Synthesis, structural characterization, in-vitro antibiogram assay and efficient catalytic activities of transition metal(II) chelates incorporating (E)-(2-((2-hydroxybenzylidene)amino)phenyl)(phenyl)methanone ligand

Article abstract of DOI:10.1016/j.molstruc.2015.01.011

A new tridentate ligand, (E)-(2-((2-hydroxybenzylidene)amino)phenyl)(phenyl)methanone and its four metal(II) chelates have been designed and synthesized. They were structurally characterized by elemental analysis, FT IR, UV-vis, ¹H NMR, ¹³C NMR, mass spectra, EPR, magnetic moment and conductivity measurements. Elemental analysis and molar conductance values reveal that all the chelates are 1:1 stoichiometry of the type [MLCl] having non-electrolytic nature. The metal chelates adopt square planar geometrical arrangements around the metal ions. The DNA-binding properties of these chelates have been investigated by electronic absorption, cyclic voltammetry, differential pulse voltammogram and viscosity measurements. The data indicate that these complexes bind to DNA via an intercalation mode. The oxidative cleavage of the metal complexes with pBR322 DNA has also been investigated by gel electrophoresis. Moreover, the antimicrobial bustle shows that all metal chelates have superior activity than the free ligand. The oxidation of toluene to benzaldehyde is effectively catalyzed by the synthesized chelates.

Full text of DOI:10.1016/j.molstruc.2015.01.011

Journal of Molecular Structure 1086 (2015) 56–63
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, structural characterization, in-vitro antibiogram assay and
efficient catalytic activities of transition metal(II) chelates incorporating
(E)-(2-((2-hydroxybenzylidene)amino)phenyl)(phenyl)methanone
ligand
Vellaichamy Muniyandi a, Narayanaperumal Pravin a, Liviu Mitu b, Natarajan Raman a,
⇑
a Research Department of Chemistry, VHNSN College, Virudhunagar 626 001, Tamil Nadu, India
b Department of Chemistry, University of Pitesti, Pitesti 110 040, Romania
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis of new DNA targeting metal
chelates.
Square planar geometrical
arrangements of the complexes.
The newly synthesized metal chelates
act as persistent intercalators.
Metal chelates display better
chemical nuclease activity.
They exhibit excellent catalytic
performance towards the oxidation of
toluene.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new tridentate ligand, (E)-(2-((2-hydroxybenzylidene)amino)phenyl)(phenyl)methanone and its four
metal(II) chelates have been designed and synthesized. They were structurally characterized by elemen-
tal analysis, FT IR, UV–vis, 1H NMR, 13C NMR, mass spectra, EPR, magnetic moment and conductivity mea-
surements. Elemental analysis and molar conductance values reveal that all the chelates are 1:1
stoichiometry of the type [MLCl] having non-electrolytic nature. The metal chelates adopt square planar
geometrical arrangements around the metal ions. The DNA-binding properties of these chelates have
been investigated by electronic absorption, cyclic voltammetry, differential pulse voltammogram and vis-
cosity measurements. The data indicate that these complexes bind to DNA via an intercalation mode. The
oxidative cleavage of the metal complexes with pBR322 DNA has also been investigated by gel electro-
phoresis. Moreover, the antimicrobial bustle shows that all metal chelates have superior activity than
the free ligand. The oxidation of toluene to benzaldehyde is effectively catalyzed by the synthesized
chelates.
Received 3 December 2014
Received in revised form 6 January 2015
Accepted 6 January 2015
Available online 13 January 2015
Keywords:
Square planar
Intercalation
Biological activity
DNA cleavage
Catalytic activity
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
For the past few decades, transition metal complexes have been
paid much attention in the study of mutation of genes in therapeu-
tic approaches [1,2]. In chemotherapeutic approach DNA will be the
⇑
Corresponding author. Tel.: +91 092451 65958; fax: +91 4562 281338.
E-mail address: ramchem1964@gmail.com (N. Raman).
http://dx.doi.org/10.1016/j.molstruc.2015.01.011
0022-2860/Ó 2015 Elsevier B.V. All rights reserved.

