Spectroscopy, structure, and electrochemistry of transition metal complexes having [M2N2OS2] coordination sphere

Article abstract of DOI:10.1080/00958972.2010.511201

Binuclear transition metal complexes of bicompartmental SNONS donors were synthesized and characterized by various physico-chemical techniques. Two different precursors with chloromethyl/formyl functionality at the 2 and 6 positions of phenolate ring are used to construct the ligands. The quinoxaline scaffolds provide SN donors incorporated at the 2 and 6 positions. Copper and zinc complexes are square pyramidal, whereas nickel and cobalt complexes are octahedral. The influence of two metal centers in terms of cooperative effect on the electronic, magnetic, electrochemical, and structural properties was investigated.

Full text of DOI:10.1080/00958972.2010.511201

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Spectroscopy, structure, and
electrochemistry of transition
metal complexes having [M N OS ]
2
2
2
coordination sphere
a
a
Naveen V. Kulkarni , Gurunath S. Kurdekar , Srinivasa
a
a
Budagumpi & Vidyanand K. Revankar
a
Department of Studies in Chemistry, Karnatak University,
Pavatenagar, Dharwad–580 003, Karnataka, India
Published online: 19 Aug 2010.
To cite this article: Naveen V. Kulkarni , Gurunath S. Kurdekar , Srinivasa Budagumpi & Vidyanand
K. Revankar (2010): Spectroscopy, structure, and electrochemistry of transition metal complexes
having [M N OS ] coordination sphere, Journal of Coordination Chemistry, 63:18, 3301-3312
2
2
2
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Journal of Coordination Chemistry
Vol. 63, No. 18, 20 September 2010, 3301–3312
Spectroscopy, structure, and electrochemistry of transition
metal complexes having [M N OS ] coordination sphere
2
2
2
NAVEEN V. KULKARNI, GURUNATH S. KURDEKAR,
SRINIVASA BUDAGUMPI and VIDYANAND K. REVANKAR*
Department of Studies in Chemistry, Karnatak University, Pavatenagar,
Dharwad – 580 003, Karnataka, India
(
Received 28 February 2010; in final form 9 June 2010)
Binuclear transition metal complexes of bicompartmental SNONS donors were synthesized and
characterized by various physico-chemical techniques. Two different precursors with
chloromethyl/formyl functionality at the 2 and 6 positions of phenolate ring are used to
construct the ligands. The quinoxaline scaffolds provide SN donors incorporated at the 2 and 6
positions. Copper and zinc complexes are square pyramidal, whereas nickel and cobalt
complexes are octahedral. The influence of two metal centers in terms of cooperative effect on
the electronic, magnetic, electrochemical, and structural properties was investigated.
Keywords: 2,6-Bis-(chloromethyl)-4-methylphenol; 2,6-Diformyl-4-methylphenol; 3-Hydrazi-
nylquinoxaline-2-thione; Bicompartmental ligand; Electrochemistry
1. Introduction
Model systems which may mimic the active sites of metallobiomolecules have been an
incentive for the development of chemistry of binuclear complexes. Binuclear systems
may bind and activate small molecules or may be used to investigate the cooperative
effect of two metal centers on electronic, magnetic, and redox properties [1, 2].
Suitably designed binuclear systems would provide new reactivity patterns and
physical properties that could not be achieved with mononuclear complexes. The
neighboring metal centers may cooperate in promoting reactions and electronic
interactions, leading to diverse physical properties [3, 4].
The symmetric and asymmetric nature of a number of homobinuclear transition
metal-derived metallobiosites [5] and the ability of the individual metal ions to have
quite distinct roles in the functioning of metalloenzymes have led to the design of
symmetric and asymmetric binucleating ligands, which make the binuclear complexes
capable of acting as models for metallobiosites.
