Spectroscopic, DNA binding ability, biological activity, DFT calculations and non linear optical properties (NLO) of novel Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) complexes with ONS Schiff base

Article abstract of DOI:10.1016/j.molstruc.2017.03.081

The reaction of Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) with the synthesized N-(2-hydroxy-1-naphthylidene)-2-aminothiophenol Schiff base ligand (H₂L) at room temperature resulted in the formation of the five complexes; [Co(HL)₂]H₂O, 1; [M(HL)₂] (M?=?Cu, Zn and Cd), (2–4) and [Hg(HL)Cl], 5. The ligand and its complexes were characterized based on elemental analyses, IR, ¹H NMR, magnetic measurement, molar conductance, and thermal analysis. Coats and Redfern method was used to compute the kinetic and thermodynamic parameters. Antimicrobial activities of H₂L and its complexes have been studied. The binding of Co(II), Cu(II) and Zn(II) complexes to calf thymus DNA (CT-DNA) has been investigated using UV–Vis and fluorescence absorption spectra. The results indicated that the ligand and its complexes may bind to DNA by intercalation modes, with a much higher binding affinity of the complexes than that of the ligand. The equilibrium geometries of the studied complexes are investigated theoretically at the B3LYP/LANL2DZ level of theory, and it was found that these geometries are non-linear. The calculated E_HOMO and E_LUMO energies of the studied complexes can be used to calculate the global properties. The calculated nonlinear optical parameters (NLO); first order hyperpolarizibility (β) of the studied complexes show promising optical properties.

Full text of DOI:10.1016/j.molstruc.2017.03.081

Accepted Manuscript
Spectroscopic, DNA Binding Ability, Biological, DFT Calculations, and Non Linear
Optical Properties (NLO) of Novel Co(II), Cu(II), Zn(II), Cd(II) and Hg(II)
Complexes with ONS Schiff base
Ayman A. Abdel Aziz, Fatma M. Elantabli, H. Moustafa, Samir M. El-Medani
PII:
S0022-2860(17)30369-1
10.1016/j.molstruc.2017.03.081
MOLSTR 23577
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
03 January 2017
02 March 2017
20 March 2017
Please cite this article as: Ayman A. Abdel Aziz, Fatma M. Elantabli, H. Moustafa, Samir M. El-
Medani, Spectroscopic, DNA Binding Ability, Biological, DFT Calculations, and Non Linear Optical
Properties (NLO) of Novel Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) Complexes with ONS Schiff base,
Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.03.081
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ACCEPTED MANUSCRIPT
High light (for review)


Spectroscopic characteriztion of synthesized ligand and its complexes.
UV-Vis spectroscopy and fluorescence studies suggest the binding of
complexes to DNA.



The complexes have antimicrobial activities and high thermal stability.
The HOMO and LUMO energies were computed and discussed.
The NLO parameters of the complexes show promising optical properties.

ACCEPTED MANUSCRIPT
Spectroscopic, DNA Binding Ability, Biological, DFT Calculations, and Non Linear Optical
Properties (NLO) of Novel Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) Complexes with ONS
Schiff base
a
b
c
b,
Ayman A. Abdel Aziz , Fatma M. Elantabli , H. Moustafa and Samir M. El-Medani *
a Department of Chemistry, Faculty of Science, Ain Shams University, Cairo 11566, Egypt.
b Department of Chemistry, Faculty of Science, Fayoum University, El-Fayoum, Egypt.
c Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt.
Abstract
The reaction of Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) with the synthesized N-(2-hydroxy-
1
-naphthylidene)-2-aminothiophenol Schiff base ligand (H L) at room temperature resulted in
2
the formation of the five complexes; [Co(HL) ]H O, 1; [M(HL) ] (M= Cu, Zn and Cd), (2-4)
2
2
2
and [Hg(HL)Cl], 5. The ligand and its complexes were characterized based on elemental
1
analyses, IR, H NMR, magnetic measurement, molar conductance, and thermal analysis. Coats
and Redfern method was used to compute the kinetic and thermodynamic parameters.
Antibacterial activities of H L and its complexes have been studied. The binding of Co(II),
2
Cu(II) and Zn(II) complexes to calf thymus DNA (CT-DNA) has been investigated using UV-
Vis and fluorescence absorption spectra. The results indicated that the ligand and its complexes
may bind to DNA by intercalation modes, with a much higher binding affinity of the complexes
than that of the ligand. The equilibrium geometries of the studied complexes are investigated
theoretically at the B3LYP/LANL2DZ level of theory, and it was found that these geometries are
non- linear. The calculated EHOMO and ELUMO energies of the studied complexes can be used to
calculate the global properties. The calculated nonlinear optical parameters (NLO); first order
hyperpolarizibility (β) of the studied complexes show promising optical properties.
Keywords: Asymmetric Schiff base; Metal complexes; DNA-binding studies; DFT calculations;
NLO properties.
*
Correspondening Author: Professor Dr. Samir M. El-Medani, Department of Chemistry,
Faculty of Science, Fayoum University, El-Fayoum, Egypt.
E-mail address: samirmedani20@gmail.com.
1

