Copper(II) complexes as superoxide dismutase mimics: Synthesis, characterization, crystal structure and bioactivity of copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2009.07.037

Two new copper(II) complexes of the type [Cu(L)X₂), where L = (E)-N-phenyl-2-[phenyl (pyridine-2-yl)methylene]hydrazinecarboxamide X = Cl/Br have been synthesized and characterized by elemental analyses, FAB (fast atomic bombardment) magnetic m

Full text of DOI:10.1016/j.ica.2009.07.037

Inorganica Chimica Acta 362 (2009) 4891–4898
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Copper(II) complexes as superoxide dismutase mimics: Synthesis,
characterization, crystal structure and bioactivity of copper(II) complexes
1
1
*
R.N. Patel , K.K. Shukla, Anurag Singh, M. Choudhary, U.K. Chauhan , S. Dwivedi
Department of chemistry, A.P.S. University, Rewa, MP 486003, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new copper(II) complexes of the type [Cu(L)X2), where L = (E)-N-phenyl-2-[phenyl (pyridine-2-
yl)methylene]hydrazinecarboxamide X = Cl/Br have been synthesized and characterized by elemental
analyses, FAB (fast atomic bombardment) magnetic measurements, electronic absorption, conductivity
measurements cyclic voltammetry (CV) and Electron paramagnetic resonance (epr) spectroscopy. The
structures of these complexes determined by single crystal X-ray crystallography show a distorted square
based pyramidal (DSBP) geometry around copper(II) metal center. The distorted CuN2OX (X = Cl/Br) basal
plane in them is comprised of two nitrogen and one oxygen atoms of the meridionally coordinated ligand
and a chloride or bromide ion and axial position is occupied by other halide ion. The epr spectra of these
complexes in frozen solutions of DMSO showed a signal at g ca. 2. The trend in g-value (g|| > g\ > 2.00)
Received 26 March 2009
Received in revised form 25 June 2009
Accepted 21 July 2009
Available online 8 August 2009
Keywords:
Copper(II) complexes
X-band EPR
Superoxide dismutase (SOD) activity
suggest that the unpaired electron on copper(II) has dx2
character. Biological activities in terms of
ꢀy2
superoxide dismutase (SOD) and antimicrobial properties of copper(II) complexes have also been mea-
sured. The superoxide dismutase activity reveals that these two complexes catalyze the fast dispropor-
tionation of superoxide in DMSO solution.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
peroxide [7–9]. All SODs employ the two step ping-pong mecha-
nism shown in Eqs. (1) and (2), where M is the redox active metal
The synthesis of low-molecular weight copper(II) complexes
mimicking the superoxide dismutase (SOD) activity [1–4], provide
models for metalloproteins active sites and lend insight toward the
design of new catalysts. Copper is a biologically relevant element
and many enzymes that depend on copper for their activity have
been identified. Among these complexes, copper(II) complexes
are known to play a significant role in naturally occurring biologi-
cal systems [5] like (Cu,Zn–SOD) superoxide dismutase. Superox-
ide dismutase (SOD) which can destroy the superoxide very
rapidly, is nature’s agent for protection of the organism from this
radical burden. In fact native SOD enzymes have been shown in
many studies to exhibit protection in animal models of inflamma-
tory diseases [6]. In a variety of scenarios, therapeutic dosage of
additional SOD enzyme has shown promise, but from a number
of view points synthetic metal complexes offer considerable prom-
ise as SOD catalysts for pharmaceutical applications. Depending on
the metal at the active center, there are three general types of SOD
enzymes, namely Cu,Zn–SOD and Mn–SOD in mammalian systems
Ni–SOD and Fe–SOD in bacterial systems. These SODs dispropor-
center capable of both oxidizing and reducing superoxide.
Mox þ Oꢀ2 ! Mred þ O2
ð1Þ
ð2Þ
Mred þ O2ꢀ þ 2Hþ ! Mox þ H2O2
The presence of copper in the active site of Cu,Zn–SOD has led
many groups to search for stable, non toxic, low-molecular weight
complexes of this metal that have SOD activity (functional model)
and could be substituted for SOD in clinical applications with
desirable qualities being cell permeability, low cost and non-
immunogenicity [10]. Since copper is a most relevant metal for
the design of synthetic SOD catalysts, our school [11–18] also em-
barked on an effect in designing of some structural and functional
Cu, Cu–SOD and Cu, Zn–SOD models. (E)-N-phenyl-2-[phenyl (pyr-
idine-2-yl)methylene]hydrazinecarboxamide is a tidentate NNO
donar ligand (Scheme 1) with donar groups suitable placed for
forming 2 five-membered chelate rings.