Phenol-based compartmental ligands of ‘‘end-off ’’ type, possessing two chelating
arms attached to 2 and 6 positions of phenolate ring, have been used to
provide phenoxo-bridged binuclear core complexes, with option for varying exogenous
*
Corresponding author. Email: vkrevankar@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.511201

3
302
N.V. Kulkarni et al.
bridges [1, 2, 6]. Recent X-ray crystallographic studies have indicated that most
bimetallic/binuclear biosites are asymmetric with respect to the donors about the
metals, the nature of the metal ions, the coordination number, and the geometric
arrangement of the donors [5]. Thus, the design of end-off compartmental ligands
providing a discrete binuclear core is of particular interest.
Quinoxaline capable of providing sulfur and nitrogen donors has been incorporated
at the 2 and 6 position of phenolate ring to construct bicompartmental ligands. The
mode of attachment is different in two ligands in order to provide different size and
flexibility for the bicompartmental SNONS cavity. Transition metal(II) chlorides are
employed for the construction of binuclear complexes. The structure as well as
electronic, magnetic, and redox properties of the prepared complexes are studied.
Several transition metal binuclear complexes with a wide variety of coordination
spheres exist [7–10]. The structural and functional properties of these complexes depend
on the size and flexibility of coordination cavity, as well as the nature of donors and the
metal ion.
2
2
. Experimental
.1. Reagents and apparatus
The chemicals used were reagent grade. Purified solvents were used for the synthesis
of ligands and complexes. Syntheses of 2,6-bis-(chloromethyl)-4-methylphenol [11],
2,6-diformyl-4-methylphenol [12], and quinoxaline-2,3-(1,4H)-dithione [13] was done
according to the method described with slight modification. Isatin was purchased from
Sigma Aldrich and the metal chlorides {CoCl Á 6H O, NiCl Á 6H O, CuCl Á 2H O, and
2
2
2
2
2
2
ZnCl } were used for complex formation. Elemental (C, H, N and S) analysis was
2
carried out on a THERMO QUEST elemental analyzer. Metal and chloride estimations
were done by following standard procedures [14]. The molar conductivity measure-
ments in DMF were made on an ELICO-CM-82 conductivity bridge with conductivity
ꢀ
1
cell having cell constant 0.51 cm . The magnetic susceptibility measurements were
made using a Faraday balance at room temperature using Hg[Co(SCN) ] as calibrant.
4
1
H NMR spectra were recorded in DMSO-d /CDCl on a Bruker-300 MHz spectro-
6
3
meter at room temperature using TMS as internal reference. Infrared (IR) spectra were
recorded in KBr using an Impact-410 Nicolet (USA) FT-IR spectrometer from 4000 to
ꢀ1
4
00 cm . Electronic spectra of the compounds in DMF were recorded on a Hitachi
50-20 spectrophotometer from 1000 to 200 nm. Cyclic voltammetric studies were
1
performed at room temperature in oxygen-free DMF created by purging with pure
nitrogen, using a CHI1110A Electrochemical analyzer (USA) comprising a three
electrode assembly of glassy carbon working electrode, platinum auxiliary electrode,
þ
and non-aqueous Ag /AgCl reference electrode. Tetramethylammonium chloride
ꢀ
1
0.01 mol L ) was used as supporting electrolyte and the instrument was standardized
(
by ferrocene/ferrocenium redox couple. FAB mass spectra were recorded from a JEOL
SX 102/DA-6000 mass spectrometer using argon/xenon (6 kV, 10 mA) as the FAB gas
and 3-nitrobenzylalcohol as matrix. TG and DTA measurements were done in nitrogen
ꢁ
ꢀ1
on a Universal V2 4F TA instrument at the scanning rate of 10 C min from 25 to
ꢁ
1
000 C.

Complexes with [M N OS ] coordination sphere
2
3303
2
2
H
N
H
N
S
N
Dry methanol
Reflux
NH2
.
NH NH H O
2 2 2
+
N
H
S
N
H
S
Quinoxaline-2,3(1,4H)-dithione Hydrazine hydrate
3-Hydrazinylquinoxaline-2(1H)-thione
Figure 1. Schematic representation of preparation of 3-hydrazinylquinoxaline-2-thione.