V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
57
target molecule in the treatment of cancer. Nowadays cis-platin and
trans-platin are used as anticancer drugs. From these two drugs,
cis-platin is an efficient chemotherapeutic agent for treating vari-
ous types of cancers such as sarcomas, small cell lung cancer, ovar-
ian cancer, lymphomas and germ cell tumors [3] but it encounters a
number of side effects such as anaemia, diarrhea, alopecia, pete-
chiae, fatigue nephrotoxicity, emetogenesis, ototoxicity and neuro-
toxicity [4]. To surmount this issue, present inorganic chemists
design more effective, cheaper, less toxic, site specific, and prefera-
bly non-covalently bound anticancer drugs. A new approach in this
task is to examine the anticancer activity of complexes containing
transition metal ions other than platinum. Late first row transition
metals viz, cobalt, nickel, copper and zinc are biologically relevant
metals as they are associated with various biomolecules related
to essential physiological activities [5]. At present, the bioactive
novel Schiff base ligands containing N, O/S donor sites with transi-
tion metal complexes have considerable attention in the treatment
of cancer cells in chemotherapeutic field [6–9].
the formation of metal-nitrogen bond. From the IR results, it is con-
cluded that the Schiff base ligand acts as tridentate (Scheme 1) and
coordinates to the metal ion through carbonyl oxygen, azomethine
nitrogen and phenolic oxygen atoms.
Molar conductivity
With a view to study the electrolytic nature of the mononuclear
metal complexes, their molar conductivities were measured in
DMF at 10ꢁ3 M. These values of all the complexes have been found
in the range 4.1–4.9
X
ꢁ1 cm2 molꢁ1 (Table 1) indicating their non-
electrolytic nature. The absence of counter (chloride) ion is con-
firmed from Volhard’s test. Elemental analyses data suggest a 1:1
ligand to metal ratio for the complexes, and all of these data have
been used to confirm the formulae of the complexes.
Electronic spectra and magnetic properties of metal(II) chelates
Benzophenone [(2-aminophenyl)(phenyl)methanone] and its
derivatives play an important role in organic chemistry, since they
are intermediates in several syntheses of pharmaceuticals, such as
benzodiazepines [10], diazocines [11] and in many photoreactions,
including photo-Fries and the photo-Claisen rearrangements [12]
which also present variable coordinating behavior toward metal
ions. In particular, it has been observed that the introduction of
another potential binding site such as an amino group modifies
both roles of benzophenone as sensitizer [13] and its co-ordinating
ability [14], suggesting a correlation between these properties. It is
familiar that DNA is the hereditary material in humans and almost
all other organisms. Although simple in structure, DNA can code
for all the complex necessities of life [15]. Interaction of DNA with
metal complexes has been investigated to design the new types of
pharmaceutical architecture, the mechanism involved in the site
specific recognition of DNA and to determine the principles gov-
erning the recognition [16–19].
In the light of the above and in continuation of our ongoing
research on DNA binding and cleavage activities of transition metal
complexes [20–22], herein we describe the synthesis and
characterization of few transition metal(II) complexes containing
tridentate ligand. In addition to this, DNA binding and cleavage
aptitude of these complexes have been evaluated. Moreover, the
catalytic activity has been explored. The antimicrobial potential
of the complexes has also been probed.
Electronic spectra of ligand and its metal(II) complexes were
measured at room temperature in DMF solution over 200–
1100 nm range. Various spectral data (Dq, B, b, b (%), m2/m1 and
LFSE) for the Ni(II) and Co(II) complexes (Table 2) were calculated
by applying band energies on Tanabe Sugano diagrams. The elec-
tronic absorption spectral data for ligand and its complexes were
obtained in DMF solution at room temperature. The free ligand
exhibited two intense bands in 32,176 and 33,487 cmꢁ1 region
due to
p ?
p⁄ and n ? p⁄ transitions, respectively. In all the metal
complexes, these transitions were shifted to blue or red frequen-
cies due to the coordination of the ligand with metal ions. The elec-
tronic spectrum of Cu(II) complex displayed the d–d transition
band at 18,914 cmꢁ1 = 1109 L Mꢁ1 cmꢁ1), due to 2B1g ? 2A1g
(e
transition. This d–d band strongly favors a square planar geometry
around the Cu(II) ion. It is further supported by the magnetic sus-
ceptibility value (1.82 BM). The electronic spectrum of Co(II) com-
plex showed band at 18,523 cmꢁ1 = 1058 L Mꢁ1 cmꢁ1), assigned
(e
to 1A1g ? 1B1g transition which was further supported by its mag-
netic moment value (2.1 BM) indicating that this complex has a
square planar configuration. The observed diamagnetic nature of
Ni(II) complex confirmed the square planar geometry. In the zinc
complex, metal to ligand charge transfer (MLCT) was also expected
but it was not observed probably due to overlapping with internal
electronic transitions of Schiff base ligand. Because of the d10 elec-
tron configuration of Zn(II), d–d electronic transitions are not
observed. Hence, based on stoichiometry of this complex and ele-
mental analysis it is four coordinated, which could be square pla-
nar geometry. Ligand field parameters such as, Racah parameter
(B), b and b° values support the covalent character of the square
planar geometry around Ni(II) and Co(II) complexes [24].
