.; LeSuer, J.; Rieger, P. H.; Weyhermuller, T.; Neese, F.;
Weighardt, K. Inorg. Chem. 2007, 46, 1100. (c) Ray, K.; George, S. D.;
Solomon, E. I.; Weighardt, K.; Neese, F. Chem.sEur. J. 2007, 13,
2783.
(33) Pariya, C.; Theopold, K. H. Curr. Sci. (Bangalore, India) 2000, 78,
1345.
(34) Ribas, X.; Dias, J. C.; Morgado, J.; Wurst, K.; Santos, I. C.; Almeida,
M.; Vidal-Gancedo, J.; Veciana, J.; Rovira, C. Inorg. Chem. 2004,
43, 3631.
5064 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
Figure 11. Packing diagram of ClPhTTF (6): (a) view of the wire frame representation, solid yellow lines indicate intermolecular S · · ·S contacts, and (b)
view of the ball and stick representation, purple dotted lines indicate intermolecular S · · ·S contacts, viewed down to the crystallographic c axis.
at negative potentials is assigned as being due to the
[NiIIIL2]1-/[NiIIL2]2- couple. Similarly, the cyclic voltam-
mogram (Figure 3b) of compound [Bu4N][Ni(NO2Phdt)2] (2)
shows a quasi-reversible oxidative response at +0.42 V
because of the [NiIV(NO2Phdt)2]0/[NiIII(NO2Phdt)2]1- couple
and a reversible reductive response at -0.31 V that can be
assigned to the [NiIII(NO2Phdt)2]1-/[NiII(NO2Phdt)2]2- couple.
Compound 2 displays two more quasi-reversible reductive
responses at -0.98 V and -0.56 V, most probably because
of the participation of the nitro groups. Similar electrochemi-
cal behavior of 5-(nitrophenyl)-1,2-dithiole-3-thiones has
been reported.37
Figure 12. Thermal ellipsoidal plot of the neutral compound [Ni(ClPhdt)2]
(8) (30% probability).
Table 7. Selected Bond Lengths [Å] and Angles [°] for Compound 8a
C(1)-C(2)
C(2)-S(2)
Ni(1)-S(2)
Ni(1)-S(1)#1
1.373(3) C(1)-S(1)
1.680(2) C(8)-Cl(1)
2.1146(6) Ni(1)-S(2)#1
2.1381(6) Ni(1)-S(1)
0.712(2)
1.737(2)
2.1146(6)
2.1382(6)
Thus, electrochemical studies indicate that the small
variation in the electron withdrawing substituents on the
dithiolene moiety can tune the redox potentials (Table 1) of
the metal dithiolene complexes. It is noteworthy that the
metal centered oxidation process (for example, from Ni3+
to Ni4+) is easier in the case of the chloro- and bromo-
substituted complexes (4 and 5, respectively) compared with
nitro-derivative (compound 2), showing that the reaction
proceeds more easily as the electron density on the dithiolene
moiety is less decreased in the chloro- and bromo-substituted
complexes than in nitro-derivative (nitro group is more
electron withdrawing, more positive E1/2, see Table 1).
On the basis of the same logic, the nitro-derivative
(compound 2) is more easily reduced (-0.31 V) compared
with the chloro- and bromo-derivatives (-0.44 V and -0.43
V, respectively; Table 1). We see the metal centered
reduction of a metal dithiolene complex very rarely. In the
present study, compounds 2, 4, and 5, having nitro, chloro,
and bromo groups, respectively, on each dithiolene ligand,
can be stabilized in their (formally) +4, +3, and +2
oxidation states, as suggested by the electrochemical studies.
X-ray Crystallographic Studies.
C(2)-C(1)-C(3)
C(3)-C(1)-S(1)
C(8)-C(7)-C(5)
C(7)-C(8)-Cl(1)
S(2)-Ni(1)-S(1)#1 88.33(2)
S(2)-Ni(1)-S(1) 91.67(2)
S(1)#1-Ni(1)-S(1) 180.0
C(2)-S(2)-Ni(1) 104.18(8)
122.29(19) C(2)-C(1)-S(1)
120.61(16) C(1)-C(2)-S(2)
117.09(16)
122.22(17)
120.6
180.0
118.9(2)
C(5)-C(7)-H(7)
119.20(18) S(2)-Ni(1)-S(2)#1
S(2)#1-Ni(1)-S(1)#1 91.67(2)
S(2)#1-Ni(1)-S(1)
C(1)-S(1)-Ni(1)
88.33(2)
104.81(8)
a Symmetry transformations used to generate equivalent atoms: #1, -x
+ 2, -y + 2, -z.
