Ni(II) and Cu(II) complexes with ONNO asymmetric tetradentate Schiff base ligand: synthesis, spectroscopic characterization, theoretical calculations, DNA interaction and antimicrobial studies

Article abstract of DOI:10.1002/aoc.3555

A novel Schiff base, namely Z-3-((2-((E)-(2-hydroxynaphthyl)methylene)amino)-5-nitrophenylimino)-1,3-dihydroindin-2-one, was synthesized from the condensation of 2-hydroxy-1-naphthaldehyde and isatin with 4-nitro-o-phenylenediamine. It was structurally characterized on the basis of ¹H NMR, ¹³C NMR and infrared spectra and elemental analyses. In addition, Ni(II) and Cu(II) complexes of the Schiff base ligand were prepared. The nature of bonding and the stereochemistry of the investigated complexes were elucidated using several techniques, including elemental analysis (C, H, N), Fourier transform infrared and electronic spectroscopies and molar conductivity. The thermal behaviours of the complexes were studied and kinetic–thermodynamic parameters were determined using the Coats–Redfern method. Density functional theory calculations at the B3LYP/6-311G++ (d, p) level of theory were carried out to explain the equilibrium geometry of the ligand. The optimized geometry parameters of the complexes were evaluated using LANL2DZ basis set. The total energy of highest occupied and lowest unoccupied molecular orbitals, Mullikan atomic charges, dipole moment and orientation are discussed. Moreover, the interaction of the metal complexes with calf thymus DNA (CT-DNA) was explored using electronic spectra, viscosity measurements and gel electrophoresis. The experimental evidence indicated that the two complexes could strongly bind to CT-DNA via an intercalation mechanism. The intrinsic binding constants of the investigated Ni(II) and Cu(II) complexes with CT-DNA were 1.02?×?10⁶ and 2.15?×?10⁶?M^?1, respectively, which are higher than that of the standard ethidium bromide. Furthermore, the bio-efficacy of the ligand and its complexes was examined in vitro against the growth of bacteria and fungi to evaluate the antimicrobial potential. Based on the obtained results, the prepared complexes have promise for use as drugs. Copyright

Full text of DOI:10.1002/aoc.3555

Full paper
Received: 21 May 2016
Revised: 7 June 2016
Accepted: 9 June 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3555
Ni(II) and Cu(II) complexes with ONNO
asymmetric tetradentate Schiff base ligand:
synthesis, spectroscopic characterization,
theoretical calculations, DNA interaction and
antimicrobial studies
a
a
b
Laila H. Abdel-Rahman , Ahmed M. Abu-Dief *, H. Moustafa
a
and Samar Kamel Hamdan
A novel Schiff base, namely Z-3-((2-((E)-(2-hydroxynaphthyl)methylene)amino)-5-nitrophenylimino)-1,3-dihydroindin-2-one, was
synthesized from the condensation of 2-hydroxy-1-naphthaldehyde and isatin with 4-nitro-o-phenylenediamine. It was structur-
1
13
ally characterized on the basis of H NMR, C NMR and infrared spectra and elemental analyses. In addition, Ni(II) and Cu(II) com-
plexes of the Schiff base ligand were prepared. The nature of bonding and the stereochemistry of the investigated complexes
were elucidated using several techniques, including elemental analysis (C, H, N), Fourier transform infrared and electronic spec-
troscopies and molar conductivity. The thermal behaviours of the complexes were studied and kinetic–thermodynamic parame-
ters were determined using the Coats–Redfern method. Density functional theory calculations at the B3LYP/6-311G++ (d, p) level
of theory were carried out to explain the equilibrium geometry of the ligand. The optimized geometry parameters of the com-
plexes were evaluated using LANL2DZ basis set. The total energy of highest occupied and lowest unoccupied molecular orbitals,
Mullikan atomic charges, dipole moment and orientation are discussed. Moreover, the interaction of the metal complexes with
calf thymus DNA (CT-DNA) was explored using electronic spectra, viscosity measurements and gel electrophoresis. The experimen-
tal evidence indicated that the two complexes could strongly bind to CT-DNA via an intercalation mechanism. The intrinsic bind-
6
6
ꢀ1
ing constants of the investigated Ni(II) and Cu(II) complexes with CT-DNA were 1.02 × 10 and 2.15 × 10 M , respectively, which
are higher than that of the standard ethidium bromide. Furthermore, the bio-efficacy of the ligand and its complexes was exam-
ined in vitro against the growth of bacteria and fungi to evaluate the antimicrobial potential. Based on the obtained results, the
prepared complexes have promise for use as drugs. Copyright © 2016 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: asymmetric Schiff base ligand; DFT studies; TGA; DNA interaction; electrophoresis
[
14]
agents.
Schiff base ligands derived from isatin exhibit many
Introduction
pharmacological effects like antimicrobial, antiviral, anticonvulsant,
anticancer, antimalarial, herbicidal and anti-inflammatory
activities.
The literature reveals that Schiff base complexes of isatin and 4-
nitro-o-phenylenediamine have not been much studied. Thus, we
describe the preparation of Ni (II) and Cu(II) complexes with an
asymmetric Schiff base ligand and the characterization of them
using various techniques. Furthermore, we describe a comparative
Schiff base ligands have played a key role in the development of co-
ordination chemistry. Schiff base ligands with N O donor atoms
2 2
[
15–17]
are well known to coordinate with a variety of metal ions. They have
attracted much interest in recent years, due to their ease of synthe-
sis, their stability under a variety of oxidative and reductive condi-
tions and their structural versatility associated with various
[1–4]
applications.
wide range of applications due to their biocidal activity against bac-
Schiff base complexes of transition metals have a
[5–8]
teria, fungi and certain types of tumours.
These compounds are
also utilized in magnetic and optoelectronic technologies for their
[
9,10]
large nonlinear responses.
The interest in the design, prepara-
*
Correspondence to: Ahmed M. Abu-Dief, Chemistry Department, Faculty of
Science, Sohag University, 82534 Sohag, Egypt. E-mail: ahmed_benzoic@yahoo.
com
tion and characterization of transition metal complexes of
unsymmetric Schiff base ligands has arisen from the recognition
that coordinated ligands around central metal ions in natural sys-
[
11–13]
a Chemistry Department, Faculty of Science, Faculty of Science, Sohag University,
2534, Sohag, Egypt
tems are unsymmetric.
As an important pharmacophore re-
8
sponsible for biological activity, isatin scaffold is an applicable
lead structure for the synthesis of efficient chemotherapeutic
b Chemistry Department, Faculty of Science, Cairo University, 12613, Giza, Egypt
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

L. H. Abdel-Rahman et al.
study of DNA binding towards the prepared complexes and their
various biological properties.
filtered using suction and washed thoroughly with aqueous etha-
nol solution. The pure compound was dried in a desiccator over
anhydrous calcium chloride. The structure of HL is shown in
Scheme 1.
