Synthesis, characterization, spectroscopy, electronic and redox properties of a new nickel dithiolene system

Article abstract of DOI:10.1016/j.ica.2010.04.004

A new dithiolene ligand with 3,5-dibromo substituted phenyl groups was designed and synthesized. The protected form of the ligand was reacted with a nickel salt providing neutral Ni(S₂C₂(3,5-C ₆H₃Br₂)₂)₂ and anionic [Ni(S₂C₂(3,5-C₆H₃Br ₂)₂)₂]^- isolated as a Bu ₄N⁺ salt. Both were characterized by UV-Vis and IR spectroscopy and compared with the similar known molecular systems. They exhibit intense low energy transitions that are characteristic of such systems. The electrochemical behavior of these molecules was investigated by cyclic voltammetry.

Full text of DOI:10.1016/j.ica.2010.04.004

Inorganica Chimica Acta 363 (2010) 2857–2864
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization, spectroscopy, electronic and redox properties
of a new nickel dithiolene system
a
a
a
b
Partha Basu a, , Archana Nigam , Benjamin Mogesa , Suzanne Denti , Victor N. Nemykin
*
a Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, PA 15282, United States
b Department of Chemistry and Biochemistry, University of Minnesota-Duluth, Duluth, MN 55812, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A new dithiolene ligand with 3,5-dibromo substituted phenyl groups was designed and synthesized. The
protected form of the ligand was reacted with a nickel salt providing neutral Ni(S2C2(3,5-C6H3Br2)2)2 and
anionic [Ni(S2C2(3,5-C6H3Br2)2)2]ꢀ isolated as a Bu4N+ salt. Both were characterized by UV–Vis and IR
spectroscopy and compared with the similar known molecular systems. They exhibit intense low energy
transitions that are characteristic of such systems. The electrochemical behavior of these molecules was
investigated by cyclic voltammetry.
Received 27 January 2010
Received in revised form 25 March 2010
Accepted 3 April 2010
Available online 11 April 2010
Dedicated to Animesh Chakravorty.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Ni-dithiolene
Synthesis
Redox chemistry
Optical properties
Electronic structures
1. Introduction
anionic, and neutral that differ in their spectroscopic signature and
magnetic properties. Significant early research has been conducted
The long standing interest in the development of sulfur coordi-
nated nickel centers stems from their relevance in the biological
systems as model complexes for the active sites of nickel enzymes
as well as their application in material chemistry. Nickel com-
plexes with sulfur donors are known to exhibit interesting proper-
ties such as metallic-conductivity [1], optical nonlinearity [2],
ability to reversibly react with olefins through a redox couple lead-
ing to olefin purification [3], and strong absorbance in the near-
infrared region suitable in laser Q-switch dyes [4]. Among these,
bis-dithiolene complexes of nickel display important and unusual
properties such as very intense electronic transitions in the near
IR region, capability to exist in different well-defined oxidation
states that can be achieved reversibly, and electrical conductivity
[5]. These square planar complexes exhibit a high degree of elec-
tron delocalization within the chelate ring involving the metal
ion that significantly contributes to the low energy electronic tran-
sition between the HOMO and the LUMO, which occurs at an
unusually low energy. This extensive electron delocalization, while
making these complexes useful as near-infrared (NIR) dyes, is irrel-
evant to intramolecular charge transfer, a crucial factor in generat-
ing second order non-linear optical properties. The delocalized
system can exist in three distinct valence states: dianionic, mono-
with unsubstituted and/or unbranched dithiolene system such as
the maleonitrile (Fig. 1: 3b and 3c). Phenyl substituted dithiolene
ligands and their nickel complexes were originally synthesized
by Schrauzer [6], which provides an entry towards substituted
dithiolene system which can be tailored towards specific function
with judicious choice of substituents [7]. A majority of the cases
the functionalization is done at the para position of the phenyl
group attached to the dithiolene ligand. Herein we report the syn-
thesis, characterization, redox and spectroscopic properties of a
new dithiolene nickel system with 3,5-dibromo substituted phenyl
groups. These bromo substitution set targets for further develop-
ment of functionalized materials. These results are complemented
with computational analyses using density functional theory. We
also report the molecular structure of 3b.
2. Experimental
Unless specified all experiments were performed with oven-
dried glassware under inert atmosphere using Schlenk (argon)
and dry box (nitrogen) techniques. Reagents used in this investiga-
tion were of analytical grade and they were purchased either from
Aldrich or from Acros Chemical Company and used as received
without further purification. Commercial grade solvents were
dried and distilled before use: dioxane, hexane, and diethyl ether
were dried and distilled over Na-benzophenone; methanol and
* Corresponding author.