Scheme 9
olene complexes, that have been summarized by McClev-
erty,35 led to recognition and experimental realization of the
three-membered, planar series {ML2} with M ) Ni, Pd, or
Pt as presented in Scheme 9, in which the neutral and
dianionic members are diamagnetic and the monoanion is
paramagnetic with a S ) 1/2 ground state. On the basis of
Scheme 9 and comparing the present electrochemical data
with other similar systems reported earlier,29c,35,36 the
oxidative response for compounds 1, 4, and 5 is attributed
to the [NiIVL2]0/[NiIIIL2]1- couple, and the reductive response
Crystal Structure Description of Compound [Bu4N][Ni-
(FPhdt)2] (3). Red crystals of the complex [Bu4N][Ni-
(FPhdt)2] (3), suitable for X-ray structure analysis, could be
grown from acetone-isopropyl alcohol (1:1) mixture. The
crystal structure shows that the relevant crystals belong to
the monoclinic system with a C2/c space group. The
(35) McCleverty, J. A. Prog. Inorg. Chem. 1968, 10, 49.
(36) (a) Gray, H. B. Transition Met. Chem. 1965, 1, 240. (b) Schrauzer,
G. N. Transition Met. Chem. 1968, 4, 299. (c) Eisenberg, R. Prog.
Inorg. Chem. 1970, 12, 295. (d) Wang, K. In Prog. Inorg. Chem.;
Stiefel, E. I., Karlin, K., Eds.; John Wiley & Sons, Inc.: New York,
2004; Vol. 52, p 267. (e) Zanello, P.; Grigiotti, E. In Trends in
Molecular Electrochemistry; Pombeiro, A. J. L., Amatore, C., Eds.;
Marcell Dekker: New York, 2004; p 3.
(37) Burgot, J. L.; Darchen, A.; Sa¨ıdi, M. Electrochim. Acta 2002, 48, 107.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5065

Madhu and Das
Figure 13. (a) Packing diagram of the [Ni(ClPhdt)2] (8) looking down to the crystallographic b axis and (b) packing diagram of the same looking down to
the crystallographic a axis in which the purple dotted lines ( · · ·) indicates the S · · ·S intermolecular interactions. Color code: Cl, green; C, gray; S, yellow;
Ni, cyan.
Figure 14. (a) View of the wire-frame representation of [Ni(ClPhdt)2] (8) looking down to the crystallographic c axis, yellow solid lines show the S · · ·S
intermolecular interactions, and (b) view of the same in ball and stick representation.
Table 2, and the selected interatomic distances and bond
angles are given in Table 3. The four sulfur atoms surround-
ing the nickel ion yield the square planar geometry with
Figure 15. π-π stacking interactions in the compound [Ni(ClPhdt)2] (8);
S-Ni-S angles of 91.30(2)°. The tetra-coordinated nickel
blue colored six membered rings indicate that these are directly involved
ion is deviated by 0.0147(8) Å out of the S4 square-plane of
in π-π stacking interactions.
two dithiolate ligands. In the five member chelate ring, the
average Ni-S, C-S, and CdC bond lengths are 2.143,
1.726, and 1.348 Å, respectively. The overall charge of this
Ni(III) complex anion [Ni(FPhdt)2]1- in compound 3 as
Figure 16. Cl· · ·Cl interactions in compound [Ni(ClPhdt)2](8). Color code:
Cl, green; C, gray; S, yellow; Ni, cyan.
expected is -1, and this anionic charge is compensated by a
asymmetric unit contains half of the molecule. Figure 4
shows the full molecule with the atom labeling scheme. The
basic crystallographic data (compound 3) are presented in
[Bu4N]+ cation as observed in the crystal structure (Figure 4).
In this complex, the outer phenyl rings are skewed with a
dihedral angle of 17.29(0.12)° with the least-squares plane
5066 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
Figure 17. Molecular structures of compound ClPhTTF (6) (shown in a) and compound [Ni(ClPhdt)2] (8) (shown in b) showing the intermolecular S · · ·S
interactions.
formed by the rest of the molecule. This accounts for both
the reduced intensity of the NIR absorption maximum and
the rhombic-shape of the ESR spectrum (vide supra).
