N-Heterocyclic based new nickel-bis(dithiolene) complexes: Synthesis, characterization and properties

Article abstract of DOI:10.1016/j.poly.2012.11.046

Five new nickel-bis(dithiolene) complexes (PPh₄) ₂[Ni^II(C₈H₂N₂S ₂R₂)₂] (R = pyridin-2-yl (1), pyridin-3-yl (2), thiophen-2-yl (3), furan-2-yl (4)) and (PPh₄)[Ni ^III(C₈H₂N₂S₂R ₂)₂] (R = naphthalen-2-yl (5)), have been prepared by the treatment of N-heterocyclic based dithiolene ligands with sodium metal in dry methanol followed by the addition of nickel chloride salt. All these dithiolene ligands and metal coordination complexes are characterized by LC-MS-, ¹H NMR- and HRMS-, IR-, UV-Vis-NIR-spectroscopy, routine elemental analysis and cyclic voltammetry. Compounds 2 and 3 are structurally characterized by single crystal X-ray crystallography. Complex 2 crystallizes in monoclinic space group P2(1)/c, whereas complex 3 crystallizes in triclinic space group P1?. Both these complexes (2 and 3) exhibit two-dimensional supramolecular networks through C-H?N hydrogen bonding interactions in their crystal structures. Nickel(II) compounds 1-4 exhibit a oxidative response in the range of E_1/2 = +0.17 to +0.21 V versus Ag/AgCl, whereas, nickel(III) complex 5 shows both oxidative and reductive responses. Compounds 1-4 show electronic absorptions in the region of 630-650 nm, while compound 5 exhibits absorption band at 890 nm. The positions of the absorption maxima in the electronic absorption spectra of compounds 1 and 2 depend on the solvent polarity, whereas absorption bands of complexes 3-5 are not influenced by the solvent polarity.

Full text of DOI:10.1016/j.poly.2012.11.046

Polyhedron 50 (2013) 612–621
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
N-Heterocyclic based new nickel–bis(dithiolene) complexes: Synthesis,
characterization and properties
⇑
Ravada Kishore, Samar K. Das
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Five new nickel–bis(dithiolene) complexes (PPh4)2[NiII(C8H2N2S2R2)2] (R = pyridin-2-yl (1), pyridin-3-yl
(2), thiophen-2-yl (3), furan-2-yl (4)) and (PPh4)[NiIII(C8H2N2S2R2)2] (R = naphthalen-2-yl (5)), have been
prepared by the treatment of N-heterocyclic based dithiolene ligands with sodium metal in dry methanol
followed by the addition of nickel chloride salt. All these dithiolene ligands and metal coordination com-
plexes are characterized by LC–MS-, 1H NMR- and HRMS-, IR-, UV–Vis–NIR-spectroscopy, routine ele-
mental analysis and cyclic voltammetry. Compounds 2 and 3 are structurally characterized by single
crystal X-ray crystallography. Complex 2 crystallizes in monoclinic space group P2(1)/c, whereas complex
Article history:
Received 9 August 2012
Accepted 27 November 2012
Available online 7 December 2012
Keywords:
Ni–dithiolene complexes
Electronic structures
Electrochemical studies
Solvatochromism
ꢀ
3 crystallizes in triclinic space group P1. Both these complexes (2 and 3) exhibit two-dimensional supra-
molecular networks through C–HÁ Á ÁN hydrogen bonding interactions in their crystal structures. Nickel(II)
compounds 1–4 exhibit a oxidative response in the range of E1/2 = +0.17 to +0.21 V versus Ag/AgCl,
whereas, nickel(III) complex 5 shows both oxidative and reductive responses. Compounds 1–4 show elec-
tronic absorptions in the region of 630–650 nm, while compound 5 exhibits absorption band at 890 nm.
The positions of the absorption maxima in the electronic absorption spectra of compounds 1 and 2
depend on the solvent polarity, whereas absorption bands of complexes 3–5 are not influenced by the
solvent polarity.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Scheme 2, for example, demonstrates two nickel dithiolene com-
plexes, in which cyano group substituted nickel complex exhibits
Metal coordination complexes of dithiolene (1,2-enedithiolate)
ligands exhibit significant and diverse properties [1–6]. Among
these, nickel based complexes, for example, [Ni(S2C2R2)2]nÀ
(n = 0–2) attract considerable attention because of their multiple
accessible redox states [7]. The redox activity of the dithiolene li-
gands can be explained by the existence of canonical forms
(Scheme 1) [8], because of which, metal can exist in variable
oxidation states (+4 to 0). This ambiguity is a classic example of
non-innocent behavior [9] of dithiolene ligands. Nickel–dithiolene
complexes exhibit interesting properties, such as, metallic-
conductivity [10], optical nonlinearity [11] and reactivity with ole-
fins leading to olefin purification [12], molecular magnetism [13],
and strong absorbance in the near-infrared region, which is impor-
tant for Q-switching laser applications [14]. Square planar dithio-
lene coordination complexes are generally characterized by high
extent of electron delocalization within the chelate ring involving
the metal ion that appreciably contributes to the low energy elec-
tronic transition between the HOMO and the LUMO. This extensive
delocalization is responsible for metal dithiolene complexes to
show an absorption band in its near-infrared (NIR) region [15].
