Influence of the substituents on the electronic and electrochemical properties of a new square-planar nickel-bis(quinoxaline-6,7-dithiolate) system: Synthesis, spectroscopy, electrochemistry, crystallography, and theoretical investigation

Article abstract of DOI:10.1021/ic301017g

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel- bis(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu ₄N]₂[Ni(X₂6,7-qdt)₂], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu₄N]₂[Ni(X₂6,7-qdt)₂]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E_1/2 = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni(X₂6,7-qdt) ₂]^-/[Ni(X₂6,7-qdt)₂]^2-. Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh₄]₂[Ni(Ph ₂6,7-qdt)₂]·3DMF (2b) for its structural characterization.

Full text of DOI:10.1021/ic301017g

Article
pubs.acs.org/IC
Influence of the Substituents on the Electronic and Electrochemical
Properties of a New Square-Planar Nickel-Bis(quinoxaline-6,7-
dithiolate) System: Synthesis, Spectroscopy, Electrochemistry,
Crystallography, and Theoretical Investigation†
Ramababu Bolligarla, Samala Nagaprasad Reddy, Gummadi Durgaprasad, Vudagandla Sreenivasulu,
and Samar K. Das*
School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad 500046, India
S
* Supporting Information
ABSTRACT: We describe the synthesis, crystal structures, electronic
absorption spectra, and electrochemistry of a series of square-planar nickel-
bis(quinoxaline-6,7-dithiolate) complexes with the general formula
[Bu4N]2[Ni(X26,7-qdt)2], where X = H (1a), Ph (2a), Cl (3), and Me (4).
The solution and solid-state electronic absorption spectral behavior and
electrochemical properties of these compounds are strongly dependent on the
electron donating/accepting nature of the substituent X, attached to the
quinoxaline-6,7-dithiolate ring in the system [Bu4N]2[Ni(X26,7-qdt)2].
Particularly, the charge transfer (CT) transition bands observed in the visible
region are greatly affected by the electronic nature of the substituent. A
possible explanation for this influence of the substituents on electronic
absorption and electrochemistry is described based on highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO)
gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in
all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at
very low oxidation potentials appearing at E1/2 = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH
solutions, corresponding to the respective couples [Ni(X26,7-qdt)2]−/[Ni(X26,7-qdt)2]2−. Compounds 1a, 3, and 4 have been
characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal
X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl
phosphonium salt of the complex anion of 2a, [PPh4]2[Ni(Ph26,7-qdt)2]·3DMF (2b) for its structural characterization.
synthesis. This is because heterocyclic based dithiolene systems
containing N and S atoms offer secondary coordination in
addition to dithiolate coordination.8 To achieve such materials,
synthetic chemists have designed and synthesized diverse
dithiolene ligands with nitrogen coordinating groups of
increasing complexity.9 We are particularly interested in
synthesizing a qdt-based system because of its use in diverse
areas. For example, qdt-based systems have been used in the
development of field-effect transistors by introducing the fused
qdt-based systems to the TTF-skeleton.10 The photophysical
(luminescence) properties of platinum complexes of qdt-type
ligand have been studied extensively by Eisenberg’s group.11
McMaster, Garner, Kirk, and their co-workers reported the first
example of valence tautomerism in a Mo-complex, which is
nothing but a qdt-ligated molybdenum complex anion
[MoO(qdt)2]−.12 We studied the acid−base properties of
nickel- and platinum-qdt system including their spectral
properties.13 Recently, we have reported the new quinoxaline
INTRODUCTION
■
Metal-dithiolene complexes have been of considerable interest
to inorganic chemists for more than four decades because of
their unique properties, and they provide redox active ligands
with the ability to form highly electron-delocalized complexes.1
The increasing attention in the design and synthesis of metal−
dithiolene complexes is due to their potential applications in
the areas of conducting-, magnetic-,2 nonlinear optical-
materials3 and near-infrared (NIR) dyes.4 The recent interest
in metal dithiolene complexes in the area of bioinorganic
modeling studies is due to the existence of a metal−dithiolene
moiety in the active sites of many metallo-enzymes
(hydroxylase-type molybdo-enzymes).5,6 This metal-dithiolene
containing active site has been characterized as “molybdopter-
in” (cofactor of the relevant metalloenzyme). The molybdop-
terin is basically a nitrogen atom containing heterocyclic based
dithiolato-metal-oxo complex. Thus quinoxaline-2,3-dithiolate
(qdt2−, Scheme 1) and its molybdenum-oxo complexes have
been investigated extensively for modeling the active sites of
molybdenum hydroxylase enzymes.7 Besides its biological
relevance, qdt2− ligand plays an important role in materials
Received: May 16, 2012
Published: December 10, 2012
© 2012 American Chemical Society
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Article
KBr pellets with a JASCO FT/IR-5300 spectrometer in the region of
Scheme 1. (a) Structural Representation of Quinoxaline
Dithiolate Ligands; (b) General Strucural Representation of
Newly Synthesized Nickel(II)-bis(quinoxaline-6,7-
dithiolate) System
1
400−4000 cm−1. H NMR spectra of compounds were recorded on a
Bruker DRX-400 spectrometer using Si(CH3)4 [TMS] as an internal
standard. Electronic absorption spectra of solutions and diffuse
reflectance spectra of solid compounds were recorded on a UV-3600
Shimadzu UV−vis−NIR spectrophotometer. A Cypress model CS-
1090/CS-1087 electro analytical system was used for cyclic
voltammetric experiments. The electrochemical experiments were
measured in MeOH containing [Bu4N][ClO4] as a supporting
electrolyte using a Ag/AgCl electrode as a reference. We have used
BAS MF-2013 model disk working platinum electrode with diameter
1.6 mm for measuring the electrode potentials. The potentials
reported here are uncorrected for junction contributions.