In the present series of compounds, the organic fluorine
atom in compound [Bu4N][Ni(FPhdt)2] (3) plays some
important role in the supramolecular aspect of its solid-state
structure. Fluorine can accept a hydrogen bond involving
C-H · · · F hydrogen bonding interactions.38 The C-H · · · F
hydrogen bond is very unique in the sense that it is very
weak but a soft donor-hard acceptor interaction of unusual
kind. Even though fluorine is the most electronegative
element in the periodic table, organic fluorine accepts a
hydrogen bond with great difficulty because of its lack of
polarizability and therefore reports on C-H · · · F hydrogen
bonds are very scarce in literature.39 In the present system
(compound 3), the existence of C-H· · ·F hydrogen bonding
interactions, that involve the [Bu4N]+cation and the [Ni-
(FPhdt)2]- anion, lead to the formation of a chainlike
structure as depicted in Figure 5. As shown in Figure 5, the
fluorine (F1) undergoes bifurcated hydrogen bonding interac-
tions with C15 and C16 of the cation. The hydrogen bonding
parameters for these C-H· · ·F interactions (C15-H15A· · ·F1:
0.97, 2.62, 3.201(3), 118.9; C16-H16C · · · F1: 0.96, 2.76,
3.339(3), 119.7) are comparable with previously reported
C-H · · · F hydrogen bonds.39
The packing diagram of compound [Bu4N][Ni(FPhdt)2]
(3) (viewed down to the crystallographic b axis) shows that
the [Ni(FPhdt)2]- molecules pack in the form of a layered
structure that includes the aggregation of linearly arranged
complex molecules on the ac plane as shown in Figure 6.
The closest Ni · · · Ni contact is of 8.949(2) Å. The large
separation between stacks of complexes (in each linear
arrangement) is probably due to the external substitution of
a phenyl ring on the dithiolene moiety, in which the phenyl
ring slightly deviates from the molecular plane. Notably, this
compound shows no short S · · ·S or F· · ·F interactions within
its crystal structure.
a CH2Cl2/hexane (1:1) mixture and crystallized in the mono-
clinic space group P2(1)/c. The molecular structure with the
appropriate label scheme is shown in Figure 7. The basic
crystallographic data are presented in Table 2, and the
selected interatomic distances and bond angles are given in
Table 4. The four sulfur atoms that surround the nickel ion
provide the square planar coordination with average S-Ni-S
angle of 90.5°, with the deviation of nickel ion of 0.0160(4)
Å from the square plane. In the five membered chelate ring
containing the nickel atom, the average Ni-S, C-S, and
CdC bond lengths are consistent with those in the crystal
structure of compound [Bu4N][Ni(FPhdt)2] (3).
The outer two phenyl rings deviate from the planarity of
the dithiolene moiety, as observed in the crystal of compound
3. In compound 4, the dihedral angles between the least-
squares planes of Ni-dithiolene chelate rings {(Ni1, S1, S2,
C1, C2) and (Ni1, S3, S4, C9, C10)} and external phenyl
rings {(C3, C4, C5, C6, C7, C8) and (C11, C12, C13, C14,
C15, C16)} are 29.57° and 22.02°, respectively. Interestingly,
in compound [Bu4N][Ni(FPhdt)2] (3), both external phenyl
rings deviate in the same fashion with a dihedral angle of
17.29 (0.12)° from the least-squares plane of the rest of the
Ni-dithiolene chelate ring, but in the compound [Bu4N]-
[Ni(ClPhdt)2] (4), both external rings deviate inconsistently
with two different dihedral angles of 29.57° and 22.02°,
respectively (from the least-squares planes of Ni-dithiolene
chelate ring). We believe that these differences might be due
to the difference in substitution of the halogen atoms namely,
F and Cl on the p-position of the external phenyl ring. In
compound 3, the fluorine atom is involved in hydrogen
bonding interactions (namely CsH · · · F), whereas in com-
pound 4, the substituted chlorine atom is not involved in
such hydrogen bond formation. Thus, it can be argued that
in similar mononuclear metal-asymmetric-dithiolene com-
plexes, the molecular arrangement can be altered by varying
the substituents on the dithiolene moiety: fluorine substitution
(compound 3) gives a layer of simple chain-like arrangement
(Figure 6); on the other hand, chlorine substitution (com-
pound 4) affords layers of wave-like zigzag chains of
complex anions as shown in Figure 8.