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1
. Introduction
Schiff bases have a great interest for their important properties, such as their catalytic activity
in the hydrogenation of olefins [1], ability to reversibly bind oxygen in epoxidation reactions [2,
], photochromic properties [4, 5] and complexing ability towards some toxic metals [6].
Moreover, Schiff base complexes displayed an important role in biological activity applications
7-10].
Many investigations have been widely studied on metal complexes of Schiff bases containing
3
[
N and/or O donor sites, [11-15], while fewer studies were performed for those having an ONS
donor atoms [16-18]. For example, complexes of salicylideneimine-2-thiophenol with some
divalent ions such as Co(II), Ni(II), Cu(II), Zn(II) and Ru(II) were investigated [19, 20].
Furthermore, the structures of ruthenium and osmium carbonyl complexes were investigated and
suggested the coordination of metals to salicylideneimine-2-thiophenol ligand via S and N atoms
leaving the OH group free [21].
In the literature, there is no systematic study of the electronic structure and
nonlinear properties of the studied complexes. Such study is important for understanding the
biological activity and NLO properties of these complexes. Non-linear optical properties (NLO)
are the ability of any compound to convert light [with intense electric field (LASER)] of longer
wave length into light of shorter wave length.
An interesting analytical application of N-(2-hydroxy-1-naphthylidene)-2-aminothiophenol
Schiff base had been previously studied by our research group [22], and in view of the
diversified roles of Schiff base metal complexes, in the present work, Co(II), Cu(II), Zn(II),
Cd(II) and Hg(II) complexes were synthesized and fully characterized by different spectroscopic
methods. The antibacterial and antifungal activities of the ligand and the complexes were
performed. Furthermore, DNA binding interaction with calf-thymus DNA (CT-DNA) have been
elucidated by electronic absorption spectral titration and steady state fluorescence competition
studies. The geometrical parameters and NBO analysis are also calculated using
B3LYP/LANL2DZ. The first order hyperpolarizability (β) values of the studied complexes has
been computed to study the NLO properties. Finally, global reactivity descriptors including
electronegativity (X), hardness (η), softness (S) of the studied complexes were calculated and
analyzed.
2
. Experimental
2

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2
.1. Reagents
The metal salts Co(NO ) .6H O, Cu(NO ) .2.5H O, Zn(NO ) .6H O, CdSO , HgCl , 2-
3
2
2
3 2
2
3 2
2
4
2
hydroxy-1-naphthaldehyde and 2-aminothiophenol were purchased from Aldrich. All chemicals
and solvents were of analytical reagent grade and they were used without further purification.
2
.2. Instrumentation
Infrared measurements were carried out on a Unicam-Mattson 1000 FT-IR spectrometer
using KBr discs. Electronic absorption spectra were measured on a Unicam UV2-300 UV-VIS
spectrometer. Nuclear magnetic resonance measurements were performed on a Spectrospin-
Bruker AC 200 MHz spectrometer. Samples were dissolved in DMSO-d6 using
tetramethylsilane as internal reference. Magnetic susceptibilities of the complexes in the solid
state (Gouy method) were recorded on a Sherwood Scientific magnetic susceptibility balance.
Elemental analyses were performed on a Perkin-Elmer 2400 CHN elemental analyzer. Mass
spectra of the solid complexes (70 eV, EI) were carried out on a Finnigan MAT SSQ 7000
spectrometer. Measurements of the thermogravimetric analysis TGA and DTG were carried out
o
under nitrogen atmosphere with a heating rate of 10 C/min using a Schimadzu DT-50 thermal.
Electronic spectra were recorded on Shimadzu UV-Vis 1800 spectrophotometer. Fluorescence
measurements were carried out on a Jenway 6270 Fluorimeter. The excitation source was a
-3
Pulsed Xenon Lamp. All conductivity measurements were performed in DMF (1x10 M) at 25
oC, by using Jenway 4010 conductivity meter.
2
.3. Synthesis of N-(2-hydroxynaphthylidene)-2-aminothiophenol Schiff base (H L)
2
The ligand H L (Scheme 1) was synthesized as reported in literature [23] by refluxing a
2
mixture of (10 mmol, 1.72 g) of 2-hydroxynaphthaldehyde and (10 mmol, 1.1 mL) of 2-
aminothiophenol in about 30 ml absolute ethanol. The reaction mixture was heated to reflux for 2
hours. The reaction mixture was left to cool at room temperature. The solid yellow product was
separated and crystallized from hot ethanol and dried on vacuum line to give yellow crystals of
the Schiff base ligand (yield 90%). TLC checked the purity of the ligand.
2
.4. Synthesis of metal complexes
All complexes were prepared according to the following general procedure. Aqueous
solution of the metal salt (1mmol, 10 mL) was added dropwise to an ethanolic solution of H2L
ligand (1mmol, 0.279 g, 25 mL) at room temperature with constant stirring for 10 minutes. The
3