The mononuclear complexes 1 and 2 show penta-coordinated
copper(II) ion with distorted square pyramidal (DSBP) coordination
geometry. Herein we report the synthesis, structure, spectroscopic,
magnetic measurement, cyclic voltammetry and superoxide dis-
mutase (SOD) activity of two new copper(II) complexes of the type
[Cu(L)X2] where L is a simple tridentate asymmetric ligand and X is
halogen (Cl and Br) atoms.
ꢀ
tionate the toxic O2 radical to molecular oxygen and hydrogen
* Corresponding author. Tel.: +91 7662 231373; fax: +91 7662 230819.
E-mail address: rnp64@yahoo.co.uk (R.N. Patel).
1
School of Environment Biology, A.P.S. University, Rewa, MP 486003, India.
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.07.037

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R.N. Patel et al. / Inorganica Chimica Acta 362 (2009) 4891–4898
sium phosphate buffer (pH 8.6) and 1 ml of 56 lM of alkaline
O
N
DMSO solution was added while string. The absorbance was then
monitered at 540 nm against a sample prepared under similar con-
dition except NaOH was absent in DMSO. A unit of superoxide dis-
mutase (SOD) activity is the concentration of complex, which
causes 50% inhibition of alkaline DMSO mediated reduction of
nitrobluetetrazolium chloride (NBT).
N
NH
NH
2.2.7. Antibacterial activity measurements
Scheme 1.
The in vitro antimicrobial (antibacterial) activities of these com-
plexes were tested using paper disc diffusion method [22,23] the
chosen strains were G(+) Staphylococcus and G(ꢀ) Proteus vulgaris,
Streptococcus, Pseudomonas and Escherichia coli. The liquid medium
containing the bacterial subcultures was autoclaved for 20 min at
121 °C and at 15 lb pressure before inoculation. The bacteria were
then cultured for 24 h at 36 °C in an incubator. Nutrient agar was
poured into a plate and allowed to solidify. The test compounds
(DMSO solutions) were added dropwise to a 10 mm diameter filter
paper disc placed at the center of each agar plate. The plates were
then kept at 5 °C for 1 h then transferred to an incubator main-
tained at 36 °C. The width of the growth inhibition zone around
the disc was measured after 24 h incubation. Four replicas were
made for each treatment.
2. Experimental
2.1. Materials
Copper(II) chloride dihydrate was purchased from S.d. fine-
chemicals, India. All other chemicals used were of synthetic grade
and used without further purification.
2.2. Physical measurements
2.2.1. FAB mass spectra
FAB mass spectra were recorded on a JEOL SX 102/DA 6000
mass spectrometer using (6 kV, 10 mA) as the FAB gas. The acceler-
ating voltage was 10 kV and the spectra were recorded at room
temperature (RT) with m-nitrobenzoyl alcohol as the matrix.
2.3. Synthesis
2.3.1. Synthesis of ligand (E)-N-phenyl-2-[phenyl (pyridine-2-yl)
methylene]hydrazinecarboxamide (L)
2.2.2. Magnetic measurements
Magnetic susceptibility measurements were made on a Gouy
balance using a mercury(II) tetrathiocynato cobaltate(II) as cali-
The Schiff base was prepared by condensation of 2-benzoylpyr-
idine and 4-phenyl semicarbazide. A solution of 4-phenylsemicar-
bazide (10.0 mmol, 1.512 g) in 10 mL ethanol was reflexed with a
ethanolic solution of 2-benzoylpyridine (10.0 mmol, 1.832 g) con-
tinuously for 6 h then 1–2 drops of acetic acid was added. On cool-
ing the solution at room temperature pale yellow crystals
separated which were filtered and washed with methanol. Anal.
Calc. for C19H16N4O(L): C, 72.06; H, 5.06; N, 17.70. Found: C,
72.08; H, 5.08; N, 17.72%.
brating agent (
v
g = 16.44 ꢁ 10ꢀ6 c.g.s. units). The molar suscepti-
bilities were corrected for the diamagnetism of the constituent
atoms by using Pascal constants.
2.2.3. Spectrometry
UV–Vis spectra were recorded at 25 °C on a Shimadzu UV–Vis
recording spectrophotometer UV-1601 in quartz cells. IR spectra
were recorded in KBr medium on a Perkin–Elmer 783 spectropho-
tometer. X-band (ꢂ9.4 GHz) epr spectra were recorded with a Vari-
on E-line Century Series Spectrometer equipped with a dual cavity
and operating at X-band with 100 kHz modulation frequency at
room temperature and at 77 K. TCNE was used as field marker.