absorption band at 863 nm and the phenyl substituted metal com-
plex reveals electronic absorption band at 935 nm. This indicates
that substituent on dithiolene moiety can influence in the shift of
position of electronic absorption band of charge transfer transition,
meaning thereby, electron-donating/electron-withdrawing sub-
stituents play an important role in the delocalization of the chelate
ring in metal–bis(dithiolene) complexes [16–21]. These facts
prompted us to design and synthesize new type of N-hetero substi-
tuent-based (pyridin-2-yl, pyridin-3-yl, thiophen-2-yl, furan-2-yl
and naphthalen-2-yl) dithiolene ligands and to synthesize corre-
sponding metal bis(dithiolene) complexes (PPh4)2[NiII(C8H2N2S2R2)2]
(R = pyridin-2-yl (1), pyridin-3-yl (2), thiophen-2-yl (3), furan-2-yl
(4)) and (PPh4)[NiIII(C8H2N2S2R2)2] (R = naphthalen-2-yl (5)), which
enable us to compare the substituent effect on the redox potentials
of the title complexes.
2. Experimental
2.1. Materials and physical methods
All the reagents for synthesis were commercially available and
used as received. 4,5-bis-thiocyanato-banezene-1,2-diamine, 5,6-
diaminobenzene-1,3-dithiole-2-thione, 1,2-di(pyridin-3-yl)ethane-
⇑
Corresponding author. Tel.: +91 40 2301 1007; fax: +91 40 2301 2460.
E-mail address: skdsc@uohyd.ernet.in (S.K. Das).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.11.046

R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
613
2.1.1. General synthetic route for the preparation of the compounds
(a–e)
A mixture of solution containing 5,6-diaminobenzo[d][1,3]-
dithiole-2-thione (1 mmol), corresponding 1,2-diketone (1 mmol),
and iodine (10 mol%) in CH3CN (10.0 mL) was stirred at room
temperature for 2 h as shown in scheme 3. The resulting yellow
colored precipitate was separated by filtration, washed with little
CH3CN and dried in an open air. The characterization data of
these compounds are described in the section of Supplementary
data.
R
R
S
S
R
R
S
S
R
R
S
S
-e
-e
+e
+e
Scheme 1. Canonical forms of a dithiolene ligand.
1
2.1.2. General synthetic route for the preparation of the compounds
(f–j)
1
NC
NC
CN
S
S
S
S
S
S
S
S
Following a literature procedure [26], to a chloroform–acetic
acid (3:1) solution of each of a–e (1 mmol), Hg(OAc)2 (3 mmol)
was added and it was stirred for 12 h at room temperature under
nitrogen atmosphere (Scheme 3). The resulting precipitate was fil-
tered off by using Celite and washed with chloroform. The filtrate
was neutralized with NaHCO3 solution, and extracted with dichlo-
romethane solvent and dried over Na2SO4. The solvent was re-
moved by rotary evaporator which produces pale yellow colored
compound as product. The characterization data for f–j are de-
scribed below:
Ni
Ni
CN
Scheme 2. Nickel dithiolene complexes.
1,2-dione and 1,2-di(naphthalen-2-yl)ethane-1,2-dione were pre-
pared according to literature procedures [22–25]. Solvents were
dried by standard procedures. Micro-analytical (C, H, N) data were
obtained with a FLASH EA 1112 Series CHNS Analyzer. Infrared spec-
tra were recorded as KBr pellets on a JASCO-5300 FT-IR spectropho-
tometer at 298 K. Electronic absorption spectra of all compounds
were recorded on a Shimadzu UV–Vis–NIR spectrophotometer.
NMR spectra were recorded on Bruker 400 and 500 MHz spectrom-
eter. The chemical shifts (d) are reported in ppm. A Cypress model
CS-1090/CS-1087 electro analytical system was used for cyclic vol-
tammetric experiments. The electrochemical experiments were
measured in dimethylsulfoxide containing [Bu4N][ClO4] (TBAP) as
a supporting electrolyte, using a conventional cell consisting of two
platinum wires as working and counter electrodes, and a Ag/AgCl
electrode as a reference. Under identical condition, Fc+/Fc couple
was observed at 0.53 V. The potentials reported here are uncorrected
for junction contributions. The electron paramagnetic resonance
(EPR) spectra were recorded on a (JEOL) JESFA200 EPR spectrometer,
and the solution state EPR spectra were recorded on a Bruker-ER073
instrument equipped with an EMX microX source for X band
measurement using XENON 1.1b.60 software provided by the
manufacturer.