Crystal Structure Description of Compound [Bu4N][Ni-
(ClPhdt)2] (4). Crystals of compound [Bu4N][Ni(ClPhdt)2]
(4), suitable for X-ray structure analysis, were grown from
The closest Ni · · · Ni separation is found to be 9.072(2) Å,
and the large separation between the molecular stacks (in a
zigzag wave-like arrangement, Figure 8) is due to the external
substitution of the phenyl ring on the dithiolene moiety, as
was described for the complex 3. The Ni · · · Ni separation in
the compound 4 is 0.123 Å larger than that in compound 3.
We have not observed any Cl · · ·Cl interactions in the crystals
(38) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565.
(39) (a) Lee, H.; Knobler, C. B.; Hawthorne, M. F. Chem. Commun. 2000,
2485. (b) Vangala, V. R.; Nangia, A.; Lynch, V. M. Chem. Commun.
2002, 1304. (c) Wiechert, D.; Mootz, D.; Dahlems, T. J. Am. Chem.
Soc. 1997, 119, 12665. (d) Thalladi, V. R.; Weiss, H. C.; Blaser, D.;
Boese, R.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120,
8702. (e) Kryachko, E.; Scheiner, S. J. Phys. Chem. A 2004, 108,
2527.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5067

Madhu and Das
of compound [Bu4N][Ni(ClPhdt)2] (4), despite the presence
of a chlorine atom as a substituent on the outer phenyl ring.
Crystal Structure Description of 4,4′-Bis(4-chlorophe-
nyl)-tetrathiafulvalene (ClPhTTF) (6). Orange crystals of
compound 6 can easily be grown from CHCl3 solvent by
slow evaporation. Compound 6 crystallizes in the monoclinic
system with space group P2(1)/c. The asymmetric unit of
compound 6 contains half of the molecule.The molecular
structure of 6 is presented in Figure 9 with the atom labeling
description. The crystallographic data are presented in Table
5, and the selected distances and bond angles are described
in Table 6. The TTF skeleton is almost planar, and both
external phenyl rings (C4, C5, C6, C7, C8, C9) are
considerably tilted, with a dihedral angle of 28.28 (0.08)°.
In the five membered dithiolate ring, the average C-S, CdC,
and S-C bond distances are 1.752 Å, 1.34 Å, and 1.761 Å,
respectively (Figure 9). The packing diagram of ClPhTTF
(6), shown in the Figure 10, shows that the molecules are
arranged in a slanting manner leading to the layered like
arrangement. As expected, we found short S · · · S contacts
in which the interplanar distances between stacks is 3.439
Å. These are the S · · · S separations that are even less than
the sum of their van der Waals radii (3.70 Å). This S · · · S
interaction is shown in Figure 11, which is another view
of the packing of the molecules in compound 6.
Crystal Structure Description of Compound [Ni(ClPh-
dt)2] (8): Comparison with Compounds [Bu4N][Ni(FPhdt)2]
(3) and [Bu4N][Ni(ClPhdt)2] (4). Dark greenish block
crystals of neutral nickel(IV) compound [Ni(ClPhdt)2] (8),
suitable for X-ray structure determination, could be grown
from a CH3CN solvent by diffusion of ether. Single crystal
X-ray structure analysis shows that the crystals belong to
the monoclinic system with a space group P2(1)/c and half
of the molecule in the asymmetric unit. Figure 12 shows
molecular structure of compound 8 with the atom labeling
scheme.