The frozen solution at 77 K used for epr spectra were in
3 ꢁ 10ꢀ3 M of DMSO solution. The epr parameters for copper(II)
complexes were determined accurately using simulation program
[19].
2.3.2. Synthesis of [Cu(L)Cl2] (1)
To a methanolic solution (20 mL) of copper(II) chloride dihy-
drate (1.0 mmol, 0.170 g) a solution of L (1.0 mmol, 0.316 g) in
methanol was added with constant stirring at ambient temp. The
blue crystal suitable for single crystal X-ray diffraction were recov-
ered from mother liquor Anal. Calc. for C19H16Cl2CuN4O (1): C,
50.58; H, 3.55; N, 12.42. Found: C, 50.55; H, 3.52; N, 12.45%. FAB
Mass (m/z). Calc.: 451. Found: 450.80.
2.3.3. Synthesis of [Cu(L)Br2] (2)
2.2.4. Electrochemistry
To
a methanolic solution (20 mL) of copper(II) bromide
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 a reference electrode,
glassy carbon as working electrode and platinum wire as an auxil-
iary electrode. NaClO4 (0.1 M) was used as supporting electrolyte
and DMSO as solvent. All measurements were carried out at
298 K under a nitrogen atmosphere.
(1.0 mmol, 0.223 g) solution of L (1.0 mmol, 0.316 g) in methanol
was added and stirred well for 1 h. The red crystals suitable for sin-
gle crystal X-ray diffraction were recorded from mother liquor
Anal. Calc. for C19H16Br2CuN4O (2): C, 42.24; H, 2.965; N, 10.37.
Found: C, 42.22; H, 2.93; N, 10.39%. FAB Mass (m/z) Calc.: 540.
Found: 539.72.
2.3.4. Crystal structure determination
2.2.5. Bioactivity
Single crystal suitable for X-ray analysis for the complexes
[Cu(L)Cl2] 1 and [Cu (L)Br2] 2 were grown from the slow evapora-
tion of the reaction mixtures at room temperature. Single crystals
suitable for single crystal X-ray of 1, and 2 were mounted on a glass
fibre and used for data collection. Crystal data were collected on
Enraf–Nonius MACH3 diffractometer using graphite monochroma-
Antimicrobial (antibacterial) and superoxide dismutase (SOD)
activities were evaluated using the following methods.
2.2.6. SOD activity
The in vitro SOD activity was measured using alkaline DMSO as
a source of superoxide radical (O2ꢀ) and nitrobluetetrazolium chlo-
tized Mo K
refinement and intensity measurements were made using the pro-
gram CAD-4PC performing -scan measurements. The structures
a radiation (k = 0.71073 Å). The crystal orientation, cell
ride (NBT) as O2ꢀ scavenger [20,21]. In general, 400
ll sample to be
assayed was added to a solution containing 2.1 ml of 0.2 M potas-
W

R.N. Patel et al. / Inorganica Chimica Acta 362 (2009) 4891–4898
4893
were solved by direct method using the program SHELXS-97 [24] and
refined by full-matrix least-square techniques against F2 using
SHELXL-97 [25]. All non-hydrogen atoms were refined anisotropi-
cally. All the hydrogen atoms were geometrically fixed and allowed
to refine using a riding model.
3. Results and discussion
3.1. Synthesis and characterization
The preparation of complexes 1 and 2 can be achieved via the
1:1 reaction of respective cupric chloride dihydrate with L in meth-
anol. In the same manner complex 2 was obtained using anhydrous
cupric bromide instead of cupric chloride dihydrate this procedure
enabled us to isolate the complex 1 and 2.
Both complexes are soluble in water, acetonitrile, methanol,
and DMF but sparingly soluble in dichloromethane and benzene
and most of the organic solvents. These complexes were character-
ized using elemental analysis and further characterized by FAB+
mass spectrometry. The molar conductivity are values in DMSO
solution are very low (i.e. 12
X X
ꢀ1 cmꢀ1 molꢀ1 for 1 and 9 ꢀ1 cmꢀ1
molꢀ1 for 2), indicating the non electrolyte nature [26] of the
complexes.
Fig. 2. Projection view of [Cu(L)Br2] (2).