2.1.2.1. 6,7-Di-pyridin-2-yl-1,3-dithia-5,8-diaza-cyclopenta[b]naph-
thal-2-one (f). (Yield: 92%, 0.344 g). 1H NMR (400 MHz, d ppm)
(DMSO-d6): 7.40–7.43 (m, 2H), 8.01–8.06 (m, 4H), 8.30 (s, 2H),
8.75 (s, 2H). LC–MS (ESI): m/z: 375 [M+H]+. Anal. Calc. for
C19H10N4OS2: C, 60.95; H, 2.69; N, 14.96. Found: C, 60.85; H,
2.75; N, 14.69%.
2.1.2.2. 6,7-Di-pyridin-3-yl-1,3-dithia-5,8-diaza-cyclopenta[b]naph-
thalen-2-one (g). (Yield: 94%, 0.351 g). 1H NMR (400 MHz, d ppm)
(DMSO-d6): 7.40–7.44 (m, 2H), 8.01–8.05 (m, 4H), 8.30 (s, 2H),
8.75 (s, 2H). LC–MS (ESI): m/z: 375 [M+H]+. Anal. Calc. for
C19H10N4OS2: C, 60.95; H, 2.69; N, 14.96. Found: C, 60.79; H,
2.78; N, 14.58%.
2.1.2.3. 6,7-Di-thiophen-2-yl-1,3-dithia-5,8-diaza-cyclopenta[b]naph-
thalene-2-one (h). (Yield: 92%, 0.353 g). 1H NMR (400 MHz, d ppm)
(DMSO-d6): 7.11–7.14 (m, 2H), 7.26–7.27 (m, 2H), 7.83–7.84 (d,
2H), 8.57 (s, 2H). LC–MS (ESI): m/z: 385 [M+H]+. Anal. Calc. for
C17H8N2OS4: C, 53.10; H, 2.10; N, 7.29. Found: C, 53.42; H, 2.17;
N, 7.68%.
O
R
O
R
N
N
N
N
H2N
H2N
S
S
R
R
R
R
S
S
S
Hg(OAc)2
S
S
O
I2, CH3CN
S
CHCl3 : AcOH
(3:1)
N
a, b
R =
f, g
h
i
c
d
S
O
j
e
Scheme 3. Synthetic route for compounds a–e and f–j.

614
R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
Scheme 4. Synthetic route for new nickel dithiolene complexes.
2.1.2.4. 6,7-Di-furan-2-yl-1,3-dithia-5,8-diaza-cyclopenta[b]naphtha-
len-2-one (i). (Yield: 91%, 0.320 g). 1H NMR (400 MHz, d ppm)
(DMSO-d6): 6.72–6.79 (m, 4H), 7.92 (s, 2H), 8.58 (s, 2H). LC–MS
(ESI): m/z: 353 [M+H]+. Anal. Calc. for C17H8N2O3S2: C, 57.94; H,
2.29; N, 7.95. Found: C, 58.26; H, 2.37; N, 8.38%.
the diffusion of diethyl ether to their acetonitrile–methanol solu-
tion and acetonitrile solution respectively. Compounds 1, 4 and 5
were crystallized also by diffusion of diethyl ether to the respective
acetonitrile solutions, but we could not obtain crystals, suitable for
single crystal X-ray crystallography because of their tiny sizes. The
characterization data for compounds 1–5 are described below.
Compounds 2 and 3 are additionally characterized by single crystal
X-ray crystallography.
2.1.2.5. 6,7-Di-naphthalen-2-yl-1,3-dithia-5,8-diaza-cyclopenta[b]-
naphthalen-2-one (j). (Yield: 91%, 0.429 g). 1H NMR (400 MHz, d
ppm) (DMSO-d6): 7.45–7.57 (m, 6H), 7.86–7.88 (m, 6H), 8.16–
8.22 (m, 3H), 8.76 (s, 1H). LC–MS (ESI): m/z: 473 [M+H]+. Anal. Calc.
for C29H16N2OS2: C, 73.70; H, 3.41; N, 5.93. Found: C, 74.03; H,
3.45; N, 6.42%.
2.1.3.1. Compound 1. Yield: 0.124 g (62% based on nickel metal).
HRMS: calc: 750.0047, found: 750.0047 (m/z). Anal. Calc. for
C
84H60N8NiP2S4: C, 70.54; H, 4.22; N, 7.83%. Found: C, 70.41; H,
4.18; N, 7.96%. IR (KBr, m
/cmÀ1): 3468, 3051, 2922, 2852, 1635,
1585, 1473, 1431, 1348, 1207, 1087, 995, 860. 1H NMR
(400 MHz, d ppm) (CD3CN): 8.12–2.11 (m, 1H), 8.04–7.89 (m,
13H), 7.77–7.65 (m, 40H), 7.62–7.60 (m, 3H), 7.47–7.42 (m, 3H).
