Spectral, structural, and superoxide dismutase activity of some octahedral nickel(II) complexes with tri-tetradentate ligands

Article abstract of DOI:10.1080/00958972.2010.515985

Three nickel(II) complexes, [Ni(L¹)₂] 3H₂O (1), [Ni(L²)₂](ClO₄)₂ (2), and [Ni(L³)(bipy)](ClO₄)₂ (3), where L¹ = N'-[phenyl(pyridin-2-y

Full text of DOI:10.1080/00958972.2010.515985

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Spectral, structural, and superoxide
dismutase activity of some octahedral
nickel(II) complexes with tri-
tetradentate ligands
R.N. Patel a , K.K. Shukla a , Anurag Singh a , M. Choudhary a ,
D.K. Patel b , J. Niclós-Gutiérrez b & D. Choquesillo-Lazarte c
a Department of Chemistry , A.P.S. University , Rewa 486003,
M.P., India
b Departamento de Química Inorgánica , Facultad de Farmacia,
Campus Cartuja, Universidad de Granada , E-18071 Granada,
Spain
c Lab. de Estudios Cristalográficos–IACT-CSIC , Edificio Inst. López-
Neyra, P.T. Ciencias de la Salud , Avenida del Conocimiento s/n,
E- 8100 Armilla, Granada, Spain
Published online: 20 Sep 2010.
To cite this article: R.N. Patel , K.K. Shukla , Anurag Singh , M. Choudhary , D.K. Patel , J. Niclós-
Gutiérrez & D. Choquesillo-Lazarte (2010) Spectral, structural, and superoxide dismutase activity
of some octahedral nickel(II) complexes with tri-tetradentate ligands, Journal of Coordination
Chemistry, 63:20, 3648-3661, DOI: 10.1080/00958972.2010.515985
To link to this article: http://dx.doi.org/10.1080/00958972.2010.515985
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Journal of Coordination Chemistry
Vol. 63, No. 20, 20 October 2010, 3648–3661
Spectral, structural, and superoxide dismutase activity of some
octahedral nickel(II) complexes with tri-tetradentate ligands
R.N. PATEL*y, K.K. SHUKLAy, ANURAG SINGHy, M. CHOUDHARYy,
´
´
D.K. PATELz, J. NICLOS-GUTIERREZz and D. CHOQUESILLO-LAZARTEx
yDepartment of Chemistry, A.P.S. University, Rewa 486003, M.P. India
zDepartamento de Quı
mica Inorganica, Facultad de Farmacia, Campus Cartuja,
´ ´
Universidad de Granada, E-18071 Granada, Spain
ficos – IACT-CSICEdificio Inst. Lopez-Neyra,
´
P.T. Ciencias de la Salud, Avenida del Conocimiento s/n, E- 8100 Armilla, Granada, Spain
xLab. de Estudios Cristalogra
´
(Received 21 November 2009; in final form 21 July 2010)
Three nickel(II) complexes, [Ni(L1)2] Á 3H2O (1), [Ni(L2)2](ClO4)2 (2), and [Ni(L3)(bipy)](ClO4)2
(3), where L1 ¼ N0-[phenyl(pyridin-2-yl)methylidene]furan-2-carbohydrazide, L2 ¼ 2,6-bis
[N,N0-(4-methoxyphenyl)ethanimidoyl]pyridine, and L3 ¼ N-(methoxy-phenyl-pyridin-2-yl-
methyl)-N-(phenyl-pyridin-2-yl-methylene)-ethane-1,2-diamine, have been synthesized and
characterized by physico-chemical and spectroscopic methods. The solid-state structures of
1–3 were determined by single crystal X-ray crystallography, which revealed distorted
octahedral geometry. In solid-state structure, 3 is self-assembled via intermolecular ꢀ ꢀ ꢀ ꢀ ꢀ
stacking and intramolecular CH(methyl) ꢀ ꢀ ꢀ ꢀ(phenyl) non-covalent interactions. Infrared
spectra, ligand-field spectra, and magnetic susceptibility measurements agree with the observed
crystal structures. These complexes have also been revealed to catalyze effectively the
dismutation of superoxide (Oꢁ2 ) in alkaline dimethyl superoxide-nitro blue tetrazolium assay.
Keywords: Nickel(II) complexes; Single crystal X-ray analysis; Electrochemistry; Magnetic
susceptibility
1. Introduction
Transition metal ions define the number of reactive sites available and modulate their
activity [1]. Investigation of structural models can lead to the understanding of
fundamental mechanistic aspects of enzymes as well as the development of structure–
activity relationships. Chemical and physical properties of transition metal complexes
may be controlled by electronic and steric factors, which may be introduced with the
backbones of the ligands. Sigel et al. [2] reported electronic effects, such as favorable
interaction between ꢀ-systems, between hydrophobic systems and between the oppo-
sitely charged side groups. These weak interactions play a crucial role in the
self-organization of molecules, supramolecular chemistry, and molecular recognition [3].
Recently, nickel-containing superoxide dismutase (NiSOD) was isolated from
*Corresponding author. Email: rnp64@yahoo.co.uk
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.515985