Results and discussion
All the metal complexes are stable at room temperature. The
free ligand is soluble in common organic solvents, but the
complexes dissolve readily only in DMSO and DMF. Analytical data
of the complexes suggest that the stoichiometry of the complex
composition is 1:1 (i.e., [MLCl]).
1H and 13C NMR spectra
1H NMR spectrum of the (Fig. S2) ligand showed a singlet at d
5.7, attributed to the phenolic OH group of 2-hydroxybenzalde-
hyde present in the ligand moiety. The absence of this peak was
noted for its zinc complex confirmed the deprotonation of AOH
proton with metal ion upon complexation. The aromatic region
was a set of multiplets in the range of d 6.8–7.3 for the ligand
and its Zn(II) complex. The ligand also showed an azomethine pro-
ton (ACH@N) signal at d 9.2. In complex, this signal was shifted
down field (d 8.6) suggesting deshielding of azomethine group
due to the coordination with metal ion. There is no appreciable
change in all other signals of the complex.
IR spectra
In the free ligand, the strong band observed at 1641 cmꢁ1 can be
assigned to the
m (ACH@N) azomethine stretching vibration. On
complexation, this band was shifted to lower frequency ca.
1610–1618 cmꢁ1 indicating the coordination of the azomethine
nitrogen atom to the central metal ion. The spectrum of the ligand
(Fig. S1) showed a broad band at 3431 cmꢁ1, which can be attrib-
uted to the stretching vibration of the OAH group [23]. Absence
of this band in all the complexes was confirmed by the coordina-
tion of phenolic oxygen to metal ions. Moreover, the chelation
was further confirmed by the formation of metal–oxygen bond in
In the 13C NMR spectra, the presence of a downfield shifted sig-
nal in the region 160.2 ppm with respect to that observed
158.1 ppm for the corresponding parent Schiff base (Fig. S3) was
in support of coordination of azomethine nitrogen to Zn atom.
the complexes in the region 556–566 cmꢁ1
observed in the complexes in the range 471–487 cmꢁ1 indicates
. The new band

58
V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
Scheme 1. Synthesis of Schiff base ligand and its metal complexes.
Table 1
Elemental and physical data of Schiff base ligand (L) and its metal(II) complexes.
Compound
Empirical formula
Formula weight
Color
Yield (%)
Elemental analysis found (Calc.)%
KM (X
ꢁ1 molꢁ1 cm2)
M
–
C
H
N
[L]
C
C
C
C
C
20H15NO2
301
399
394
394
401
Pale yellow
Brown
74
68
61
65
68
79.6
(79.7)
59.9
(60.1)
60.7
(60.9)
60.7
4.8
(5.0)
3.3
(3.5)
3.5
(3.6)
3.4
4.6
(4.6)
3.4
(3.5)
3.3
(3.5)
3.4
–
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
20H14ClNO2Cu
20H14ClNO2Ni
20H14ClNO2Co
20H14ClNO2Zn
15.7
(15.9)
14.7
(14.8)
14.7
(14.9)
16.1
4.1
4.6
4.7
4.9
Pale green
Pale pink
Yellow
(60.8)
59.9
(3.6)
3.4
(3.5)
3.3
(16.3)
(59.8)
(3.5)
(3.5)
The signal which appeared in 159.4 ppm region for the carbon atom
adjacent to the phenolic oxygen in the spectrum of free ligand, has
been observed in the complex at 157.1 ppm, indicating the coordi-
nation of phenolic oxygen to metal ion [25]. Comparison of all car-
bon peaks of the ligand with those of zinc complex showed some
upfield and downfield shifts, but these shifts were not large.
Mass spectra
The mass spectrum of Schiff base ligand (Fig. S4) showed peak
at m/z 301 corresponding to [C20H14NO2]+ ion. Also the spectrum
exhibited peaks for the fragments at m/z 195, 105 and 77 corre-
sponding to [C13H9NO]+, [C7H5O]+ and [C6H5]+. respectively

V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
59
Table 2
Electronic absorption spectral data (in DMF) and magnetic susceptibility of the Schiff base metal(II) complexes.
Compound kmax (cmꢁ1
leff (BM)
)
Band assignments Geometry
Ligand field parameter
Dq (cmꢁ1
–
)
B (cmꢁ1
)
b
b (%)
LFSE (kJ molꢁ1
–
)
m2
/m1
[CuLCl]
[NiLCl]
41,315
31,942
18,914
41,112
31,214
20,406
17,996
41,587
31,123
18,523
41,654
31,929
INCT
INCT
Square planar 1.82
–
–
–
–
2B1g
?