Synthesis of 6,7-Dimethyl-1,3-dithia-5,8-diaza-cyclopenta-
[b]naphthalen-2-thione (D). 5,6-Diamino-benzo[1,3]dithiole-2-thi-
one (C) (150 mg, 0.7 mmol) and butane-2,3-dione (0.67 mL, 0.77
mmol) were dissolved in acetic acid (15.0 mL), and the reaction
mixture was refluxed for 1.5 h. The resulting yellow precipitate was
washed with MeOH several times and dried in vacuum to give a yellow
colored solid. This compound can be directly used for the preparation
of ligand L4. Yield: 0.180 g (96.0%); Yellow solid; IR (KBr, cm−1):
3059, 1450, 1398, 1321, 1182, 1068 (CS), 883, 844, 758, 509, 424;
1H NMR (400 MHz, CDCl3): δ 8.08 (s, 2H), 2.77 (s, 6H) ppm; LC−
dithiolate-ligand [{6,7-qdt}2− = quinoxaline-6,7-dithiolate,
Scheme 1a], and its nickel-square planar bis(dithiolene)
complex [Bu4N]2[Ni(6,7-qdt)2] (1a),14 which has been useful
to demonstrate the comparison of its electronic and electro-
chemical properties with those of existing well-established
system [Bu4N]2[Ni(qdt)2]. As a continuation, in the present
contribution we wish to describe the effect of different
substituents of the {6,7-qdt}2− ligand on the electronic
properties of the {NiS4} group of [Ni(6,7-qdt)2]2−. We are
prompted to undertake this work by the landmark/classic
publication of Gray and co-workers.15a They investigated
systematically the effect of different substituents of dithiolato
ligands on the electronic structure of {MS4} group in a series of
metal complexes containing benzene 1,2-dithiolate and related
ligands.15a A recent review article has described the 50-year old
debate regarding the true electronic structure of transition
metal bis(dithiolene) complexes that has been revolutionized
by involvement of sulfur K-edge X-ray absorption spectroscopy
to directly probe the sulfur composition of the frontier
orbitals.15b We have reported three new quinoxaline based
dithiolate-ligands ({Ph26,7-qdt}2−, {Cl26,7-qdt}2−, and{Me26,7-
qdt}2−) along with the reported dithiolate-ligand {6,7-qdt}2−
(Scheme 1a) and their nickel square-planar bis(dithiolene)
complexes. This new series of square-planar bis(dithiolene)
complexes {[Ni(X26,7-qdt)2], Scheme 1b} give us an
opportunity to carry out their comparative studies of electronic
and electrochemical properties: [Bu4N]2[Ni(6,7-qdt)2] (1a),
[PPh4]2[Ni(6,7-qdt)2] (1b), [Bu4N]2[Ni(Ph26,7-qdt)2] (2a),
[PPh4]2[Ni(Ph26,7-qdt)2]·3DMF (2b), [Bu4N]2[Ni(Cl26,7-
qdt)2] (3), [Bu4N]2[Ni(Me26,7-qdt)2] (4).
MS (positive mode): m/z = 265 [M++H]+; Anal. Calc. for C11H8N2S3:
C, 49.97; H, 3.05; N, 10.60. Found: C, 50.23; H, 2.84; N, 10.85%.
Synthesis of 6,7-Dimethyl-1,3-dithia-5,8-diaza-cyclopenta-
[b]naphthalen-2-one (L4). To a solution of 6,7-dimethyl-1,3-dithia-
5,8-diaza-cyclopenta[b]naphthalen-2-thione (D) (135 mg, 0.511
mmol) in chloroform and acetic acid (3:1, v/v, 40 mL), Hg(OAC)2
(521 mg, 1.63 mmol) was added, and it was stirred for 12 h at room
temperature under N2 atmosphere. The precipitate was filtered off
using Celite and washed with chloroform. The filtrate was extracted
with saturated NaHCO3 solution (3 × 33 mL) and water (50 mL). To
remove any traces of water, anhydrous Na2SO4 was added. The solvent
was removed in vacuum, which produces pale yellow colored
compound L4 as product. Yield: 115 mg (91.0%); pale yellow solid;
IR (KBr, cm−1): 3057, 2916, 1745 (CO), 1653, 1454, 1398, 1319,
1178, 1093, 869, 844, 756, 582, 439; 1H NMR (400 MHz, CDCl3): δ
8.10 (s, 2H), 2.75 (s, 6H) ppm; LC−MS (positive mode): m/z = 249
[M++H]+; Anal. Calc. for C11H8N2OS2: C, 53.20; H, 3.25; N, 11.28.
Found: C, 53.67; H, 3.01; N, 10.98%.
Synthesis of [PPh4]2[Ni(6,7-qdt)2] (1b). The dianion of 6,7-qdt
was generated, in situ, by the reaction of quinoxaline-6,7-dithiol (L1)
(0.104 g, 0.536 mmol) with NaOH (0.060 g, 1.5 mmol) in MeOH (10
mL) under nitrogen atmosphere. To the resulting clear red solution,
solid NiCl2·6H2O (0.064 g, 0.269 mmol) was added; the resulting dark
red solution was stirred for 15 min at room temperature. Dark brown
micro crystals were precipitated by adding tetraphenylphosphonium
bromide (0.292 g, 0.696 mmol); the micro crystals were filtered,
washed with water followed by a wash with diethyl ether, and dried at
room temperature. It was recrystallized from acetonitrile solution by
vapor diffusion with diethyl ether. Yield: 0.350 g (71.3% based on Ni).
Anal. Calc. for C64H48N4S4P2Ni: C, 68.51; H, 4.31; N, 4.99. Found: C,
68.23; H, 4.12; N, 5.36%. IR (KBR pellet) (υ/cm−1): 3040, 1660,
1583, 1483, 1435, 1412, 1340, 1174, 1107, 1078, 1024, 925, 779, 752,
721, 688, 524; 1H NMR (400 MHz, δ ppm) (CD3CN): 7.67−7.76 (m,
40H), 7.90−7.93 (m, 8H).
EXPERIMENTAL SECTION
Synthesis of [Bu4N]2[Ni(Ph26,7-qdt)2] (2a). The dianion of 2,3-
diphenyl-6,7-qdt was generated, in situ, by the treatment of 2,3-
diphenyl-quinoxaline-6,7-dithiol (L2) (0.052 g, 0.15 mmol) with
NaOH (0.020 g, 0.5 mmol) in methanol (7.0 mL). To the resulting
clear red solution, solid NiCl2·6H2O (0.018 g, 0.075 mmol) was
added; the resulting dark brown solution was stirred for 15 min at
room temperature. Black colored micro crystals were precipitated by
adding tetrabutylammonium bromide (0.07 g, 0.215 mmol); the micro
crystals were filtered, washed with water followed by washing with
diethyl ether, and dried at room temperature. It was recrystallized from
acetonitrile solution by vapor diffusion with diethyl ether. Yield: 0.200
g (74.1% based on Ni). Anal. Calc. for C72H96N6S4Ni: C, 70.16; H,
7.85; N, 6.82. Found: C, 70.57; H, 7.29; N, 6.49%. IR (KBR pellet)
■
Materials and Methods. All the chemicals for the synthesis were
commercially available and used as received. 1,2-diaminobenzene-
bis(thiocyanate) (A),16 2,3-diphenyl-6,7-bis-thiocyanato-quinoxaline
(B),17 5,6-diamino-benzo[1,3]dithiole-2-thione (C),18 quinoxaline-
6,7-dithiol (H26,7-qdt) (L1),14 2,3-diphenyl-quinoxaline-6,7-dithiol
(L2),17 6,7-dichloro-1,3-dithia-5,8-diaza-cyclopenta[b]naphthalen-2-
one (L3),19 and [Bu4N]2[Ni(6,7-qdt)2] (1a)14 were prepared
according to literature procedures. Syntheses of metal complexes
were performed under N2 using standard inert-atmosphere techniques.
Solvents were dried by standard procedures.