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produced precipitate was isolated by filtration, then washed several times with 3:1 ethanol-water
mixture followed by ether.
2
2
.5. Antimicrobial activity
.5.1 Bacterial susceptibility screening
In vitro antibacterial activity of synthesized compounds was studied against Gram- positive
and Gram- negative bacteria by the agar well diffusion [24]. Mueller Hinton agar was used as the
bacteriological medium. The compounds were dissolved in 10% aqueous dimethylsulfoxide
(DMSO) to a final concentration of 100 g/100 L. Pure DMSO was taken as the negative
control and 100 g/100 L Streptomycin as the positive control. 100 L of inoculum was
aseptically introduced onto the surface of sterile agar plates and sterilized cotton swabs were
used for the even distribution of the inoculum. Wells were prepared in the agar plates using a
sterile cork borer of 6.0 mm diameter. 100 L of the test and control compound were introduced
in the well. The same procedure was used for all the strains. The plates were kept in laminar flow
o
for 30 min for pre-diffusion of compound and then incubated aerobically at 35 C and examined
after 24 h [25, 26]. The diameter of the zone of inhibition produced by each agent were measured
with a ruler and compared with those produced by the commercial antibiotic Streptomycin.
2
.5.2 Fungal susceptibility screening
The antifungal activity of the synthesized compounds was tested using agar well diffusion
method. The potato dextrose agar plates were inoculated with each fungal culture (10 days old)
by point inoculation. A well of about 6.0 mm diameter with sterile cock borer was aseptically
punched on each agar plate. The synthesized test compound (100 g/100 L) was introduced
into the well; a negative control well was too made with 100 L of the solvent DMSO and 100
g/100 L Nystatin as the positive control. The plates were kept in laminar flow for 30 min for
o
pre-diffusion of compound to occur and then incubated at 28 C for 48 h. Resulting zone of
inhibition (in mm) was measured using a Hi media zone scale [27].
2
.6. CT-DNA interaction studies
All experimental involving CT DNA were performed in HCl/NaCl (5:50 mM) buffer solution
(pH = 7.24). Tris-HCl was prepared using deionized and sonicated triple distilled water and kept
o
at 4 C for 3 days. The absorption ratio of CT DNA solutions A /A was 1.9:1, indicating that
2
60
280
the DNA was sufficiently free from protein [28]. The CT DNA concentration was determined via
4