2
2
.2. Synthesis
.2.1. Synthesis of 3-hydrazinylquinoxaline-2(1H)-thione. Hydrazine hydrate in meth-
ꢀ1
anol (0.6 mL, 0.1 mol L ) was added dropwise to a methanolic solution of quinoxaline-
ꢀ
1
,3-(1,4H)-dithione (1.94 g, 0.1 mol L ) with constant stirring. The mixture was then
2
refluxed for 2 h over a steam bath. Product formed as a shiny dark yellow solid was
ꢁ
filtered, thoroughly washed with methanol and air dried (Yield: 83%, m.p.: 182–184 C)
(figure 1).
1
2
.2.2. Synthesis of HL and HL . Hot methanolic solution of 3-hydrazinylquinoxa-
2
line-2(1H)-thione (1.92 g, 0.1 mol L ) was added dropwise to a stirring solution of
ꢀ
1
ꢀ
2
1
1
,6-diformyl-4-methylphenol (0.82 g, 0.05 mol L ) (for HL )/2,6-bis-(chloromethyl)-4-
2
ꢀ1
methyl phenol (1.08 g, 0.05 mol L ) (for HL ) in dry methanol. The mixture was
refluxed for 2 h on a heating mantle and the solid separated was isolated by filtration.
1
The solid was thoroughly washed with methanol and dried {(HL , m.p.: 169–172 C,
ꢁ
2
ꢁ
Yield: 78%) (HL , m.p.: 173–176 C, Yield: 79%)}. The reaction pathway is given in
figure 2.
1
.2.3. Preparation of complexes. A solution of ligand HL (0.524 g, 0.01 mol L ) in
ꢀ1
2
ꢀ
1
methanol was added to methanolic solution of CuCl ꢂ 2H O (0.341 g, 0.002 mol L
)
2
2
with constant stirring and refluxed on a heating mantle for 1 h. The copper complex,
separated as greenish black solid, was filtered, washed thoroughly with methanol and
dried.
Similar procedure was followed for the preparation of the other complexes with
2
1
appropriate amounts of ligands and metal salts [{HL (0.524 g, 0.01 mol L ), HL
ꢀ1
ꢀ
1
ꢀ1
(
0
0
0.538 g, 0.01 mol L )}, {CoCl Á 6H O (0.475 g, 0.02 mol L ), NiCl Á 6H O (0.474 g,
2
2
2
2
ꢀ1
ꢀ1
.02 mol L ), CuCl Á 2H O (0.341 g, 0.002 mol L ), and ZnCl
(0.271 g,
2
2
2
ꢀ1
.02 mol L )}].
3
. Results and discussion
The phenoxo-bridged binuclear complexes obtained in the present investigation
were non-hygroscopic, amorphous solids insoluble in ethanol, water, and chloro-
hydrocarbons, but soluble in DMF and DMSO. The composition and coordina-
tion geometry of these complexes have been established based on elemental analyses,

3
304
N.V. Kulkarni et al.
H
N
N
H N
2
2
S
N
H
CH3
OH
CH3
OH
3
-Hydrazinyl-3,4-
dihydroquinoxaline-2(1H)-thione
Cl
Cl
O
O
Reflux in methanol
2
,6-bis-(Chloromethyl)-
2,6-Diformyl-
4
-methyl phenol
4-methyl phenol
CH3
CH3
HN N
OH
N NH
N
HN NH
N
OH HN NH
N
N
S
S
S
S
NH
HN
NH
HN
HL2
HL1
1
Figure 2. Schematic representation of synthesis of HL and HL .
2
IR, NMR, mass spectral analyses, and TG studies. All the experimental results are
in good agreement with the molecular formulae suggested for the compounds.
The elemental analyses data of the ligands and their complexes along with molar
conductivity values are compiled in table 1.
3.1. Molar conductivity measurements
The molar conductance values of the complexes measured at room temperature
ꢀ3
ꢀ3
in DMSO solution with 10 mol dm
ꢀ1
concentration are in the range
3.3–16.8 Ohm cm mol , indicating non-electrolytic nature of the complexes [15].