E-mail address: basu@duq.edu (P. Basu).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.004

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P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
x
Br
Br
x
x
S
S
S
S
NC
NC
S
S
CN
CN
S
S
S
S
S
S
Br
Br
Br
Br
Ni
Ni
Ni
Br
Br
2a (x = 0)
2b (x = -1)
1a (x = 0)
1b (x = -1)
3b (x = -1)
3c (x = -2)
Fig. 1. The compounds used in this investigation.
ethanol were distilled over Mg-turnings; methylene chloride
(CH2Cl2) was dried and distilled over CaH2; CH3CN was distilled
over P2O5. Anhydrous N,N-dimethylformamide (DMF) was ob-
tained from Acros chemicals. 3,5-dibromobenzaldehyde [8], 2a
[6], 2b, 3b and 3c [9], (Fig. 1) were synthesized following reported
procedures.
ammonium bromide (0.32 g, 1.0 mmol) and KCN (0.015 g,
0.211 mmol) were added into the reaction mixture sequentially
and the resulting solution was stirred at room temperature for
18–20 h. The solvent was removed by distillation and the resulting
residue was poured into water (100 mL) and extracted with
CH2Cl2. The organic extracts were washed with saturated NaCl
solution and dried over anhydrous MgSO4. The gummy material
obtained after evaporation of the solvent was crystallized at a
low temperature by keeping the material in a refrigerator. The
product was recrystallized using CH2Cl2/ethanol to obtain pure
benzoin as a yellow solid. Yield: 30%, 1.87 g (3.56 mmol).
Room temperature 1H and 13C NMR spectra were recorded
using
a Bruker ACP-300 spectrometer at 300.133 MHz and
75.469 MHz frequencies, respectively. Mass spectra were recorded
in a Waters LCQ ESI/APCI Quadrupole Mass spectrometer in ESI
and/or APCI modes. Electronic spectra were recorded in Cary 3 or
Cary 14 spectrophotometers. Infrared spectra were recorded on a
Perkin–Elmer FT-IR 1760X spectrometer on NaCl plates or in KBr
pellets. Electrochemical investigations were performed at room
temperature using CV-50W voltammetric analyzer with a typical
three-electrode system: platinum or graphite-working electrode,
Ag/Ag+-reference electrode, and Pt wire-auxiliary electrode. 0.1 M
Tetra butyl ammonium perchlorate (TBAP) was used as a support-
ing electrolyte. Redox potentials were referenced internally with
ferrocenium/ferrocene (Fc+/Fc) couple. Magnetic susceptibility
was measured on a Johnson-Matthey Evans balance, and conduc-
tivity was recorded on a VWR conductivity meter. Elemental anal-
yses were performed at Midwest Microlab, LLC, Indianapolis, IN.
2.2. Synthesis of Ni(S2C2(3,5-C6H3Br2)2)2 (1a)
A mixture of 3,30,5,50-tetrabromobenzil or tetrabromobenzoin
0.32 g (0.61 mmol), 2 equivalents of P4S10 (0.53 g, 1.19 mmol)
and dry dioxane (7 mL) was placed in a 250 mL Schlenk flask.
The reaction mixture was heated at 110 °C for 5–6 h, cooled, fil-
tered to remove unreacted P4S10 and washed with minimum diox-
ane. To the filtrate, an alcoholic solution (5 mL) of NiCl2ꢁ6H2O
(0.16 g, 0.68 mmol) was added whereby the color changed to red-
dish brown. The solution was refluxed with stirring at 90 °C for 2 h,
and the reaction flask was capped and kept in refrigerator for 20 h.
Excess P4S10 precipitated out and was centrifuged and a clear red-
dish-brown solution was evaporated. The residue was dissolved in
a minimal quantity of CH2Cl2, and the target compound was pre-
cipitated as a green crystalline material by adding methanol. Yield:
12%, 0.082 g (0.07 mmol). Absorption spectrum: (CH2Cl2) kmax, nm
2.1. Syntheses of 3,30,5,50-tetrabromobenzoin and 3,30,5,50-
tetrabromobenzil
2.1.1. First method
3,5-Dibromobenzaldehyde (300 mg, 1.14 mmol) was dissolved
in ethanol (7 mL), which was mixed with an aqueous solution
(4 mL) of KCN (36 mg, 0.57 mmol) and stirred for 10 min. The solu-
tion was heated overnight at 80 °C in a water bath, and the result-
ing solution was treated with aqueous NaHCO3 and extracted with
chloroform followed by diethyl ether. Both organic layers were
washed with saturated aqueous NaHSO3 solution and dried over
anhydrous Na2SO4. Evaporation of the chloroform layer yielded a
pale yellow solid (3,30,5,50-tetrabromobenzil, 110 mg, 0.21 mmol)
and the ether layer yielded a yellow solid (3,30,5,0-tetrabromobenz-
oin, 50 mg, 0.094 mmol). 3,30,5,50-tetrabromobenzil: 1H NMR
(CDCl3): d = 8.05 (s, 2H, aromatic), 7.8 (s, 1H, aromatic); 13C NMR
(CDCl3): d = 164 (C@O), 138, 134, 131, 123 (aromatic); IR (KBr):
1642 (C@O); yield = 38%. 3,30,5,50-tetrabro = mobenzoin: 1H NMR
(CDCl3): d = 8.0 (s, aromatic), 7.8 (s, aromatic); 7.5 (s, aromatic),
6.4 (br, s, CHOH), 5.9 (br, s, CHOH); 13C NMR (CDCl3): d = 164
(C@O), 142, 138, 134, 133, 131, 128, 122 (aromatic), 72 (CHOH);
IR (KBr): 1727 (C@O). Yield: 17%.