Experimental
The physical and spectroscopic data for ligand HL are as fol-
lows. Dark orange; m.p. 228°C; yield 80%. H NMR (400 MHz,
1
Materials
6
DMSO-d , δ, ppm): 14.58 ppm (s, 1H, OH), 10.82 (s, 1H, CH N),
9.76 (s, 1H, H-C17), 8.94 (d, J = 8.5 Hz, 1H, H-C23), 8.64 (d, 1H,
All the starting materials of chemicals used in this investigation
J = 9.1 Hz, H-C15), 8.19 (d, 1H, J = 9.1 Hz, H-C14), 8.14 (d, 1H,
J = 8.0 Hz, H-C25), 8.05 (d, 1H, J = 8.5 Hz, H-C22,), 7.99 (d, 1H,
J = 8.5 Hz, H-C28), 6.67 (dd, 1H, J = 8.0, 6.1 Hz, H-C26), 6.71 (dd,
such
as
2-hydroxy-1-naphthaldehyde,
isatin,
4-nitro-o-
phenylenediamine and the metal salts Cu(CH COO) ꢁH O and Ni
3
2
2
(
NO ) ꢁ6H O, calf thymus DNA (CT-DNA) and Tris were purchased
3
2
2
1H, J = 8.5, 6.1 Hz, H-C27), 7.91 (d, 1H, J = 7.7 Hz, H-C6), 7.60
from Sigma-Aldrich Chemie (Germany). Spectroscopic grade etha-
nol, dimethylsulfoxide (DMSO), dimethylformamide (DMF) and
HCl were used.
(
dd, 1H, J = 7.7, 5.9 Hz, H-C7), 7.43 (dd, 1H, J = 8.5, 5.9 Hz, H-
13
C8), 7.25 (s, 1H, NH), 7.23 (d, 1H, J = 8.5 Hz, H-C9). C NMR
(
(
(
100 MHz, DMSO-d , δ, ppm) (Scheme 1): (C3) = 234.7,
C2) = 181.8, (C5) = 137.3, (C7) = 133.1, (C9) = 128.5,
C8) = 124.4, (C6) = 120.0, (C4) = 116.2, (C13) = 160.1, (C16)
6
Instrumentation
=
149.8, (C12)
(C17) = 121.4, (C19) = 168, (C21) = 163.7, (C24) = 160.1,
C23) = 133.3, (C29) = 133.1, (C25) = 128.5, (C27) = 127.9,
= 148.2, (C15) = 121.6, (C14) = 124.1,
NMR spectra were recorded with an FT-NMR spectrometer (Bruker
1
13
ARX 400.1) at 400 MHz ( H) and 100.6 MHz ( C) in DMSO-d
6
at Cen-
(
tral Lab, Chemistry Department, Faculty of Science, Sohag Univer-
1
13
(C28) = 113.8, (C26) = 113.9, (C22) = 111.4, (C20) = 110.5. Anal.
Calcd for (C H N O ) (%): C, 68.81; H, 3.67; N, 12.73. Found (%):
sity. The H and C chemical shifts are presented in ppm.
1
25 16 4 4
Coupling constants indicate JHH in H unless denoted otherwise.
ꢀ
1
C, 68.69; H, 3.55; N, 12.73. Selected IR data (KBr disc, cm ):
437 (b), 3199 (m), 3021 (w), 3070 (w), 1740 (s), 1634 (vs), 1600
s), 1212 (m).
The melting point of the prepared Schiff base ligand and decompo-
sition temperatures of its complexes were determined using a melt-
ing point apparatus (Gallenkamp, UK). The prepared Schiff base
ligand and its complexes were subjected to (C, H, N) elemental anal-
ysis which was carried out with a PerkinElmer model 240c elemen-
tal analyser. The molar conductivity measurements of freshly
3
(
Preparation of metal complexes
ꢀ
3
ꢀ4
ꢀ3
A solution of Cu(CH COO) ꢁH O (1.00 g, 5 mmol, 30 ml of ethanol)
3
2
2
prepared solutions of 1 × 10 and 4 × 10 mol cm in DMF were
measured using a Jenway 4320 conductivity meter at 298 K. The
magnetic susceptibility measurements of the investigated com-
plexes were evaluated at room temperature with a Gouy balance
or Ni(NO
3 2 2
) ꢁ6H O (1.45 g, 5 mmol, 30 ml of ethanol) was added to
a solution of HL (2.18 g, 5 mmol, 30 ml of ethanol). The resulting
mixture was stirred and refluxed with stirring for 1 h. A dark brown
precipitate in the case of copper and dark red precipitate in the
case of nickel was formed, which was filtered off, washed with eth-
[
1,2,6,7]
by making diamagnetic corrections using Pascal’s constant.
The effective magnetic moments, μ , per metal atom were calcu-
eff
qffiffiffiffiffiffiffiffi
2
anol and dried in vacuo over anhydrous CaCl .
′
lated from the equation μeff ¼ 2:83 χ T, where χ′
M
is the molar
M
NiL. Dark red; m.p. > 300°C; yield 75%. Anal. Calcd for [Ni
(C H N O )]ꢁ½H O (%): C, 51.49; H, 2.92; N, 12.01. Found (%): C,
susceptibility. Infrared (IR) spectra of the ligand and its metal che-
lates were monitored using a Shimadzu model 8101 spectrometer
25
17
5
8
2
ꢀ
1
ꢀ1
2
51.35; H, 2.77; N, 11.93. Molar conductance Λ (Ω mol cm )
m
ꢀ
1
ꢀ1
in the region 400–4000 cm using KBr discs with high resolution
in DMF: 2.07. Selected IR data (KBr disc, cm ): 3468 (b), 3207 (m),
3056 (w), 3028 (w), 1699 (s), 1618 (vs), 1569 (s), 1167 (m), 668 (w),
575 (w).
ꢀ
1
of 2 cm . UV–visible spectra of the prepared ligand and its
complexes were monitored using 10 mm matched quartz cells with
a PG model T+80 spectrophotometer. Thermal analyses were
recorded using a Shimadzu 60H for recording the thermody-
namic curves (thermogravimetric analysis (TGA), differential
thermogravimetry (DTG)) of the investigated solid compounds
CuL. Dark brown; m.p. > 300°C; yield 78%. Anal. Calcd for [Cu
(C H N O )]ꢁH O (%): C, 54.59; H, 3.71; N, 9.43. Found (%): C,
27
20
4
7
2
ꢀ
1
ꢀ1
2
51.48; H, 3.60; N, 9.31. Molar conductance Λ (Ω mol cm ) in
m
1
ꢀ
DMF: 4.46. Selected IR data (KBr disc, cm ): 3451 (b), 3199 (m),
3039 (w), 3050 (w), 1689 (s), 1600 (vs), 1570 (s), 1172 (m), 641(w),
581(w).