Micro analytical (C, H, N) data were obtained with a FLASH EA
1112 Series CHNS Analyzer. Infrared (IR) spectra were recorded on
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Table 1. Crystal Data and Structural Refinement for Complexes 1b, 2b, 3, and 4
1b
2b
3
4
empirical formula
formula weight
temperature (K)
crystal size (mm)
crystal system
space group
Z
C64H48N4P2S4Ni
1121.95
C97H85N7O3P2S4Ni
1645.61
C48H76N6S4Cl4Ni
1065.94
C52H88N6S4Ni
984.23
100(2)
100(2)
100(2)
100(2)
0.46 × 0.20 × 0.08
monoclinic
C2/c
0.50 × 0.26 × 0.04
monoclinic
C2/c
0.24 × 0.10 × 0.06
monoclinic
P21/c
0.22 × 0.14 × 0.08
triclinic
P1
̅
4
8
2
2
wavelength (Å)
a [Å]
0.71073
0.71073
0.71073
0.71073
26.993 (2)
13.2302(9)
16.4171(11)
90.000
19.6540(12)
19.3493 (12)
41.897(3)
90.000
13.8484(15)
8.5484(9)
22.521(3)
90.000
13.0326(11)
14.3918(12)
15.8859(13)
114.6950(10)
94.4320(10)
100.2510(10)
2625.3(4)
1.245
b [Å]
c [Å]
α [deg]
β [deg]
117.476(2)
90.000
102.767(10)
90.000
101.239(3)
90.000
γ [deg]
volume [Å3]
calculated density (Mg/m−3
reflections collected/unique
R(int)
5201.6(7)
1.433
15539.2(17)
1.407
2614.9(5)
1.262
)
27521/5393
0.0548
79828/15262
0.0862
23914/4580
0.0771
27356/10289
0.0554
F(000)
2328
6896
988
1068
max. and min transmission
0.9504 and0.7563
0.9819 and0.8029
1.50 to 26.03
0.9552 and0.8366
1.50 to 24.94
0.9559 and 0.8850
1.43 to 26.08
Θ range for data collection (deg) 1.70 to 26.48
refinement method
full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2
data/restraints/parameters
goodness-of-fit on F2
R1/wR2 [I > 2σ(I)]
5393/0/340
1.112
15262/0/972
1.112
4580/0/286
1.249
10289/0/580
1.089
0.0804/0.1970
0.0921/0.2052
] 1.834 and −0.449
0.0875/0.1805
0.1120/0.1924
0.821 and −0.984
0.1690/0.3640
0.1830/0.3733
1.377 and −0.791
0.0786/0.1801
0.1049/0.1942
1.910 and −0.451
R1/wR2 (all data)
largest diff. peak and hole [e Å−3
(υ/cm−1): 2959, 2918, 2860, 1595, 1483, 1423, 1340, 1192, 1072,
1024, 858, 821, 698. 1H NMR (400 MHz, δ ppm) (CD3CN): 0.949 (t,
24H), 1.32−1.37 (m, 16H), 1.60 (bs, 16H), 3.10 (t, 16H), 7.15−7.37
(m, 24H).
(0.100 g, 0.401 mmol) with Na (0.046 g, 2.0 mmol) in MeOH (10
mL) under nitrogen atmosphere. To the resulting clear red solution,
NiCl2·6H2O (0.048 g, 0.201 mmol) was added, and the dark red
solution obtained was stirred for 15 min at room temperature. Dark
red micro crystals were precipitated by adding tetrabutylammonium
bromide (0.2 g, 0.621 mmol); the micro crystals were filtered, washed
with water followed by washing with diethyl ether, and finally
microcrystals were dried at room temperature. It was recrystallized
from acetonitrile solution by vapor diffusion with diethyl ether. Yield:
0.295 g (71.2% based on Ni). Anal. Calc. for C52H88N6S4Ni: C, 63.46;
H, 9.01; N, 8.54. Found: C, 63.25; H, 8.89; N, 8.81%. IR (KBR pellet)
(υ/cm−1): 2957, 2924, 2866, 1736, 1635, 1570, 1448, 1423, 1325,
1168, 1076, 989, 839, 748, 588; 1H NMR (400 MHz, δ ppm)
(CD3CN): 0.93 (bs, 24H), 1.32 (bs, 16H), 1.58 (bs, 16H), 2.56 (s,
12H) 3.09 (bs, 16H), 8.15 (s, 4H).
Synthesis of [PPh4]2[Ni(Ph26,7-qdt)2]·3DMF (2b). This com-
pound was prepared by using above procedure, used for the
preparation of compound 2a, but tetraphenylphosphonium bromide
was added in place of tetrabutylammonium bromide. The solid thus
obtained was recrystallized from DMF solution by vapor diffusion with
diethyl ether. Yield: 79.4% based on Ni. Anal. Calc. for
C97H85N7O3P2S4Ni: C, 70.79; H, 5.21; N, 5.96. Found: C, 70.25; H,
5.34; N, 6.18%. IR (KBR pellet) (υ/cm−1): 3059, 2953, 2924, 1670,
1
1425, 1340, 1195, 1107, 1082, 1020, 723, 690, 520; H NMR (400
MHz, δ ppm) (CD3CN): 7.31−7.48 (m, 20H), 7.70−7.77 (m, 40H),
7.90−7.93 (m, 4H).
Synthesis of [Bu4N]2[Ni(Cl26,7-qdt)2] (3). The dianion of 2,3-
dichloro-6,7-qdt was generated, in situ, by the treatment of 6,7-
dichloro-1,3-dithia-5,8-diaza-cyclopenta[b]naphthalen-2-one (L3)
(0.080 g, 0.277 mmol) with Na metal (0.020 g, 0.877 mmol) in
methanol (10 mL). To the resulting clear brown solution, solid
NiCl2·6H2O (0.034 g, 0.143 mmol) was added, and it was stirred for
15 min at room temperature. To this, tetrabutylammonium bromide
(0.1 g, 0.310 mmol) was added followed by the addition 30 mL of
deionized water that results in the precipitation of black colored micro
crystals; the micro crystals were filtered, washed with water followed
by washing with diethyl ether, and dried at room temperature. It was
recrystallized from acetonitrile solution by vapor diffusion with diethyl
ether. Yield: 0.190 g (52.9% based on Ni). Anal. Calc. for
C48H76N6S4Cl4Ni: C, 54.09; H, 7.19; N, 7.88. Found: C, 54.41; H,
7.25; N, 7.51%. IR (KBR pellet) (υ/cm−1): 2959, 2872, 1562, 1433,
1377, 1315, 1194, 1149, 1078, 983, 862, 738, 570, 518. 1H NMR (400
MHz, δ ppm) (CD3CN): 0.946 (t, 24H), 1.34 (bs, 16H), 1.59 (bs,
16H), 3.08 (t, 16H), 7.42 (s, 4H).
X-ray Crystallography. Single crystals, suitable for facile structural
determination for all the compounds 1b, 2b, 3, and 4, were measured
on a three circle Bruker SMART APEX CCD area detector system
under Mo−Kα (λ = 0.71073 Å) graphite monochromatic X-ray beam.
The frames were recorded with an ω 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.20 Empirical absorption
corrections were done using equivalent reflections performed program
SADABS.20 The structures were solved by direct methods and least-
squares refinement on F2 for all the compounds by using SHELXS-
97.21 All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms were included in the structure factor calculation by
using a riding model. The crystallographic parameters, data collection,
and structure refinement of the compounds 1b, 2b, 3, and 4 are
summarized in Table 1. Selected bond lengths and angles for the
compounds 1b, 2b, 3, and 4 are listed in Table 2.