(
e, Mꢀ1 cmꢀ1): 270 (43 000), 316 (60 000), 383 (sh, 11 800) 582
(2200), 834 (30660); ESI-MS (ꢀ): 1173 (100%), calculated for
28H12S4Br8Ni (M), 1173.
C
2.3. Synthesis of (Bu4N)[Ni(S2C2(3,5-C6H3Br2)2)2] (1b)
The reaction was carried out similar to that for the synthesis of
1a. A mixture of 3,30,5,50-tetrabromobenzil or tetrabromobenzoin
(0.94 g, 1.79 mmol), 2 equivalents of P4S10 (1.58 g) and dry dioxane
(7 mL) was placed in a 250 mL Schlenk flask. The reaction mixture
was heated at 101 °C for 5–6 h, cooled and filtered to remove unre-
acted P4S10 and washed with minimum dioxane. To the filtrate, an
aqueous solution (1 mL) of NiCl2ꢁ6H2O (0.47 g, 1.99 mmol) was
added. The mixture was refluxed at 90 °C for 2 h and allowed to
cool down to room temperature. Tetrabutylammonium bromide
was added to the reaction mixture and stirred overnight. The sol-
vent was evaporated, and the oily residue was washed with petro-
leum–ether several times. The residue was recrystallized from
warm CH2Cl2 and isopropanol by overnight standing yielding dark
reddish violet crystals, which were collected by filtration. Yield:
2.1.2. Second method
3,5-Dibromobenzaldehyde (3.14 g, 11.89 mmol) was dissolved
in 12 mL DMF in a Schlenk flask at room temperature. Tetra butyl
(48%, 0.6 g). Absorption spectrum (CH2Cl2) kmax, nm (
280 (23000), 320 (25000), 380 (sh, 13000), 518 (2000), 680 (500),
e
, Mꢀ1 cmꢀ1):

P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
2859
934 (5000). ESIMS(ꢀ): 1173 (100%) calculated for C28H12S4Br8Ni
2.7. Synthesis of [Bu4N]2[Ni(S2C2(CN)2)2] (3c)
([M-Bu4N]ꢀ), 1173.
1.71 g, 9.24 mmol Na2mnt (Na2S2C2(CN)2) was dissolved in
30 mL of ethanol/water (1:1) and stirred. This solution was added
slowly to another flask containing a solution of NiCl2ꢁ6H2O (1 g,
4.2 mmol) in 7.5 mL of H2O with stirring. A dark red solution was
formed, and the stirring continued for another 5 min after the addi-
tion was complete followed by filtration. To the filtrate, an etha-
nolic solution of t-Bu4NBr was slowly added to the filtrate, which
separated orange red crystals immediately. The crystals were sep-
arated by filtration, washed with H2O (10 mL) followed by ethanol/
water (10 mL, 1:1) and dried. The crude product was recrystallized
from hot acetone (80 mL) and isobutyl alcohol (35 mL) for over-
night, the crystals were washed with isobutyl alcohol and hexane.
Yield: (45.2%, 1.89 mmol.). Absorption Spectrum (CH3CN) kmax, nm
2.4. Synthesis of Ni(S2C2(C6H5)2)2 (2a)
In a 500 mL Schlenk flask, benzil or benzoin (5 g, 23.8 mmol),
P4S10 (21.1 g, 47.6 mmol) and dry dioxane (7 mL) were added under
argon atmosphere and the mixture was refluxed at 110 °C for 5–6 h.
After refluxing, the reaction was filtered to remove unreacted P4S10
and washed with minimal quantity of dioxane. To the filtrate,
NiCl2ꢁ6H2O (6.27 g, 26.4 mmol) in distilled water (1 mL) was added.
The mixture was refluxed at 90 °C for 2 h. A green colored compound
was isolated, which was filtered and washed with methanol, water.
The crude product was recrystallized using CH2Cl2/MeOH. Yield:
(30%, 7.19 mmol). Absorption spectrum (CH2Cl2) kmax, nm (eM,
molꢀ1 cmꢀ1): 312 (44074), 371 (sh, 10785), 437 (sh, 2643), 596
(1920), 852 (27475). ESIMS(ꢀ): 542 (100%) calc. for C28H20S4Ni
(Mꢀ), 542. Anal Calc. C, 61.89; H 3.71. Found: C, 61.52; H 3.81%.
(
eM, molꢀ1 cmꢀ1): 377 (8258), 468 (4474), 514 (1736), 855 (80).
The corresponding dianionic Et4N+ salt was synthesized simi-
larly using Et4NBr as a source of the cation instead of Bu4NBr. From
this dianionic compound, X-ray quality single crystal of the mono-
anionic complex was prepared in air by slow evaporation of a
CH3CN solution.