ꢀ
1
in nitrogen atmosphere (40 ml min ) at a heating rate of 10°-
ꢀ1
C min
from ambient temperature to 750°C. The values of
ꢀ
5
ꢀ3
absorbance of the complexes of 4 × 10 mol dm were mea-
sured at different pH values and were adjusted using a series of
Magnetic moment measurements
[18]
Britton universal buffer.
A HANNA211 pH meter was applied
for pH measurements at 298 K.
Magnetic susceptibility measurements supply information to sup-
port the electronic structure of the investigated complexes.
Preparation of Schiff base ligand
The Schiff base ligand, namely Z-3-((2-((E)-(2-hydroxynaphthyl)
methylene)amino)-5-nitrophenylimino)-1,3-dihydroindin-2-one
(
(
HL), was prepared as follows. 4-Nitro-o-phenylenediamine
5 mmol, 0.77 g) in 25 ml of ethanol was slowly added to ethanol
solution (25 ml) containing isatin (5 mmol, 0.74 g) followed by
slow addition of 2-hydroxy-1-naphthaldehyde (5 mmol, 0.86 g)
dissolved in 25 ml of ethanol. A dark orange precipitate was ob-
[
19]
tained on refluxing the solution for 3 h.
The precipitate was
Scheme 1. Structural formula of the prepared Schiff base ligand HL.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Ni(II) and Cu(II) complexes: synthesis and biological studies
Magnetic susceptibility was calculated according to the following
relation:
ꢀ
ꢁ
ꢀ
ꢂ
ꢃꢁ
[1,2,7,20,21]
logðW∞=ðW∞ ꢀ WÞÞ
AR
2RT
log
¼ log
1 ꢀ Eꢂ
2
ꢂ
T
φE
qffiffiffiffiffiffiffiffi
ꢂ
E
′
μeff ¼ 2:83 χ T
M
(1)
ꢀ 2
(4)
:303RT
∞
where W is the mass loss at the completion of the decomposition
′
reaction, W is the mass loss up to temperature T, R is the universal
gas constant and φ is the heating rate. Since 1 ꢀ 2RT/E* ≈ 1, a plot
of the left-hand side of equation (4) against 1/T would give a
straight line. E* was then calculated from the slope and the Arrhe-
nius constant, A, was obtained from the intercept. The other kinetic
parameters, S*, H* and G*, were calculated using the following
equations:
χ
¼ χ ꢀ ðdiamag:corr:Þ
(2)
M
M
where μeff is the magnetic moment (in Bohr magnetons), χ’
molar magnetic susceptibility, T the absolute temperature (K) and
after correction.
M
the
χ
M
Evaluation of stoichiometry of prepared complexes
The stoichiometry of the tested complexes was determined by ap-
Ah
[1,2,7,22]
ꢂ
plying the spectrophotometric molar ratio method
and con-
ΔS ¼ 2:303R log
(5)
KBT
[
5,6,24]
tinuous variation method,
as shown in Figs S4 and S5.
Evaluation of apparent formation constants of synthesized
complexes
ꢂ
ꢂ
ΔH ¼ E ꢀ RT
(6)
(7)
f
The apparent formation constants (K ) of the synthesized com-
plexes formed in solution were determined from spectrophotomet-
ꢂ
ꢂ
ꢂ
[
2,5,20,25]
ΔG ¼ H ꢀ TΔS
ric measurements using the continuous variation method,
according to the following relation:
where K
B
and h are Boltzmann’s and Planck’s constants,
respectively.
A=Am
Kf ¼
(3)
2
ð1 ꢀ A=AmÞ C
Computational aspects
where A is the arbitrarily chosen absorbance value on either side of
the absorbance mountain col. (pass), C is the initial concentration of
the metal and Am is the absorbance at the maximum formation of
With the increasing development of computational chemistry, den-
sity functional theory (DFT) has been extensively utilized due to its
accuracy and low computational cost to calculate a wide variety of
molecular properties and it has provided authoritative results which
are in accordance with experimental data. We attempted to recon-
noitre the optimized geometrical parameters (bond lengths, bond
angles and dihedral angles), net charges on active centres and en-
ergetics of the ground state for the unsymmetric tetradentate Schiff
base ligand HL and its complexes with Ni(II) and Cu(II) metals. In all
the complex. The obtained K values (Table 1) indicate the stability
f
of these complexes.
Kinetic studies of prepared complexes
Kinetic and thermodynamic studies of the thermal degradation
process are a powerful tool for providing sufficient knowledge
about Arrhenius parameters, i.e. frequency factor (A), entropy of ac-
tivation (ΔS*), enthalpy of activation ( H*) and free energy of activa-
tion (ΔG*). From TGA/DTG curves, the Coats–Redfern method was
[
27]
[28]
calculations, DFT was used at the B3LYP level, 6-311G ++ (d, p)
[
29]
as a basis set for the ligand and LANL2DZ as a basis set for the
complexes. All calculations were performed using the Gaussian
[
6,7,26]
[30]
employed to calculate the mentioned kinetic parameters:
09 W program.
f
Table 1. Electronic absorption spectral data, magnetic data, formation constant (K ) and stability constant for the prepared Schiff base ligand and its
complexes
Schiff base ligand/complex
HL
λ
max (nm)
Assignment
n → π*
μ
eff (BM)
Proposed geometry
K
f
Log K
f
460
—
—
—
—
3
3
2
96
n → π*
π → π*
π → π*
14
72
a
5
CuL
NiL
737 (b)
d–d band
d–d band
LMCT band
Intraligand band
π → π*
d–d band
d–d band
LMCT band
π → π*
1.87
3.10
Octahedral
Octahedral
8.24 × 10
5.92
5
4
3
3
12
84
93
25
5
741
4.44 × 10
5.65
550
437
251
a
b = broad
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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L. H. Abdel-Rahman et al.
transilluminator and photographed with a Panasonic DMC-LZ5
Lumix digital camera.
Biological activity
DNA studies
[
2,7,20,34]
Antimicrobial activity
Electronic spectra
The prepared Schiff base ligand and its complexes were screened
for their bactericidal and fungicidal activities by agar and potato
dextrose agar diffusion methods, respectively, against Staphylococ-
cus aureus, Escherichia coli and Pseudomonas aeruginosa bacteria
and Aspergillus flavus, Candida albicans and Trichophyton rubrum
fungi. Each compound was dissolved at different concentrations
The interaction of the studied complexes with DNA was conducted
in Tris–HCl buffer (50 mM, pH = 7.2) at room temperature to inves-
tigate the binding affinity between CT-DNA and the complexes. Be-
fore employing CT-DNA, it was purified by centrifugal dialysis. CT-
DNA solution in the buffer solution at pH = 7.5 gives a ratio of UV
absorbance >1.86 at 260 and 280 nm, indicating that the DNA
ꢀ
1
[
1,2,9,30,31]
of 10 and 25 mg ml in DMSO solvent. The bacteria were sub-
cultured in agar medium. Petri dishes were incubated for 24 h at
was sufficiently free from protein contamination.
centration of CT-DNA per nucleotide was determined by observa-
The con-
ꢀ
1
2
37°C. Standard antibacterial drug tetracycline was also tested under
tion the UV absorption at 260 nm using ε260 = 6600 mol cm .