3.2. Crystal structures
Cu–Br bond lengths are the longest in the coordination sphere
[Cu(1)–Cl(1) = 2.205(3) Å, Cu(1)–Cl(2) = 2.5138(16) Å, for 1 and
Cu(1)–Br(1) = 2.3488(5) Å, Cu(1)–Br(2) = 2.6544(4) Å for 2] and
are comparable to those reported for other mononuclear dihalo
copper(II) complexes [28–30]. The Cu–Npy bond distances 2.024
and 2.018 Å in complex 1 and 2, respectively are similar and can
be viewed as strongly coordinated to the metal [31] and are also
in the range of Cu–Npy distance determined for other mononuclear
copper(II) complexes 2. The C(6)–N(2) bond distances (1.291(10) Å
for complex 1 and 1.281(4) Å for complex 2) are close to the theo-
retically predicted value of a double bond C=N confirms the forma-
tion of the Schiff base.
In complex 2, the molecules are packed in the lattice in an or-
dered manner along a/b/c axis. The intermolecular N(3)–
H(3 N)ꢃ ꢃ ꢃBr(2), N(4)–H(4 N)ꢃ ꢃ ꢃBr(2) and C(2)–H(2)ꢃ ꢃ ꢃBr(2) as well
as the intramolecular C(1)–H(1)ꢃ ꢃ ꢃBr(2) and C(19)–H(19)ꢃ ꢃ ꢃO(1)
hydrogen bonds (Table 4) of the neighboring molecules stabilize
the distorted square based pyramidal geometry around copper(II).
However, no significant hydrogen bonding interactions are found
in the packing for complex 1. The main difference in the packing
is shown by a weak interactions in the crystal structure of complex
2 not present in 1.
Complexes 1 and 2 have been characterized by the single crystal
X-ray diffraction technique. The ORTEP views of the complexes are
shown in Figs. 1 and 2. Crystallographic data and structure refine-
ment parameters are given in Table 1 and selected bond length and
bond angles are listed in Tables 2 and 3 The structures of the these
complexes are quite similar. The geometry around copper(II) ion
can be best described as trigonal bipyramidal distorted square
based pyramid (TBSBP) as indicated by the value of the trigonal in-
dex
s
= 0.14 for 1 and 0.25 for 2 (
s
= (b ꢀ
a
)/60), where
a and b are
the largest coordination angles. The value of
s
is 1 for perfect trigo-
nal bipyramidal geometry and is zero for a perfect square pyrami-
dal geometry [27]. All the three donor atoms two nitrogen and one
oxygen atoms (N1N2O1) of the meridionally coordinated ligand
occupy the three corners of the square plane and a chloride
(Clꢀ)/bromide (Brꢀ) at the fourth one, the remaining chloride/bro-
mide ions occupying the axial position. As expected the Cu–Cl and
3.3. Magnetic moment measurements
The magnetic moment measurements in the solid state show
that present complexes are paramagnetic at room temperature.
The observed magnetic moments of these complexes are 1.78
and 1.89 BM for complexes 1 and 2 respectively. These values sug-
gest planarity in copper(II) complexes and characteristics of d9
electronic configuration of Cu(II). These values are quite close to
the values expected for copper(II) complexes [32,33] and is in fair
agreement with the spin only system S = 1/2.
3.4. Epr spectra
Epr spectra of complexes 1 and 2 were recorded in polycrystal-
line and in solution. Representative epr spectra are shown in Figs. 3
and 4. Derived epr parameters are given in Table 5. The epr spectra
of polycrystalline samples recorded at room and liquid nitrogen
Fig. 1. Projection view of [Cu(L)Cl2] (1).

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R.N. Patel et al. / Inorganica Chimica Acta 362 (2009) 4891–4898
Table 1
Crystal data and structure refinement for complexes [Cu (L)2](Cl)2 1, [Cu(L)2](Br)2 2.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C19H16Cl2CuN4O
C19H16Br2CuN4O
539.725
150(2)
0.71073
monoclinic
Cc
450.80
150(2)
0.71073
triclinic
ꢀ
P1
9.2657(10)
9.7851(7)
11.6199(3)
13.3819(2)
b (Å)
c (Å)
11.5208(16)
104.268(9)
102.792(10)
99.696(7)
959.32(18)
1.561
13.4763(2)
90
114.631(2)
90
1904.85(6)
1.882
a
(°)
b (°)
c
(°)
V (Å3)
Dcalc (Mg/m3)
Z
2
4
Absorption coefficient (mmꢀ1
F(0 0 0)
)
1.433
458
5.359
1060
Crystal size (mm)
h range for data collection (°)
0.27 ꢁ 0.22 ꢁ 0.17
3.31–25.00
0.7927 and 0.6982
1.187
0.33 ꢁ 0.27 ꢁ 0.21
3.33–24.98
Maximum and minimum transmission
0.3991 and 0.2708
1.033
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0777, wR2 = 0.1686
R1 = 0.0953, wR2 = 0.1765
R1 = 0.0176, wR2 = 0.0424
R1 = 0.0193, wR2 = 0.0428
Table 2
Selected bond length (Å) and bond angels (°) of complex 1.