2A1g
INCT
INCT
Square planar Diamagnetic 963
903.16 (971 for free ion)
0.87 12.31 158.72
1.50
1A1g
?
?
1A2g
1B1g
1A1g
[CoLCl]
[ZnLCl]
INCT
INCT
Square planar 2.1
1085
–
841.02 (1030 for free ion) 0.86 13.38 103.96
1.63
–
1A1g
?
1B1g
d–d envelope
Square planar Diamagnetic
–
–
–
–
LMCT (n ? p⁄
)
Table 3
The spin Hamiltonian parameters of Cu(II) complex in DMSO solution at 77 K.
Complex
g-Tensor
g||
A ꢃ 10ꢁ4 (cmꢁ1
)
f
G
a2
b2
K||
K\
g\
giso
A||
A\
26
Aiso
[CuLCl]
2.21
2.05
2.14
138
63
162
4.23
0.72
0.96
0.83
0.61
(Scheme S1, supplementary file). The mass spectrum of Cu(II) com-
plex showed peak at m/z 400, corresponding to its molecular ion
peak and also the isotopic peaks of all the complexes were
observed with the weak relative intensities at (M + 2). In all the
complexes, the observed fragment ion [C20H14NO2M]+ was due to
the elimination of one coordinated chlorine atom. The Cu(II) com-
plex gave a fragment ion peak with loss of one chlorine atom at m/z
365. All these fragments leading to the formation of the species
[ML]+ which further experienced de-metallation to yield the spe-
cies [L+] giving the fragment ion peak at m/z 301.
evidence to the conclusion obtained on the basis of electronic spec-
trum and magnetic moment value.
DNA binding studies
Absorption spectral titrations
Electronic absorption spectroscopy is a useful technique in DNA
binding studies of molecules. DNA usually exhibits hypochromism
and red shift (bathochromism) as a consequence of the intercala-
tion mode, which involves a strong stacking interaction between
an aromatic chromophore and the base pairs of DNA. This strong
stacking interaction is due to the contraction of calf thymus CT
DNA in the helix axis and its conformational changes. On the other
hand, hyperchromism results from the secondary damage of DNA
double helix structure, in which the extent of hyperchromism is
indicative of partial or non-intercalative binding modes [29].
The absorption spectrum of [CuLCl] in the absence and presence
of CT DNA is given in Fig. 1. As the DNA concentration is increased,
the MLCT transition band of [CuLCl] complex at 335 nm exhibits
hypochromism of about 10% and bathochromism of 4 nm respec-
tively, further the intense absorption bands with maxima of 336,
335 and 336 nm for [NiLCl], [CoLCl] and [ZnLCl] exhibit hypochro-
mism of about 5%, 5%, 6% and bathochromism of 4, 3, and 3 nm,
respectively. These spectral characteristics may suggest a mode
of binding that involves a stacking interaction between the aro-
matic chromophore and the DNA base pairs. The hypochromisms
observed for the bands of these four compounds are accompanied
by a small red shift by less than 4 nm. The above phenomena imply
that these compounds interact with CT DNA by intercalative bind-
ing mode. To compare quantitatively the affinity of these com-
pounds binding to DNA, the intrinsic binding constant (Kb) has
been estimated. From absorption data, Kb is determined using the
following equation [30]:
Electron paramagnetic spectrum of the Cu(II) complex
The EPR spectrum of Cu(II) complex is very important to under-
stand the metal ion environment in the complex [26] i.e., the geom-
etry, nature of the donating atoms from the ligand and degree of
covalency of the Cu(II)-ligand bonds. The EPR spectra of the Cu(II)
complex were recorded in DMSO at liquid nitrogen temperature
(LNT) and at room temperature (RT). The spin Hamiltonian param-
eters of Cu(II) complex are listed in Table 3. The observed spectral
parameters for this compound are A|| (138) > A\ (26); g|| = 2.21
and g\ = 2.05 and giso = 2.14 which indicate that the unpaired elec-
tron is localized in d2x–y orbital of the Cu(II) ion with 3d9 configuration
with considerable covalent nature [27] present in Cu-L.
The parameter G, determined as G = (g||ꢁge)/(g\ꢁge), which
measures the exchange interaction between the metal centers in
a polycrystalline solid, has been calculated. According to Hathaway
[28] if G > 4, the exchange interaction is negligible, but G < 4 indi-
cates considerable exchange interaction in the solid complex. The
Cu(II) complex gives the G value 4.23 which is greater than 4 indi-
cating that the exchange interaction is absent in solid complex.