Computational Details. Electronic structure calculations of the
ground state of the 1a, 2b, 3, and 4 complex anions have been
performed using density functional theory (DFT) methods employing
the G03 suite of program.22 The functional used throughout this study
is the B3LYP,23 consisting of the nonlocal hybrid exchange functional
Synthesis of [Bu4N]2[Ni(Me26,7-qdt)2] (4). The dianion of 2,3-
dimethyl-6,7-qdt was generated, in situ, by the reaction of 6,7-
dimethyl-1,3-dithia-5,8-diaza-cyclopenta[b]naphthalen-2-one (L4)
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been shown in Scheme 2. The synthetic procedure of
dithiolate-ligands L1 and L2 has been reported in our earlier
publications as shown in Scheme 2a.14,17 In our previous
letter,19 the synthesis of dithiolate-ligand L3 has been described;
the relevant synthesis starts from 5,6-diamino-benzo[1,3]-
dithiole-2-thione as shown in Scheme 2b. Likewise, dithiolate-
ligand L4 has been obtained in two steps, starting from 5,6-
diamino-benzo[1,3]dithiole-2-thione (C). In the first step, 6,7-
dimethyl-1,3-dithia-5,8-diaza-cyclopenta[b]naphthalen-2-thione
(D) was prepared according to a modifed literature procedure24
that is described by the condensation reaction of 5,6-diamino-
benzo[1,3]dithiole-2-thione (C) with butane-2,3-dione in acetic
acid at refluxing conditions (Scheme 2b). In the second step of
the synthesis, the 1,3-dithia-2-thione derivative (D) is
converted into corresponding 1,3-dithia-2-one derivative (L4)
by using Hg(OAc)2 in CHCl3:CH3COOH (3:1) as shown in
Scheme 2b. All these synthesized organic compounds have
been characterized by NMR spectroscopy including elemental
and LC-MS analyses (see Supporting Information for relevant
plots).
Syntheses of metal complexes have been performed using a
general procedure as shown in Scheme 3. The {X26,7qdt}2−
ions are generated, in situ, by the reaction of ligands (L1−L4) in
basic medium containing MeOH solution, which are
subsequently reacted with nickel chloride salt resulting in the
formation of the respective nickel complexes and precipitated/
isolated as tetrabutylammonium salts by the addition of [Bu4N]
Br [compounds 1a, 2a, 3, and 4]. Tetraphenylphosphonium
bromide is added instead of adding tetrabutylammonium
bromide for the precipitation of nickel complexes of ligands
L1 and L2 resulting in the isolation of compounds 1b and 2b,
respectively as shown in Scheme 3a. Metal complexes 1a, 1b,
2a, 3, and 4 are recrystallized from acetonitrile solution by the
vapor diffusion of diethyl ether. Compound 2b is recrystallized
from DMF solution by the vapor diffusion of diethyl ether.
Crystals of compounds 1a, 1b, 2b, 3, and 4 have been
characterized by single crystal X-ray structure determination.
The crystals of compound 2a are not suitable for single crystal
X-ray structure analysis. All the metal coordination complexes
have been further characterized by NMR spectroscopy
including their elemental analysis.
Electronic Absorption Spectra. The electronic absorp-
tion spectra of complexes 1a, 2a, 3, and 4 in MeOH solutions
are shown in Figure 1a. The broad bands are observed in the
visible region centered at ∼588 nm (ε = 35000 L mol−1 cm−1),
∼607 nm (ε = 21,000 L mol−1 cm−1), ∼610 nm (ε = 34,600 L
mol−1 cm−1), and ∼547 nm (ε = 24,200 L mol−1 cm−1) for
complexes 1a, 2a, 3, and 4, respectively. The corresponding
solid state diffuse reflectance spectra are shown in Figure 1b.
Molecular orbital calculations, reported for metal bis-
(dithiolato) complexes, indicate the lowest energy charge
transfer (CT) state involves a highest occupied molecular
orbital (HOMO) which is a mixture of dithiolate (π) and metal
(d) orbital character; the relative contribution depend on the
metal ion, type of dithiolate chelate and charge of the
concerned complex.25 For the complex anion [Pt(mnt)2]2−,
the CT transition was assigned to involve a HOMO (having a
large dithiolate ligand contribution and small metal “d” orbital
contribution) and a lowest unoccupied molecular orbital
(LUMO) of π* (dithiolate) participation.11a,25g However, in
the case of [Ni(mnt)2]2−, the HOMO is characterized by
dithiolate-centered orbitals without metal-d contribution, and
the LUMO is a mixture of dithiolate (π) and metal (d)
Table 2. Selected Bond Lengths and Bond Angles of the
Compounds Presented in the Study
a
Compound 1b
Ni(1)−S(2)#1
Ni(1)−S(1)#1
2.1578(11)
2.1664(12)
Ni(1)−S(2)
Ni(1)−S(1)
S(2)#1−Ni(1)−S(1)#1
2.1578(11)
2.1665(12)
91.35(4)
S(2)#1−Ni(1)− 180.00(6)
S(2)
S(2)−Ni(1)−
S(1)#1
88.65(4)
91.35(4)
104.89(15)
S(2)#1−Ni(1)−S(1)
S(1)#1−Ni(1)−S(1)
C(2)−S(2)−Ni(1)
88.65(4)
S(2)−Ni(1)−
180.00(4)
104.92(15)
S(1)
C(1)−S(1)−
Ni(1)
Compound 2b
Ni(1)−S(1)
Ni(1)−S(2)
2.1592(13)
2.1746(13)
178.36(6)
Ni(1)−S(4)
Ni(1)−S(3)
S(1)−Ni(1)−S(2)
2.1622(13)
2.1746(13)
91.54(5)
S(1)−Ni(1)−
S(4)
S(4)−Ni(1)−
88.53(5)
S(1)−Ni(1)−S(3)
S(2)−Ni(1)−S(3)
C(21)−S(3)−Ni(1)
C(2)−S(2)−Ni(1)
88.51(5)
S(2)
S(4)−Ni(1)−
91.48(5)
177.81(6)
105.61(15)
105.24(16)
S(3)
C(1)−S(1)−
105.29(16)
105.56(16)
Ni(1)
C(22)−S(4)−
Ni(1)
Compound 3
Ni(1)−S(2)
Ni(1)−S(1)#2
2.160(3)
2.163(3)
179.998(1)
Ni(1)−S(2)#2
Ni(1)−S(1)
S(2)−Ni(1)−S(1)#2
2.160(3)
2.163(3)
88.42(10)
S(2)−Ni(1)−
S(2)#2
S(2)#2−Ni(1)− 91.58(10)
S(1)#2
S(2)−Ni(1)−S(1)
S(1)#2−Ni(1)−S(1)
C(2)−S(2)−Ni(1)
91.58(10)
180.0
S(2)#2−Ni(1)− 88.42(10)
S(1)
C(1)−S(1)−
105.8(4)
105.4(4)
Ni(1)
Compound 4
Ni(1)−S(4)
Ni(1)−S(1)
2.1599(12)
2.1699(12)
177.89(5)
Ni(1)−S(2)
Ni(1)−S(3)
S(4)−Ni(1)−S(1)
2.1675(12)
2.1786(12)
87.93(4)
S(4)−Ni(1)−
S(2)
S(2)−Ni(1)−
90.83(4)
S(4)−Ni(1)−S(3)
S(1)−Ni(1)−S(3)
C(6)−S(2)−Ni(1)
C(11)−S(3)−Ni(1)
91.22(4)
S(1)
S(2)−Ni(1)−
90.15(4)
175.30(5)
105.55(14)
105.20(15)
S(3)
C(1)−S(1)−
105.77(14)
105.91(15)
Ni(1)
C(16)−S(4)−
Ni(1)
a
Symmetry transformations used to generate equivalent atoms: #1 −x
+1/2,−y+3/2,−z+1; #2 −x+2,−y+2,−z.