2.5. Synthesis of [Et4N][Ni(S2C2(C6H5)2)2] (2b)
2.5.1. First method
2.8. Synthesis of [Bu4N][Ni(S2C2(CN)2)2] (3b)
0. 1g, 0.18 mmol of 2a was dissolved in THF. An acetonitrile
solution of Et4NBH4 (0.03 g, 0.18 mmol) was gradually added to
the solution Ni-complex. The color of mixture changed from bluish
green to reddish violet. The mixture was stirred for 21 h. The sol-
vent was evaporated to dryness and the residue was redissolved
in CH3CN and filtered to get reddish violet filtrate and orange col-
ored compound as a residue. The filtrate was concentrated and lay-
ered with ether to get reddish violet compound, which was
dissolved in THF and filtered to get clear reddish violet solution.
The solvent was evaporated to give a reddish violet compound.
The crude product was further recrystallized from hot acetone
and methanol. The reddish purple colored compound was obtained
which was washed with methanol and dried in vacuum. Yield:
12% (0.02 mmol). Absorption Spectrum (CH2Cl2) kmax, nm (eM,
molꢀ1 cmꢀ1): 264 (38320), 313 (43640), 367 (sh, 12640), 528
(1624), 680 (500), 935 (15217). ESI-MS (ꢀ): 542 (100%) Calc. for
An iodine solution (0.3 g, 1.2 mmol) in methyl sulfoxide (1 mL)
was added to the solution of dianionic complex of [Bu4N]2[-
Ni(S2C2(CN)2] (0.5 g, 0.75 mmol) in methyl sulfoxide (4 mL). The
mixture was immediately added to ethanol (12.7 mL) and stirred
for 20–30 min at room temperature. The black crystalline com-
pound was isolated, filtered and washed with ethanol (5–6 times)
followed by ether. The compound was recrystallized by dissolving
the compound in hot acetone and adding ether. Keeping the mix-
ture overnight at room temperature gives black crystalline com-
pound. Yield: (65.3%, 0.4 mmol). Absorption spectrum (CH3CN)
kmax, nm (eM, molꢀ1 cmꢀ1): 346 (sh, 11372), 475 (2510), 538
(670), 602 (461), 863 (9700). mass spectra (m/z) (ESI-): 338
(100%, m/z) [C8S4N4Ni ([M-Bu4N]ꢀ), 338].
C
28H20S4Ni ([M-Et4N]ꢀ), 542. Anal Calc. C, 64.18; H 5.98; N, 2.08.
3. X-ray crystallography
Found. C, 64.23; H 6.00; N, 2.18%.
X-ray quality single crystals were selected under the micro-
scope, and mounted on glass fibers. X-ray intensities were mea-
2.5.2. Second method
sured on
graphite-monochromatized Mo K
room temperature using the
a
Rigaku AFC-7R four-circle diffractometer using
Complex 2a (0.500 g, 0.92 mmol) and p-phenylene diamine
(0.25 g, 2.3 mmol) were dissolved in dry methyl sulfoxide
(10 mL). The reaction mixture was stirred and color of mixture
changed from dark green to reddish brown. After 2 h of stirring,
a
radiation (k = 0.71069 Å) at
x
ꢀ2h scan technique. Three standard
reflections were measured every 150 reflections; a linear correc-
tion factor was applied. The structure of complex was initially
the mixture was poured into
a solution of Et4NBr (0.42 g,
2.01 mmol) in EtOH (15 mL). Red crystalline compound was
recrystallized using hot acetone/MeOH to give dark purple red
compound, which was washed with cold methanol and dried un-
der reduced pressure. Yield: 55% (0.51 mmol).
Table 1
Crystallographic data for 3b.
3b
Formula
Fw
Crystal system
Space group
a (Å)
C16H20N5S4Ni
2.6. Reduction of Ni(S2C2(C6H5)2)2 (2a)
469.34
monoclinic
P21/n
8.180(3)
18.411(3)
14.435(3)
92.79(2)
2171.3(10)
4
1.44
4238
0.0476
0.1176
0.51
In a Schlenk flask, fresh sodium (0.01 g, 0.37 mmol) was added
to a solution of anthracene (0.065 g, 0.37 mmol) in 2 mL of THF in
anaerobic condition. The dark blue solution was stirred for 6–7 h
and then filtered. The filtrate was added drop-wise by a cannula
to a solution of 2a (0.1 g, 0.18 mmol) in THF (2 mL). The reaction
mixture became reddish violet. The solution was stirred overnight
and then a solution of Et4NBr (0.08 g, 0.37 mmol) in CH3CN was
added. The solvent was evaporated and the residue was washed
with diethyl ether. The residue was dissolved in minimal quantity
of acetonitrile, filtered and layered with ether. The dark purple col-
ored compound 2b was isolated (yield was not measured).
b (Å)
c (Å)
b (°)
V (Å3)
Z
qcalc (g cmꢀ1
)
Data collected
R(F2)
wR(F2)
Maximum residual electron density

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P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
solved by Patterson method using TeXsan crystallographic soft-
ware package of Molecular Structure Corporation [10] then they
were refined using Crystals for Windows package [11]. The cut
tetrabromobenzoin (30%) was achieved when 3,5-dibromobenzal-
dehyde was reacted directly with KCN and Bu4NBr in DMF [14].