Octahedral nickel(II)
3649
Streptomyces species [4, 5]. The enzymatic activity of NiSOD [6] is comparable to that
of Cu–ZnSOD. Superoxide dismutase (SOD), which can destroy superoxide very
rapidly, is nature’s agent for the protection of the organism from this radical. Native
SOD enzymes have been shown to exhibit protection in animal models of inflammatory
diseases [7]. In a variety of scenarios, therapeutic dosage of additional SOD enzyme has
shown promise, but from a number of viewpoints, namely stability, tissue permeability,
oral bioavailability, specific-tissue-targeting immunogenicity; synthetic metal complexes
offer considerable promise as SOD catalysts for pharmaceutical applications. SODs are
an important class of redox enzymes responsible for the disproportionation of
superoxide ion (O2ꢁ) produced by one-electron reduction of oxygen. All SODs employ
two steps shown in equations (1) and (2).
Mox þ Oꢁ2
o Mred þ O2,
ð1Þ
Mred þ Oꢁ2 þ Hþ
o Mred þ H2O2,
ð2Þ
where M is the redox-active metal center capable of both oxidizing and reducing
superoxide. X-ray crystallographic studies [8] of NiSODs from Streptomyces illustrate
the nature of the enzyme-active site. In both structures, square-pyramidal coordination
geometry exhibited by the Ni center in the oxidized form (NiSODox) is converted to
square-planar geometry in the reduced state (NiSODred), as the axial imidazole group
derived from the His1 residue dissociates upon metal reduction. The four equatorial
ligands include two thiolates from Cys2 and Cys6 in a cis arrangement, the
deprotonated amide of the Cys2 backbone, and the N-terminal –NH2 group of His1.
Synthesis and structural investigation for a series of nickel(II) octahedral complexes
have been reported [9–16]. We have focused our efforts on the design of nickel(II)-based
complexes as SOD mimics and have achieved success in generating stable nickel(II)
complexes with outstanding catalytic SOD activity. Only a few synthetic complexes
have been reported [6, 12] and these have been shown to possess modest catalytic SOD
activity.
Octahedral nickel(II) complexes show relatively simple magnetic behavior. All these
complexes show room temperature (RT) magnetic moment value in the range
2.84–2.97 B.M. This range depends on the magnitude of the orbital contribution. In
continuation of our work [11, 12] on nickel(II) octahedral complexes, here we describe
the syntheses, crystal structures, and spectroscopic characterizations of three new
nickel(II) octahedral complexes namely, [Ni(L1)2] Á 4H2O (1), [Ni(L2)2](ClO4)2 (2), and
[Ni(L3)(bipy)](ClO4)2 (3). We have also investigated SOD activities of these complexes.
2. Experimental
2.1. Materials
Nickel(II) chloride hexahydrate, sodium azide, and ammonium thiocyanate were
purchased from SD fine-chemicals, India. The Schiff bases were prepared as reported
[17]. All other chemicals were of synthetic grade and used as received.