as defined by Becke 3-Parameter (exchange) and nonlocal Lee, Yang,
and Parr correlation functional. The LanL2DZ basis set is used for the
Ni atom including LanL2 effective core-potential (ECP), and for the
remaining atoms the 6-311G** basis set is used. The ground-state
anionic complexes are obtained in the methanol phase by full
geometry optimization and also frequency calculations. The optimized
structures located as saddle points on the potential energy surfaces
were verified by the absence of imaginary frequencies. Vertical excited
state calculations are performed with the TD-B3LYP method using the
same basis functions mentioned in the above. The details of
computational output are described in the Supporting Information.
RESULTS AND DISCUSSION
Synthesis and General Characterization. The synthetic
routes for the synthesis of four dithiolate-ligands L1−L4 have
■
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Scheme 2. Synthesis of Quinoxaline Dithiolate Ligands (a) L1 and L2; (b) L3 and L4
orbitals.26 Cummings and Eisenberg reported electronic
transition for [Pt(qdt)2]2− (a complex anion with a ligand,
related to the present study) and described the lowest-energy
excited state as a mixture of MLCT and dithiolate intraligand
(IL) character.11a We performed ground state electronic
structure calculations for [Ni(6,7-qdt)2]2− (in 1a), [Ni-
(Ph26,7-qdt)2]2− (in 2b), [Ni(Cl26,7-qdt)2]2− (in 3), and
[Ni(Me26,7-qdt)2]2− (in 4) using density functional theory
(DFT) methods (see computational details in Experimental
Section). From the vertical excitation energy data for these
complex anions, the first absorption originates from respective
HOMO to LUMO, where HOMOs are of π type character and
LUMOs are of π* type character. The resulting HOMO and
LUMO diagrams are shown in Figure 2. By careful investigation
Scheme 3. Synthesis of Nickel 6,7-Quinoxaline-dithiolate
Complexes (a) 1a, 1b, 2a, and 2b; (b) 3 and 4
Figure 1. (a) Electronic absorption spectra in MeOH solutions and (b) diffuse reflectance spectra of solid compounds 1a, 2a, 3, and 4.
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particular substituent on an aromatic system can usually be
understood in terms of inductive as well as mesomeric effects.27
The mesomeric effect comes from the delocalization of the π-
electrons between aromatic core and the substituent. Thus in
the present study, the mesomeric effect is important in the case
of compounds [Bu4N]2[Ni(Ph26,7-qdt)2] (2a) and
[PPh4]2[Ni(Ph26,7-qdt)2]·3DMF (2b), where substituted
phenyl group can share π-electrons (by resonance) with
aromatic core having dithiolato ligands. The inductive effect is
fully associated with the σ-electron system of the aromatic
molecule. The electron withdrawing group (for example, Cl
atom as a substituent) pulls electron density by the inductive
effect and lowers the energies of the highest occupied and
lowest unoccupied π-MOs. This is because the electron
acceptors withdraw some electron density from the ligands,
thereby reducing the repulsive Coulombic interaction between
the electrons occupying the ligand-localized π-MOs and the
electrons of the σ-system. In the present system, we have
established (by molecular orbital calculations and comparing
with other similar systems) that the HOMO is a mixture of
dithiolate (π) and metal (d) orbital character and the LUMO is
a π* orbital of the dithiolate ligand. Thus an electron
withdrawing substituent on the heterocyclic dithiolate ligand
(present system) causes lowering the energy of LUMOs
because these are purely π-MOs of dithiolate ligand without
lowering the energy of HOMOs considerably (these are
mixture of nickel d-orbital character and dithiolate π-type). This
results in decreasing the energy gap between HOMO and
LUMO. This explains why electron withdrawing groups
(substituents −Cl and −C6H5 in compounds 3 and 2a
respectively) cause red shift in their electronic absorption
spectra (Table 3). The red shift of the absorption maximum of
the phenyl-substituted compound 2a indicates that the
inductive effect is dominant over mesomeric effect. On the
other hand, electron donating group (for example substituent
−CH3 in compound 4) increases the electron density toward
dithiolate ligand and thereby increases the repulsive Coulombic
interaction between ligand centered π-MOs and σ-electrons. As
a result, the energy of LUMOs will rise and the energy gap
between HOMO and LUMO will increase. This is why the
methyl substituted dithiolate complex exhibits blue shift in its
electronic absorption spectrum (Table 3). As shown in Figure
1a, there are some small features beyond 800 nm (820 nm for
compound 1a, 865 nm for compound 2a, 890 nm for
compound 3 and 800 nm for compound 4). These small
features might be due to the radical anion species formed by
aerial oxidation of the corresponding [Ni(X2-6,7-qdt)2]
dianions, which is not surprising because of low oxidation
potential of the present system.
Figure 2. HOMO and LUMO diagrams originated from molecular
orbital calculations for the anions of compounds 1a, 2b, 3, and 4.
on the MO picture (Figure 2), we found a small contribution of
the central metal in the HOMOs and in the LUMOs, the
electron density is high in the π* orbitals of dithiolate. Since the
HOMO includes metal-mixed ligand-based orbitals, we state
that the transitions are “mixed-metal ligand-to-ligand” CT, as
also mentioned for the (diimine)Pt(dithiolene) complexes.11a
The calculated vertical transition wavelengths for the complex
anions [Ni(6,7-qdt)2]2− (in 1a), [Ni(Ph26,7-qdt)2]2− (in 2b),
[Ni(Cl26,7-qdt)2]2− (in 3), and [Ni(Me26,7-qdt)2]2− (in 4) are
490.13 nm, 514.09 nm, 537.87, and 471.99 nm, respectively.