ꢀ
1
2
1
3
.2. IR spectral studies
Key IR absorption bands are summarized in table 2. Both ligands exhibit absorption
ꢀ1
bands of medium intensity in the 3100–3330 cm region assigned to ꢀ(NH) [16].
In spectra of complexes these bands shift to higher frequency and broaden in
some complexes due to overlapping ꢀ(OH) of coordinated water molecules at
ꢀ
1
ꢀ
1
3
band, which disappears upon complexation, indicating deprotonation and coordination
390–3450 cm . The absorption due to phenolic ꢀ(OH) is at 3050 cm as a weak
of phenoxide [17]. This is further supported by the shift of phenolic ꢀ(C–O)
1
ꢀ
ꢀ1
at ꢃ1300 cm in ligands to higher frequency by 9–45 cm in the spectra of complexes.
The thioamide bands having a major contribution from C¼S chromophore are

Complexes with [M N OS ] coordination sphere
2
3305
2
2

3
306
N.V. Kulkarni et al.
ꢀ1
Table 2. IR spectral data in cm
.
Compound
ꢀ(OH) water
ꢀ(NH)
ꢀ(C¼N)
ꢀ(C¼N) ring
Thioamide bands
ꢀ(C–O)
1
HL
–
3162m
3
–
1613s
1388s
858s
1300m
106m
1
CoL
NiL
3421b
3446b
–
3200b
3200b
3150b
3180b
3250b
3291m
3200b
3327s
3333s
–
–
–
1614m
1617m
1624m
1617s
1606s
1540s
1618s
1551s
1578s
1350 w
1345w
1371m
1350w
1382s
1377m
1377m
1370m
1388m
830w
830w
840w
835w
850m
831m
830w
820m
848m
1326m
1326m
1320m
1321m
1302m
1320m
1309m
1334s
1
1
CuL
ZnL
1
–
–
2
HL
CoL
–
3399b
3422b
–
1640s
1631s
1634m
1638s
1626s
2
2
NiL
CuL
2
2
ZnL
–
1346m
s – strong absorption, m – medium, w – weak, and b – broad.
CH3
CH3
HN NH OH HN NH
HN NH
OH HN NH
N
N
N
N
S
S
SH
HS
N
NH
HN
N
L1H
L1H
1
Figure 3. Thioketo-enol tautomerism in HL .
ꢀ
1
observed around 1385 and 855 cm in ligands, which later reduce in intensity and shift
to lower frequency upon complexation, suggesting the coordination of thione sulfur to
2
ꢀ1
metal [18]. For HL , a sharp band at 1640 cm is assigned to ꢀ(C¼N) [19]. In the
complexes it shifts to lower frequency region due to the azomethine nitrogen
1
coordination [19]. Low-frequency non-ligand bands at 500–600 and 450–500 cm are
ꢀ
assigned to ꢀ(M–N) and ꢀ(M–S), respectively [18].
1
.3. H NMR studies
3
NMR studies provide significant conclusion regarding the nature of bonding and
1
coordination. Ligand HL has a thio-enol tautomeric equilibrium in solution (CDCl )
3
(figure 3), shown by the proton resonance at 4.61 ppm (D O exchangeable). But in the
2
zinc complex spectrum, absence of this peak and appearance of a new broad peak at
1.50 ppm suggest the thioketo form and sulfur coordination [19]. The phenolic –OH
observed at 8.40 ppm in the ligand (D O exchangeable) disappears upon complexation,
1
2
indicating deprotonation and phenoxide coordination [17]. The peak at 7.89 ppm is
attributed to hydrazine protons [18] and the peak at 3.61 is assigned to methylene amine
protons, shifted downfield upon complexation. The aromatic protons are resonated in

Complexes with [M N OS ] coordination sphere
2
3307
2
2
Table 3. The electronic spectra and magnetic moment data.