The stock solution was stored at 4 °C and used within one day.
Absorption titration was carried out by varying the concentration
of CT-DNA (3–30 μM), while keeping the metal complex concentra-
similar conditions for comparison. The fungi were sub-cultured in
potato dextrose agar medium. Standard antifungal drug flucona-
zole was utilized for comparison. Petri dishes were incubated for
ꢀ
5
48–72 h at 35°C. The minimum inhibitory concentration for each
tion (10
M) constant. The absorption of free CT-DNA was
tested substance was determined by microscopic observation of
microbial growth. It corresponds to the well with the lowest con-
centration of the tested substance where microbial growth was
clearly inhibited. The zones of inhibition based upon zone size
around the wells were measured. The activities of the prepared
complexes were determined by calculating the activity index (Table
eliminated by adding equimolar CT-DNA to pure buffer solution
in the reference compartment and the resulting spectra were be-
lieved to result from the metal complexes and the DNA–metal
complex aggregates. The titration processes were repeated until
there was no change in the spectra, indicating binding saturation
had been achieved. The absorption data were estimated for an
[
1,2,20,35]
S5) according to the following relation:
b
evaluation of the intrinsic binding constant (K ) of the complexes
[32]
with CT-DNA.
Inhibition zone of complex ðmmÞ
Activity index ðAÞ ¼
ꢃ100
Viscosity measurements
Inhibition zone of standard drug ðmmÞ
(8)
Viscosity experiments were accomplished using an Ostwald micro-
viscometer and a water bath maintained at constant temperature
(
25 ± 1°C). The DNA concentration was kept constant in all samples,
but the complex concentration was increased each time Results and discussion
10–250 μM). Mixing of the solution was achieved by bubbling ni-
trogen gas through viscometer. The mixture was left for 10 min at
7°C after addition of each aliquot of complex. The flow time was
repeatedly measured with an accuracy of ±0.2 s with a digital stop-
(
Characterization of Schiff base ligand
3
The tetradentate Schiff base ligand HL was prepared by the reac-
tion of 4-nitro-o-phenylenediamine with isatin and 2-hydroxy-1-
naphthaldehyde in ethanol. The structure of the ligand was con-
firmed using H NMR, C NMR, IR and UV–visible spectroscopies, el-
emental analysis and theoretical studies. The structure of the ligand
is represented in Scheme 1. Unfortunately, single crystals of the
studied ligand could not be obtained because of lack of solubility
in most organic solvents.
0
1/3
watch. Data are presented as (η/η ) versus the ratio [complex]/
0
1
13
[DNA] = R, where η and η are the specific viscosities of DNA in
the presence and absence of complex, respectively. The values of
0
η and η were calculated using the following equation:η = (t ꢀ t
o o
)/t ,
where t
o
is the observed flow time of DNA-containing solution and t
[
1,5–7,33,34]
is the flow time of buffer alone.
were calculated from the ratio (η/η ).
Relative viscosities of DNA
0
1
13
H NMR and C NMR spectra
Agarose gel electrophoresis of DNA interaction with investigated
complexes
1
The synthesized ligand was characterized from its H NMR spec-
trum (400 MHz) in DMSO-d . The data are presented in the Experi-
6
mental section and in Figs S1 and S2. All the spectra are in good
Agarose was purchased from Fischer-Biotech (GC Health care). CT-
DNA and Ready-load 100 bp DNA ladder were used as the native-
size DNA and were purchased from Bio Labs. The samples were
subjected to electrophoresis on 1% agarose gel prepared in TBE
buffer (45 mM Tris, 45 mM boric acid and 1 mM EDTA; pH = 7.3),
with 20 μl of each of the incubated complexes and 20 μl of DNA.
The mixture was incubated at room temperature for 30 min at
1
agreement with the proposed structure of the ligand. The
H
NMR spectrum of the synthesized Schiff base ligand shows
distinguishing NMR signals. The spectrum of HL shows a singlet sig-
nal at 14.58 ppm for OH proton. The signal due to the azomethine
1
proton ( HC N) is found at 10.82 ppm. The H NMR spectra of the
complexes cannot be obtained due to interference of their para-
magnetic properties.
37°C. After that, it was loaded on the gel with tracking dye (0.25%
bromophenol blue). The electrophoresis was performed at constant
voltage (100 V) for about 2 h (until the bromophenol blue had
passed through 50% of the gel) in TBE buffer. At the end of electro-
phoresis, i.e. the end of DNA migration, the electric current was
switched off. Then, the gel was stained by immersing it in water
Characterization of metal complexes
Elemental analyses and molar conductance measurements
Interaction of copper acetate and nickel nitrate with HL in ethanol
gives dark brown and red powder complexes, respectively. The iso-
lated solid complexes were characterized using elemental analyses,
ꢀ
1
containing ethidium bromide (0.5 μg ml ) for 30–45 min at room
temperature and later visualized under UV light using a
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Ni(II) and Cu(II) complexes: synthesis and biological studies
IR and UV–visible spectroscopies, molar conductance, TGA and
magnetic susceptibility. All complexes are stable in air at room tem-
perature, non-hygroscopic, insoluble in water and soluble in polar
coordinating solvents such as DMSO and DMF. The stoichiometries
and symmetric stretching vibration motions Δν (= νasy(COO) ꢀ νsy
ꢀ1
(COO)) is found to be 179 cm , which matches with monodentate
[
41]
ligation.
The characteristic frequencies of the coordinating ni-
ꢀ
1
trate group in the nickel complex appear at 1423 cm (ν(NO )asy),
2
ꢀ1
of the complexes derived from elemental analyses correspond to
1311 cm (ν(NO
two highest frequency bands ( ν = νasy ꢀ νsy) is 112 cm , indicating
that nitrate ion (NO ) in the solid complex coordinates to Ni(II) ion in
a unidentate manner. These complexes also show weak bands at
820–890 cm due to the presence of rocking mode of coordinated
water molecules.
2
)sy) and 1085 (ν(NO)). The difference between the
ꢀ
ꢀ1
the general formula [M(L)(X)(H
NO
3
2
O)]ꢁnH
2
O, where X = CH
3
COO or
ꢀ
and n = number of hydrated water molecules (Scheme 2).