These findings follow the same trend as we observed in the
experiment (the electronic absorption spectra of complexes 1a,
2a, 3, and 4 in MeOH solutions as shown in Figure 1a). Thus
the origin of the electronic transition (broad band in the visible
region) of the present system ([Ni(X26,7-qdt)2]2−; X = H, Ph,
Cl, and Me) is analogous to that of [Pt(qdt)2]2−, as reported by
Cummings and Eisenberg.11a
It is evident from the electronic absorption spectral studies
(Figure 1, Table 3) in solution as well as in solid state that
Table 3. Influences of the Nature of the Electron Donating
and Withdrawing Groups on the Shifting of Absorption
Position Maxima of the CT Bands in the Visible Region
CT band of the compounds, λmax (nm)
S.No
solvent/solid
1a
2a
3
4
1
2
3
4
5
6
MeOH
DCM
588
593
593
619
619
595
607
623
624
662
660
654
609
642
640
707
709
728
547
560
566
593
589
533
Furthermore, the CT bands of all these complexes are
sensitive to the solvent polarity of the solvents as shown in
Table 3. In DMF and acetone solvents, the CT bands are
observed in the lower energy region; on other hand in DCM
and acetonitrile solvents, the CT bands absorb at the higher
energy region for all the complexes 1a, 2a, 3, and 4.
acetonitrile
DMF
acetone
solid-state
Electrochemistry. Interestingly, complexes 1a, 2a, 3, and 4
undergo reversible oxidation at very low oxidation potentials in
MeOH solutions as shown in Figure 3. We have shown in our
electron donating groups (e.g., methyl group of compound 4)
alter the position of the absorption maxima toward the blue
region and electron-withdrawing groups (e.g., chloro and
phenyl groups in compounds 3 and 2a, respectively) shift the
position of the absorption maxima to the red region. This trend
is consistent in the solid state even more profoundly as shown
Figure 1b (diffuse reflectance spectra). The influence of a
earlier communication14 that complex 1a undergoes E1/2
=
+0.12 V vs Ag/AgCl (ΔE = 74 mV) corresponding to [Ni(6,7-
qdt)2]−/[Ni(6,7-qdt)2]2− redox couple, which is less as
compared to the first oxidation potential for the complex
(Bu4N)2[Ni(qdt)2] appearing at E1/2 = +0.41 V vs Ag/AgCl
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only the phenyl-substituted system (compounds 2a and 2b)
should have a mesomeric effect because the π-electrons of the
phenyl group can undergo delocalization with π-electrons of
the aromatic ring. On the other hand, the substituents of
compounds 3 and 4 should have an inductive effect because in
compound [Bu4N]2[Ni(Cl26,7-qdt)2] (3) there is an electron
withdrawing substituent (chlorine atom), which can pull
electron density from the {NiS4} core and in compound
[Bu4N]2[Ni(Me26,7-qdt)2] (4), the substituted methyl group is
electron donating, that will push/increase the electron density
toward the {NiS4} core. Thus expectedly, the first oxidation
potential of compound 3 is higher than that of compound 1a
(the relevant {NiS4} moiety is difficult to oxidize, because the
metal electron will have a tendency to shift toward the electron
withdrawing substituent). For the same reason, compound 4
will have relatively less potential (because the relevant {NiS4}
core is easy to oxidize because the electron donating methyl
group would increase the electron density toward the metal
ion). In the case of compound 2a (or 2b), both inductive- and
mesomeric- effects are possible; however in the electrochemical
cell, the mesomeric effect dominates over the inductive effect
and that is why it exhibits a relatively lower oxidation potential
(relevant metal center is easy to oxidize because of electron
pushing tendency toward metal ion through delocalization of π-
electrons between phenyl group and aromatic ring). This is in
contrast to the electronic absorption spectroscopy where the
inductive effect in compound 2a (or 2b) prevails (vide supra).
Description of the Crystal Structures. The structural
details of compound [Bu4N]2[Ni(6,7-qdt)2] (1a) have been
reported in our earlier communication.14
[PPh4]2[Ni(6,7-qdt)2] (1b). The crystals of compound 1b,
suitable for single crystal X-ray structure determination, were
obtained from acetonitrile solution by the vapor diffusion of
diethyl ether. Crystallographic analysis revealed that compound
1b crystallizes in the monoclinic system with space group C2/c
whereas compound 1a crystallizes in P2(1)/c.14 The asym-
metric unit in the crystal structure of compound 1b contains a
half molecule of [Ni(6,7-qdt)2]2− anion and one tetraphenyl-
phosphonium cation. A thermal ellipsoid plot of compound 1b
containing the anion complex unit is shown in Figure 4a. Even
though, both complex anions in 1a and 1b form almost square
planar geometry around the Ni2+ ion, which is coordinated by
four sulfur atoms from two 6,7-qdt2− ligands between the two
SMS planes, there is more deviation in the planarity of the
Figure 3. Cyclic voltammograms of compounds 1a, 2a, 3, and 4 in
MeOH solutions at scan rate 50 mV s−1, V vs Ag/AgCl.
(ΔE = 89 mV), that corresponds the [Ni(qdt)2]−/[Ni(qdt)2]2−
redox couple in the electrochemical scale. We have also
mentioned that complex 1a is easily oxidized from dianion to
monoanion compared to the (Bu4N)2[Ni(qdt)2] complex. To
answer the question of why [Ni(H2-6,7-qdt)2]2− has much
lower oxidation potential than that of [Ni(qdt)2]2−, we argue
that the electron accepting ability of the ring N atoms in
[Ni(qdt)2]2− should be more than that of the ring N atoms in
[Ni(H2-6,7-qdt)2]2− because the ring N atoms are closer to
{NiS4} core in [Ni(qdt)2]2− than those in [Ni(H2-6,7-qdt)2]2−.
If the electron density of the {NiS4} core is pulled more toward
the ligand (as in the case of [Ni(qdt)2]2−), the concerned
{NiS4} moiety should not be oxidized easily. On the other
hand, in the case of [Ni(H2-6,7-qdt)2]2−, because of the longer
distance of the ring N acceptors from the nickel ion, the
electron density in {NiS4} core is not pulled toward the ligand
that effectively. Consequently, the [Ni(H2-6,7-qdt)2]2−can be
relatively easily oxidized. Thus the difference of ring N position
between 6,7-qdt and normal qdt explains the shift of oxidation
potential.
The first oxidation potentials for the complexes 2a, 3, and 4
appear at E1/2 = +0.033 V (ΔE = 65 mV), E1/2 = +0.18 V (ΔE
= 77 mV) and E1/2 = +0.044 V (ΔE = 89 mV) vs Ag/AgCl,
respectively, as shown in Figure 2. These oxidations correspond
to the dianionic complex to monoanionic complex of 2a, 3, and
4, respectively. In addition, compounds 1a, 2a, 3, and 4 show
some irreversible oxidations as shown in Table 4, and the
relevant CV diagrams are shown in the Supporting Information,
Figures S1, S2, S3, S4.
The effect of substituents of the aromatic ring having
dithiolate ligands attached to the {NiS4} core on the oxidation
potential of the complex anion can be explained by inductive-
and mesomeric-effects of the substituents. In the present study,
Table 4. Electrochemical Data of the Compounds 1a, 2a, 3,
and 4
1a
2a
3
4
oxd
E1
(V)
+0.12(rev)
+0.27(irr)
+0.58(irr)
+0.033(rev)
+0.18(rev)
+0.32(irr)
+0.49(irr)
+0.044(rev)
1/2
Figure 4. (a) Thermal ellipsoidal diagram of compound 1b containing
complex unit (40% probability); sideview representation of complexes
(b) 1b and (c) 1a.