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–Cl(2)
N(1)–C(1)
N(2)–C(6)
N(3)–C(13)
N(4)–C(14)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(2)–Cu(1)–Cl(1)
O(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(2)
C(13)–O(1)–Cu(1)
C(1)–N(1)–Cu(1)
1.965(6)
2.024(9)
2.5138(16)
1.344(10)
1.291(10)
1.359(10)
1.402(10)
78.5(3)
Cu(1)–O(1)
Cu(1)–Cl(1)
O(1)–C(13)
N(1)–C(5)
N(2)–N(3)
N(4)–C(13)
2.081(5)
2.205(3)
1.248(8)
1.345(14)
1.358(7)
1.342(8)
C(5)–N(1)–Cu(1)
C(6)–N(2)–Cu(1)
N(3)–N(2)–Cu(1)
N(1)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(2)
O(1)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–Cl(2)
114.4(5)
120.4(5)
114.6(5)
99.0(3)
98.75(18)
95.24(13)
100.78(7)
78.3(2)
154.2(3)
162.37(16)
99.59(17)
96.85(15)
111.8(5)
125.3(10)
Table 3
Selected bond length (Å) and bond angles (°) of complex 2.
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–Br(2)
N(1)–C(1)
N(2)–C(6)
N(3)–C(13)
N(4)–C(14)
N(2)–Cu(1)–N(1)
C(6)–N(2)–Cu(1)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–Br(1)
O(1)–Cu(1)–Br(1)
N(1)–Cu(1)–Br(2)
O(1)–Cu(1)–Br(2)
C(1)–N(1)–Cu(1)
1.975(3)
2.018(3)
2.6554(4)
1.333(4)
1.281(4)
1.379(4)
1.414(4)
78.66(10)
119.9(2)
153.96(10)
100.02(8)
98.60(6)
101.58(7)
91.33(6)
127.0(2)
Cu(1)–O(1)
Cu(1)–Br(1)
O(1)–C(13)
N(1)–C(5)
N(2)–N(3)
N(4)–C(13)
2.043(2)
2.3488(5)
1.243(4)
1.345(4)
1.351(4)
1.326(4)
N(2)–Cu(1)–O(1)
N(3)–N(2)–Cu(1)
N(2)–Cu(1)–Br(1)
N(2)–Cu(1)–Br(2)
Br(1)–Cu(1)–Br(2)
C(13)–O(1)–Cu(1)
C(5)–N(1)–Cu(1)
78.21(9)
114.6(2)
163.00(8)
92.80(8)
104.016(16)
112.9(2)
114.2(2)
Fig. 3. EPR spectra of [Cu(L)Cl2] (1) at 298 K.
Table 4
Hydrogen bonding interactions for Cu(II) complexes 1 (Å and °).
D–Hꢃ ꢃ ꢃA
D–H (Å) Hꢃ ꢃ ꢃA (Å) Dꢃ ꢃ ꢃA (Å) \D–Hꢃ ꢃ ꢃA (°)
1 N(3)–H(3 N)ꢃ ꢃ ꢃBr(2)
1 N(4)–H(4 N)ꢃ ꢃ ꢃBr(2)
Intra 1 C(1)–H(1)ꢃ ꢃ ꢃBr(1)
1 C(2)–H(2)ꢃ ꢃ ꢃBr(2)
0.74(4)
0.92(4)
0.95
2.92(4)
2.34(4)
2.91
2.93
2.28
3.523(3)
3.245(3)
3.489(3)
3.708(3)
2.886(4)
140(3)
168(3)
120
140
121
temperature are in agreement with geometry observed in the X-
ray analysis. There is no apparent difference in the epr spectral fea-
tures or in spectral parameters. In polycrystalline state, the epr
spectra are slightly orthorhombic with gx < gy < and gz (Table 5).