3
[42]
These proposals are also in accord with molar conductance, IR,
TGA and UV–visible results.
Conductivity measurements have frequently been used in struc-
ture elucidation of metal chelates, i.e. the possible mode of bonding
within the limit of their solubility; they also provide a method of
testing the degree of ionization of complexes. The molar conductiv-
ꢀ
1
[43]
The presence of coordinated water is also
established and supported from the TGA results for these
complexes.
ities (Λ ) of Cu(II) and Ni(II) complexes in DMF solution at 25 °C were
Electronic spectra and magnetic properties of metal complexes
m
measured. The low values of Λm reveal that the acetate and nitrate
groups remain in the coordination sphere.
The UV–visible spectra of HL and its metal complexes were re-
corded in DMF solution at 298 K. Relevant electronic spectral data
are presented in Table 1 and Fig S3. The UV–visible spectrum of
HL shows three bands at 314, 396 and 460 nm. The first band at
[
36]
IR spectroscopy
IR spectra of HL and its complexes were recorded in KBr pellets
3
14 nm is assigned to π → π* transitions which are due to transi-
tions from the benzene ring. The second band observed at
96 nm is most probably due to n → π* transition for the electrons
ꢀ
1
from 400 to 4000 cm . These bands give fundamental information
about the nature of the functional groups attached to the metal
3
[44]
[37]
localized on the (C N) chromophore. The third band observed at
atoms. The IR spectra of HL and its complexes are found to be
quite complex as they in general exhibit a large number of bands
of varying intensities. The spectrum of HL shows peaks of
4
60 nm is assigned to n → π* transition originating from donating
[45]
groups of the Schiff base ligand. On complexation this band dis-
appears, suggesting the coordination of azomethine nitrogen to
the metal ion, as the formation of the metal–nitrogen bond stabi-
ꢀ
1
azomethine group CH N at 1634 and 1600 cm , which are shifted
to lower frequencies in the spectra of the complexes within the
[46]
ꢀ
1
lizes the electron pair on the nitrogen atom.
ranges 1600–1618 and 1569–1573 cm . This result indicates that
Thus, addition of metal ion to ligand solution causes characteris-
the nitrogen atom of the two CH N shares the coordination with
[
1,2,7,20,26,37]
tic changes in the visible absorption spectrum of the ligand, sug-
metal ions.
C O) in the free ligand is located at 1740 cm , which is shifted
to lower frequencies in the spectra of the complexes indicating
The stretching vibration motion of indole
[47]
ꢀ
1
gesting an immediate complex formation in solution.
The
(
designed complexes display a characteristic band centred at
λmax = 251–393 nm, due to an intramolecular charge transfer tran-
[
31]
the involvement of (C O) in the coordination. The ligand shows
ꢀ
1
sition taking place in the complexed ligand. Moreover, there is a
band appearing in the region 437–484 nm, which can be attributed
to charge transfer from ligand to metal. Furthermore, the d → d
transition band is observed from 515 to 741 nm. This band can be
mainly attributed to transitions in an octahedral structure of the
prepared complexes. The Cu(II) complex shows bands at 737 and
a peak at 3199 cm which is assigned to ν(NH) of amide group;
upon complex formation this peak remains at the same frequency
or slightly shifted from its position suggesting no participation of
[38]
ꢀ1
(
NH) in bonding.
The appearance of a band at 3437 cm in
the IR spectrum of HL can be assigned to ν(OH). This band disap-
pears in the spectra of the complexes but is replaced in the same
2
2
ꢀ
1
515 nm, which can be attributed to a E
istic of tetragonally distorted octahedral geometry.
complex exhibits two bands at 741 and 550 nm, attributed to
g g
→ T transition character-
place by other bands within 3451–3468 cm assigned to the
[
48]
The Ni(II)
stretching motion of uncoordinated water molecules. The phenolic
ꢀ
1
(
C O) stretching vibration appears at 1212 cm for the free ligand,
3
3
3
3
ꢀ
1
A g → T g(F) and A g → T g(F) transition geometry around
2 2 2 2
shifted towards lower frequencies (1167–1172 cm ) for the com-
[
49]
the Ni(II) ion.
plexes. This shift confirms the involvement of the oxygen atom of
Magnetic measurements were recorded at room temperature.
[
39]
phenolic group in the chelation with C O M. In the case of the
metal complexes, there are number of medium to weak bands in
The effective magnetic moment (μ ) values of the complexes are
eff
ꢀ
1
given in Table 1. The μeff value at room temperature of the Ni(II)
complex is 3.10 BM, which is in the normal range observed for oc-
tahedral Ni(II) complexes. This indicates that the Ni(II) complex is
the two ranges 641–668 and 475–581 cm which are assigned
to the ν(M O) and ν(M N) stretching frequencies, respectively. Two
essentially strong to medium bands are seen at 1520 and
[
49]
ꢀ
1
probably octahedral.
On the other hand, the μeff value (1.87
1
341 cm which can be assigned to ν (COO) and ν (COO) vibra-
asy sy
[40]
BM) for the Cu(II) complex falls within the range normally observed
tions of carboxylate ion. The difference between the asymmetric
[
50]
for tetragonally distorted octahedral Cu(II) complexes.
Evaluation of stoichiometry of prepared Schiff base ligand complexes
The stoichiometry of the tested complexes was determined by ap-
[2,20]
plying the spectrophotometric continuous variation
ratio methods.
and molar
The curves of continuous variation method
display maximum absorption at mole fraction Xligand = 0.56 for Ni
II) and Cu(II) complexes, which indicates the formation of 1:1 metal
[
5,6,20,23]
(
ion to ligand complexes as presented in Scheme 2. The maximum
change will occur when the mole ratio of the reactants is close to
the optimum ratio which is the stoichiometric ratio in the chemical
Scheme 2. Suggested structures of CuL and NiL.
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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L. H. Abdel-Rahman et al.
equation. Moreover, the data resulting from applying the molar ra-
tio method support the same metal ion to ligand ratio of the pre-
pared complexes (Figs S4 and S5).
iii. The positive values of G* for the investigated complexes reveal
increases significant for the subsequently decomposition stages
due to the increasing values of T S* from one step to another
which override the values of H*. This increase reflects that the
rate of removal of the subsequent ligand will be lower than that
Evaluation of apparent formation constants of synthesized complexes
[53]
The obtained apparent formation constants indicate the high sta-
of the preceding ligand.
bility of the prepared complexes. The values of K
complexes decrease according to the following sequence:
CuL > NiL. Moreover, the values of log K (stability constant) of
f
for the prepared
Theoretical calculations
From the elemental analysis and spectroscopic data, Cu(II) and Ni
f
the investigated complexes are presented in Table 1.