2.1.3. General procedure for the synthesis of nickel dithiolene
coordination complexes 1–5
Sodium metal (30 mg) was added to the 10 mL methanol solu-
tion of f–g (2 mmol); after getting clear solution, NiCl2Á6H2O
(1 mmol) was added and it was stirred for 10 min, followed by
the addition of [PPh4]Br (2 mmol). The stirring was continued for
an additional 15 min by the addition of 10 mL water, whereby
the product was obtained as precipitate, which was filtered and
air dried (Scheme 4). The crystals of compounds 2 and 3, suitable
for single crystal X-ray structure determination, were obtained
by crystallizing the above mentioned precipitates of 2 and 3 by
2.1.3.2. Compound 2. Yield: 0.103 g (58% based on nickel metal).
HRMS: calc. 750.0047, found: 750.0047 (m/z). Anal. Calc. for
C
84H60N8NiP2S4: C, 70.54; H, 4.22; N, 7.83%. Found: C, 70.65; H,
4.32; N, 7.79%. IR (KBr, m
/cmÀ1): 3435, 3049, 2922, 2851, 1583,
1481, 1327, 1205, 1078, 997, 850. 1H NMR (500 MHz, d ppm)
(CD3CN): 8.06–8.03 (m, 12H), 7.89–7.85 (m, 21H), 7.83–7.78
(m, 23H), 7.72 (s, 4H).
Table 1
Crystal data and structural refinement for compounds 2 and 3.
2
3
Empirical formula
Formula weight
T (K)
C
80H60NiN8S4P2
C80H56N4NiP2S8
1450.42
100
1430.29
298
Crystal size (mm)
Crystal system
Space group
Z
0.24 Â 0.22 Â 0.18
monoclinic
P2(1)/c
0.28 Â 0.26 Â 0.22
triclinic
ꢀ
P1
2
1
k (Å)
0.71073
0.71073
a (Å)
b (Å)
c (Å)
15.506(5)
15.916(5)
15.090(4)
90.00
9.364(10)
10.592(11)
17.983(19)
78.44(2)
a
(°)
b (°)
90.97(2)
76.54(2)
c
(°)
90.00
3723.95(19)
1.276
73.63(2)
1647.0(3)
1.462
V (Å3)
Dcalc (Mg/m3)
Reflections collected/unique
Rint
14890/6451
0.0357
15756/5783
0.0485
F(000)
1484
750
Maximum and minimum transmission
h (°)
0.9208 and 0.8964
2.70–25.00
6451/0/448
0.821
0.8705 and 0.8393
2.03–25.00
5783/0/430
1.208
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
R1/wR2 [I > 2
r
(I)]
0.0550/0.1484
0.0911/0.1761
0.330 and À0.247
0.0704/0.1401
0.0858/0.1464
0.962 and À0.474
R1/wR2 (all data)
Largest diff. peak and hole (e ÅÀ3
)

R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
615
Table 2
(500 MHz, d ppm) (CD3CN): 8.06–8.01 (m, 15H), 7.86–7.54 (m,
33H), 7.43–7.21 (m, 3H), 7.16–7.13 (m, 5H).
Bond lengths (Å) and angles (°) for the compounds 2 and 3.
Compound 2
Ni(1)–S(2)
Ni(1)–S(1)
S(2)–C(2)
N(2)–C(8)
C(1)–S(1)
2.1668(9)
2.1798(9)
1.742(4)
1.333(4)
1.740(4)
S(2)–Ni(1)–S(1)
C(2)–S(2)–Ni(1)
C(31)–P(1)–C(37)
C(31)–P(1)–C(25)
C(1)–S(1)–Ni(1)
91.55(3)
105.29(12)
109.6(2)
109.2(2)
104.84(11)
2.1.3.4. Compound 4. Yield: 0.122 g (58% based on nickel metal).
HRMS: calc: 706.9486, found: 705.9408 [MÀH]+. Anal. Calc. for
C
80H56N4NiO4P2S4: C, 69.31; H, 4.07; N, 4.04%. Found: C, 69.55;
H, 4.13; N, 4.49%. IR (KBr, m
/cmÀ1): 3416, 2922, 2851, 1581, 1481,
1425, 1327, 1251, 1205, 1157, 1078, 997, 887. 1H NMR
(400 MHz, d ppm) (CD3CN): 7.93–7.86 (m, 5H), 7.77–7.70 (m,
15H), 7.69–7.65 (m, 7H), 7.63–7.60 (m, 5H), 7.47–7.42 (m, 14H),
7.26–7.22 (m, 8H), 7.17–7.15 (m, 2H).
Compound 3
Ni(1)–S(2)
Ni(1)–S(1)
S(2)–C(2)
2.1550(11)
2.1719(11)
1.726(4)
S(2)–Ni(1)–S(1)
C(1)–S(1)–Ni(1)
C(2)–S(2)–Ni(1)
C(7)–N(1)–C(5)
91.12(4)
104.85(15)
105.63(16)
118.3(4)
S(4)–C(16)
1.707(5)
2.1.3.5. Compound 5. Yield: 0.128 g (57% based on nickel metal).
HRMS: calc: 946.0863, found: 946.0863 (m/z). Anal. Calc. for
C
80H52N4NiPS4: C, 74.65; H, 4.07; N, 4.35%. Found: C, 74.52; H,
4.12; N, 4.43%. IR (KBr, m
/cmÀ1): 3439, 3053, 2962, 2924, 2852,
Table 3
Hydrogen bonding parameters for compounds 2 and 3 (Å and °).