As benzil and benzoin are reported to form the same thiophos-
phoric ester [15], both 3,30,5,50-tetrabromobenzoin and 3,30,5,50-
tetrabromobenzil were reacted with P4S10 under inert atmosphere
forming the thiophosphoric ester, which was used in preparing the
nickel complex.
off limit for the reflection was I P 2r(I). Selected crystal data for
3b are presented in Table 1.
4. Computational methods
Complex 1a and the alkyl ammonium salt, 1b were obtained
following the method initially developed by Schrauzer et al. [6]
and later modified by Sung et al. [15]. The mechanistic details of
this reaction has recently been elucidated by Arumugam et al.
through isolation and structural characterization thiophosphoryl
compound [16]. Reaction of thiophosphoric ester with Ni(II) salts,
hydrated NiCl2, gave a reddish-brown solution (Scheme 1). The
green colored neutral 1a was isolated from CH2Cl2 at a low temper-
ature (5 °C). Complex 1b was synthesized by using an analogous
procedure for 1a i.e., direct reaction between the metal and the
pro-ligand. Here thiophosphoric ester was refluxed with NiCl2 fol-
lowed by precipitation of the tetrabutylammonium salt using
Bu4NBr as a precipitant. The compound was initially obtained as
a reddish violet oil, which was washed with petroleum ether and
recrystallized from CH2Cl2 to get a reddish violet needle shaped
crystalline compound. A neutral nickel complex, Ni(C2S2Me2)2,
has been successfully reduced to dianionic [Ni(C2S2Me2)2]2- using
sodium anthracide [17]. Similar attempts to synthesize dianionic
[Ni{C2S2(3,5-C6H3Br2)2}2]2ꢀ were unsuccessful, although neutral
2a could be reduced to monoanaionic 2b. Similarly, synthesis of
neutral maleonitrile complex (i.e., neutral 3a) through oxidation
of 3b, or 3c was also not successful. Thus, poising of redox poten-
tials play a major role in stabilizing a specific oxidation state, and
the redox potentials can be modulated by suitable substituents
(vide infra) [18].
All calculations were carried out using Gaussian 03W software
package [12]. For the electronic structure calculations, all geome-
tries were optimized at DFT level of theory using BP86 exchange
correlation functional and 6-311G(d) basis set for all atoms. Fre-
quency calculations were conducted on all optimized structures
in order to ensure the optimized structures represent a local min-
imum. The percentages of atomic orbital contributions to their
respective molecular orbitals were calculated by using the VMOdes
program [13].
5. Results and discussion
5.1. Syntheses and characterization
The synthetic strategy for 3,5-dibromophenyl dithiolene ligand
and its nickel complexes [Ni{C2S2(3,5-C6H3Br2)2}2]0,ꢀ1 (1a and 1b)
is presented in Scheme 1. The starting material 3,5-dibromobenz-
aldehyde was synthesized from commercially available 1,3,5-trib-
romobenzene following a reported method in good yield [8].
3,30,5,50-tetrabromo benzoin as well as 3,30,5,50-tetrabromo benzyl
were synthesized from 3,5-dibromobenzaldehyde following ben-
zoin condensation reaction, which were purified by extraction with
diethyl ether and chloroform, respectively, and were characterized
by 1H, 13C NMR and IR spectroscopy. The 1H NMR spectrum for
benzil revealed a symmetric structure, while that of benzoin exhib-
its two CHOH resonances at 6.4 and 5.9 ppm. The yields for both
benzil and benzoin were moderate (17% for benzoin and 38% for
benzil) due to the deactivating nature of the Br-group at the meta
positions of the phenyl ring. However, a better yield for 3,30,5,50-
The compositions of the complexes were confirmed by electro-
spray ionization mass spectrometry (ESIMS) in the negative ion
mode. The characteristic molecular ion peaks for complexes were
detected with ꢂ100% intensity. Both 1a and 1b gave peak clusters
with a base peak with mass to charge ratio at 1173 consistent for
Br
HO
H
Br
Br
Br
Br
Br
Br
KCN
n-BuLi
(CH3)2NC(O)H
O
Br
+
Br
Br
O
H
O
Br
Br
O
x
Br
Br
Br
Br
P5S10
S
S
S
S
S
S
Ni2+
Br
Br
Br
Br
Br
Br
P
Ni
S
S
Br
Br
Br
1a (x = 0)
1b (x = -1)
Scheme 1.

P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
2861
[M]ꢀ for 1a and [M-Bu4N]ꢀ for 1b respectively. Conductivity of
60000
50000
40000
30000
20000
10000
0
complexes was measured in nitromethane solutions at room tem-
perature (Table 2). 1b exhibits characteristic conductivity for a 1:1
complex like 3b [19], while 1a was non-conducting indicating its
electroneutrality. Solid state magnetic susceptibilities of 1b at
293 K showed paramagnetism with spin only value corresponds
to one unpaired electron (Table 2), whereas 1a is diamagnetic.