3650
R.N. Patel et al.
2.2. Synthesis of ligands
Schiff bases were prepared by standard literature procedure and recrystallized from
ethanol or methanol.
2.2.1. Synthesis of L1. The ligand HL1 was synthesized by refluxing 2-benzoylpyridine
(1.832 g, 10 mmol) and 2-furoicacidhydrazide (1.261 g, 10 mmol) in ethanol for 1 h;
crystals appeared immediately after cooling to RT. The crystalline solid was filtered off,
washed with ethanol, and dried in air. Yield: 80%. Anal. Found (%): C, 70.02; H, 4.46;
N, 14.41. Calcd (%): C, 70.08; H, 4.49; N, 14.46.
2.2.2. Ligand L2. The ligand HL2 was synthesized by refluxing 4-methoxyaniline
(2.463 g, 20 mmol) and 2,60-diacetylpyridine (1.832 g, 10 mmol) in ethanol for 4 h. The
solution was filtered, washed with a small amount of ethanol, and allowed to stand at
RT for a few days. Yield: 75%. Anal. Found (%): C, 73.84; H, 6.15; N, 11.23.
Calcd (%): C, 73.89; H, 6.18; N, 11.28.
2.2.3. Ligand L3. The ligand L3 was synthesized by refluxing 2-benzoylpyridine
(3.664 g, 20 mmol) and ethylene diamine (0.59 g, 10 mmol) in ethanol for 3–4 h.
On cooling to RT, white crystalline product was filtered off and washed with methanol.
Yield: 85%. Anal. Found (%): C, 76.75; H, 6.20; N, 13.26; Ni, 8.18. Calcd (%): C,
57.40; H, 4.50; N, 11.81; Ni, 8.26.
2.3. Synthesis of complexes
The complexes were prepared by the following general procedures:
1
2
.
2.3.1. [Ni(L )2] 4H2O (1) and [Ni(L )2](ClO4)2 (2). To an MeOH solution (20 mL) of
Ni(ClO4) ꢀ 6H2O (0.365 g, 1.0 mmol) an MeOH solution (20 mL) of L1 (0.582 g,
2.0 mmol) was added with stirring for 30 min at 25ꢂC. The resulting red solution was
allowed to slowly concentrate by evaporation at RT for 2 days. A red microcrystalline
solid deposited was collected by filtration, washed with methanol, and stored in CaCl2
desiccators at RT. Anal. Found (%): C, 58.58; H, 4.68; N, 12.01; Ni, 8.44. Calcd (%)
for C34H32N6NiO7: C, 58.37; H, 4.60; N, 12.07; Ni, 8.43. FAB mass (m/z): Obs. (Calcd)
623.37 (623.37). [Ni(L2)2](ClO4)2 (2) was also prepared in a similar fashion by
employing L2 (0.75 g, 2.0 mmol) in place of L1. Anal. Found (%): C, 54.82; H, 4.61; N,
8.21; Ni, 5.84. Calcd for C46H46N6NiCl2O12 (%): C, 54.95; H, 4.57; N, 8.36; Ni, 5.82.
FAB mass (m/z): Obs. (Calcd) 806.5 (806.5).
2.3.2. [Ni(L3)(bipy)](ClO4)2 (3). To an MeOH solution (20 mL) of NiCl2 Á 6H2O
(0.237 g, 1.0 mmol), L3 (0.391 g, 1.0 mmol) in MeOH solution (20.0 mL) was added with
stirring for 30 min. To the above reaction mixture, a methanolic solution (20 mL) of
bipy (0.156 g, 1.0 mmol) was added with stirring for 30 min at RT. After completion of
reaction a light-pink microcrystalline solid deposited was collected by filtration
and washed with methanol. The obtained solid was dried in air and stored in a

Octahedral nickel(II)
3651
CaCl2 desiccator. Anal. Found (%): C, 53.14; H, 4.10; N, 10.05; Ni, 7.02. Calcd for
C35H34N6NiCl2O9 (%): C, 53.09; H, 4.06; N, 10.00; Ni, 7.00. FAB mass (m/z): Obs.
(Calcd) 638.31 (638.31).
Caution! Although no problems were encountered in this work, perchlorate salts of
metal complexes are potentially explosive. They should be synthesized in small quantity
and handled with great care.
2.4. Physical measurements
FAB mass spectra were recorded on a JEOL SX 102/DA 6000 Mass Spectrometer
using xenon (6 kV, 10 mA) as the FAB gas. The accelerating voltage was 10 kV and
the spectra were recorded at RT with m-nitrobenzoyl alcohol as the matrix. RT
magnetic susceptibilities were measured by the Gouy balance using a mercury(II)
tetrathiocyanato cobaltate(II) as calibrating agent (ꢁg ¼ 16.44 ꢃ 10ꢁ6 c.g.s. units).
Diamagnetic corrections were estimated from Pascal tables. Ligand-field spectra were
recorded at 25ꢂC on a Shimadzu UV-Vis recording Spectrophotometer UV-1601 in
solution. Infrared (IR) spectra were recorded in KBr medium on a Perkin-Elmer 783
Spectrophotometer. X-band EPR spectra were recorded on a Varian E-line Century
Series Spectrometer equipped with a dual cavity and operating at X-band with
100 kHz modulation frequency. TCNE was used as field marker. Cyclic voltammetry
was carried out with a BAS-100 Epsilon electrochemical analyzer having an
electrochemical cell with a three-electrode system. Ag/AgCl was used as reference
electrode, glassy carbon as working electrode, and platinum wire as an auxiliary
electrode. NaClO4 (0.1 mol Lꢁ1) was used as supporting electrolyte in DMSO. All
measurements were carried out at 298 K under nitrogen. The solution was
deoxygenated by purging nitrogen gas. The in vitro SOD activity was measured
using alkaline DMSO as a source of superoxide radical (Oꢁ2 ) and nitrobluetetra-
zolium chloride (NBT) as Oꢁ2 scavenger [18, 19]. In general, 400 mL sample to be
assayed was added to a solution containing 2.1 mL of 0.2 mol Lꢁ1 potassium
phosphate buffer (pH 8.6) and 1 mL of 56 mmol Lꢁ1 alkaline DMSO solution with
stirring. The absorbance was then monitored at 540 nm against a sample prepared
under similar condition in DMSO (except NaOH). A unit of SOD activity is the
concentration of complex, which causes 50% inhibition of alkaline DMSO-mediated
reduction of NBT.
Crystals suitable for single-crystal X-ray analysis for [Ni(L1)2] Á 3H2O,
[Ni(L2)2](ClO4)2, and [Ni(L3)(bipy)](ClO4)2 were grown from slow evaporation of the
reaction mixtures at RT in CH3OH : DMSO (1 : 1). Single crystals of 1, 2, and 3 suitable
for X-ray study were mounted on a glass fiber and used for data collection. The crystal
orientation, cell refinement, and intensity measurements were made using CAD-4PC
performing -scan measurements. Data were collected with Bruker X8 Proteum
diffractometers. The data for 2 were processed with APEX2 and corrected for
absorption using SADABS. The structures were solved by direct methods
using SHELXS-97 [20] and refined by full-matrix least-squares against F2 using
SHELXL-97 [21]. All non-hydrogen atoms were refined anisotropically. All hydrogens
were geometrically fixed and allowed to refine using a riding model.