max in nm (ꢃ" in L cmꢀ mol
1
ꢀ1
)
ꢃeff (BM)
Compound
ꢂ
1
HL
CoL
250(10000), 280(9000), 320(9000), 330(8500)
321(9800), 350(1100), 425(25000), 530(110)
320(10100), 421(24000), 460(200), 650(150)
300(12000), 373(10000), 423(26000), 450(150), 546(100)
280(12000), 350(1000), 400(1000), 420(23000)
242(12000), 278(9000), 316(8500), 327(8500), 362(9200)
316(10000), 350(10000), 420(25000), 520(100)
340(9800), 420(25000), 471(250), 645(200)
–
1
3.35
2.81
1.08
1
NiL
CuL
1
1
ZnL
HL
Diamagnetic
2
–
3.31
2.80
1.06
2
CoL
NiL
2
2
CuL
ZnL
300(9800), 360(8700), 420(23000), 440(130), 552(100)
280(10000), 350(9800), 380(9500), 422(22000)
2
Diamagnetic
the range 6.50 to 7.60 ppm, methylene protons resonate at 2.25 ppm, and methyl
2
protons of cresol absorb at 1.65 ppm [20]. In the proton NMR spectrum of ligand HL ,
broad peaks at 11.77 and 10.37 ppm (D O exchangeable) are attributed to the
2
quinoxaline ring –NH and phenolic –OH, respectively. The azomethine protons
resonate at 8.64 ppm and hydrazine NH at 7.70 ppm. The aromatic protons are
observed at 6.60–7.49 ppm and methyl protons are at 2.25 ppm. The peak at 10.37 ppm
corresponding to the phenolic –OH disappears in the zinc complex. All other protons
shift downfield upon complexation.
3
.4. Electronic spectral studies
1
2
Electronic spectral data are summarized in table 3. Ligands HL and HL exhibit
similar spectral features in the UV-Vis region with peaks at 250, 280, and 320 nm, with
molar absorptivity of " ꢃ 10,000 L cmꢀ mol . The sharp intense peak at 250 nm
assigned to intraligand ꢁ–ꢁ* transition remains unaltered even after complexation.
Absorption at 300–320 nm is assigned to n–ꢁ* transition from phenoxide to aromatic ꢁ
system (intraligand charge-transfer transition) [21]. The absorptions at 320–350 nm
1
ꢀ1
2
were assigned to the n ! ꢁ* transitions of the thioamide chromophore. HL exhibits a
broad peak at 350 nm corresponding to the n ! ꢁ* transition associated with
azomethine [22]. On complexation, this peak shows red shift due to the donation of
electrons to the metal through coordination [21]. All the complexes have an absorption
peak at 350–380 nm (" ꢃ 10,000 L cmꢀ mol ), which is assigned to the charge-transfer
1
ꢀ1
transition from the filled pꢁ orbital of the bridging phenolic oxygen to the vacant d
ꢀ
1
ꢀ1
orbital of the metal [21]. An intense peak at 420 nm (" ꢃ 25,000 L cm mol ) is
assigned to S!M(II), ligand to metal charge-transfer transition (LMCT) [18].
The cobalt complexes have a medium intensity absorption in the visible region
ꢀ1
ꢀ1
around 500 nm (" ꢃ 100 L cm mol ), assigned to d–d transition corresponding to
4
4
T ! A , suggesting an octahedral geometry [23, 24]. The absorption corresponding
1
g
2g
4
4
to T (F) ! T (P) transition is obscured due to the strong CT absorptions. The
1
g
1g
nickel(II) complexes exhibit many absorptions in the visible region with molar
ꢀ
1
ꢀ1
absorptivity in the range 60–80 L cm mol . Appearance of these spin-allowed
transitions (around 460 and 650 nm) suggest an octahedral geometry with the
3
3
3
3
absorptions assigned to A ! T (P) and A ! T , respectively [25, 26].
2
g
1g
2g
1g
Electronic spectra of copper complexes display a broad absorption at 500–650 nm

3
308
N.V. Kulkarni et al.