The observed values of a2 and b2 parameters indicate that the
complex has some covalent character and there is interaction in
the out-of-plane
to b2 (0.96) indicates that the in-plane
than the in-plane -bonding. For the present Cu(II) complex, the
observed order K|| (0.83) was greater than K\ (0.61) implying a
greater contribution from out-of-plane -bonding than from in-
plane -bonding in metal-ligand -bonding. Based on these obser-
p
-bonding. The lower value of
a
2 (0.72) compared
r
-bonding is more covalent
½DNAꢂ=ðea
ꢁ ef Þ ¼ ½DNAꢂ=ðeb ꢁ ef Þ þ ½Kbðeb ꢁ
ef Þꢂꢁ1
p
p
where [DNA] is the concentration of DNA in base pairs, the apparent
absorption coefficients ea f, and b are the apparent, free and bound
metal complex extinction coefficients, respectively. A plot of [DNA]/
f) vs [DNA], gives a slope of 1/(eb f) and a y-intercept equal to
p
p
,
e
e
vations, a square planar geometry is proposed for Cu(II) complex.
The EPR study of the Cu(II) complex has provided supportive
(eb–e
–e

60
V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
Fig. 2. Effect of increasing amount of [EB] ( ), [CuLCl] ( ), [CoLCl] ( ), [NiLCl] ( ),
[ZnLCl] ( ) and L ( ) on the viscosity of DNA. R = [complex]/[DNA] or [EB]/[DNA].
Fig. 1. Absorption spectrum of [CuLCl] in buffer pH = 7.2 at 25 °C in presence of
increasing amount of DNA. The arrow indicates increasing the amount of DNA.
Table 4
Intrinsic binding constant (Kb) values of Schiff base metal(II) complexes.
Complexes kmax (nm)
Free Bound
D
(nm)
k
% H Kb ꢃ 104
DG
(Mꢁ1
)
(KJ molꢁ1
)
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
331
332
332
333
335
336
335
336
4
4
3
3
10
5
5
1.68
1.60
1.66
1.47
ꢁ24.07
ꢁ23.96
ꢁ24.07
ꢁ23.73
6
[Kb/(eb–e
f)]ꢁ1. Kb is the ratio of the slope to the y-intercept. Intrinsic
binding constants of 1.68 ꢃ 104, 1.60 ꢃ 104, 1.66 ꢃ 104, 1.47 ꢃ 104
have been determined for [CuLCl], [NiLCl], [CoLCl] and [ZnLCl]
respectively. The results suggest that the interaction of metal(II)
complexes with DNA is a significant intercalative binding mode.
The Kb values (Table 4) obtained here is lower than that reported
Fig. 3. Cyclic voltammogram of [CuLCl] in buffer pH = 7.2 at 25 °C in presence of
increasing amount of DNA. The arrow indicates increasing the amount of DNA.
for a few metallointercalators [Ru(bpy)2(dppz)]2+ (4.90 ꢃ 106 Mꢁ1
)
[31] and [Ru(bpy)2(HBT)]2+ (5.71 ꢃ 107 Mꢁ1) [32].
complement to the previously used methods of investigation, such
as UV–vis spectroscopy and viscosity studies [35]. Small molecules,
which are not amenable to such methods either because of weak
absorption bands or because of overlap of electronic transition with
those of the DNA molecules, can potentially, be studied via voltam-
metric techniques. Cyclic voltammogram of Cu(II) complex in the
absence and presence of CT DNA is shown in Fig. 3. As seen in this
figure, in the absence of CT DNA the cyclic voltammogram featured
anodic peak Epa (ꢁ0.172 V) and cathodic peak Epc (ꢁ0.786 V) at
0.1 V sꢁ1. The reduction and oxidation potential observed at Epc = -
ꢁ0.767 V and Epa = ꢁ0.166 V is assigned to the redox couple, Cu(II)/
Cu(I). The ratio of Ipa/Ipc peak current is 0.30 (less than unity), indi-
cating a quasi-reversible redox process.