In solution both complexes show an axial symmetry (g|| > g\) with
0.95
Intra1 C(19)–H(19)ꢃ ꢃ ꢃO(1) 0.95
a dx2
ground state in copper(II) located in the square based
complexes fall in the range for CuN3O chromophore [36,37] in
the g|| versus A|| plot [27]. This kind of observations suggests that
the weak axial and even the equatorial chloride ions are possibly
ꢀy2
geometries [34,35]. The g|| (2.23 for 1 and 2.24 for 2) and A||
(0.0135 for 1 and 0.0121 cmꢀ1 for 2) values observed for these

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4895
a2 ¼ ðA =0:036Þ þ ðg ꢀ 2:0023Þ þ 3=7ðg? ꢀ 2:0023Þ þ 0:04
jj
jj
The orbital reduction factors K|| and K\ were estimated from the
expression [41].
K2jj ¼ ðgjj ꢀ 2:0023ÞEd-d=8k0
K2? ¼ ðg? ꢀ 2:0023ÞEd-d=2k0
where K|| = a2b2, K\
=
a2c2 and k0 represents the one electron spin–
orbit coupling constant for the free ion, equal to ꢀ828 cmꢀ1. Signif-
icant information about the nature of bonding in the copper(II)com-
plexes can be derived from the magnitude of K|| and K\. In case of
pure
r
bonding K|| ꢄ K\ ꢄ 0.77 whereas K|| < K\ implies consider-
able in-plane bonding, while for out-of-plane bonding, K|| > K\ In
all the present copper(II) complexes, it is observed that K|| < K\
which indicates the presence of significant in-plane bonding. The
evaluated values of
with both strong in-plane
a
2, b2 and c2 of the complexes are consistent
r
and in-plane p bonding. The computed
value of a2 and b2 (Table 5) are compared with other copper(II)
complexes while are ionic in nature [42]. Therefore present com-
plexes may be regarded as ionic complexes. The empirical factor
f = g||/A|| cmꢀ1 is an index of tetragonal distortions and its value
may vary from 105–135 for small to extreme distortions in square
planar complexes and it depends on the nature of the coordinated
atoms [43]. The f values of these complexes are found 165 for 1
and 185 for 2, indicating significant distortion from planarity.
Fig. 4. EPR spectra of [Cu(L)Cl2] (1) in DMSO (0.003 molꢀ1 dmꢀ3) at 77 K.
3.5. Ligand field spectra
Table 5
Epr spectral parameters of the copper(II) complexes.
The room temperature ligand field spectra of these complexes
have been recorded in 100% DMSO solution. The electronic spectra
of these two complexes are very similar to each other and show a
low energy ligand field (LF) band (705 5 nm) and a high energy
Epr parameter
1
2
Polycrystalline state (298 K)
ligand bands (300 and 310 nm) n ?
p of pyridine ring in agree-
gz
gy
gx
giso/ave
2.110
2.048
2.022
2.06
2.139
2.061
2.020
2.073
ment with its X-ray structural analysis [44]. The complex also
show a band at 400 nm due to Cl–Cu charge transfer band .The li-
gand field band energy of complex 1 is lower than that for 2 due to
the higher electronegativity of Cl atoms than Br atoms.
DMSO (77 K)
g||
2.233
2.069
145
3.377
0.676
1.044
1.123
0.758
0.705
165
2.240
2.076
130
3.156
0.647
1.090
1.220
0.792
0.710
185
g\
AG (G)
G
3.6. IR spectra
a2
The IR spectra of the complexes also give expected band shapes
and positions. The (>C@N) band of the ligand at 1623 cmꢀ1 is found
to be slightly shifted to lower energies (1624 for 1 and 1618 cmꢀ1
for 2) in the spectra of complexes, indicating the coordination via
the azomethine nitrogen (C@N) [45,46] The bands in the regions
508–535 and 280–332 cmꢀ1 are attributed to the (Cu–N) an-
d)(Cu–X) (X = Cl/Br) band, respectively [47]. Vibrations at
455 cmꢀ1 (weak) can be attributed to M–O bonds [48].
b2
c2
K\
K||
f
replaced by solvent molecules [34] to form the CuN3O chromo-
phore in solution. The g|| and A|| values for complex 2 are higher
and lower, respectively, than for 1 is retained even in solution,
which is consistent with the ligand field spectral results and single
crystal X-ray structures. The geometric parameter G, which is mea-
sure of the exchange of interaction between the copper centers in a
polycrystalline solid has been calculated. According to Hathaway
[38,39] if G > 4 the exchange interaction is negligible and if G < 4,
indicates exchange interaction. The value of G for present com-
plexes are 3.38 for 1 and 3.16 for 2 indicating exchange interaction
in solid state.