The basic crystallographic data are presented in Table 5,
and the selected interatomic distances and bond angles are
presented in Table 7. The four sulfur atoms surrounding the
nickel ion provide a perfect square planar coordination
(deviation of the nickel ion from the {S4} square plane )
0.000 Å) with average S-Ni-S angle of 90°. In the five
membered chelate ring containing the nickel atom, the
average Ni-S, C-S, and CdC bond distances are 2.126 Å,
1.696 Å, and 1.373 Å, respectively (Table 7). The corre-
sponding average C-S values in the crystal structures of
compounds [Bu4N][Ni(FPhdt)2] (3) and [Bu4N][Ni(ClPhdt)2]
(4) are considerably larger (1.726 Å for compound 3 and
1.724 Å for compound 4, Tables 3 and 4, respectively). On
the other hand, the CdC bond in neutral compound 8 is
larger (1.373 Å) than average CdC bonds in compounds 3
and 4 (1.349 Å for compound 3 and 1.337 Å for compound
4). In this context, it is worth mentioning that monoanionic
compounds [Bu4N][Ni(FPhdt)2] (3) and [Bu4N][Ni(ClPhdt)2]
(4) have nickel ions in formally +3 oxidation states, and
neutral compound 8 is formally a Ni(IV) complex that was
isolated by iodine oxidation of compound 4 (vide supra).
Thus, it is possible that electron delocalization due to the
higher oxidation state of nickel in compound 8 results in a
shorter C-S bond and a relatively longer CdC bond. The
average Ni-S bond length, found in the crystal structure of
compound 8, is typical of a neutral Ni(IV)-dithiolene
complex.40
The packing diagram of the neutral compound [Ni-
(ClPhdt)2] (8) viewed down to the crystallographic b axis is
depicted in Figure 13a, which shows that molecules pack in
a layered structure on the crystallographic ac plane. We
found that the closest Ni · · · Ni distance is 6.362(1) Å and
the shortest interstack S· · ·S contact is 3.616(2). This distance
(between S(1) and S(2) in the crystal structure) is comparable
to the sum of their van der Waals radii (3.70 Å). Similar
neutral systems with such S · · ·S contacts have been reported
earlier.41
The short S · · · S contacts (in compound 8) are shown in
Figures 13b and 14. Further, the π-π stacking interactions
among the external aromatic rings on the dithiolene part of
compound [Ni(ClPhdt)2] (8) lead to a chain type arrangement
as shown in Figure 15. The relevant centroid-centroid
(Cg-Cg) distance is 4.3032(17) Å, and the mean plane
separation is 3.450 Å between aromatic rings.
Supramolecular Cl · · · Cl Interactions in Compounds
(ClPhTTF) (6) and [Ni(ClPhdt)2] (8). The chlorine sub-
stitution on the outer phenyl ring on the dithiolene moiety
(in compounds 6 and 8) suggests the possibility of the
existence of Cl · · ·Cl interactions in the relevant crystals. The
weak intermolecular interactions between chlorine atoms
have been a subject of interest and debate for many
decades.42 Two hypotheses have been proposed that account
for the short Cl · · · Cl contacts in the crystal structure of
chlorinated molecules. It was originally suggested that the
atomic charge density of chlorine has a nonspherical shape,
producing a decreased repulsion and thus closer Cl · · ·Cl
contacts in certain directions.43 Afterward, an alternative
hypothesis was given according to which the Cl· · ·Cl
associates are the results of a specific attractive force (for
example, “donor-acceptor”, “secondary”, or “charge-
transfer” interaction, or “incipient electrophilic and nucleo-
philic attack”, involving a weak form of covalent bonding)
that produces short Cl · · · Cl contacts in certain directions.44
Desiraju and Parthasarathy reported Cl · · · Cl close contacts
in the crystal structures of chlorinated hydrocarbons.45
Afterward, several reports on Cl · · ·Cl interactions appeared.46
In many cases, intermolecular interactions associated with
these Cl · · · Cl contacts are shorter than the sum of the van
(40) (a) Rabaca, S.; Duarte, M. C.; Santos, I. C.; Fourmigue´, M.; Almeida,
M. Inorg. Chim. Acta 2007, 360, 3797. (b) Charlton, A.; Hill, C. A. S.;
Underhill, A. E.; Malik, K. M. A.; Hursthouse, M. B.; Karaulov, A. I.;
Moller, J. J. Mater. Chem. 1994, 4, 1861. (c) Charlton, A.; Kilburn,
J. D.; Underhill, A. E.; Webster, M. Acta Crystallogr. 1996, C52, 2441.
(41) Kobayashi, A.; Fujiwara, E.; Kobayashi, H. Chem. ReV. 2004, 104,
5243.