(
II) metals were expected to be coordinated to the ligand through
Stability range of investigated complexes
a nitrogen atom (N1) and an oxygen atom (O9) forming a six-
member ring, through a nitrogen atom (N1) and a nitrogen atom
(N10) forming a five-member ring and through a nitrogen atom
(N10) and an oxygen atom (O13) forming a five-member ring
(Fig. 1).
The stability range of the studied complexes was found to be in the
pH range of 4–10 for CuL and 4–11 for NiL according to the ob-
tained pH–absorbance curves (Fig. S6). This means that M(II) ion
greatly stabilizes the Schiff base in this range. Accordingly, these li-
gands can be used as masking reagents of M(II) ions in that range of
pH.
Geometric parameters of ligand
Thermal analysis and kinetic and thermodynamic parameters of
complexes
The optimized structure of the ligand (anion), numbering system
and vector of the dipole moment are depicted in Fig. 1(a). The
energy of the highest occupied molecular orbital EHOMO, energy
of the lowest unoccupied molecular orbital ELUMO, energy gap
The thermal decomposition data for the two complexes are sum-
marized in Table 2. The results show good agreement with the for-
mulas suggested from the analytical data. TGA of Cu(II) and Ni(II)
complexes exhibits a weight loss at 110 and 125°C, respectively, in-
dicating the elimination of lattice water molecules from the com-
plexes. Whereas the coordinated water molecules are removed at
g
E and the dipole moment computed by B3LYP/6-311G++(d, p)
for the neutral ligand and its anion are presented in Table S1. In
addition, the computed geometric parameters were compared
with the corresponding values obtained from the experimental
data. The optimized bond length (Table S3) of C C in phenyl rings
of the ligand falls in the range 1.379–1.425 Å which is greater
than the experimentally determined value of 1.339 Å; for the
C O bond, the optimized length of 1.236 Å is slightly shorter than
the experimental value of 1.243 Å; and the optimized C N bond
length of the ring fall in the range 1.397–1.399 Å which is shorter
[51]
2
15 and 220°C. Then, the ligand decomposes gradually to the
corresponding metallic residue at higher temperature. These results
are consistent with the composition of the two complexes.
In continuation of the thermal investigation, the thermo-kinetic
parameters of the decomposition steps for the metal complexes
are listed in Table 2. From the obtained data, the following remarks
can be made:
[
54]
than the experimental value of 1.344 Å.
The bond angles for
DFT-B3LYP/6-311G++(d, p) reported are slightly better than those
for the HF method compared to the experimental results. The
computation underestimates the ∠NCC and ∠OCO angles. The li-
gand and its anion are non-planar with naphthyl and indolin-2-
one moieties out of the molecular plane of the 5-nitrophenyl ring
by 55°. The differences between calculated and measured values
i. The S* values for the complexes are found to be negative. This
indicates that the activated complex is more ordered than the
[
52]
reactants and/or the reactions are slow.
ii. The negative values of H* mean that the decomposition pro-
cesses are exothermic.
Table 2. Thermal, kinetics and thermodynamic parameters for metal complexes
a
b
ꢀ1
4
ꢀ1
ꢀ1
ꢀ1 ꢀ1
ꢀ1
Compound
CuL
T (°C)
Weight loss (%) Proposed segment E* (kJ mol ) A (× 10 s
)
H* (kJ mol
)
S* (kJ mol
K
)
G* (kJ mol
)
35–110
10–215
15–367
3.05 (3.03)
3.06 (3.03)
9.96 (9.94)
H
2
O
O
0.11
0.71
ꢀ1.06
ꢀ1.36
ꢀ2.22
ꢀ3.33
ꢀ3.78
ꢀ0.167
22.43
28.75
45.87
68.71
78.14
1
2
3
4
H
2
ꢀ0.169
ꢀ0.171
ꢀ0.173
ꢀ0.174
CH
3
COO
NO
6 3 2
O + C H NO
67–513 28.50 (28.47)
59–749 44.90 (44.82)
C H
11 7
C
8
H
5
N
2
c
Final residue (%)
NiL
>749
39–125
25–222
10.53 (10.71) Cu
1.60 (1.54)
3.20 (3.09)
0.5H
2
O
0.11
1.93
ꢀ1.63
ꢀ1.61
ꢀ1.64
ꢀ1.65
ꢀ1.66
ꢀ1.69
ꢀ0.159
ꢀ0.161
ꢀ0.164
ꢀ0.165
ꢀ0.166
ꢀ0.169
31.96
38.19
55.60
61.77
72.69
102.97
1
2
3
4
5
2
H O
22–395 10.82 (10.64) NO
3
95–478 24.94 (24.88)
78–566 29.11 (29.00)
66–749 20.05 (20.77)
C
C
C
8
5 2
H N O
11
7
H NO
6
H
NO
3 2
c
Final residue (%) >749
10.28 (10.08) Ni
a
Temperature range of decomposition pathways
b
Weight loss found (calculated)
c
Residue found (calculated)
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Appl. Organometal. Chem. (2016)

Ni(II) and Cu(II) complexes: synthesis and biological studies
Geometric parameters of complexes
Tables S1–S3 and Figs 1(b) and (c) present the optimized geometry,
numbering system, vector of the dipole moment, energetics, dipole
moment, energy gap, net charges on active centres, bond lengths,
bond angles and dihedral angles of all metal complexes studied in
8
this work. The electronic configuration for Ni(II) is 3d and for Cu(II)
9
is 3d . In all complexes, the metal ion coordinates with N1 and O9
to form a six-member ring and with N10 and O13 to form a five-
member ring. Therefore, distortion from regular octahedral geome-
try is expected for all the studied complexes.
The geometric changes that are observed in the ligand moiety it-
self are interesting. Thus, most bonds show elongation upon com-
plexation with the metal ion. The length of the coordinate covalent
bonds between metal and ligand site, i.e. M N and M O, are too long
compared to the typical M X bond lengths which have the values
[
54]
Cu O 1.62 Å, Cu N 1.66 Å and Ni N 1.44 Å.
The too long M O
and M N bonds in the complexes mean that the ionic character of
these bonds is small. Also, the charge on the metal ion in the com-
plex is much less than 2 in Ni(II) and Cu(II); hence, the comparison
between the calculated (B3LYP/6-31G++(d, p) and the typical M
(II) N and M(II) O is not very precise.
The calculated values of bond angles (between metal ion and
binding sites) (Table S3) O9MN1, N1MO25, O25MN10, N10MO13
and O13MO21 vary between 85° and 132° which compare nicely
with the experimental data as obtained from X-ray analysis for O
h
[54]
Cu complexes, which indicate a distorted octahedral geometry.