These measurements probed the neutral and monoanionic nature
of 1a and 1b respectively, indicating that metal ion is formally in
+3 oxidation state in 1b, dithiolene acts as a monoanionic ligand
i.e., in its oxidized form. ESI-MS of 3b gave a molecular ion at
ꢂ338 peak due to anionic [Ni(S2C2(CN)2)2]ꢀ whose isotope distri-
bution pattern matches well with that of the calculated molecular
ion. For the corresponding dianionic complex (3c) a peak cluster
was observed at the same m/z value even though the electronic
spectra of the solution is different and showed a 2:1 electrolyte.
This suggests that during the mass spectral analyses the complex
is oxidized. Such oxidation chemistry under mass spectrometric
conditions has been observed before in transition metal chemistry
[20].
1a
1b
2a
2b
3b
3c
400
500
600
700
800
900 1000 1100 1200
Wavelength,nm
Fig. 2. Electronic spectra of Ni-complexes recorded at room temperature. 1a, 1b, 2a,
and 2b were recorded in CH2Cl2. 3b and 3c were recorded in CH3CN.
5.2. Electronic spectroscopy
is only an 8 nm shift. However, this band shifts to 863 nm in the
case of 3b. With the exception of 3c no other dianionic species
was isolated, where this band is absent. Thus, the low energy band
maxima follow the order: 1b ꢂ 2b > 3b. This is consistent with the
reported results on neutral complexes of general formula,
Ni{(S2C2(p-C6H4X)2}2 [15]. The intensities of these bands in three
compounds are different, the band in 3b being the least intense.
Because the intensity comes from the mixing of the orbital coeffi-
cients involved in the optical transition, it is suggestive that the
ground and the excited states do not mix as efficiently, and inclu-
sion of the phenyl ring increases the mixing perhaps by lowering
the symmetry. Thus, while significant differences exist going from
localized dithiolene to phenyl substituted dithiolene, there is little
difference with substitution on the phenyl ring.
The electronic spectroscopy is an excellent tool to define the
neutral or anionic nature of the Ni-dithiolene complexes. A strong
absorption band at the low energy region (near IR region) is a char-
acteristic of neutral complex, which shifts to a lower energy with
less intensity in the monoanionic complex and this band is not
present in the dianionic complex. Electronic spectra of 1a and 1b
were recorded in CH2Cl2 (Fig. 2). Compounds 3b and 3c were insol-
uble in CH2Cl2 and spectra were recorded in CH3CN. Fig. 2 also
shows electronic spectra 2a, 2b, 3b, and 3c.
High level density functional calculations [21] on Ni(S2C2Me2)2
0
suggest the ground state to be . . .(18a1g)2(4b3g)2(6b1u)2(5b2g
)
(13b1g)0. . .In the neutral species, the HOMO, the b1u orbital, is fully
occupied which is primarily ligand based with minor contribution
from the metal. The intense charge transfer absorption is due to
b1u ? b2g transition. The b2g orbital in the one-electron reduced
5.3. Electrochemical properties
monoanionic species is singly occupied. The contributions from
nickel to this b2g orbital increase with reduction, and the band po-
sition also shifts. Because of the involvement of the dithiolene orbi-
tals, the position of the low energy transition is dependent on the
substituent on the dithiolene ligand itself. For example, in mono-
anionic complexes, the low energy band shifts to lower energy
with increasing electron-releasing character of the substituent.
The results presented here indicate that the effect of distal substit-
uents (ca. the substituents on the phenyl ring that is attached to
the dithiolene) is small. The electronic spectra show variation:
the first band near 852 nm for the 2a shifts to 834 nm, a shift of
18 nm, while 935 nm band in 2b moved to 927 nm in 1b, which
Electrochemical properties of all complexes were investigated
by cyclic voltammetry in CH2Cl2 (compounds 1a and 1b) or CH3CN
(3b and 3c). Cyclic voltammogram of 1a is shown in Fig. 3. Two
quasi-reversible one-electron redox couples were observed for 1a
E
= -1065 mV
½
ΔE = 130 mV
E
= -288 mV
½
ΔE = 114 mV
Table 2
Characterization data for the nickel complexes used in this investigation.
Complex Base
peak in
UV–Vis
spectra
E1/2
(
D
E) leff
,
KM
B.M. Ohmꢀ1 cm2 molꢀ1
in nitromethane
e
(k,
mV vs.
ESI-MS
nm) Mꢀ1cmꢀ1 Fc+/Fc
1a
1b
2a
2b
1173
30 660 (834)
6500 (927)
ꢀ288
(114)
ꢀ1065
(130)
ꢀ258
(94)
ꢀ1275
(133)
ꢀ262
(68.5)
0
non-conducting
1173
542
1.77 63
27 500 (852)
15 200 (935)
0
non-conducting
542
1.82 68
-1400 -1200 -1000
-800
-600
-400
-200
0
200
mV, vs. Fc+/Fc
3c
3b
338
338
80 (855)
9700 (863)
0
187
1.54 69
Fig. 3. Room temperature cyclic voltammogram of 1a in CH2Cl2.