1658, 1624, 1593, 1433, 1352, 1261, 1176, 1082, 906, 860. 1H
NMR (500 MHz, d ppm) (CD3CN): 8.24–8.22 (m, 6H), 8.17–8.13
(m, 7H), 8.09–8.08 (m, 2H), 8.05–8.02 (m, 5H), 7.88–7.84 (m,
12H), 7.82–7.78 (m, 10H), 7.75–7.70 (m, 5H), 7.67–7.60 (m, 5H).
D–H...A
d(D–H)
d(HÁ Á ÁA)
d(DÁ Á ÁA)
<(DHA)
Compound 2
C(10)–H(10)Á Á ÁN(3)#1
0.93
0.93
2.52
2.92
3.363(6)
3.653(7)
150.2
136.5
C(39)–H(39)Á Á ÁN(3)#2
2.2. Crystal structure determination
Compound 3
C(31)–H(31)Á Á ÁN(1)#3
C(22)–H(22)Á Á ÁN(2)#2
C(27)–H(27)Á Á ÁN(1)#2
C(11)–H(11)Á Á ÁS(4)#4
0.93
0.93
0.93
0.93
2.85
2.85
2.86
3.02
3.529(6)
3.656(6)
3.699(6)
3.862(5)
131.1
145.9
151.3
151.0
Crystal data of compound 2 were measured at 298(2) K on Ox-
ford Gemini Diffractometer equipped with EOS CCD detector.
Monochromatic Mo K
measurements. Absorption corrections using multi
a
radiation (k = 0.71073 Å) was used for the
–scans were
w
Symmetry transformations used to generate equivalent atoms:
#1 Àx, y + 0.5, Àz + 0.5 #2 Àx, Ày + 1, Àz + 1 #3 Àx + 1, Ày, Àz + 1 #4 Àx, Ày, Àz + 1.
applied. Single crystals, suitable for facile structural determination
for the compound 3, was measured at 100(2) K on a three circle
Bruker SMART APEX CCD area detector system under Mo K
(k = 0.71073 Å) graphite monochromatic X-ray beam, 2400 frames
were recorded with an scan width of 0.3°, each for 10 s, crystal-
detector distance 60 mm, collimator 0.5 mm. Data reduction was
performed by using SAINTPLUS [27]. Empirical absorption corrections
were performed by using equivalent reflections performed pro-
gram SADABS [27]. The structures were solved by direct methods
a
2.1.3.3. Compound 3. Yield: 0.135 g (68% based on nickel metal).
HRMS: calc: 770.8572, found: 769.8494 [MÀH]+. Anal. Calc. for
x
C
80H56N4NiP2S8: C, 66.24; H, 3.89; N, 3.86%. Found: C, 66.42; H,
3.96; N, 4.19%. IR (KBr,
m
/cmÀ1): 3437, 2922, 2851, 1641, 1581,
1514, 1483, 1431, 1358, 1203, 1080, 995, 848. 1H NMR
N
N
N
N
N
N
N
N
N
N
S
S
S
S
S
S
N
N
N
N
N
N
(PPh )
(PPh )
Ni
Ni
4 2
4 2
S
S
(2)
(1)
S
O
N
N
N
N
N
N
S
S
S
S
S
S
O
(PPh )
4 2
Ni
(PPh )
Ni
4 2
S
O
S
S
S
S
O
N
N
(4)
(3)
N
N
N
S
S
S
(PPh )
Ni
4
S
N
(5)
Scheme 5. Structural representation of synthesized nickel dithiolene complexes 1–5.

616
R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
Fig. 1. Thermal ellipsoidal diagrams of (a) compound 2 (20% probability), (b) compound 3 (40% probability, hydrogen atoms are omitted for clarity).
Fig. 2. (a) Mean plane (M1) of the half dithiolene unit (Ni1S1C1C6C5N1C7C8N2C4C3C2S2) in compound 2 and (b) mean plane (M2) of the half dithiolene unit
(Ni1S1C1C6C5N1C7C8N2C4C3C2S2) in compound 3.
and least-square refinement on F2 for the compounds 2 and 3 by
using SHELXS-97 [28]. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were included in the structure
factor calculation by using a riding model. The crystallographic

R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
617
Fig. 3. (a) Molecular packing diagram of the compound 2 characterized by CÀHÁ Á ÁN weak interactions resulting in two dimensional supramolecular layer, (b) C–HÁ Á Á
interactions between pyridyl rings from the anion moiety and C–HÁ Á Á interactions between centroid of the metal chelate ring and cation (tetraphenylphosphonium), (c)
Two-dimensional supramolecular network through CÀHÁ Á ÁN hydrogen bonding interactions in the crystal structure of compound 3.
p
p
parameters, data collection and structure refinement of the com-
pounds 2–3 are summarized in Table 1. Selected bond lengths
and bond angles for the compounds 2 and 3 are listed in Table 2.