E2 oxd (V)
E3 oxd (V)
+0.36(irr)
+0.57(irr)
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Figure 5. (a) View of the asymmetric unit of [PPh4]2[Ni(6,7-qdt)2] (1b) (thick solid lines) (some of the hydrogen atoms are removed for clarity)
showing all the H-bonding intermolecular cation−anion and anion−anion contacts (dotted lines) with other surrounding moieties (thin solid lines);
(b) One dimensional supramolecular chain of compound 1b characterized by C−H···N and C−H···S weak interactions.
a
dithiolate-chelate present in the complex 1b; on the other hand,
the crystal structure of compound 1a shows less deviation as
shown in Figures 4b and 4c. The coordination angles are in the
range of 88.41(13)°−91.59(13)° in complex 1a and
88.65(4)°−91.35(4)° in complex 1b, which are very close to
90.0°, and other coordination angles are 180.0° in both the
complexes 1a and 1b, which are not deviated. The bending
deviation (η) between the SMS plane and SCCS plane are
characterized by the angle of 11.93° present in the
{Ni1S1S2C1C2} chelate of the complex 1b, whereas in
complex 1a this bending deviation is 1.88°. This deviation in
planarity of the dithiolate-chelate might be due to the
involvement of S atoms in C−H···S weak interactions with
cation−anion contacts, as shown in Figure 5a. In the crystal
structure of compound 1b, both cation [Ph4P]+ and anion
[Ni(6,7-qdt)2]2− are involved in C−H···S and C−H···N
hydrogen bonding interactions resulting in a one-dimensional
supramolecular chainlike arrangement (Figure 5b).The relevant
hydrogen bonding geometrical parameters are listed in the
Table 5. This supramolecular chain engages C−H···S weak
hydrogen bonding interactions with surrounding tetraphenyl-
phosphonium cations as shown in Figure 5b.
[PPh4]2[Ni(Ph26,7-qdt)2]·3DMF (2b). The crystals of com-
pound 2b, suitable for single crystal X-ray structure
determination, were obtained from DMF solution by the
vapor diffusion of diethyl ether. Compound 2b crystallizes in
monoclinic space group C2/c. The asymmetric unit of
compound 2b contains one molecule of [Ni(Ph26,7-qdt)2]2−
and two tetraphenylphosphonium cations as shown in
Supporting Information, Figure S5a. In addition to this, three
DMF solvent molecules are crystallized in the crystal lattice of
the compound 2b, in which, two of them are present in 2-fold
axis of symmetry. These two DMF solvent molecules suffer
from significant disorder; thus we could not perform
anisotropic refinement for these two solvent molecules as
shown in the Supporting Information, Figure S5b. Thermal
ellipsoidal diagram of the complex anion [Ni(Ph26,7-qdt)2]2−
in compound 2b is shown in Figure 6a. The dihedral angle
between the two SMS planes is 2.70° and the geometry around
the Ni2+ ion, which is coordinated by four sulfur atoms from
two {Ph26,7-qdt} ligands, shows slightly distorted from square
planar geometry as shown in Figure 6b. The coordination
angles are in the range of 177.81(6)°−178.36(6)° and
88.51(5)°−91.51(5)°, which are slightly deviated from angles
of 180.0° and 90.0°, respectively. In addition to the dihedral
angle, complex 2b shows bending deviations (from planarity)
of the dithiolate-chelates. The bending deviations (η) between
Table 5. Hydrogen Bonds for the Complexes 1b, 2b, and 4
d(D-
H)
D−H···A
d(H···A)
d(D···A)
∠(DHA)
Compound 1b
b
C(3)−H(3)···N(2)
0.93
0.93
2.61
2.88
3.531(6)
3.650(6)
170.1
141.4
C(22)−H(22)···S(2)
Compound 2b
c
c
c
C(76)−H(76)···S(2)
C(61)−H(61)···S(1)
C(60)−H(60)···S(3)
C(54)−H(54)···O(1)
C(48)−H(48)···O(1)
C(50)−H(50)···N(4)
C(12)−H(12)···O(1)
0.93
0.93
0.93
0.93
0.93
0.93
0.93
2.99
3.769(6)
3.801(5)
3.722(5)
3.469(6)
3.583(6)
3.576(6)
3.316(6)
142.9
154.8
147.3
152.9
164.6
153.8
125.8
2.94
2.91
2.62
2.68
2.72
2.68
d
d
e
f
Compound 4
g
C(25)−H(25A)···S(2)
C(21)−H(21A)···S(4)
C(46)−H(46B)···S(1)
C(49)−H(49B)···S(4)
C(38)−H(38A)···N(3)
C(37)−H(37B)···S(2)
0.97
0.97
0.97
0.97
0.97
0.97
0.97
2.86
3.814(4)
3.778(4)
3.669(4)
3.780(4)
3.584(6)
3.609(4)
3.785(5)
167.1
154.4
156.9
166.9
163.6
169.5
152.1
g
2.88
2.76
2.83
2.64
2.65
2.90
h
h
i
j
j
C(31)−H(31B)···S(2)
a
Symmetry transformations used to generate equivalent atoms are
b
c
d
e
given in footnotes b−j. −x,y,−z+1/2. x,y−1,z. x,−y+1,z+1/2. x+1/
2,y−1/2,z. x,y,z+1. −x+1,−y+1,−z+1. −x+1,−y+1,−z. x,y+1,z. x−
1,y+1,z.
f
g
h
i
j
the {S1Ni1S2} and {S1C1C2S2} planes, and {S3Ni1S4} and
{S3C21C22S4} planes, are characterized by the angles of
5.80°and 0.26° present in {Ni1S1S2C1C2} and
{Ni1S3S4C21C22} dithiolate-chelates, respectively. The crystal
structure of the compound 2b is characterized by three C−
H···S weak interactions and one C−H···N weak interaction
between the anion and the cations resulting in a one-
dimensional supramolecular chain as shown Figure 6c. The
weak C−H···S interactions may cause the deviation of dihedral
angles around the metal ion and bending deviation of
dithiolene-chelated ring.
[Bu4N]2[Ni(Cl26,7-qdt)2] (3). The crystals of compound 3, for
single crystal X-ray structure determination, were recrystallized
from acetonitrile solution by the vapor diffusion of diethyl
ether. X-ray crystal structure analysis shows that compound 3
crystallizes in monoclinic space group P2(1)/c. The relevant
asymmetric unit of the compound 3 contains half a molecule of
[Ni(Cl26,7-qdt)2]2− anion and one molecule of tetrabutylam-
monium cation. The thermal ellipsoidal diagram of [Ni(Cl26,7-
qdt)2]2− anion, present in the compound 3, is shown in Figure
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could not get a better quality of the single crystals of this
compound. In the anion of the compound 3, the geometry
around the Ni2+ ion, which is coordinated by four sulfur atoms
from two {Cl26,7-qdt} ligands, shows approximately square
planar geometry, because the associated coordination angles are
in the range of 88.42(10)°−91.58(10)° which are very close to
90.0° and other coordination angles are 180.0° without any
deviation as shown in Figure 7b. However, there is a deviation
in the planar nature of the dithiolene-ligand (chelate) present in
the compound 3. The bending deviation (η) between the SMS
plane and SCCS plane is characterized by the angle of 5.32°
present in the {Ni1S1S2C1C2} chelate in 3.