(42) (a) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids;
Elsevier: New York, 1989; pp 175-201. (b) Price, S. L.; Stone, A. J.;
Lucas, J.; Rowland, R. S.; Thomley, A. E. J. Am. Chem. Soc. 1994,
116, 4910.
(43) Nyburg, S. C.; Wong-Ng, W. Proc. R. Soc. London 1979, A367, 29.
(44) Ramasubbu, N.; Parthasarathy, R.; Murray-Rust, P. J. Am. Chem. Soc.
1986, 108, 4308.
(45) Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, I l l, 8725.
5068 Inorganic Chemistry, Vol. 47, No. 12, 2008

Substituted Bis(1,2-dithiolato)-nickel(III) Complexes
der Waals radii of the contacting chlorine atoms. In the
present work, strong Cl · · · Cl interactions are evidenced by
the Cl · · ·Cl separations of 3.279 Å and 3.275 Å in the crystal
structures of compounds ClPhTTF (6) and [Ni(ClPhdt)2] (8),
respectively. These interactions in compound 8 are shown
in Figure 16. Each of these Cl · · · Cl contacts is considerably
shorter than the sum of the van der Waals radii of the
concerned chlorine atoms (van der Waals radius of chlorine
is 1.76 Å). Compound 6 is a tetrathiafulvalene derivative.
Among the numerous halogenated tetrathiafulvalenes the
synthesis of which has been described so far, only a few
have been crystallographically characterized.47 Thus, com-
pound 6 is one of rare structurally characterized tetrathi-
afulvalene that shows short Cl · · · Cl contacts. To the best of
our knowledge, compound [Ni(ClPhdt)2] (8) exhibits the first
structure of a transition metal dithiolene complex in which
short Cl · · · Cl contacts have been observed.
Structural Relationships between Neutral [Ni(ClPhdt)2]
(8) and ClPhTTF (6): Emergence of an Inorganic Coun-
terpart of Tetrathiafulvalene. We have synthesized two
TTF derivatives, ClPhTTF(6) and BrPhTTF (7); one of
these (compound 6) has been characterized unambiguously
by crystal structure analysis. In this section, we have
described a very interesting structural relationship between
the neutral compound [NiIV(ClPhdt)2] (8) and tetrathiaful-
valene 6 based on their structural similarities in terms of
their S · · · S contacts and packing diagrams. Moreover, as
discussed in the preceding section, compounds 6 and 8
show Cl · · · Cl contacts of almost the same distances (3.279
Å for compound 6 and 3.275 Å in compound 8). Figures
17a and 17b depict molecular structures of compounds
ClPhTTF (6) and [Ni(ClPhdt)2] (8), respectively. At the
first glance, it appears that both structures become identical
if the metal center (Ni4+ ion) in compound 8 is replaced
by a CdC double bond or the central CdC double bond
in compound 6 is substituted by a Ni4+ ion. Both
operations would give identical molecular structures.
In the case of compound [Ni(ClPhdt)2] (8), the C-S (the
average bond lengths of C(1)-S(1) and C(2)-S(2)) and
CdC (C(1)-C(2)) bond lengths of nickel chelate five
membered rings are 1.696 Å and 1.373 Å, respectively
(Figure 12). Similarly, the five membered ring of the
tetrathiafulvalenes has average C-S and CdC bond lengths
of 1.752 Å and 1.34 Å, respectively (Figure 9). Thus, for
compound [Ni(ClPhdt)2] (8), the average C-S bond distance
is 0.056 Å less than that of compound 6. This clearly
indicates that the degree of delocalization is more in
compound 8 than that in compound 6.
dihedral angle, in compound 8, is 4.53° larger that in
compound 6. This might be the reason for the slightly larger
S · · · S contact for compound 8 than that in compound 6.
Thus, on the basis of the remarkable similarity between
compounds 6 and 8 (see Figure 17) in terms of molecular
structures, bond distances, and angles, S · · · S contacts, and
overall molecular packing diagrams along with strong
Cl · · · Cl interactions, we have classed compound [Ni-
(ClPhdt)2] (8) as the “inorganic counterpart to the organo-
sulfur donor tetrathiafulvalene”. We believe that Cl· · ·Cl
interactions play an important role in this common structur-
ing. Similar analogy was offered previously for F · · ·F
interactions.48
Conclusion
We have presented a rational synthetic strategy for the
preparation of a new series of asymmetrically substituted
bis(dithiolene) monoanionic Ni(III) complexes [Bu4N]-
[Ni(XPhdt)2] (where X ) H, F, Cl, Br, and NO2) and a
neutral Ni(IV) complex [Ni(ClPhdt)2]. We have demonstrated
that, by varying the substituents of the dithiolene moiety of
the complexes, one can tune their NIR absorption maxima.