CH
3
CH
3
Cl
M
Cl
M
OH2
HN NH
HN NH
O
HN NH
HN NH
S
O
N
M
N
N
M
N
S
Cl
S
S
Cl
Cl
H2O
Cl
NH
HN
NH
HN
M=Cu and Zn
M=Co and Ni
CH
CH
3
3
Cl
M
Cl
M
OH2
HN
NH
N
S
O
N
NH
HN
HN
NH
N
S
O
N
NH
HN
N
M
Cl
N
N
M
Cl
N
Cl
S
Cl
S
H2O
M=Cu and Zn
M=Co and Ni
Figure 4. Proposed structures of complexes.
along with the intraligand and charge-transfer transitions. Because of the Jahn–Teller
distortion and low symmetry environment around Cu(II), detailed interpretation is
complicated [27]. The more intense charge transfer bands, which extend deep into the
visible region, also prevent the analysis of d–d transitions in the complexes. Both the
zinc complexes have shown a strong absorption peak around 370–400 nm, due to
LMCT, along with intra-ligand transitions in the 280–320 nm range.
3.5. Magnetochemistry
Magnetic susceptibility measurements were done at room temperature (values given in
1
table 3). The magnetic moments are found to be 3.35 and 3.31 BM for CoL and CoL ,
2
1
.81 and 2.81 BM for NiL and NiL , and 1.08 and 1.06 BM for CuL and CuL ,
2
1
2
2
respectively. The subnormal values of magnetic moment are attributed to the spin
exchange interaction between the closely spaced metal ions by phenoxide endogenous
bridging [28, 29].
3
.6. FAB mass spectral studies
1
2
The mass spectra of the complexes CuL and CuL exhibit intense peaks for the
molecular ions at m/z ¼ 749 and 744, respectively. The molecular ion peak corresponds
to the mass of entire binuclear complex including bridged chloride and coordinated
chloride ions. The general formula [Cu L(mCl)Cl ] is hence assigned for both
2
2
complexes, which agree well with elemental analysis data. The proposed structures of
complexes are given in figure 4.

Complexes with [M N OS ] coordination sphere
2
3309
2
2
3.7. TG-DTA analysis
1
1
2
2
Thermal behaviors of CuL , NiL , CuL , and NiL complexes were examined from
ꢁ
1
5 to 1000 C. The thermogram of CuL shows a plateau region up to 230 C, indicating
the absence of water in the complex. Disintegration takes place at 230–290 C with
weight loss (15.81%) due to the removal of three chlorides as HCl. The exothermic
nature of decomposition was indicated by the DTA curve. Further, gradual decom-
ꢁ
2
ꢁ
1
position at higher temperature is assigned to ligand and the residue was CuO. For NiL ,
ꢁ
decomposition takes place in three steps. Weight loss (4.74%) at 50–120 C corresponds
to the elimination of two water molecules. The corresponding DTA curve indicates the
ꢁ
endothermic nature of the process. The second step of decomposition at 150–300 C was
assigned to the loss of three chlorides as HCl (14.34%). Third step was the
2
decomposition of ligand and the final product was NiO. CuL decomposed in two
steps corresponding to the loss of three chlorides (14.30% weight loss at 200–300 C)
and the fragmentation of the ligand. Final decomposition product was copper oxide.
2
NiL has exhibited three-step decomposition as NiL , assigned to elimination of
ꢁ
1
ꢁ
coordinated water (4.74% weight loss, 80–110 C), evolution of three chlorides (14.95%
ꢁ
weight loss, ꢃ225 C), and ligand decomposition.
3
.8. Electrochemistry
The electrochemistry of oxo-bridged binuclear copper complexes is of interest because
of the biological significance of the Cu(II)/Cu(I) redox couple. Studies reveal that
copper electron transfer proteins and the mechanisms of copper enzymes are due to the
ready accessibility of the Cu(I) oxidation state [30, 31]. Although the oxidation state þ3
is unusual for copper, the existence of Cu(III) complexes as intermediates in various
enzymatic and non-enzymatic reactions have raised interest in Cu(III) complexes in
biological systems. To understand the electrochemistry of metallobiomolecules, studies
on the electrochemical behavior of copper complexes are necessary [32]. The cyclic
voltammetric study of molecules that contain two or more chemically equivalent
electroactive sites is a point of attention. Cyclic voltammograms for molecules with
multiple, non-interacting redox centers will be similar to those of the corresponding
species with a single center, and DEp will be 58 mV [33]; departure from this
generalization may be due to electronic interactions between metal centers.