2862
P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
and 2a, while only one redox couple was observed for 3b. A de-
tailed spectroscopic study on 3b (monoanionic) suggests that the
singly occupied molecular orbital (SOMO) of this compound largely
dominated by sulfur from the dithiolene ligand, which underscores
the highly delocalized nature of the metal-sulfur orbitals in these
molecules [21]. Our electronic structure calculations, detailed later,
are in good agreement with the previous findings. Thus, assigning
the redox potentials only to metal based orbitals is ambiguous at
best. We therefore designate the redox potentials in terms of the
overall charge of the molecule – neutral (N), monoanionic (M) to
dianionic (D), and these species are related by Eq. (1)
clearly redox orbital (b1u) in this case is influenced significantly by
the substitution at the phenyl ring. However, the direction of shift
for the N/M couple is counter intuitive, which becomes more diffi-
cult by 30 mV for 1 compared to 2. Taking together, the redox
properties of these complexes are influenced by the substituents
on the dithiolene ligands because of the ligand based nature of
the HOMO.
5.4. Molecular structure
X-ray quality single crystals of [Et4N][Ni(S2C2(CN)2)2] were ob-
tained by slow evaporation of an CH3CN solution of [Et4N]2[-
Ni(S2C2(CN)2)2] in air. It is interesting to point out that the
structure of this compound has been solved before with different
cell parameters and space groups [22]. The structure of this com-
plex was as expected with very little unusual features and thus
the discussion will be limited. The Ni–S distances are as expected
2.14 Å, and the bite angles of the two ligands are ꢂ92°. The molec-
ular structure is shown in Fig. 4. The nickel center shows a square
planar geometry coordinated by the four sulfur donors of the two
dithiolene ligands. We were pleased with no disorder problem
with the ethyl groups of the cation, which often exhibit a positional
disorder. The packing diagram of the complex (Fig. 5) shows that
the planar Ni-complexes are stacked and in between the layers,
the tetraethyl ammonium ions are positioned, which may explain
rigidity of the tetraethyl ammonium groups.
þe
e
N ꢀ M ꢀ D
ð1Þ
ꢀe
þe
It is interesting to note that maleonitrile complexes (3) exhib-
ited only one redox couple at ꢀ260 mV, which is due to the M/D
couple. This couple is moved cathodically in the phenyl substituted
compounds by 800 mV and 1013 mV for molecular systems 1 and
2, respectively. Thus, introduction of the highly electron withdraw-
ing nitrile group moves the N/M couple beyond the solvent win-
dow. Interestingly this couple has been proposed to lie beyond
1 V [17], which is consistent with the difficulty in the synthetic
procedures described here.
Compared to the unsubstituted phenyl derivative, introduction
of electron withdrawing Br-groups also influences the redox
potentials. The direction of the change in the M/D couple is how-
ever, in the expected order. Introduction of eight Br-groups in
the molecule shifts the redox potential towards more positive
direction. Interestingly, the difference between the two couples
N/M–M/D is smaller in the case of Br-substituted compound. Thus,
5.5. Electronic structure
The electronic structures of the Ni-complexes were investigated
by density functional theory using optimized geometries. The
[Ni(S2C2(CN)2)2]ꢀ1/ꢀ2 exhibited a D2h symmetry while that for the
phenyl complexes was converged to D2 point group. The monoan-
ionic complexes were calculated using spin unrestricted model,
while neutral and dianionic complexes were calculated with a spin
restricted model.
The compositions of the frontier orbitals are listed in Table 3
and energy levels are shown in Figs. 6 and 7, and pictorial repre-
sentations of selected orbitals are presented in Fig. 8. The compo-
sitions the HOMO and the LUMO of 1b, 2b and 3b show small
difference. In all cases metal and the sulfur orbitals contribute sig-
nificantly, however, incorporation of phenyl rings in the ligand
architecture reduces the metal electron density by ꢂ10%. There is
virtually no difference in the orbital compositions with and with-
out the Br-substitution in the phenyl ring. The high S contributions
in the frontier orbitals indicate a facile electron delocalization
N(4)
C(8)
C(12)
C(11)
S(4)
C(13)
C(6)
C(9)
C(10)
C(14)
S(2)
N(3)
C(5)
Ni(1)
N(5)
C(2)
C(4)
C(7)
C(1)
C(15)
N(2)
S(3)
C(16)
C(3)
N(1)
S(1)
Fig. 4. Thermal ellipsoid (50% probability) representation of molecular structure of
[NEt4] [Ni(S2C2(CN)2)2].
Fig. 5. Packing diagram of [NEt4][Ni(S2C2(CN)2)2].

P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
2863
Table 3
6
The composition of selected orbitals.
Compound
Orbital
Symmetry Energy (eV) % Ni
% S
1b
a
b
HOMO-1
HOMO
LUMO
LUMO+1
HOMO-1
HOMO
B3
B2
B1
A
B1
B3
B2
B1
ꢀ2.796
ꢀ1.951
ꢀ0.754
ꢀ0.426
ꢀ2.804
ꢀ2.587
ꢀ1.579
ꢀ0.621
49.6
13.4
16.6
0.40 12.5
0.70 46.9
48.2
21.9
16.5
15.1
49.2
33.1
3
0
(b)
(c)
13.7
43.2
30.4
LUMO
LUMO+1
(a)
-3
-6
2b
a
b
HOMO-1
HOMO
LUMO
LUMO+1
HOMO-1
HOMO
B3
B2
B1
A
B1
B3
B2
B1
ꢀ1.919
ꢀ1.088
0.130
51.5
14.0
17.7
0.5
15.7
51.0
36.3
13.3
45.9
14.3
45.2
34.2
0.583
ꢀ1.921
ꢀ1.712
ꢀ0.719
0.270
0.7
50.2
23.0
17.9
Fig. 7. Molecular orbital diagram of 3c (a), 2a (b) and 1a (c) calculated in spin
restricted DFT method. The dashed line represents the separation between the
HOMO and the LUMO.