Moreover, small changes of substitution (for example, from
fluorine to chlorine) on the dithiolene results in a diversity
of structural arrangements. We have observed that the nature
of the substituents on the dithiolene moiety plays an
important role in tuning the redox potentials. The nickel
complexes 1-5 in this present study are in a Ni3+ oxidation
state as confirmed by the ESR studies. We have presented
the synthesis and characterization of two new asymmetrically
substituted halogen derivatives of tetrathiafulvalene. Such
tetrathiafulvalene derivatives as precursors to new substituted
tetrathiafulvalene derivatives with C-C bond formation can
be synthesized, which are otherwise difficult to prepare using
normal synthetic routes. Finally, we have discussed the
structural analogy between a neutral unsymmetrical Ni(IV)
dithiolene complex and a TTF derivative and argued that,
the neutral compound [Ni(ClPhdt)2] can be classed as the
“inorganic counterpart to tetrathiafulvalene”. We have
reported a C-H · · · F hydrogen bonding interaction in the
solid-state structure of compound 3, which is rarely known
in metal dithiolene chemistry. Furthermore, we have de-
scribed noncovalent strong Cl · · · Cl interactions (compound
8), which is a new addition to metal dithiolene chemistry.
Acknowledgment. We thank Department of Science and
Technology, Government of India, for financial support
(Project No. SR/SI/IC-23/2007). The National X-ray dif-
fractometer facility at the University of Hyderabad by the
Both compounds exhibit the intermolecular S · · ·S contacts
in their solid states, and interestingly, all four sulfur atoms
of each compound interact with sulfur atoms of adjacent
molecules along the stacking direction (Figures 11, 14, and
17). In the compound [Ni(ClPhdt)2] (8), the average S · · · S
contact distance is 0.177 Å larger than that in compound
ClPhTTF (6). In both cases, the external phenyl rings are
twisted considerably from their respective least-squares
planes of the rest of the molecules, e.g., nickel bis-dithiolene
chelate in compound [Ni(ClPhdt)2] (8). The concerned
(46) (a) Broder, C. K.; Howard, J. A. K.; Keen, D. A.; Wilson, C. C.;
Allen, F. H.; Jetti, R. K. R.; Nangia, A.; Desiraju, G. R. Acta
Crystallogr. 2000, B56, 1080. (b) Reddy, C. M.; Kirchner, M. T.;
Gundakaram, R. C.; Padmanabhan, K. A.; Desiraju, G. R. Chem.sEur.
J. 2006, 12, 2222. (c) Devic, T.; Evain, M.; Moe¨lo, Y.; Canadell, E.;
Auban-Senzier, P.; Fourmigue´, M.; Batail, P. J. Am. Chem. Soc. 2003,
125, 3295. (d) Me´zie´re, C.; Fourmigue´, M.; Canadell, E.; Cle´rac, R.;
Bechgaard, K.; Auban-Senzier, P. Chem. Mater. 2000, 12, 2250.
(47) Fourmigue´, M.; Batail, P. Chem. ReV. 2004, 104, 5379.
(48) Dautel, O. J.; Fourmigue´, M.; Canadell, E.; Auban-Senzier, P. AdV.
Funct. Mater. 2002, 12, 693.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5069

Madhu and Das
Supporting Information Available: Synthetic procedures (along
with synthesis schemes) for thione derivatives which have been
used as starting precursors for the preparation of compounds 1
through 5 (PDF) and complete X-ray crystallographic files in CIF
format for four compounds 3, 4, 6, and 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
Department of Science and Technology, Government of
India, is gratefully acknowledged. We are grateful to UGC,
New Delhi, for providing the infrastructure facility at
University of Hyderabad under a UPE grant. V.M. thanks
CSIR, New Delhi, for a fellowship. We thank to Professor
S. Pal for helpful discussion and Mr. B. Ramababu for his
help in preparing this manuscript.
IC701408M
5070 Inorganic Chemistry, Vol. 47, No. 12, 2008