3652
R.N. Patel et al.
3. Results and discussion
The nickel(II) complexes have been synthesized by the following synthetic routes:
O
OCH 3
O–
CH3
H3
C
C
N
H3CO
N
N
N
N
(ClO 4
)
2
N
N
Ni
O–
Ni
N
N
N
N
N
OCH 3
CH3
H3
O
H
3CO
[Ni(L 1)2] (1)
[Ni(L2)2](ClO4)2 (2)
MeOH
L1
MeOH
L2
Ni2+
L3
MeOH
CH3
O
H
C
N
N
N
(ClO4
)
2
N
Ni
H2
C
H2
C
N
N
C
[Ni(L3)2](ClO4)2 (3)
All the complexes gave satisfactory elemental analysis and were further character-
ized by FAB mass spectrometry. The molar conductivity (Ãm) measurements in
10ꢁ3 mol Lꢁ1 DMF solution show non-electrolyte [22] (11 ꢀꢁ1 cm2 molꢁ1) for 1 and a
2 : 1 electrolyte for 2 and 3 (140 ꢄ 2 ꢀꢁ1 cm2 molꢁ1).
3.1. Crystal structure
The molecular structures of 1–3 are shown in figures 1–3. Crystal data and structural
refinement parameters of the complexes are presented in table 1. Selected bond
distances and angles are collected in tables 2–4. The nickel(II) centers are in a distorted
octahedral geometry with a N4O2/N2 donor environment. The distortions from ideal
octahedral geometry about Ni are of the same extent as those commonly observed for
nickel(II) complexes containing multidentate ligands [11, 12, 23]. Individual mean
distances and angles compare well with those found for other octahedral nickel(II)
complexes involving nitrogen donors [11, 12, 24]. The cis and trans angles (table 5)

Octahedral nickel(II)
3653
Figure 1. Projection view of [Ni(L1)2] Á 4H2O (1).
reflect the degree of distortion from ideal octahedral geometry. The five-membered
chelate ring is planar in these complexes. These observations are comparable to those
found in similar nickel(II) complexes [25].
Complex 1 crystallizes in the monoclinic space group P21/n with four molecules in the
unit cell. Nickel is in a distorted axially compressed octahedron (table 2). The two neutral
ligands are tridentate via two pyridyl nitrogens, two hydrazone nitrogens, and two
carbonyl oxygens, thus forming two fused five-membered rings. Both ligands are
coordinated meridional and would be perpendicular in an idealized octahedron.
However, the disposition of the ligands deviates noticeably from perpendicularity as
indicated by the decreased value of the trans angles (N2–Ni(1)–N(5) ¼ 178.98(18)) and by
a significant difference in the values of the interligand cis angle associated with the same
Ni–N(hydrazide) bond [102.73(14) and 101.13(16) for the angles involving the Ni–N5
and 100.11(17) and 103.60(16) for the Ni–N2 bond]. The Ni–N(hydrazide) bonds
˚
(1.982(4) and 1.984(4) A) occupying the equatorial positions are significantly shorter
˚
than the Ni–O/N (2.075(4)–2.096(4) A) contacts. This difference is indicated by the bis
(chelating) mode of the ligands in which Ni–N (hydrazide) bonds appear to be shared by
two condensed five-membered chelate rings. Such observation has been made by other
workers [24, 43]. The C–N bond length in 1 (C23–N5 ¼ 1.299(8), C18–N4 ¼ 1.349(7), and
C(1)–N(1) ¼ 1.360(7)) is inconsistent with partial double-bond character. These factors

3654
R.N. Patel et al.
Figure 2. Projection view of [Ni(L2)2](ClO4)2 (2).
confirm coordination through the enolate form by deprotonation after enolization of the
˚
ligand. The approximate C–N bond distances of 1.32 A, close to the value of double bond
˚
C¼N (1.28 A) [26], confirm the formation of the Schiff base [27].
Complex 2 crystallized in the monoclinic space group P21/c with four molecules in
the unit cell. Two tridentate L2 are coordinated with nickel(II) similar to that of 1. The
nickel resides in a distorted octahedral environment comprising six nitrogens of two
2
˚
tridentate L . The Ni–N(pyridine) bonds (1.9591(4) and 1.9653(4) A) occupying
˚
equatorial position are significantly shorter than the Ni–N (2.1136(14)–2.1311(14) A)
contacts. Again this difference (similar to that of 1) is indicated by the bis (chelating)
mode of the ligands, in which Ni–N(py) bonds appear to be shared by four condensed
five-membered chelate rings and compare well with those reported for other nickel(II)
˚
complexes [28]. The C–N bond lengths of 1.28 A [26] confirm the formation of the
Schiff base [27].
In 3, the coordination polyhedron around nickel(II) consist of atoms contributed by
tetradentate L3 and bidentate (bipy). Both ligands coordinate via the pyridine nitrogens,
imino nitrogens, and bipyridyl imine nitrogens with a common Ni–N(py) bond.