LUMO
LUMO+1
3b
a
b
HOMO-1
HOMO
LUMO
LUMO+1
HOMO-1
HOMO
B1g
B2g
B3g
B1g
B3u
B1g
B2g
ꢀ3.426
ꢀ2.566
ꢀ1.371
ꢀ0.682
ꢀ3.435
ꢀ3.215
ꢀ2.178
ꢀ1.221
64.9
16.5
18.6
2.7
17.3
62.7
35.9
19.1
55.9
15.7
56.6
34.2
0.7
64.5
27.6
19.0
HOMO
HOMOLUMO+1 B3g
1a
2a
3c
HOMO-1
HOMO
LUMO
B3
B1
B2
B1
ꢀ6.042
ꢀ5.932
ꢀ4.977
ꢀ3.821
ꢀ5.345
ꢀ5.212
ꢀ4.283
ꢀ3.125
1.159
1.916
2.771
3.049
48.6
0.7
13.5
18.3
17.0
36.9
49.6
37.0
LUMO+1
HOMO-1
HOMO
LUMO
B3
B1
B2
B1
50.1
0.8
13.8
18.7
17.6
35.8
50.4
38.3
LUMO+1
HOMO-1
HOMO
LUMO
Ag
85.1
47.6
20.6
5.6
7.7
43.0
32.9
21.3
B2g
B3g
B1g
LUMO+1
Fig. 8. Pictorial representation of the HOMOs. The complexes are arranged
vertically in order of increasing charge. The left panel shows complex 3 while the
right panel shows complex 2.
2b
3b
1b
LUMO separation is not significantly different, the maximum dif-
ference being 0.2 eV. This indicates that introduction of the phenyl
ring while does not significantly alter the orbital composition, but
the overall energies are affected, which of course can influence not
only the reduction potentials but also the optical spectra. A similar
situation has been observed in X-ray absorption spectroscopy
[21,23].
2
0
-2
The diamagnetic compounds are different such that compound
1a and 2a are neutral, compound 3c is dianionic. The frontier orbi-
tals in 1a and 2a are much more stabilized than those in 3c,
although the HOMO–LUMO gap differs only by 0.1 eV. The orbitals
compositions are significantly different: metal contribution to
HOMO in 1a and 2a is virtually negligible, but that in 3c substan-
tial. Contribution from the S orbitals to the HOMO is 35% in 1a and
2a which is ꢂ10% than that in 3c. In 3c both Ni and S contributes
equally to the HOMO. These differences not only affect the physical
properties but also impact their chemical reactivity.
-4
-6
β
β
α
α
β
α
Fig. 6. Molecular orbital diagram of 3b, 2b, and 1b calculated in spin unrestricted
DFT method. The dashed line represents the separation between the HOMO and the
LUMO. The formal oxidation state of Ni in these complexes is considered to be +3.
pathway involving the Ni and sulfur orbitals underscoring the
importance of the non-innocent nature of the dithiolene ligand.
The presence of the phenyl ring impacts the energetic more signif-
icantly; the HOMO in 3b is stabilized by 1.5 eV due to the electron
withdrawing cyano group. The destabilization caused by phenyl
ring is attenuated (by ꢂ0.8 eV) in 1b due to the presence of the
electron withdrawing Br-derivative. Interestingly, the HOMO–
6. Summary
We have designed and synthesized new nickel dithiolene com-
plexes containing 3,5-C6H3Br2 substituted phenyl groups. Com-
pounds (1a and 1b) have been characterized by NMR, UV–Vis,
and IR spectroscopy. The redox potentials exhibit exquisite sensi-
tivity with respect to the substituents and the optical spectra show

2864
P. Basu et al. / Inorganica Chimica Acta 363 (2010) 2857–2864
[7] V. Madhu, S.K. Das, Inorg. Chem. 47 (2008) 5055.
little variation (in similarly charged molecular systems like 1a and
2a). The differential sensitivity is a result of different alignment of
the redox orbitals. We determined the molecular structure of the
3b, and the electronic structures of 1b, 2b, and 3b show a variation
in the energies of the orbitals without significantly effecting the
composition. Similar calculations on diamagnetic 1a, 2a and 3c
indicate not only a difference in energy but also compositional var-
iation of frontier orbitals.
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Acknowledgements
We thank Dr. Raghvendra S. Sengar for insightful discussions.
Financial assistance from the National Institutes of Health
(GM061555) is gratefully acknowledged. We also acknowledge
the Minnesota Supercomputing Institute for the generous support
of computer time.
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