Viscosity measurements
Hydrodynamic measurements (i.e., viscosity and sedimentation)
that are sensitive to length changes are the most critical and least
ambiguous tests of binding in solution without crystallographic
data [33]. A classical intercalation model results in the lengthening
of the DNA helix as the base pairs are separated to accommodate the
binding molecule leading to an increase in the DNA viscosity. How-
ever, a partial and/or non-classical intercalation of ligand may bend
(or kink) DNA helix, resulting in the decrease of its effective length
and concomitantly its viscosity [34]. The effects of all the com-
pounds and together with ethidium bromide [EB] on the viscosity
of CT DNA are shown in Fig. 2. On increasing the amount of metal
complexes, the relative viscosity of DNA moderately increased stea-
dily, which is similar to the behavior of ethidium bromide. The
results suggest that all the metal complexes bind to DNA with inter-
calative mode. The viscosity measurements clearly show that all the
compounds can intercalate between adjacent DNA base pairs, caus-
ing an extension in the DNA helix and thus increasing the viscosity
of DNA with an increasing concentration of the complexes. On the
basis of all the spectroscopic studies together with the viscosity
measurements, we find that the metal complexes can bind to CT
DNA via an intercalative mode.
For the cyclic voltammogram of Ni(II) complex in the absence of
DNA, the anodic peak Epa (1.021 V) and cathodic peak Epc
(ꢁ0.671 V) appeared at 0.1 V sꢁ1 for Ni(II) ? Ni(I), (Epa = 1.101 V,
Epc = ꢁ0.651 V,
DEp = 1.752 V and E1/2 = 0.225 V). The ratio of Ipa/
Ipc peak current is 0.66, indicating a quasi-reversible redox pro-
cess. The cyclic voltammogram of Co(II) complex featured anodic
peak Epa (0.96 V) and cathodic peak Epc (ꢁ0.718 V) at 0.1 V sꢁ1
.
The reduction and oxidation potential observed at Epc = ꢁ0.637 V
and Epa = 1.021 V is assigned to the redox couple Co(II)/Co(I). The
ratio of Ipa/Ipc peak current is 0.63, indicating a quasi-reversible
redox process. In Zn(II) complex, in the absence of DNA the anodic
peak (0.906 V) and cathodic peak (ꢁ0.735 V) at 0.1 V sꢁ1 for
Zn(II) ? Zn(0), (Epa = 1.012 V, Epc = ꢁ0.701 V,
DEp = 1.713 V, and
Electrochemical behavior
E1/2 = 0.155 V). The ratio of Ipa/Ipc peak current is less than unity
(0.38), indicating a quasi-reversible redox process (Table 5). The
incremental addition of CT DNA to the complexes revealed that
the redox couples caused a more positive shift in E1/2 and decrease
The application of electrochemical methods to the study of
organic and metallointeraction to DNA provides
a useful

V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
61
Table 5
DNA cleavage efficacy
Electrochemical parameters for the interaction of DNA with Cu(II), Ni(II), Co(II) and
Zn(II) complexes.
There are number of agents which exert their effect by inhibit-
ing enzymes that act upon DNA. These inhibitions result from the
binding of such agents to the enzyme site of interaction on the
DNA rather than to direct enzyme inactivation. Transition metals
have been reported to inhibit DNA repair enzymes. The DNA cleav-
age efficiency of complex is attributed to the different binding
affinity of complex to DNA. There has been considerable interest
in DNA cleavage reactions activated by transition metal complexes.
The delivery of metal ion to the helix, in locally generating oxygen
or hydroxide radicals, yields an efficient cleavage reaction [37]. Gel
electrophoresis is a technique based on the migration of DNA
under the influence of an electric potential. When the original
supercoiled form (Form I) of plasmid DNA is nicked an open circu-
lar relaxed form (Form II) will exist in the system and the linear
form (Form III) can be found upon further cleavage. When circular
plasmid DNA is run on horizontal gel by electrophoresis, the com-
pact Form I migrates relatively faster while the nicked Form II
migrates slowly, and the linearized Form III migrates between
Forms I and II.
Compound Redox couples aE1/2 (V)
Free
bDEp (V)
Ipa/Ipc
Bound
Free
Bound
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
Cu(II) ? Cu(I)
Ni(II) ? Ni(I)
Co(II) ? Co(I)
Zn(II) ? Zn(0)
ꢁ0.479 ꢁ0.466 0.614 0.601
0.30
0.66
0.63
0.38
0.164
0.121
0.085
0.225
0.192
0.155
1.67
1.752
1.678 0.384
1.641 1.713
Data from cyclic voltammetric measurements.
a
E1/2 is calculated as the average of anodic (EPa
)
and cathodic (Epc) peak
a
potential; E1/2 = (Epa + Epc)/2.
b
D
Ep = Epa ꢁ Epc.
of
DEp. The Ipa/Ipc values also decreased in the presence of DNA.