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-1
-1
absorption spectroscopy using the molar absorption coefficient of 6600 M cm (260 nm) for
CT DNA [29].
Stock solutions of metal complexes were prepared by dissolving them in dimethyl sulphoxide
(DMSO) and suitably diluting them with the corresponding buffer to the required concentrations
for all experiments. The extent of DMSO in the final concentration didn’t exceed 0.1 % in the
tested solutions. At this concentration, DMSO was not found to have any effect on CT DNA
conformation.
Absorption titration experiments were carried out by varying the CT DNA concentration in
the range of (0-10 µM) and maintaining the H L and its Co(II) Cu(II) and Zn(II) complexes
2
concentration constant at (100 µM). Upon measuring the absorption spectra, equal amount of
DNA was added to both the complex solution and the reference solution to eliminate the
absorbance of DNA itself. The reference solution was the corresponding buffer solution.
Absorbance values were recorded after each successive addition of CT DNA solution and
equilibration for ~ 10 min. Each sample was measured three times and an average value was
calculated. The absorption data were analyzed for an evaluation of the intrinsic binding constant
K of the complexes with CT DNA.
b
DNA competitive binding studies with ethidium bromide solution (EB) were carried out
keeping the concentration of EB (30 µM) and CT DNA (200 µM) constant and varying the
concentration of the complexes (0-100 µM). Before measurements, the resulting solutions were
shaken up and incubated for 30 min. The emission spectra were recorded in the wavelength
range of 500-700 nm at 595 nm (478 nm excitation wavelength).
2
.7. Computational methods:
All computations were carried out using Gaussian 09W software package [30]. Molecular
geometries of all the studied complexes were fully optimized using B3LYP/LANL2DZ [31-33].
No symmetry constrains were applied during the geometry optimization [34, 35] [. The choice of
this basis set was due to its flexibility and the fact that the diffused p functions on the H-atom
tend to compensate the anharmonic effects of the CH and NH stretches. By using HOMO and
LUMO energy values for complexes, electronegativity and chemical hardness can be calculated
as follows: X=(I+A)/2 (electronegativity), ɳ=(I-A)/2 (chemical hardness), S=1/2ɳ (chemical
softness) where I and A are ionization potential and electron affinity, and I= -EHOMO and A= -
ELUMO, respectively [36, 37]. NBO calculations have been performed at the B3LYP/ LANL2DZ
5

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level using NBO 3.1 program as implemented in the Gaussian 09W software package in order to
qualitatively measure the intramolecular delocalization in the studied system. Throughout this
work MOs were constructed using the Gauss-view 5.08 visualization program [38]. The total
static dipole moment (µ), the mean polarizability <α>, the anisotropy of the polarizability ∆α and
the mean first hyperpolarizability <β> using the x, y, z components were calculated by using the
following equations [39-41]:
ퟏ
ퟐ
풙
ퟐ
풚
ퟐ ퟐ
풛
µ = (µ + µ + µ )
(ퟏ)
(ퟐ)
훂풙풙 + 훂 + 훂풛풛
풚풚
훂 =
ퟑ
ퟐ
ퟐ
ퟐ
(
훂풙풙 ‒ 훂풚풚)) + (훂 ‒ 훂풛풛)) + (훂 ‒ 훂풙풙))
풚풚 풛풛
∆
훂 =
(3)
ퟐ
ퟐ
풙
ퟐ
풚
ퟐ
풛
<
훽 > = 훃 + 훃 + 훃
(ퟒ)
where
훃풙 = 훃풙풙풙 + 훃풙풚풚 + 훃풙풛풛
훃풙 = 훃풚풚풚 + 훃풙풙풚 + 훃풚풛풛
훃풙 = 훃풛풛풛 + 훃풙풙풛 + 훃풚풚풛
(ퟓ)
(ퟔ)
(ퟕ)
3
. Results and Discussion
.1. Schiff base characterization
The elemental analyses (C, H, N, S) data with molecular formula of the Schiff base ligand,
3
H L, are presented in Table 1. The results are in good agreement with the suggested molecular
2
formula, C H NOS. The structure of the Schiff base H L is represented in Scheme 1. The mass
1
7
13
2
+
spectrum of the ligand (Fig. 1) shows a well-defined parent peak at m/z = 280 (P ), with a
relative intensity of 17%. The IR spectrum (KBr pellets) of the solid H L free ligand (Fig. 2)
2
-1
+
displayed a strong, sharp band at 3262 cm corresponded to the stretching frequency of NH
group [18, 42] where a proton transferred from SH group to the nitrogen of azomethine as
6