Octahedral nickel(II)
3655
Figure 3. Projection view of [Ni(L3)(bipy)](ClO4)2 (3).
This complex crystallizes in the monoclinic space group C2/c with eight molecules in the
unit cell. The bond distances between the donor nitrogens of L3 and metal center in
the complex are Ni(1)–N(3) ¼ 2.104(2), Ni(1)–N(4) ¼ 2.026(6), Ni(1)–N(5) ¼ 2.144(2),
˚
and Ni(1)–N(6) ¼ 2.079(2) A and bond distances of bipy and metal center are
˚
Ni–N(1) ¼ 2.061(2) and Ni(1)–N(2) ¼ 2.097(2) A. Individual mean bond distances and
angles within the coordination polyhedron compare well with those found in other
octahedral nickel(II) complexes involving N donors [29]. Cis and the trans angles
(table 5) reflect the degree of distortion from octahedral geometry. The trans axial sites
are occupied by two nitrogens N(5) and N(3) of tetradentate L3 (figure 3). The r.m.s
˚
deviation from planarity is in the range 0.0558–0.1526 A. The C(16)–N(4) bond distance
˚
(1.282(2) A) also confirm the formation of the Schiff bases, like 1 and 2. The C(26)–N(5)
˚
bond distance of 1.473(3) A is close to the predicted value of a single bond and also the
presence of singly bonded group with C(26).
In the lattice structure of 1 all nickel(II)s form rectangular boxes when viewed along
˚
the a-axis with Ni–Ni distance of 9.017 and 13.696 A, respectively. The crystal water in
1 is a single acceptor and double donor, stabilizing the network. The structures of 2 and
˚
3 have CH ꢀ ꢀ ꢀ ꢀ non-covalent interactions, giving a H(14) ꢀ ꢀ ꢀ Cg distance of 3.296 A in 2
˚
and 3.208 A in 3 (Cg is the centroid of the phenyl ring). A similar type of CH ꢀ ꢀ ꢀ ꢀ
interaction is reported in vanadium and copper complexes of the Schiff base [30, 31].
Another significant structural observation is the intermolecular ꢀ–ꢀ stacking interac-
tion between two individual molecules in 3. The intermolecular stacking distance
˚
between centroids of bipy ring is 5.256 A. A combination of coordination and weak

3656
R.N. Patel et al.
Table 1. Crystal data and structure refinement for 1–3.
Empirical formula
C34H32N6NiO8
C46H46Cl2N6NiO12
C37H34Cl2N6NiO9
Formula weight
Crystal system
Space group
710.71
Monoclinic
P21/n
1004.50
Monoclinic
P21/c
836.31
Monoclinic
C2/c
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
ꢂ
ꢃ
ꢄ
8.9461(3)
27.0834(9)
14.1034(4)
90
94.317(3)
90
3407.43(19), 4
120(2)
1.355
0.625
0.71073
1448
0.32 ꢃ 0.27 ꢃ 0.22
2.99 to 25.00
31551/5993
[R(int) ¼ 0.0805]
25.00 (99.8%)
5993/474
1.139
R1 ¼ 0.0774,
wR2 ¼ 0.1517
R2 ¼ 0.1174,
wR2 ¼ 0.1691
17.1368(17)
16.6817(17)
16.6000(18)
90
109.342(4)
90
4477.6(8), 4
100(2)
1.490
2.331
1.54178
2088
0.15 ꢃ 0.10 ꢃ 0.07
3.81 to 66.53
56611/7712
[R(int) ¼ 0.0492]
66.53 (97.6%)
7712/613
26.4345(6)
9.8187(2)
28.5550(7)
90
99.283(2)
90
7314.5(3), 8
120(2)
1.519
0.742
0.71073
3456
0.23 ꢃ 0.19 ꢃ 0.16
3.01 to 25.00
30265/6443
[R(int) ¼ 0.0496]
25.00 (99.8%)
6443/501
0.956
R1 ¼ 0.0316,
wR2 ¼ 0.0700
R1 ¼ 0.0604,
wR2 ¼ 0.0801
3
˚
Volume (A ), Z
Temperature (K)
Dcalcd (mg mꢁ3
)
ꢅ (mmꢁ1
Wavelength (A)
)
˚
F (000)
Crystal size (mm)
ꢆ range for data collection (ꢂ)
Reflections collected/unique
[Rint]
Completeness to ꢆ
Reflection used/parameters
Goodness-of-fit on F2
R1/wR2 (observed data)
1.077
R1 ¼ 0.0356,
wR2 ¼ 0.0965
R1 ¼ 0.0362,
wR2 ¼ 0.0971
R2/wR1 (all data)
1
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) for [Ni(L )2] ꢀ 4H2O.
Ni(1)–N(2)
Ni(1)–N(6)
Ni(1)–O(1)
Ni(1)–N(3)
C(23)–N(5)
C(18)–N(4)
1.983(4)
2.082(4)
2.095(3)
2.096(4)
1.299(8)
1.349(7)
C(1)–N(1)
C(6)–N(2)
N(2)–C(6)
Ni(1)–O(3)
Ni(1)–N(5)
1.360(7)
1.293(7)
1.293(6)
2.075(4)
1.984(4)
N(2)–Ni(1)–N(5)
N(2)–Ni(1)–O(3)
N(5)–Ni(1)–O(3)
N(2)–Ni(1)–N(6)
N(5)–Ni(1)–N(6)
N(1)–N(2)–Ni(1)
N(2)–Ni(1)–O(1)
N(5)–Ni(1)–N(3)
178.98(18)
103.60(16)
77.36(15)
100.11(17)
78.93(17)
118.2(3)
N(6)–Ni(1)–N(3)
O(1)–Ni(1)–N(3)
O(3)–Ni(1)–N(6)
N(5)–Ni(1)–O(1)
N(2)–Ni(1)–N(3)
N(6)–Ni(1)–O(1)
O(3)–Ni(1)–O(1)
94.49(16)
156.13(15)
156.28(15)
102.73(14)
78.55(15)
90.57(15)
93.93(14)
77.60(14)
101.13(16)
intermolecular interactions are responsible for the supramolecular architecture [32].
These non-covalent interactions are very important in crystal engineering and
supramolecular frameworks [33].
3.2. Magnetic measurement
Magnetic moment values and electronic spectral data of the complexes are presented
in table 6. The predominant contribution to the magnetic susceptibility of octahedral