In binuclear copper(II) complex the sequential electron transfer processes are
represented as
ꢀ
ꢀ
e
e
II
II
II
IIꢄI
IIꢄI
ꢀ CuIIꢄI
ꢀ
ꢀꢀꢀ ꢀꢀ *
ꢀꢀꢀꢀ ꢀꢀ *
Cu ꢀ Cu )ꢀꢀꢀꢀ ꢀꢀ Cu ꢀ Cu
)ꢀꢀꢀꢀ ꢀꢀ Cu
f
E
1
f
E
2
f
f
If for particular case E ꢅ E , the net result would be transfer of two electrons at a
1
2
single potential [34, 35]. The redox behavior of binuclear copper(II) complexes can be
interpreted by the consideration of the comproportionation constant, Kcom, which
relates mixed valence pair with native-valence system as follows:
Kcom
II
II
IIꢄI
ꢀ CuIIꢄI )ꢀꢀꢀꢀ ꢀꢀ 2Cu ꢀ Cu
II
IIꢄI
ꢀ
ꢀꢀꢀ ꢀꢀ *
Cu ꢀ Cu þ Cu
ꢀ
ꢁ
f
f
log Kcom ¼ E ꢀ E =0:0591
1
2

3
310
N.V. Kulkarni et al.
Table 4. Cyclic voltammetry data.
Scan
a
1
b
c
d
rate
Epa
(V)
1
Epa
(V)
2
Epc
(V)
2
Epc
(V)
1
DEp
(V)
DEp
(V)
2
E
1/2(1)
(V)
E
1/2(2)
(V)
ꢀ1
Compound (V s
)
Ipc
1
/Ipa
1
Ipc
2
/Ipa
2
1
CuL
CuL
0.15
0.20 0.50 0.34 0.02
0.18 0.48 0.36 0.04
0.16 0.46 0.38 0.06
0.12 0.52 0.33 0.03
0.1 0.50 0.35 0.05
0.08 0.48 0.37 0.07
0.18
0.14
0.10
0.09
0.05
0.01
0.16
0.12
0.08
0.21
0.15
0.09
0.11
0.11
0.11
0.07
0.07
0.07
0.42
0.42
0.42
0.43
0.43
0.43
0.88
0.89
0.92
0.91
0.92
0.94
0.87
0.87
0.88
0.88
0.89
0.9
0
0
.1
.05
2
0.15
0
0
.1
.05
a
DEp
DEp
1
¼ Epa
¼ Epa
1/2(1) ¼ (Epa
1
ꢀ Epc
1
.
b
2
2
ꢀ Epc2.
c
E
1
þ Epc
1
)/2.
)/2.
d
E
1/2(2) ¼ (Epa
2
ꢀ Epc
2
If the mixed-valence species are unstable with respect to disproportionation, K
f
com
f
tends to zero and E ꢅ E , which represent the two electron transfer at a single
1
2
potential. But in other cases where mixed-valent species are stable, the cyclic
voltammogram should give two successive reversible or quasireversible redox poten-
tials. The stability of the mixed-valent species increases as the separation in potential
between the two successive reduction or oxidation steps increases [36].
In the present work, copper complexes were investigated for electrochemical behavior
1
ꢀ
in DMSO (0.01 mol L ). The compounds were scanned from ꢀ1 to þ1 V with three
ꢀ1
different scan rates (0.05, 0.1, and 0.15 V s ). The numerical data are presented in
table 4 and representative voltammograms are provided in Supplementary material.