The values of the dihedral angles around metal ion, i.e. MN2C28C30,
MO14C13N17, MO23N24O25 and MO10C9C4 (Table S3), are far
from 0° or 180° which indicate that the metal ion is not in the same
plane as the donating sites. All the studied complexes are polar, as is
evident from the magnitudes of their dipole moment (Table S1). As
the energy gap of the studied complexes decreases, the reactivity of
the complexes increases. The reactivity of the Cu(II) complex is
greater than that of the Ni(II) complex.
Charge distribution analysis
In the NiL and CuL complexes, Ni and Cu metal ions end up with
net charges of 0.5562 and 0.6772 indicating that the two metal
ions receive 1.4438 e and 1.3228 e from the surrounding ligand,
respectively. This donation takes place mainly through the
σ-orbital. The amount of electronic charge received by each
metal ion in the studied complexes is presented in Table S2.
The relative charges accumulated on each metal ion in the com-
plexes are expected in terms of the size and electronegativity of
the metal ion. There is no electron back-donation from the metal
ion to the donating sites (O9, O13, N1 and N10) as indicated
from the net charge on the donating sites before and after
complexation (Table S2).
Figure 1. Optimized geometry, numbering system and vector of the dipole
moment of compounds using B3LYP/6-31G++(d, p): (a) Schiff base ligand
anion; (b) NiL complex; (c) CuL complex.
can be explained by the fact that the calculations assume an iso-
lated molecule where the intermolecular columbic interactions
with neighbouring molecules are absent, where the experimental
results correspond to interacting molecules in the crystal lattice
for a similar compound. The ligand in its anion form is consid- Biological evaluation of synthesized
ered as the most electron-donating as indicated from computed
complexes
g
EHOMO (Table S1). The computed energy gap E , which measures
the reactivity, shows that free ligand in its anion form is the most
reactive ligand (smallest value). The computed dipole moment
shows that the anion form of the ligand is less polar than the
neutral form by 1.33 D (Table S1). From the computed net
charge on active centres of the ligand, free ligand in its anion
form, it is found that the most negative centres are N27, O35,
N7 and O13 which are the centres chelated with the metal to
form the complex.
DNA binding studies
Electronic spectral studies
The interactions of metal complex with DNA have been a topic of
interest for the development of effective chemotherapeutic agents.
Transition metal centres are particularly attractive moieties for such
research since they exhibit well-defined coordination geometries
and also often have distinctive electrochemical or photophysical
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

L. H. Abdel-Rahman et al.
properties, thus enhancing the functionality of the binding agent.
Currently, the spectrophotometric method for DNA interaction ap-
pears to be the most commonly used for determination of DNA-
binding constants of metal complexes.
the lowest energy bands at about 403 nm are attributed to the
metal-to-ligand charge transfer transitions. The absorption spec-
tra show clearly that the addition of DNA to the Cu(II) complex
yields a significant hypochromism and a slight blue shift at the
ligand-to-metal charge transfer (LMCT) band. Distinctly, these
spectral characteristics suggest that all compounds have some
interaction with DNA. These spectral characteristics indicate that
the complex interacts with DNA most likely through a mode that
involves electrostatic or hydrophobic interaction. As stated
before, CuL complexes have a replaceable acetate ligand in solu-
Transition metal complexes can bind to DNA via covalent (re-
placement of a labile ligand of the complex by a nitrogen base
of DNA, e.g. guanine N7) and/or non-covalent (intercalation, elec-
[55]
trostatic or groove binding) interactions.
The binding of an
intercalative complex molecule to DNA has been well elucidated
by notable intensity decrease (hypochromism) and shift of the
electronic spectral bands because of strong stacking interaction
between the aromatic chromophore of the ligand and DNA base
pairs. The degree of hypochromism and the amount of the shift
[
58]
tion, which may be replaced with H O molecules in solution.
2
So the complex contains a positive charge on the copper atom
in the middle. Due to the removal of acetate ligand from the
complex in solution, the CuL complex will have a flat part in
the middle (tetradentate square planar). Therefore, possible inter-
action of the Cu(II) complex with DNA could be as follows
(Scheme 3):
[
56]
depend on the strength of the intercalative interaction.
the other hand, metal complexes which do not intercalate or in-
On
[
57]
teract electrostatically with DNA may exhibit hyperchromism.
ꢀ
5
Fixed amounts (10 M) of metal complexes were titrated with
increasing amounts of CT-DNA in the range from 3 to 30 μM.
The electronic spectra of the Cu(II) complex in the absence
and presence of CT-DNA are shown in Fig. 2(a). The sharp bands
at about 246 nm are assigned to intraligand π–π* transitions and
•
•
First, interaction of Cu(II) complex with base backbone of DNA or
even coordination of Cu with base pairs of DNA.
2+
Second, insertion of the flat part of the complex between the
2+
base pairs and consequently coordination of Cu with base
pairs of DNA.
In order to quantitatively compare the binding affinity of the
compounds with CT-DNA, the intrinsic binding constants (K ) of
b
the compounds were determined by monitoring the changes oc-
curring in absorption at the LMCT band of the Cu(II) complex with
increasing concentration of CT-DNA using the following functional
[
1,2,7,20]
equation:
½
DNAꢄ
½DNAꢄ
εb ꢀ εf
1
¼
þ ½Kbðεb ꢀ εfÞꢄꢀ
(9)
εa ꢀ εf
ꢀ
1
where [DNA] is the concentration of DNA in M (nucleotide), ε is
a
the molar absorption coefficient of the compound at a given DNA
concentration, ε is the molar absorption coefficient of the com-
f
pound in free solution, ε is the molar absorption coefficient of
b
the compound when fully bound to DNA and K is the equilibrium
b
ꢀ
1
binding constant in M . In a plot of [DNA]/(ε ꢀ ε ) versus [DNA]
a
f
(
Fig. 2(b)), K is given by the ratio of the slope to the intercept.
b
The K values of Ni(II) and Cu(II) complexes are determined to be
b
6
6
ꢀ1
1
.06 × 10 and 2.15 × 10 M , respectively. However, the deter-
mined K values are higher than those observed for typical classical
intercalators (ethidium bromide: K
HCl/40 mM NaCl buffer, pH = 7.2). Hence, it is obvious that the
present metal complexes are involved in strong intercalative
interactions.
b
6
ꢀ1
b
[
= 1.4 × 10 M in 25 mM Tris–
59]
Viscosity studies
Although spectral techniques are necessary, they are not sufficient
to support a binding mode. In the absence of crystallographic struc-
tural data, hydrodynamic measurements are considered as least
obscure and the most critical test for determining the binding
[
60]
mode of DNA with small molecules.