The supramolecular hydrogen bonding interactions in the crystal
structures of compounds 2 and 3 are presented in Table 3. In the
crystal structure of complex 2, the disordered solvent contribu-
tions were removed by the SQUEEZE [29] command in the PLATON
program.

618
R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
Fig. 3. (continued)
yield. Diagrammatic representations of the compounds 1–5 have
been shown in Scheme 5.
3.2. Molecular and crystal structures
The block-shaped-crystals of compound (PPh4)2[Ni(C18H10N4S2)2]
(2) were crystallized in monoclinic space group P(2)/c, whereas
compound (PPh4)2[Ni(C16H8N2S4)2] (3) was crystallized in triclinic
ꢀ
space group P1. The asymmetric units in the crystal structures of
compounds 2 and 3 (represented as labeled atoms in Fig. 1) contain
half molecule of nickel–dithiolene moiety and one tetraphenyl-
phosphonium cation. The average Ni–S bond distances in the crys-
tal structure of compound 2 is 2.173 Å, whereas in compound 3, it
is 2.163 Å. The distance between two mean planes of chelate rings
Ni1S1C1C2S2 in [Ni(C18H10N4S2)2]2À (2) and [Ni(C16H8N2S4)2]2À (3)
are 0.149 Å and 0.172 Å respectively. The dihedral angle between
two S1NiS2 planes in both the compounds 2 and 3 is 0°, that is,
the c1–Ni–c2 angle is 180° (c1 and c2 are the midpoints of the sulfur
atoms in the five-membered chelate rings), which indicates the
geometry around nickel metal centre is perfectly square planar in
both compounds. The angle between two pyridin-3-yl rings
(C14C15C16C17N3C18 and C9C10N4C11C12C13) in compound 2
is 62.20°, and angle between two thiophen-2-yl rings
(C9C10C11C12S3 and C13S4C14C15C16) is 51.96° in compound
3. This indicates that, due to the large size of the pyridin-3-yl rings
(steric effect), the concerned angle is larger in compound 2 com-
Fig. 4. Z-shaped nonplanar geometry.
pared to that in compound 3. The bending angles (g) between
{S1Ni1S2} and {S1C1C2S2} planes present in the {Ni1S1C1C2S2}
dithiolate-chelated ring are 9.78° and 11.31° in compounds 2 and
3 respectively. The distance between two mean planes in com-
pound 2 is 0.423 Å (M1 in Fig. 2), whereas in compound 3 this dis-
tance (M2) between two mean planes is 0.526 Å. As shown in
Fig. 2, the half moiety of M1, which is present in compound 2, is
more planar than the half moiety of M2 which is present in com-
pound 3.
The weak interactions between pyridine moieties with a dis-
tance 2.52 Å (HÁ Á ÁA) of C(10)–HÁ Á ÁN(3) hydrogen bond in com-
pound 2 lead to a two dimensional supramolecular network
(Fig. 3a). In the crystal structure of compound 2, there are addi-
Fig. 5. Normalised electronic absorption spectra for compounds 1–5 in acetonitrile
solution.
3. Results and discussion
3.1. Synthesis
Compounds 1–5 have been synthesized by the reaction of one
equivalent NiCl2Á6H2O with two equivalents of di-substituted-
1,3-dithia-5,8-diaza-cyclopenta[b]naphthalen-2-ones (f–j) in
MeOH treated with sodium metal at an ambient condition and
then precipitated by adding tetraphenylphosphonium bromide,
yielding the microcrystalline form of compounds 1–5 in reasonable
tional C(16)–HÁ Á Á
p
(H16Á Á ÁCg, Cg = C9C10C11C12C13N4) interac-
tions between pyridine moieties with a distance of 3.032 Å; these
interactions involve four anionic moieties around one anionic moi-
ety, as shown in Fig. 3(b). In addition to this, there are other C–
HÁ Á Á
p
interactions C–H(29)Á Á ÁCg and C–H(34)Á Á ÁCg (HÁ Á ÁCg = 3.012
and 2.950 Å respectively), that are present between phenyl moiety

R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
619
the absorption maximum from 714 nm in chloroform to 639 nm
in acetonitrile. This solvatochromic behavior of compounds 1 and
2 indicates more charge transfer nature of the ground state than
the excited state [34–36]. Increase in polarity of the solvent stabi-
lizes the ground state more than the excited state, which further
increases the energy gap between the HOMOs and LUMOs in
dithiolene complexes (for example, in the present study, com-
plexes 1 and 2). The increase in this energy gap with increase in
solvent polarity results in blue shift of the observed absorption
maximum. Unlike compounds 1 and 2, complexes 3–5 did not
show any solvatochromic behavior.