3.7. Electrochemical studies
The electrochemical properties of the two complexes have been
studied by cyclic voltammetry (CV) under a nitrogen atmosphere
in DMSO solution in the potential range +1.00 to ꢀ9.00 V versus
Ag/AgCl reference and the results are presented in Table 6. A rep-
resentative CV is shown in Fig. 5. The cyclic voltammograms of
these complexes exhibit only one irreversible CuII/CuI redox couple
[49]. The difference in CuII/CuI redox potential between complexes
1 and 2 demonstrates that the coordination environments around
the copper(II) ions of the two complexes are different and afford a
substantial explanation for the difference in SOD activity of them.
It is known that an adequate CuII/CuI redox potential for effective
catalysis of superoxide radical must be required between
The EPR parameters and d–d transition energies were used to
evaluate the bonding parameter
garded as measure of the covalency of the in-plane
and the in-plane - and out-of-plane bonding, respectively.
The in-plane
bonding parameter a2 was calculated by using
the expression [40]:
a c
2, b2 and 2, which may be re-
r
bonding
p
p
ꢀ
ꢀ
r
ꢀ0.405 V versus SCE for O2/O2 +0.645 V versus SCE for O2
/
H2O2. Though it is inappropriate to compare the E0 values with

4896
R.N. Patel et al. / Inorganica Chimica Acta 362 (2009) 4891–4898
Table 6
Cyclic voltammetric data for 1 mM solution of the Cu(II) complexes in DMSO containing 0.1 M NaClO4 as supporting electrolytes.
Scan rate (mV/s)
Epc (mV)
Ipc
(l
A)
Epa (mV)
Ipa
(lA)
D
Ep (mV)
E0’ (mV)
Ipa/Ipc (lA)
[Cu(L)(Cl)2]
100
200
14.0
5
6.384
7.0985
526
549
1.2543
1.2390
512
544
270
277
0.196
0.175
[Cu(L)(Br)2]
100
200
132
107
7.3152
10.8400
934
951
20.7279
27.4113
802
844
533
529
2.833
2.529
D
Ep = Epa ꢀ Epc; E°0 = (Epa + Epc)/2.
Table 7
Superoxide dismutase activity of some copper (II) complexes.
S. No.
Complex
IC50
(l
mol dmꢀ1
)
Ref.
1
2
2
3
4
5
6
7
8
9
10
11
12
Native SOD
[Cu(L*)Cl2]
0.04
0.09
149
125
132
32
50
51
52
52
20
20
20
11
11
53
53
[Cu(PMDT)(H2O)](ClO4)2
[Cu(PMDT)(ImH)](ClO4)2
[Cu(glygly)]ꢃ3H2O
[Cu(glygly)(phen)]ꢃ3H2O
[Cu(glygly)(bipy)]ꢃ3H2O
[Cu(PMDT)(bipy)](ClO4)2
[Cu(PMDT)(phen)]ꢃ(ClO4)2
[Cu(SAA)H2O]
25
105
111
63
35
22
[Cu(SAA)(MeImH)]
[Cu(L)Cl2]
[Cu(L)Br2]
this work
this work
18
PMDT = N,N,N0,N00,N00-pentamethyldiethylenetriamine; glygly = glycylglycine; SAA =
salicylidineanthranilic acid, L* = dichloro[(4,5-dihydroimidazole-2-yl)-1H-benzimi-
dazole-N,N0].
mechanism of dismutation of O2ꢀ by native SOD. Complex 2 shows
lower IC50, and exhibits higher SOD activity than other mononu-
clear complexes except for two systems including native SOD so
for reported [50–53]. Important factors for SOD like activity are a
medium strength donar and the ability of the ligand to accommo-
date the reduced copper(I) in a tetrahedral-like environment [54–
56].
Fig. 5. Cyclic voltammogram (0.003 molꢀ1 dmꢀ3) of [Cu(L)Cl2] (1) in DMSO (0.1 M
NaClO4 as supporting electrolyte, scan rate: 200 mV/s).
This reasonable correlation allow us to propose that the higher
dissociation of this complex lead of the transition DSBP geometry
to the distorted square planer (like tetrahedral), a structure which
is more accessible to the reactions with superoxide [57].
the reduction potentials of the two complexes in DMSO should be
in the allowed range of an SOD mimic, since the obtained results of
IC50 shows that both of them have relatively low activity. Two
reduction waves at high negative potential are assigned to the
reduction of ligand.