Viscosity measurements
are sensitive to changes in the DNA molecule. The sensitivity of this
method largely relies on the changes in the DNA length that occur
as result of its different binding modes with guest molecules. In
classical intercalations, the DNA helix lengthens as base pairs are
disconnected to accommodate the bound guest leading to increas-
ꢀ
5
Figure 2. (a) Spectrophotometric titration of CuL complex (10 M) in
.01 M Tris buffer (pH = 7.4, 25 °C) with CT-DNA from 3 to 30 μM. The
[1,2,61]
ing DNA viscosity,
whereas in groove binding or electrostatic
0
mode, the length of the helix is unchanged resulting in no apparent
alteration in DNA viscosity. In contrast, compounds that bind
arrow indicates increasing amount of DNA. (b) Plot of [CT-DNA]/(ε
a
f
ꢀ ε )
versus [CT-DNA] for the titration of CT-DNA with CuL complex.
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Copyright © 2016 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2016)

Ni(II) and Cu(II) complexes: synthesis and biological studies
Scheme 3. Suggested mechanism for interaction of CuL with DNA (a) via electrostatic binding and (b) via intercalation binding.
exclusively in the DNA groove by partial and/or non-classical inter-
calation under the same conditions, typically give rise less pro-
basis of viscosity results, the compounds bind with DNA through in-
tercalation mode. The increased degree of viscosity which may de-
pend on affinity to DNA follows the order CuL > NiL. Thus, the
viscosity measurements are consistent with the results of electronic
spectra.
[
62]
nounced or no change in the DNA solution viscosity. A classical
intercalation mode causes a significant increase in viscosity of
DNA due to an increase in separation of base pairs at intercalation
sites and hence an increase in overall DNA length. In order to fur-
ther clarify the nature of the interaction between the complexes
and DNA, viscosity measurements were carried out and are shown
Gel electrophoresis
1/3
in Fig. 3. Plots of relative viscosity (η/η
0
)
versus [complex]/[DNA]
Gel electrophoresis is a widely used technique for studying the
binding of compounds with nucleic acids: in this method separa-
tion of molecules will be on the basis of their relative rate of move-
ment through a gel under the influence of an electric field. DNA is
negatively charged and when it is placed in an electric field, it mi-
grates towards the anode; the extent of migration of DNA is
ratio (0–0.6) illustrate a significant increase in the relative viscosity
of DNA on increasing concentration of complex, which is similar
[
20,26]
to that for intercalator ethidium bromide.
This may be ex-
plained by the insertion of the complexes between base pairs at in-
tercalation sites and thus an increase in overall DNA length. On the
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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L. H. Abdel-Rahman et al.
Table 3. Spectral parameters for DNA interaction with the prepared
complexes
Complex
λ
max
λ
max
n
K
b
(×
Type of Chromism G*
a
6
ꢀ1
(
free) (bound)
10 M ) chromism
(%)
(kJ mol
ꢀ1
(
nm)
245
65
246
03
(nm)
)
NiL
243
439
243
401
2
1.00 hypo
hypo
32.80 ꢀ33.51
60.75 ꢀ36.81
11.15 ꢀ33.51
ꢀ36.12
4
26
1
CuL
2.15 hypo
hypo
4
2
36.26
a
Chromism (%) = (Afree ꢀ Abound)/Afree
Antimicrobial activity
The free ligand and metal complexes were tested against
bacterial and fungal species by measuring the size of the
bacteriostatic diameter. The results are presented in Table S4,
and it is evident that the tested compounds show marked
bactericidal and fungicidal activities against various types of
bacteria (S. aureus, E. coli and P. aeruginosa) and fungi (A. flavus,
C. albicans and T. rubrum).
Figure 3. Effect of increasing amount of synthesized complexes on relative
viscosities of DNA at [DNA] = 0.5 mM at 298 K.
determined by the strength of the electric field, buffer, density of
agarose gel and size of DNA. Generally it is seen that mobility of
DNA is inversely proportional to its size. Photographs of the gel
show bands with different band widths and brightnesses com-
pared to the control. The interaction of the prepared complexes
with DNA was studied by agarose gel electrophoresis and the re-
sults are represented in Fig. 4 (lanes 2 and 3 are CT-DNA + metal
complex; while lanes 1 and 4 are CT-DNA and blank). The diversity
in DNA-cleavage efficiency of the investigated complexes is attrib-
Antibacterial activity
The evolution of new bacterial strains resistant to current antibi-
otics has become critical problem in public health. Therefore,
there is a strong stimulus to develop new bactericides. So, nowa-
days pharmaceutical industries are seeking for alternative com-
pounds which behave as drugs. Currently much attention has
been focused on the synthesis of new metal complexes and esti-
mating their antibacterial activity. In the present work, we ex-
plored the activity of the Schiff base ligand and its metal(II)
complexes against three different human pathogens. The diame-
ter of the zone of inhibition (mm) was used to contrast the antimi-
crobial activity of the complexes with that of a commercial drug
[
20,63]
uted to their difference in binding ability to DNA.
The intensity
of lanes is decreased in the sequence CuL > NiL and these results
are in a good agreement with values of binding constant of the
tested complexes with CT-DNA (Table 3).
(tetracycline). The results show that complexes exhibit varying de-
grees of inhibitory effects on the growth of bacterial strains, which
may be due to the effect of the metal ion on cell metabolism.
Even though DMSO is an antimicrobial solvent, here we per-
formed our antibacterial tests by employing DMSO as a negative
control. Further, control experiments using DMSO alone do not
show any antibacterial effect. Hence, it is noticed that the com-
plexes are more potent bactericides than the free Schiff base. This
higher antimicrobial activity of the metal complexes compared to
the Schiff base may be due to the alteration in structure due to
coordination and chelating tends to make metal complexes act
as more powerful and potent bacteriostatic agents, thus blocking
[
6,20,27,63]
the growth of the microorganisms.
Moreover, coordina-
tion reduces the polarity of the metal ion mainly because of the
partial sharing of its positive charge with the donor groups within
[
64]
the chelate ring system created during the coordination.
This
process, in turn, increases the lipophilic nature of the central metal
atom, which supports its permeation more efficiently through the
lipid layer of microorganisms, thus destroying them more aggres-
sively. The results (Table S4; Fig. 5) show that the Cu(II) complex
has higher antibacterial activity than the Ni(II) complex. The
diversity in the activity of the complexes against different microor-
ganisms depends either on the impermeability of the cells of the
microbes or differences in ribosomes in microbial cells.
Figure 4. Gel electrophoresis pattern showing the interaction of the
investigated complexes with DNA. Lane 1: CT-DNA; lane 2: CT- DNA + CuL;
lane 3: CT-DNA + NiL; lane 4: blank.
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Appl. Organometal. Chem. (2016)

Ni(II) and Cu(II) complexes: synthesis and biological studies
been estimated using the Coats–Redfern method. The synthesized
complexes interact with DNA via intercalation mode and exhibited
higher binding constant than a classical intercalator. The antimicro-
bial screening assay of the compounds revealed variable activities
towards the studied bacterial and fungal organisms.
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