ACCEPTED MANUSCRIPT
suggested in literature [22]. This fact is identical with the present work where no stretching
frequency band due to SH group is displayed. In addition, the IR spectrum of the solid ligand
showed no band due to the OH stretching frequency referred to the presence of intramolecular
hydrogen bonding between the OH and S-moieties [43] (Scheme 2). Furthermore, the IR
-1
spectrum displayed stretching bands at 1622, 1267 and 708 cm due to C=N, C-O and C-S
1
moieties, respectively [44]. The H NMR spectrum of a solution of the Schiff base ligand in
DMSO displayed singlets at 10.54 and 8.44 ppm due to OH and CH=N protons, respectively
1
(
Fig 3-a, Table 3). The OH singlet was disappeared in H NMR spectrum in the presence of D O
2
(Fig 3-b). The singlet of SH signal was expected to be confused through the aromatic multiplets.
So, the 1H NMR spectrum was expanded in the aromatic region. Accordingly, in the absence of
D O (Fig 4-a) a broad singlet signal at 7.00 ppm was appeared due to SH proton. This signal
2
disappeared in presence of D O (Fig. 4-b). These results confirmed the presence of SH and OH
2
1
protons at 7.00 ppm and 10.54 ppm in the H NMR spectrum of the ligand solution.
3
.2.
Complexes characterization
Reaction of the metal ions Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) with N-(2-
hydroxynaphthylidene)-2-aminothiophenol (H L) in ethanol resulted in the formation of the five
2
complexes [Co(HL) ]H O, [Cu(HL) ], [Zn(HL) ], [Cd(HL) ] and [Hg(HL)Cl] (1-5). The
2
2
2
2
2
conductivity measurements of the ligand and its complexes in DMF proved non-electrolyte
character (Table 1). The IR spectra of the metal complexes showed suitable shifts in the
characteristic bands of the ligand as result of complex formation (Table 2). The IR spectra of the
studied complexes exhibited (OH) stretching frequency band in the range 3410-3463 cm-1
indicating the presence of the OH group in the complex. The broad stretching band at 3463 cm-1
in the cobalt complex was referred to the presence of H O moiety. The (OH) band of the OH
2
1
group in cobalt complex may be confused within the broad band of the H O molecule. The H
2
NMR spectra of the three diamagnetic complexes 3, 4 and 5 displayed OH singlets at 9.45, 9.27
and 9.50 ppm, respectively. The highier field shift of these signlets with respect to that of the
ligand (10.54 ppm) indicates the coordination of the metal ions to H L ligand via the oxygen of
2
the OH group without deprotonation. Furthermore, the IR spectrum of the complexes exhibited
-1
-1

(C=N), (C-O) and (C-S) bands in the ranges 1602-1616 cm , 1209-1216 cm and 679-720
-1
cm , respectively, with lower shifts referred to the corresponding values for the ligand. These
results indicate the coordination of azomethine nitrogen, hydroxyl oxygen and sulfur of thiol
7

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group of the H L Schiff base to the metal ions in the complexes [14]. This is confirmed by the
2
appearance of stretching frequency bands, in the IR spectra of the metal ion complexes,
-1
-1
corresponding to M-O, M-N and M-S bonds in the ranges 487-505 cm , 450-469 cm and 407-
-1
4
27 cm , respectively, (Table 2).
The cobalt and copper complexes showed paramagnetic properties and gave no H NMR
spectrum. Magnetic susceptibility measurements of these two complexes at 298 K showed values
1
-6
-1
-6
-1
of 8.5610 e.m.u.g for cobalt complex and 1.3110 e.m.u.g for copper complex with an
effective magnetic moment (μ ) of 3.61 BM and 1.40 BM, respectively. The μ value of the
eff
eff
7
cobalt complex indicated a high spin d electronic configuration. This electronic configuration is
in accordance with +2 formal oxidation state of the cobalt in the complex. Moreover, the μeff
9
value of the copper complex indicated a high spin d electronic configuration. This electronic
configuration is in accordance with +2 formal oxidation state of copper in the complex.
Accordingly, the studied metal complexes may have the structures represented in Scheme 3.
3
.3. Thermogravimetric analysis
The thermal studies of the cobalt, copper, zinc, cadmium, and mercury complexes were
carried out using thermogravimetry (TG) and differential thermogravimetry (DTG) techniques.
The TG data of the reported complexes were listed in Table 4 and plotted as shown in Figures
(
S1-S5). The TG plot of CoC H N O S (H O) displayed three resolved and well-defined
34 24 2 2 2 2
o
decomposition steps in the temperature range 92-879 C. The first decomposition step occurred
o
in the temperature range 92-190 C, with a net weight loss of 2.63%, could be due to the
elimination of one H O molecule which confirms the presence of a hydrated water molecule in
2
the complex. The second decomposition step occurred in the temperature range 206-548 oC with
a net weight loss of 59.56 % was due to the loss of a C H N SO moiety. On the other hand, the
2
4
12
2
o
third decomposition step (548-879 C, 28.38 %) was due to the elimination of C H SO species
1
0
12
to leave metallic residue of cobalt.
The TG plot of the CuC H N O S complex displayed also three decomposition steps in
3
4
24
2
2 2
o
the temperature range 200-968 C. The first decomposition step occurred in the temperature
o
range 200-387 C with a net weight loss of 30.87 % corresponded to the elimination of a C H
1
5
12
moiety. On the other hand, the second decomposition peak occurred in the temperature range
o
3
89-512 C with a weight loss of 19.63 % and corresponded to the material decomposition of
8