Bard has reported [36] that if E1/2 is shifted to more positive value,
the interaction mode is intercalative binding. The above results of
metal-DNA interaction by the cyclic voltammogram studies con-
firm that Cu(II), Ni(II), Co(II) and Zn(II) complexes bind to DNA
via intercalation binding mode.
To assess the DNA cleavage ability of our synthesized metal(II)
complexes, supercoiled (SC) pBR322 DNA was incubated with the
complexes in 5 mM Tris-HCl/50 mM NaCl buffer at pH 7.2 for 2 h.
The cleavage experiments were carried out in the presence of acti-
vating agent, H2O2 under aerobic conditions and are shown in
Fig. 5. The intensity of supercoiled SC (Form I) diminished gradu-
ally and completely converted to nicked form (NC) (Form II). The
intensity of the NC (Form II) band increased whereas the produc-
tion of linear form L (Form III) of DNA also increased in the pres-
ence of H2O2 [38]. Control experiments have suggested that
untreated DNA did not show any cleavage (lane 1). Moreover,
uncoordinated ligand did not show any apparent cleavage (lane
2). These experimental facts demonstrate that a combination of
all the metal(II) complexes and activating agent (H2O2) are
required to show effective cleavage of plasmid DNA. It is obvious
that Cu(II) (Fig. 5; lane 4) has more ability to cleave the supercoiled
plasmid DNA when compared to that of other complexes. It is
believed that the cleavage ability of complexes is due to the reac-
tion of metal ions with H2O2 which produces diffusible hydroxyl
radicals or molecular oxygen at ease which in turn damage DNA
through Fenton-type chemistry [39].
Differential pulse voltammogram of the Cu(II) complex in the
absence and presence of varying concentration of DNA is shown
in Fig. 4. Increase in concentration of DNA causes a negative poten-
tial shift along with significant decrease of current intensity. The
shift in potential is related to the ratio of binding constant:
E0 ꢁ E0 ¼ 0:0591 logðK =K2þÞ
þ
b
f
where E0b and E0f are formal potentials of the Cu(II), Ni(II), Co(II) and
Zn(II) complex couples in the bound and free form respectively. In
the present study, all the complexes exhibited one electron transfer
during the redox process. The ratio of the binding constants (K[red]
/
K[oxd]) for DNA binding of synthesized complexes was calculated
and found to approximately less than unity. The above electro-
chemical experimental results indicate the preferential stabilization
of Cu(II), Ni(II), Co(II) and Zn(II) forms over other forms on binding
to DNA. In this study, the synthesized complexes reveal that both
anodic and cathodic peak potential shifts are either positive or neg-
ative. In this regard, the present Cu(II) complex shows a positive
potential shift along with significant decreasing of current intensity
during the addition of increasing amount of DNA. It indicates that
copper ion stabilizes the duplex (GC pairs) by intercalating way.
Antimicrobial activity
The biological activities of the Schiff base ligand (L) and its
metal(II) complexes were tested against few bacteria and fungi.
All of the tested compounds showed a remarkable biological activ-
ity against different types of Gram-positive and Gram-negative
bacteria and against fungi species. The minimal inhibitory concen-
trations of tested compounds against certain bacteria and fungi are
Fig. 5. Gel electrophoresis pattern showing cleavage of pBR322 supercoiled DNA
(10
lM) by Cu(II), Ni(II), Co(II) and Zn(II) complexes (60
lM) in the presence of
Fig. 4. Differential pulse voltammogram of [CuLCl] in buffer pH = 7.2 at 25 °C in
presence of increasing amount of DNA. The arrow indicates increasing the amount
of DNA.
H2O2 (100
l
M). Lane 1: DNA alone; lane 2: DNA + [L] + H2O2; lane 3: [CuL-
Cl] + DNA + H2O2; lane 4: [CoLCl] + DNA + H2O2; lane 5: [NiLCl] + DNA + H2O2; lane
6: [ZnLCl] + DNA + H2O2.

62
V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
Table 6
Minimum inhibitory concentration of the synthesized compounds against the growth
of bacteria ( M).
l
Compound
Minimum inhibitory concentration (MIC) (ꢃ104
lM)
S.
P.
E.
S.
K.
aureus
aeruginosa
coli
epidermidis
pneumoniae
[L]
16.9
7.1
9.4
8.4
11.3
1.7
15.8
8.1
9.6
7.1
10.8
1.9
16.4
6.2
9.0
8.6
13.5
1.8
16.2
7.7
9.7
6.3
12.1
1.3
14.5
7.3
9.2
7.9
11.7
2.3
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
aStreptomycin
a
Streptomycin is used as the standard.