[Bu4N]2[Ni(Me26,7-qdt)2] (4). The crystals of compound 4,
suitable for single crystal X-ray structure determination, were
obtained from acetonitrile solution by the vapor diffusion of
diethyl ether. Crystallographic analysis reveals that compound 4
crystallizes in triclinic space group P1. The relevant asymmetric
̅
unit contains one molecule of [Ni(Me26,7-qdt)2]2− anion and
two molecules of tetrabutylammonium cations. The thermal
ellipsoidal plot of [Ni(Me26,7-qdt)2]2− anion, present in
compound 4, is shown Figure 8a. In the anion of the
Figure 6. (a) Thermal ellipsoidal diagram of compound 2b containing
complex unit (50% probability); and its (b) sideview representation;
(c) one-dimensional supramolecular chain of compound 2b
characterized by C−H···N and C−H···S weak interactions.
Figure 8. (a)Thermal ellipsoidal diagram of compound 4 containing
complex unit (40% probability); and its (b) side view representation.
7a. Because of the poor quality of the crystal, tetra-
butylammonium cation, present in the crystal structure of the
compound 3, suffers from significant disorder as shown in
Supporting Information, Figure S6. Because of this, we could
not study weak interactions between the cations and the anions
in the relevant crystal structure. Although few attempts of
recrystallization with various techniques have been made, we
compound 4, the geometry around the Ni2+ ion, which is
coordinated by four sulfur atoms from two Me26,7-qdt ligands,
shows distortion from square planar geometry; the relevant
distortion is between the two SMS planes with a dihedral angle
of 4.87°, because the coordination angles are in the range of
175.30(5)°−177.89(5)° and 87.93(4)°−91.22(4)° which are
deviated from 180.0° and 90.0°, respectively. The distortion of
the {NiS4} core from square planar geometry is clearly shown
in the side view of complex anion in compound 4 (Figure 8b),
whereas complexes 1a and 1b do not show similar distortion.
The reason for this distortion from the square planar geometry
(compound 4) may be due to the involvement of the sulfur
atoms in extensive C−H···S hydrogen bonding interactions.
The bending deviations (η) between the {S1Ni1S2} and
{S1C1C6S2} planes, and {S3Ni1S4} and {S3C11C16S4}
planes, are characterized by the angles of 6.24°and 4.53°
present in {Ni1S1S2C1C6} and {Ni1S3S4C11C16} dithiolate-
chelates, respectively.
In the crystal structure of the compound 4, three out of four
sulfur atoms in each dithiolate anionic complex are
characterized by six C−H···S hydrogen bonding contacts with
four surrounding [Bu4N]+ cations and one C−H···N hydrogen
bonding contact with one [Bu4N]+ cation as shown the
Supporting Information, Figure S7. In the asymmetric unit of
Figure 7. (a) Thermal ellipsoidal diagram of compound 3 containing
complexic unit (20% probability); and its (b) side view representation.
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the crystal structure, there are two types of [Bu4N]+ cations
characterized by six C−H···S hydrogen-bond contacts with
three surrounding anions and one C−H···N hydrogen-bonding
contact with one anion as shown in Figure 9a. The relevant
new series of square-planar nickel-bis(quinoxaline-6,7-dithio-
late) complexes with the general formula [Bu4N]2[Ni(X26,7-
qdt)2], where X = H (1a), Ph (2a), Cl (3), and Me (4).
Solution and solid state electronic absorption spectra of all
these compounds show broad bands in the visible region that
are due to the CT transitions involving electronic excitation
from a HOMO which is a mixture of dithiolate (π) and metal
(d) orbital character to a LUMO which is a π* orbital of the
dithiolate. The present ligand system (6,7-qdt)2−, in compar-
ison with the existing well-studied ligand (qdt)2−, is unique.
The change in position of ring nitrogen atoms from sulfur-
attached ring to adjacent ring makes a lot of difference in terms
of oxidation potential of the resulting coordination complexes,
for example, [Ni(6,7-qdt)2]2− has much lower oxidation
potential than that of [Ni(qdt)2]2−. Therefore, the effect of
substitution of this new class of ligands on the electronic
structures of the relevant metal-coordination complexes is an
important approach not only in metal-dithiolene chemistry but
also in bioinorganic chemistry (modeling active sites of
molybdenum hydroxylases). The active sites of molybdenum
hydroxylases contain a similar ligand system, in which the ring
nitrogens are not present in the sulfur attached ring but in the
adjacent second ring as observed in (6,7-qdt)2−. The
electrochemical properties (the first oxidation potentials) and
the electronic absorption spectral behavior of the present
system [Bu4N]2[Ni(X26,7-qdt)2] (X = H, Ph, Cl, and Me) are
dependent on the nature of the electron donating/withdrawing
groups (X) attached to the 6,7-quinoxaline dithiolene core
moiety. Thus the present study serves to understand the role of
substituents of a particular ligand on electron/CT in frontier
orbitals. Moreover, the present system has potential to be a
good oxidation catalyst as indicated by its low oxidation
potential. Presently, we are working on synthesis, character-
ization, and reactivity of oxo-molybdenum/tungsten complexes
with the same ligands with relevance to the active sites of oxo-
transferase metalloenzymes.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format and computational outputs
(coordinates of the G03 optimized structures) for compounds
1a, 2b, 3, and 4. Further details are given in Figures S1−S21.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
Figure 9. (a) View of the asymmetric unit of compound 4 (thick solid
lines) (some of the hydrogen atoms are removed for clarity) showing
all the H-bonding intermolecular cation−anion contacts (dotted lines)
with other surrounding moieties (thin solid lines); (b) two-
dimensional supramolecular network of compound 4 is characterized
by C−H···N and C−H···S weak interactions.
*E-mail: skdsc@uohyd.ernet.in, samar439@gmail.com. Fax:
+91-40-2301-2460. Tel: +91-40-2301-1007.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
hydrogen bonding geometrical parameters are listed in the
Table 5. The effective combination of C−H···S and C−H···N
weak hydrogen bonding interactions between the cation and
the anions result in a two-dimensional supramolecular network
as shown in Figure 9b.
Our special thanks to Professor S. Mahapatra, School of
Chemistry, University of Hyderabad, for his help in theoretical
calculations. 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 India, is gratefully acknowledged.
We are grateful to UGC, New Delhi, for providing infra-
structure facility at University of Hyderabad under a UPE grant.
CONCLUSIONS
With the aim of designing new quinoxaline dithiolate ligands
and their metal coordination complexes, we have synthesized a
■
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R.B., S.N.R., and G.D. thank CSIR India for their fellowships,
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DEDICATION
(14) Bolligarla, R.; Durgaprasad, G.; Das, S. K. Inorg. Chem. Commun.
2011, 14, 809.
■
†Dedicated to Professor Sabyasachi Sarkar on the occasion of
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