3.4. Electrochemistry
The electrochemical behavior of the complexes 1–5 were
performed in dimethylsulfoxide solution, which contained
[Bu4N][ClO4] (TBAP) as supporting electrolyte and platinum as
working electrode. We have characterized compounds 1–4 as Ni(II)
coordination complexes and compound 5 as Ni(III) complex. Com-
pound 1 exhibits a reversible oxidative response at E1/2 = 0.17 V
Fig. 6. Absorption spectra of compound 2 in different solvents at room tempera-
ture, (a) acetonitrile, (b) DMSO, (c) DMF, (d) acetone, (e) chloroform.
(D
E = 92 mV) versus Ag/AgCl (under identical condition, Fc+/Fc
couple is observed at 0.53 V, see in the section of Supplementary
materials). Compound 2 shows an reversible oxidative response
(from tetraphenylphosphonium cation) and the centroid (Cg) of
the Ni1S1C1C2S2 chelate ring system. Compound 3 also offers a
2-D network through C–HÁ Á ÁN supramolecular interactions as
shown in Fig. 3(c). A prominent feature of the structural aspect
for these two complexes is that [Ni(C8H2N2S2R2)2]2À (R = pyridin-
3-yl and thiophen-2-yl) exhibit non-planar Z-shaped geometry
with the ligand fragments, that are bent away from the {NiS4}
planes with a dihedral angle of 11.14° in compound 2 and 14.28°
in compound 3 (Fig. 4); these values are comparable to those of
known copper dithiolene complexes [30].
at E1/2 = +0.21 V (
DE = 83 mV) versus Ag/AgCl. Compounds 3 and
4 exhibit quasi-reversible oxidative responses at E1/2 = 0.18 V
3.3. UV–Vis absorption spectra for compounds 1–5
Fig. 5 shows the electronic absorption spectra of the complexes
1–5 in acetonitrile. The absorption maxima of the compounds 1–5
are observed at 635, 639, 665, 649 and 892 nm respectively. These
absorption bands can be attributed to the charge transfer transi-
tions (CT) involving electronic excitation from a HOMO which is
a mixture of dithiolate (p) and metal (d) orbital character to a
LUMO which is a p⁄ orbital of the dithiolate [31,32]. The extent
of the relative shift in the absorption maximum of each complex
is explained based on the extent of delocalization (in the chelate
ring) induced by the substituent. The absorption spectra of com-
plexes 1 and 2 did not deviate much from one another and the
close proximity of their maxima is due to the identical nature of
the pyridin-2-yl and pyridin-3-yl substituents, present in com-
pounds 1 and 2 respectively. However, it is noticed that simple ex-
change of furan-2-yl (compound 4) by thiophen-2-yl (compound
3) enhances the extent of electron delocalization (in compound
3), which is responsible for the red shift of the absorption maxi-
mum of complex 3 (665 nm) compared to that of complex 4
(649 nm). Putting more p–electron cloud by introducing naphtha-
lene substituent in complex 5 drastically shifts the absorption
maximum of the complex into the near IR region with absorption
maximum at 892 nm. The influence of different substituents with
different electron delocalization capabilities on the spectral prop-
erties of the nickel complexes is known in literature and the results
observed in the present study are in accord with the mechanisms
explained (in literature) that are based on the delocalization effects
of the substituents [15,33].
Interestingly, complexes 1 and 2, in the present work, show
negative solvatochromic behavior with a blue shift in the spectral
maximum with increase in solvent polarity. Fig. 6 represents the
negative solvatochromic behavior of complex 2 with a change in
Fig. 7. (a) Cyclic voltammogram of compound
2 in TBAP/DMSO at scan rate
50 mVsÀ1. (b) Cyclic voltammogram (reductive response) of the complex 5.

620
R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
Fig. 8. EPR spectra of the compound 5: (a) powder form at room temperature, (b) frozen state (in DMF) at liquid nitrogen temperature.
(
D
E = 120 mV) and 0.17 V (
D
E = 132 mV) versus Ag/AgCl respec-
uration, probable for a 3+ formal oxidation state of nickel. The gav
value of the compound 5 is 2.158, which implies that nickel metal
in this compound has the 3+ oxidation state. The anisotropy in the
ESR spectra implies some contribution of the d-orbitals of nickel in
the total spin density, which is usually, in this class of compouds
[47,48], distributed 40% on nickel and 60% on the ligands.