1
In the case of CuL , during the anodic scan, two sequential oxidation peaks were
observed at Ea ¼ 0.16 V and Ea ¼ 0.48 V, representing the oxidation reactions. In the
1
2
corresponding reverse scan (cathodic scan), two responses were observed at
2
Ec ¼ 0.36 V and Ec ¼ 0.04 V, representing the consequent reduction reactions. CuL
1
2
exhibits similar redox behavior with Ea ¼ 0.10 V and Ea ¼ 0.50 V, and Ec ¼ 0.35 V
1
2
1
and Ec ¼ 0.05 V. The electrochemical responses exhibited by both complexes were
2
purely metal based, since the corresponding ligands and zinc complexes are redox
inactive. Hence, the two-step, two-electron redox process is assigned according to the
sequence
ꢀ
ꢀ
þe
þe
II
II
II
IIꢄI
IIꢄI
ꢀ CuIIꢄI
Cu ꢀ Cu ꢀ) ꢀꢀꢀꢀꢀꢀꢀꢀꢀ * ꢀꢀ Cu ꢀ Cu
ꢀ) ꢀꢀꢀꢀꢀꢀꢀꢀꢀ * ꢀꢀ Cu
ꢀ
ꢀ
ꢀ
e
ꢀe
o
o
E
2
E
1
Although the chemical environment around the two metals in binuclear complexes is
same, the overall two-electron exchanges appear as separate one-electron processes
indicating that oxidation/reduction at one site induces steric and electronic influence on
the other site, and hence induces a positive/negative shift to the oxidation/reduction
potential of metal ion. The large difference between the redox potentials observed
1
2
{
considerable stability of the mixed-valence species (K
[DE ¼ E (1) ꢀ E (2)] ¼ 310 and 360 mV, respectively, for CuL and CuL } indicates
1
/2
1/2
6
ꢃ 10 ). In both complexes the
com
redox processes are chemically quasireversible, as evidenced by the following criteria:
(
i) DEp is greater than 59 mV, (ii) Ip /Ip ꢃ 1, and (iii) Ep shifts negatively with
a
c
c
increasing scan rate. DE values of redox couples vary with scan rate and decrease
p

Complexes with [M N OS ] coordination sphere
2
3311
2
2
significantly with decrease in scan rate, indicating possible reversibility of redox
II
III
II
I
reactions at slow rate. The appearance of Cu /Cu and the absence of Cu /Cu redox
couple can be explained in terms of flexibility and size of the coordination cavity in the
complexes and the geometric requirements and the size of the metal ions in different
II
9
I
10
oxidation states. The reduction of Cu (d ) to Cu (d ) involves an increase in the metal
radius and a configuration changes from square planar to tetrahedral. On the other
III
hand, Cu (d ) tends to assume square-planar coordination geometry with a low spin
8
II
ground state and the oxidation of Cu to Cu involves reduction in ionic radius [37].
III
III
III
The constrained cavity fits Cu better and thus stabilizes the Cu complex. The extra
stability achieved by the complex on oxidation will influence the redox reaction and
slow the rate of reduction.
4
. Conclusion
1
2
HL and HL are monobasic binucleating chelates and provide bicompartmental
SNONS cavities which fit transition metal(II) ions. Copper and zinc complexes are
square pyramidal, while nickel and cobalt complexes are octahedral. Chloride binds two
metal ions as exogenous bridge. Mutual influence of two metal centers in terms of
cooperative effect on the electronic, magnetic, and structural properties occur. The
sequential two step redox processes exhibited by copper complexes are correlated to
stereochemical arrangements. The differences of SNONS cavity in the two ligands have
very little influence over electronic, magnetic, and electrochemical properties of
complexes.
Acknowledgments
The authors thank Department of Chemistry and USIC, Karnatak University,
Dharwad for providing spectral and analytical facility. Recording of FAB mass
spectra (CDRI Lucknow) is gratefully acknowledged. The authors Naveen Kulkarni
and Srinivasa Budagumpi thank Karnatak University, Dharwad and UGC, New Delhi
for the Nilekani Scholarship and RFSMS.
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