3.9. Antimicrobial activity of complexes against pathogens
3.8. Superoxide dismutase activity
Present complexes were evaluated against different species of
bacteria as the test organism in an antimicrobial study. Antimicro-
bial assessment of the complexes was tested as a function of con-
centration of theses complexes and results are presented in Table
8. Four concentration of the complexes were taken, i.e. 5 mM,
Biological activities (SOD mimetic activities) of the complexes
have also been measured. The SOD mimetic activities of the pres-
ent complexes were examined by the NBT assay [20,21] following
kinetically the reduction of NBT to MF+ at 560 nm. Superoxide was
enzymatically supplied from alkaline DMSO. The count fraction
causing 50% inhibition of NBT reduction is called IC50. Concentra-
tion equivalent to one unit of SOD activity (IC50 values), together
with the IC50 value of native SOD are given in Table 7. Obtained re-
Table 8
Antibmicrobial activity of copper(II) complexes.
Complex (mM)
Diameter of inhibition zone(in mm)
sults of IC50 for present complexes remain in the range 20
2 lM.
Streptococcus aureus
E. Coli
The observed IC50 values of the present complexes are comparable
with the various reported values for the copper(II) complexes
[11,20], but are less active than the native SOD. The difference in
IC50 values for two complexes should be ascribed to the evidence
discrepancy in the electronegativity of halo atoms. The good activ-
ities of two complexes may be attributed to the flexible L ligand,
which is able to accommodate the geometrical change from CuII
[Cu(L)Cl2] 1
5
10
15
20
6
8
10
16
6
12
20
30
[Cu(L)Br2)] 2
5
10
15
20
4
8
10
20
8
to CuI, specially the two labile halogen atoms, which are proposed
16
20
30
ꢀ
to be easily substituted by the substrate O2 in the catalytic pro-
,
cess, just like the O2ꢀ, in place of H2O bound to copper site in the

R.N. Patel et al. / Inorganica Chimica Acta 362 (2009) 4891–4898
4897
35
30
25
20
15
10
5
certain strains of bacteria towards the present copper(II) com-
plexes were determined by measuring the size of inhibition diam-
eter. The growth inhibitory effects were observed against the
following bacterial pathogens, E. coli, Staphylococcus aureus and Sal-
monella typhi. Complexes 1 and 2 were tested for their antimicro-
bial activity. Both complexes are effective against two micro-
organisms, i.e. E. coli and S. aureus. Both the Bacteria are pathogens
for humans, which causes dysentery and food poisoning, respec-
tively. Results of these antimicrobial assessments of complexes
are presented in Table 8. The area of zone of inhibition is less in
the concentration of 5 mM in both micro-organisms and more in
20 mM concentration (Figs. 6 and 7). This kind of observation is
suggestive of that both complexes are effective against both patho-
gens. In case of complex 1 diameter of inhibition zone (30 mm) is
highest for E. coli. It was noted that complex 1 is more effective
against E. coli than the S. aureus. Similar observations were found
for complex 2. Both complexes are also tested in S. typhi which
causes typhoid in humans and the results showed that both com-
plexes are also effective against this pathogens. Similar antimicro-
bial results were reported by Tarafder et al. [58] and also by our
school [15,59] on simple copper(II) binary and ternary complexes.
E.Coli
S.aureus
4. Conclusion
Our aim when beginning this work was to design low-molecu-
lar weight copper(II) complexes as potential SOD mimics. The crys-
tal structures of these complexes have been successfully
determined and are turn out to be CuN3OCl2 or CuN3OBr2 trigonal
bipyramidal distorted square based pyramid (TBSBP) geometry
around copper(II) metal centers. These two complexes exhibit sig-
nificant SOD activity being the superior catalysts.
In conclusion a combined magnetic and spectroscopic approach
is useful not only to give the complete characterization of the com-
plexes, but also to correlate their structural features with biological
activity. These complexes can be considered as good model for SOD
activity.
0
5
10
15
20
Concentration (mM)
Fig. 6. Antibmicrobial activity of [Cu(L)Cl2] (1).
35
30
25
20
15
10
5
S.aureus
E.Coli
Acknowledgements
Our grateful thanks are due to the National Diffraction Facility,
X-ray Division, and RSIC (SAIF), IIT Mumbai for single crystal data
collection and epr measurements, respectively. The Head RSIC
(SAIF), Central Drug Research Institute, Lucknow is also thankfully
acknowledged for providing analytical and spectral facilities.
Appendix A. Supplementary material
CCDC 698930 and 698931 contain the supplementary crystallo-
graphic data for [Cu(L)Cl2] and [Cu(L)Br2)]. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2009.07.037.
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