tively. These oxidative responses correspond to the respective
[NiIII(C8H2N2S2R2)2]1À/[NiII(C8H2N2S2R2)2]2À redox couples. A rep-
resentative cyclic voltammogram of compound
2 is shown
Fig. 7(a) (see the section of Supplementary data for the CV dia-
grams of compounds 1, 3, 4). According to literature [37–40], usu-
ally Ni(II)–bis(dithiolene) complexes exhibit oxidative responses
within the range of E1/2 = +0.12 to +0.41 V. Thus compounds 1–4,
having lower oxidation potential values, can be relatively easily
oxidized to their respective Ni(III) species. Compound 5 (a nicke-
l(III) complex) shows a quasi-reversible oxidative response (see
4. Conclusion
We have described here N-heterocyclic based nickel–bis(dithio-
lene) complexes (PPh4)2[Ni(C8H2N2S2R2)2] (R = pyridin-2-yl (1),
pyridin-3-yl (2), thiophen-2-yl (3), furan-2-yl (4)) and (PPh4)
[Ni(C8H2N2S2R2)2] (R = naphthalen-2-yl (5)). Complexes 2 and 3
have been characterized by X-ray crystallography; the relevant
analysis shows supramolecular weak interactions in the crystal
structures and the distortion in square-planar geometry around
the metal ion in terms of dihedral angle (k) between two SMS
the section of Supplementary data for the relevant diagram) at E1/2
0.09 V (
=
D
E = 111 mV) which corresponds the [NiIV(C8H2N2S2R2)2]0/
[NiIII(C8H2N2S2R2)2]1À (R = naphthalen-2-yl). The very low oxida-
tion potential of compound 5 indicates that this compound is very
susceptible to oxidation to corresponding Ni(IV) complex. Probably
the electron drift from metal center to the substituent naphthalene
ring by resonance effect is responsible for this low oxidation poten-
tial of compound 5. For the same reason, compound 5 would be dif-
ficult to reduce to its corresponding Ni(II) species. Thus, compound
5 exhibits a quasi reversible reductive response at E1/2 = À0.93 V
planes and bending angle (g) (between the SMS and SCCS planes)
of dithiolene chelate ring system. We have compared the effect
of dithiolene-substituents on the electronic absorption spectral
shift. Charge transfer absorption bands in compounds 1 and 2
(635 and 639 nm) are influenced by the solvent polarity, and hence
it shows a negative solvatochromism (absorption band is shifted
towards blue shift for the more polar solvent). An important aspect
of the present study is that compound 5 (a Ni(III) complex) is oxi-
dized electrochemically at a very low oxidation potential meaning
that complex (PPh4)[NiIII(C8H2N2S2R2)2] (5) (R = naphthalen-2-yl)
would be easily oxidized to its corresponding Ni(IV) compound
[NiIV(C8H2N2S2R2)2]. The isolation and further studies of such
Ni(IV) complexes are in progress in our laboratory.
(DE = 173 mV) versus Ag/AgCl, which correspondence to the
[Ni(C8H2N2S2R2)2]1À/[Ni(C8H2N2S2R2)2]2À redox couple (Fig. 7(b)).
It is worth mentioning that compounds 3 and 4 have the similar
oxidation potentials indicating that both these compounds have
identical stabilities towards the chemical oxidation. Based on the
literature [41–43], thiophene-containing nickel dithiolene com-
plexes are more favorable towards electropolymerization. In the
present study, compound 3 (a thiophene-containing nickel dithio-
lene complex) can be easily oxidized (considering its oxidation
potential value) to a neutral species [Ni(C8H2N2S2R2)2] (R = thiophen-
2-yl), which might not be suitable for the electropolymerization.
3.5. ESR spectroscopy
Acknowledgements
The electrochemical studies of compound 5, suggesting that it
should be Ni(III) complex at its resting state, prompted us to per-
form ESR studies of compound 5. The ESR spectra of solid samples
of compound 5 were recorded at room temperature and at liquid
nitrogen temperature; room temperature solution state ESR spec-
trum of compound 5 has also been recorded (see Section-8 in Sup-
plementary data). Compound 5 exhibits a rhombic type signal
(gx = 2.237, gy = 2.195 and gz = 2.044) at liquid nitrogen tempera-
ture as shown in Fig. 8. The ESR studies indicate that compound
5 is a Ni(III) complex. Based on the literature [44–46], the gav val-
ues are in the range of 2.12–2.16 for a d7 low-spin electron config-
We thank Department of Science and Technology, Government
of India, for financial support (Project No. SR/SI/IC-23/2007). The
National X-ray Diffractometer facility at University of Hyderabad
by the Department of Science and Technology, Government of In-
dia, is gratefully acknowledged. We are grateful to UGC, New Delhi,
for providing infrastructure facility at University of Hyderabad un-
der UPE grant. RK thanks CSIR, New Delhi, for their fellowship. We
thank to Mr. K. Santhosh, Mr. Suresh (EPR) for helpful discussion in
preparing this manuscript. We are grateful to Professor S. Pal and
his co-workers for providing us the facility of electrochemical
experiments.

R. Kishore, S.K. Das / Polyhedron 50 (2013) 612–621
[20] Y. Tian-Ming, Z. Jing-Lin, Y. Xiao-Zeng, Polyhedron 14 (1995) 1487.
621
Appendix A. Supplementary material
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(1996) 1633.
CCDC 893972 and 893973 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.poly.2012.11.046.
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