Table 7
Minimum inhibitory concentration of the synthesized compounds against the growth
of fungi ( M).
l
Fig. 6. The catalytic behavior of Cu(II) complex. (a) Absorption peak of toluene
without Cu(II) complex, (b) reaction in the presence of Cu(II) complex at 70 °C (5 h)
and (c) reaction in the presence of Cu(II) complex at 70 °C (6 h).
Compound
Minimum inhibitory concentration (MIC) (ꢃ104
lM)
A. niger
F. solani
C. lunata
R. bataticola
C. albicans
[L]
13.9
9.3
10.2
9.9
12.3
1.1
19.3
9.8
10.9
10.2
12.3
1.6
16.5
13.7
13.5
12.9
12.1
1.2
11.9
10.1
11.7
10.3
12.9
1.0
14.9
11.6
10.9
11.8
11.3
1.5
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
aNystatin
a
Nystatin is used as the standard.
shown in Tables 6 and 7. The remarkable activity of the Schiff base
ligand may be arise from the hydroxyl group which may play an
important role in the antibacterial activity, as well as the presence
of one azomethine group which imports in elucidating the mecha-
nism of transformation reaction in biological systems. The
obtained results indicate that the complexes have higher activity
than the ligand against the same microorganisms under identical
experimental conditions. This suggests that the chelation could
facilitate the ability of a complex to cross a cell membrane and
can be explained by Tweedy’s chelation theory [40]. Chelation con-
siderably reduces the polarity of the metal ion because of partial
sharing of its positive charge with donor groups and possible elec-
tron delocalization over the whole chelate ring. Such a chelation
could enhance the lipophilic character of the central metal atom,
which subsequently favors its permeation through the lipid layer
of the cell membrane.
Scheme 2. The catalytic activity of Cu(II), Ni(II), Co(II) and Zn(II) Schiff base
complexes in the conversion of toluene to benzaldehyde.
Table 8
Catalytic efficiency of Cu(II), Ni(II), Co(II) and Zn(II) complexes.
Compound
[CuLCl]
[NiLCl]
[CoLCl]
[ZnLCl]
@ 70 °C
68
58
64
65
@ Room temperature with catalyst
@ 70 °C without catalyst
NR
NR
NR
NR
NR
NR
NR
NR
NR – No Reaction.
reduction of M(II) facilitates the ligand around the metal ion. This
catalytic system does not require any oxidant to carry out the reac-
tion. Hence, this is an efficient and environmental friendly catalyst.
Catalytic activity
Conclusion
The catalytic oxidation of toluene under aerobic conditions is
significantly a fascinating reaction, because the direct functionali-
sation of inactivated ACAH bonds in hydrocarbons usually
requires drastic reaction conditions such as high temperature
and pressure. Fig. 6 shows the oxidation of toluene into benzalde-
hyde. The peak at 350 nm is the characteristic peak for the benzal-
dehyde [41]. The conversion percentage of toluene at room
temperature and 70 °C is 0 and 68 respectively (Scheme 2). There
is no product obtained at room temperature. It is noted that tem-
perature has a significant effect on the efficiency of present cata-
lytic system [42]. The results also explore that no oxidation takes
place without the catalyst. According to the results obtained from
the UV analyses, the peak may be the oxidation of toluene to benz-
aldehyde. Copper complex is the most efficient catalyst with the
68% conversion of toluene. Other three complexes have consider-
ably low conversion percentages only at 70 °C viz., 58%, 64% and
65% respectively (Table 8). Very important point in the oxidation
of toluene is the reduction of M(II) to M(I) of the complexes. This
Few metal(II) chelates have been synthesized from 2-amino-
benzophenone analogue. The physico-chemical and spectral data
reveal that all the metal chelates are mononuclear and adopted
square planar geometry around the metal ions. The results of
DNA binding studies suggest that the complexes bind with CT
DNA through intercalation mode. The agarose gel electrophoresis
studies show that the complexes can promote the oxidative cleav-
age of plasmid DNA. However, these observations and a more
extensive study would be necessary in order to assert that the
complexes act as cleavage agents. All the metal chelates show con-
siderable catalytic efficiency towards the oxidation of toluene.
Cu(II) complex exhibits the highest activity as compared with
other three complexes. Moreover, the compounds show antimicro-
bial activity against selected kinds of bacteria and fungi. The
results of this antimicrobial work can show that the approach of
coordinating 2-aminobenzophenone analogues with pharmacolog-
ically interesting metals such as copper, nickel and zinc could be a

V. Muniyandi et al. / Journal of Molecular Structure 1086 (2015) 56–63
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Appendix A. Supplementary material
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