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o
C2H8N2. In addition, a third decomposition step (514-968 C, 39.83 %) was due to the
elimination of C H S O species to give finally CuO + 3C solid residue.
1
4
4 2
The ZnC H N O S complex was found to thermally decomposed in three well-defined
3
4
24
2
2 2
o
steps in the temperature range 150-999 C. In the first decomposition step occurred in the
o
temperature range 150-387 C with a net weight loss of 15.68% and corresponded to the
o
elimination of two C H N species. The second decomposition step (388-528 C, 30.69 %) was
6
9
due to the elimination of two C H NS species. The third decomposition peak occurred in the
9
6
2
o
temperature range 528-999 C (weight loss 24.47 %) could be due to the elimination of a C H O
1
1
9
moiety to leave ZnO + 8C as a residue.
The CdC H N O S complex decomposed in three well-defined steps with a total mass
3
4
24
2
2 2
o
loss of 100 %. The first decomposition peak occurred in the temperature range 150-378 C with
mass loss of 58.05% corresponded to the loss of C H N moiety. The second decomposition
2
9
11
2
o
step (379-578 C, 25.33 %) was due to the elimination of C H O S species. On the other hand,
5
13
2 2
o
the third decomposition step occurred in the temperature range 578-999 C with a weight loss of
1
6.62 % corresponded to the elimination of metallic cadmium leaving no solid residue.
The TG plot of the HgC H NOSCl complex displayed two decomposition steps in the
1
7
12
o
temperature range 150-858 C. The first decomposition step occurred in the temperature range
o
1
50-261 C with a net weight loss of 35.89 % corresponded to the elimination of a C H O
1
3
10
o
moiety. A second decomposition peak occurred in the temperature range 261-858 C with a
weight loss of 63.88 % corresponded to the material decomposition of [C H NSCl + Hg] giving
4
2
no solid residue.
.
3
.4. Kinetic data
The Coats-Redfern relation [45] (equation 1) was employed in this work to evaluate the
kinetic and thermodynamic activation parameters (E*, H*, S* and G*) of the decomposition
processes of the reported complexes.
퐖∞
퐥퐨퐠
∗
퐖 ‒ 퐖
∞
퐀퐑
ퟐ퐑퐓
퐄
퐥퐨퐠
= 퐥퐨퐠 [
ퟏ ‒
] ‒
(ퟖ)
ퟐ
∗
(
∗
)
ퟐ.ퟑퟎퟑ퐑퐓
[
퐓
]
∅퐄
퐄
9

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W is the total mass loss of the decomposition reaction, W is the mass loss up to temperature