Octahedral nickel(II)
3657
2
Table 3. Selected bond lengths (A) and angles ( ) for [Ni(L )2](ClO4)2.
ꢂ
˚
Ni(1)–N(31)
Ni(1)–N(1)
Ni(1)–N(5)
Ni(1)–N(35)
Ni(1)–N(50)
1.9591(14)
1.9653(14)
2.1136(14)
2.1226(14)
2.1311(14)
Ni(1)–C(17)
C(3)–N(5)
C(18)–N(20)
C(33)–N(35)
C(48)–N(50)
1.339(2)
1.288(2)
1.292(2)
1.288(2)
1.286(2)
N(3)–Ni(1)–N(1)
N(31)–Ni(1)–N(5)
N(1)–Ni(1)–N(5)
N(31)–Ni(1)–N(35)
C(17)–N(1)–Ni(1)
C(2)–N(1)–Ni(1)
N(1)–Ni(1)–N(20)
N(35)–Ni(1)–N(20)
N(31)–Ni(1)–N(50)
N(1)–Ni(1)–N(50)
177.78(6)
N(5)–Ni(1)–N(50)
N(35)–Ni(1)–N(50)
N(20)–Ni(1)–N(50)
N(1)–Ni(1)–N(35)
N(5)–Ni(1)–N(35)
N(31)–Ni(1)–N(20)
N(5)–Ni(1)–N(20)
C(3)–N(5)–Ni(1)
C(6)–N(5)–Ni(1)
86.73(5)
115.15(5)
100.67(5)
103.10(5)
98.01(5)
104.38(5)
155.45(5)
114.45(11)
120.69(10)
100.05(5)
77.75(6)
77.41(5)
119.05(11)
118.72(11)
77.83(5)
85.12(5)
77.74(5)
101.75(5)
3
Table 4. Selected bond lengths (A) and angles ( ) for [Ni(L )(bipy)](ClO4)2.
ꢂ
˚
Ni(1)–N(4)
Ni(1)–N(1)
Ni(1)–N(6)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(5)
2.026(2)
2.061(2)
2.079(2)
2.097(2)
2.104(2)
2.144(2)
O(9)–C(26)
N(4)–C(16)
N(4)–C(24)
N(5)–C(26)
N(5)–C(25)
C(16)–N(4)
C(26)–N(5)
1.423(3)
1.282(3)
1.457(3)
1.473(3)
1.497(3)
1.282(3)
1.473(3)
N(4)–Ni(1)–N(1)
N(4)–Ni(1)–N(6)
N(1)–Ni(1)–N(6)
N(4)–Ni(1)–N(2)
Ni(1)–N(1)–N(3)
N(6)–Ni(1)–N(3)
N(2)–Ni(1)–N(3)
168.30(8)
88.81(8)
97.47(8)
96.51(8)
91.90(5)
98.18(8)
89.23(8)
N(6)–Ni(1)–N(5)
N(2)–Ni(1)–N(5)
N(3)–Ni(1)–N(5)
N(1)–Ni(1)–N(2)
N(6)–Ni(1)–N(2)
N(4)–Ni(1)–N(3)
N(4)–Ni(1)–N(5)
N(1)–Ni(1)–N(5)
79.21(8)
95.09(8)
160.02(8)
78.43(8)
171.69(8)
77.38(8)
82.74(8)
108.07(8)
Table 5. The cis and trans angles (ꢂ) of nickel(II) complexes.
Complex
Cis angle
Trans angle
[Ni(L1)2] ꢀ 4H2O
77.36(15)–90.75(15)
77.41(5)–103.10(5)
77.38(8)–108.07(8)
93.93(14)–178.98(18)
155.15(5)–177.78(16)
160.02(8)–171.69(8)
[Ni(L2)2](ClO4)2
[Ni(L3)(bipy)](ClO4)2
Table 6. Magnetic moment and absorption maxima Dq, ꢃ, B, and ꢃꢂ of 1–3 (ꢇmax in cmꢁ1).
3A2g
!
3A2g
!
3A2g
!
B
ꢅeff
(B.M.)
Complex
3T2g(ꢇ1) 3T1g(F)(ꢇ2) 3T1g(P)(ꢇ3) ꢇ2/ꢇ1
Dq
ꢃ
(cmꢁ1
)
ꢃꢂ
[Ni(L1)2] ꢀ 4H2O
11,050
10,200
10,150
18,200
17,860
18,520
26,310
26,300
26,300
1.65 1105 0.74
1.75 1020 0.87
1.76 1015 0.92
757
903
957
26.5
13
8
2.97
2.84
2.89
[Ni(L2)2](ClO4)2
[Ni(L3)(bipy)](ClO4)2
B ¼ (ꢇ2 þ ꢇ3) ꢁ 3ꢇ1; ꢃ ¼ B/B0 [B0(free ion) ¼ 1030]; ꢃꢂ ^ (1 ꢁ ꢃ) ꢃ 100.