T,  is the heating rate while R is the gas constant. The plot of the left-hand side of equation (8)
against 1/T, considering 1-2RT/E*  1, would give a straight line with definite slope and
intercept. The activation energy (E*) and the pre-exponential factor, A, could be calculated
from the slope and the intercept of this line, respectively. On the other hand, the entropy (S*),
enthalpy (H*) and free energy change (G*) of activation, were calculated using the following
equations:
퐀퐡
∗
퐒 = ퟐ.ퟑퟎퟑ (퐥퐨퐠 )퐑
(ퟗ)
퐤퐓
∗
∗
퐇 = 퐄 ‒ 퐑퐓
(ퟏퟎ)
(ퟏퟏ)
∗
∗
∗
퐆 = 퐇 ‒ 퐓퐒
where, (h) is Planck’s constant and (k) is Boltzman constant. The kinetic parameters are listed in
-1
Table 5. The activation energies of decomposition were in the range 4.60-218.34 kJ mol . The
high positive values of the G* reflect the high stability of the investigated complexes. This
behavior corresponded to the covalent character of the complexes [46, 47]. On the other hand,
-1
the negative values of S* for the degradation process (-5.40 to -283.26 kJ mol ) reveal more
ordered activated complex and lower rate for the decomposition reactions than normal ones [48,
4
9].
3
.5. Antimicrobial activity
The ligand (H L) and its metal complexes were screened against the bacterial strains B.
2
subtilis, S. aureus, E. coli, K. pneumoniae and fungal strains C. albicans, A. niger. The results of
inhibition are compared with standard antibacterial drug Streptomycin and antifungal drug
Nystatin. The antimicrobial activity of H L ligand and its complexes bacteria and fungi is
2
summarized in Table 6. The antibacterial activities of Cd(II) and Hg(II) complexes weren’t
studied for their toxicity. It was clear that, Co(II), Cu(II) and Zn(II), complexes exhibit
significant activity against all bacterial and fungal strains, this may be due to the presence of the
imine group may be one of the reasons for the biocidal action of both the ligand and metal
complexes. Among the reported complexes, Cu(II) complex shows the highest biocidal activity
than other complexes. Generally, all complexes show higher antimicrobial properties compared
to the free H L ligand. This behavior was explained by Overtone’s concept and Tweedy’s
2
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0

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chelation theory [50-52]. Upon chelation operates, the polarity of the metal ion decrease
extensively and the delocalization of π-electrons increase on the whole chelation ring for the en
towards enhancement lipophilicity of complexes. This could results in better penetration of the
complexes into lipid membranes. This can further block the metal binding sites on the enzymes
of the microorganisms. Thus, in general, the complexes show better activity than their Schiff
base ligand.
3
3
.6. DNA binding studies
.6.1. Absorption spectral studies
The application of UV-Vis absorption spectroscopy technique is one of the most metod to
study the binding mode of DNA with small molecules [53]. The electronic absorption spectra of
the investigated ligand and its complexes consist of two or three well-resolved bands in the range
of 259-500 nm. The high-energy absorption band appeared in the spectra of the ligand and its
complexes below 350 nm are assigned to -* and n-* transitions [54] and the band observed
above 400 nm in the complexes spectra may be assigned to LMCT transition, usually observed in
metal Schiff base complexes containing phenolated ligands due to “phenolate-to-M” charge
transfer [55] or d-d transition. The absorption spectra of H L and its Co(II) and Cu(II) complexes
2
in the absence and presence of CT DNA are given in Fig 6. With increasing concentration of CT
DNA, all the complexes showed hypochromicity and a red-shifted charge transfer peak maxima,
indicating the complexes are actively in associating with CT DNA. These spectral characteristics
obviously suggested that the titled complexes most likely interact with DNA through a mode of
stacking interaction between the aromatic chromophore of the complexes and the base pairs of
DNA. For the quantitative investigation of the binding strength of the ligand and its complexes to
CT DNA, the intrinsic binding constants K of the complexes with CTDNA were calculated
b
using the following function equation [56]:
[
퐂퐓 퐃퐍퐀] [퐂퐓 퐃퐍퐀]
ퟏ
=
+
(ퟏퟐ)
(
훆 ‒ 훆 )
(훆 ‒ 훆 )
퐊 (훆 ‒ 훆 )
퐚
퐟
퐛
퐟
퐛
퐚
퐟
where [CT DNA] is the concentration of CT DNA in base pairs,  is the extinction coefficient
a
obtained by calculating Aobs/[compound], f corresponds to the extinction coefficient of the
compound in its free form and b refers to the extinction coefficient of the in the fully bound
form, respectively. In plots [CT DNA]/(a - f) versus [CT DNA] (Fig.5, inset), the intrinsic
binding constants (K ) value, given by the ratio of the slope to the intercept of the linear fit of the
b
1
1