3658
R.N. Patel et al.
Table 7. IC50 values and kinetic constant of 1, 2, and 3.
Complex
IC50 (mmol)
KMccF ((mol L
)
ꢁ1 ꢁ1 sꢁ1) ꢃ 106
[Ni(L1)2] ꢀ 4H2O
35
55
60
2.71
1.73
1.58
[Ni(L2)2](ClO4)2
[Ni(L3)(bipy)](ClO4)2
KMccF were calculated by K ¼ kNBT ꢃ (pH 7.8) ¼ 5.94 ꢃ 104 (mol L
)
ꢁ1 ꢁ1 sꢁ1 [40].
Ni2þ complex is given by the spin-only term (equation 3):
3ðS þ 1ÞNꢃ2g2
,
ð3Þ
3kT
where N is Avogadro’s number, ꢃ the Bohr magneton, g ¼ 2 and S ¼ 1. A small
3
contribution because of spin orbit coupling between the first excited T2g and the
3
ground state A2g leads to a magnetic moment above the spin only value [34].
The measured magnetic moment of the Ni(II) complexes at RT lies in the range
2.95–2.96 B.M. These values are in tune with a high-spin configuration and show the
presence of an octahedral environment for Ni(II) in all the complexes reported in this
article. These values lie within the range normally found for nickel(II) complexes [4, 10].
3.3. Electronic spectra
The RT electronic spectra of 1–3 recorded in DMSO show three absorptions
3
3
3
3
3
[3A2g ! T2g(ꢇ1), A2g ! T1g(F)(ꢇ2), A2g ! T1g(P)(ꢇ3)], characteristics of octahedral
complexes [35]. Electronic spectra of the complexes in DMSO also show a broad band
around ꢅ820 nm. This broad band seems to be an overlapping of the spin-allowed
3
3
3
3
transition with the spin-forbidden transition A2g ! T1g. The A2g ! T1g(P) band is
masked by the high-intensity charge transfer band around 380 nm. Additionally, there
are charge-transfer bands and intraligand transition present in the UV region for all the
complexes. The ligand-field parameter Dq (3A2g ! T2g) lowest energy transition and ꢃ
3
(Racah parameter) values (table 6) were calculated according to the average
environment rule in the range found for octahedrally coordinated nickel(II) [36].
3.4. SOD activity
The SOD activities for the complexes were measured. Superoxide was enzymatically
supplied from alkaline DMSO and SOD activity was evaluated by the NBT assay
[19, 20] following the reduction of NBT to MFþ kinetically at 560 nm. These complexes
exhibit significant catalytic activity toward the dismutation of superoxide anions. The
concentration causing 50% inhibition of NBT reduction is IC50. The observed IC50
values of the present complexes were compared with earlier reported values for
nickel(II) complexes [37–40]. The catalytic activity of NiSOD [6], however, is on the
)
same high level as that of Cu–ZnSOD at about 109 (mol Lꢁ1 ꢁ1Sꢁ1 per metal center.
The IC50 data of the SOD activity assay along with kinetic catalytic constants of 1–3
[18, 41] are presented in table 7. Complex 3 shows highest SOD activity

Octahedral nickel(II)
3659
Table 8. Cyclic voltammetric data for 1 mmol Lꢁ1 solution of the nickel(II) complexes in DMSO containing
0.1 mol Lꢁ1 NaClO4 as supporting electrolytes.
Scan rate (mV sꢁ1
)
Epc (mV)
Ipc (mA)
Epa (mV)
Ipa (mA)
DEp (mV)
E ꢂ0 (mV)
[Ni(L1)2] Á 4H2O
ꢁ767
ꢁ783
1.048
0.795
ꢁ652
ꢁ668
0.785
0.588
115
115
ꢁ709.5
ꢁ725.5
[Ni(L2)2](ClO4)2
100
200
ꢁ890
ꢁ895
1.020
1.920
ꢁ776
ꢁ770
0.856
1.650
114
125
ꢁ833
ꢁ832.5
[Ni(L3)(bipy)](ClO4)2
100
200
ꢁ782
ꢁ790
1.048
1.218
ꢁ652
ꢁ670
0.917
1.044
130
120
ꢁ717
ꢁ730
DEp ¼ Epa ꢁ Epc; Eꢂ0 ¼ (Epa þ Epc)/2.
(IC50 ¼ 35 mmol dmꢁ3) whereas the other two show IC50 ¼ 55 for 1 and 60 mmol dmꢁ3
for 2.
3.5. Electrochemistry
Electron-transfer properties of the complexes were studied in 10ꢁ3 mol Lꢁ1 DMSO by
cyclic voltammetry. The electrochemical data for all the nickel(II) complexes are
presented in table 8. Voltammograms of all complexes exhibit one-step redox in DMSO
at RT under nitrogen. Voltammograms consist of two well-separated peaks, one
cathodic peak potential (Epc), and one anodic peak potential (Epa). This reduction is
quasireversible with a peak-to-peak separation of 120 ꢄ 5 mV and the cathodic peak
current (Ipc) is almost equal to the anodic peak current (Ipa). The one-electron nature of
this reduction has been established by comparing its current with that of a standard
ferrocene/ferrocenium couple under the experimental condition. The pertinent redox
couple is represented in the following electron transfer:
I
Ni :
NiII
ꢁꢁꢁꢁꢁꢁ*
)ꢁꢁꢁꢁꢁꢁ
Such electrochemistry for octahedral nickel(II) complexes is earlier reported
[11, 13, 28].
3.6. IR spectra
IR data of these complexes are compared in Supplementary material. The IR spectra
show a strong band at ca 1600–1583 cmꢁ1 characteristic of the azomethine, indicating
coordination of Schiff bases through nitrogen to the metal [42]. Complex 3 displayed
sharp bands at 3065 and 1555 cmꢁ1 associated with ꢇ(N–H) [43]. Complex 1 showed a
stretching frequency at 1206 cmꢁ1 due to ꢇ(C–O) band and at 3433 cmꢁ1 from lattice
water. A broad intense band due to ClO4ꢁ shows no splitting in 2 and 3, indicating
uncoordinated ClOꢁ4 in both complexes. New bands at ꢅ500, ꢅ465, and 400 cmꢁ1 fall
in the metal-oxygen and metal-nitrogen stretching region.

3660
R.N. Patel et al.
4. Conclusions
We have described the synthesis and characterization of three new mononuclear
nickel(II) complexes, namely [Ni(L1)2] Á 4H2O (1), [Ni(L2)2](ClO4)2 (2), and
[Ni(L3)(bipy)](ClO4)2 (3). All were structurally characterized by single crystal X-ray
analysis. Complexes 2 and 3, in the solid state, form supramolecular frameworks via
weak intra- and inter-molecular CH ꢀ ꢀ ꢀ ꢀ and ꢀ ꢀ ꢀ ꢀ ꢀ stacking interactions. Magnetic
moment values and spectroscopic data of these complexes show a paramagnetic
nickel(II) center. Complex 3 shows higher SOD activity than 1 and 2.
Supplementary material
CCDC 700762, 700763, and 700764 contain the supplementary crystallographic data
for [Ni(L1)2] Á 4H2O (1), [Ni(L2)2](ClO4)2 (2), and [Ni(L3)(bipy)](ClO4)2 (3), respectively.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; Fax (þ44) 1223-336-033; or E-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgments
Our grateful thanks are due to the National Diffraction Facility, X-ray Division, IIT
Mumbai and IACT-CSIC Armilla (Granada) Spain for single crystal data collection.
The Head RSIC, Central Drug Research Institute, Lucknow is also thankfully
acknowledged for providing analytical and spectral facilities. Financial assistance from
CSIR [Scheme No. 01(2094)\07\EMR-II] and UGC [Scheme No. 36-28/2008 (SR)]
New Delhi are also thankfully acknowledged.
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