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4
-1
4
-1
-1
4
-1
data, were found to be 0.72210 M , 2.68510 M , 3.078104 M , 1.99910 M , for H L
2
and its Co(II), Cu(II) and Zn (II) complexes, respectively, indicating a moderate intercalation
between the complexes and CT DNA. These (K ) values are much smaller than the typical
b
6
-1
classical intercalators (e.g. EB-DNA, ~10 M ) [57] and have the same level as those of some
4
well-established intercalation agents (~10 ) [58, 59]. The K value (Table 7) revealed that the
b
comparative binding strength of the complexes with CT-DNA were in the order Cu(II) complex
>
Co(II) complex > H L, suggesting that Cu(II) intercalates more strongly than other complexes.
2
3
.6.2. Fluorescence spectral studies
Ethidium bromide (EB) was used to investigate the potential DNA binding mode of the
ligand and its Co(II), Cu(II) and Zn (II) complexes. EB emits intense fluorescence at 595 nm in
the presence of CT DNA due to high intercalation between the adjacent DNA base pairs. The
addition of a second small molecule which binds to DNA better than EB would supress the
DNA-induced EB emission [60]. The extent of quenching of the fluorescence of EB bound to
DNA would reflect the extent of the DNA binding of the second molecule. The ligand, the
complexes, and CT DNA independently or in combination do not give luminescence spectra in
the buffer and DMSO. The emission spectra of DNA-bound EB in the absence and the presence
of the ligand and its Co(II) Cu(II) and Zn(II) complexes are shown in Fig. 6. It was clearly seen
that, the addition of the reported compounds to CT-DNA pretreated with EB caused an
appreciable reduction in emission intensity, indicating that the complexes bind to DNA at the
sites occupied by EB. The quenching of the EB bound to DNA by the complex is in agreement
with the linear Stern–Volmer Eq. [61]:
퐈퐨
[퐂퐨퐦퐩퐨퐮퐧퐝]
[퐂퐓 퐃퐍퐀]
=
ퟏ + 퐊퐒퐕
(ퟏퟑ)
퐈
where I and I represent the fluorescent intensities in the absence and presence of the complex,
o
respectively, and Ksv is the linear Stern-Volmer quenching constant. The K value is obtained
sv
from the slope of the I /I versus [Compound]/[CT DNA] linear plot (Fig. 6, inset) and is found to
o
be 0.67, 1.45, 1.98 and 1.03 for H L, Co(II), Cu(II) and Zn(II) complexes, respectively,
2
indicating that, the CT DNA binding affinity of Cu(II) complex is higher than those of other
complexes. Also, the data revealed that DNA-bound EB can be more readily replaced by the
complexes in the order Cu(II) complex > Co(II) complex > Zn(II) complex > H L, which is
2
consistent with the above UV-Vis observations.
1
2

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3
.7.
Molecule orbital calculations
The optimized geometrical parameters (bond lengths, bond angles and dihedral angles),
natural charges on active centers and energetic of the ground state for the studied complexes
were computed and analysed. From the elemental analysis and spectroscopic data, metal ions
coordinated to the ligand via N, O and S-atoms forming the complexes. Two ligand moieties
used in the coordination with Co(II), Cu(II), Zn(II), Cd(II) forming [M(HL) ] complexes,
2
whereas, Hg(II) coordinated with one moiety of the ligand to form [Hg(HL)Cl] complex.
3
.7.1 Geometry of the complexes
Tables 8, 9 and Fig. 7 present the optimized geometry, numbering system, vector of the
dipole moment, energetic, dipole moment, bond lengths, bond angles and dihedral angles of all
metal complexes studied in this work. In Co(II), Cu(II), Zn(II) and Cd(II) complexes, the metal
ion coordinates with O40,C28,C27,C31,N32 and N52,C11,C10,C5,O19 to form a six-member
ring and with S39,C32,C33,N32 and N52,C53,C54,S59 to form a five member ring. In case of
Hg-complex, the ion coordinated with S27,C22,C21,N20 to form five member ring and with
0
19,C5,C10,C11,N20 to form six member ring. Therefore, distortion from regular octahedral
geometry is expected for all the studied complexes. Most M-N, M-S and M-O bonds show
elongation upon complexation. The length of the coordinate covalent bonds between metal and
ligand site, i.e. M-N, M-S and M-O, are too long compared to the typical MX bond lengths [62].
The too long M-O, M-S and M-N bonds in the studied complexes mean that the ionic character
of these bonds is small. The calculated values of bond angles between metal ion and binding
sites (Tables 8 and 9) N32MS39, N32MO40, O40MS59, S59MN52, N52MO19 and O19MS39
vary between 45° and 129° which compare nicely with the experimental data as obtained from
X-ray analysis for Oh complexes [63], which indicate a distorted octahedral geometry. The
dihedral angles around metal ion, i.e.MN32C31C27, MO40C28C27, MO40C28C29 and
MN32C31H47, are far from 0° or 180° which indicate that the metal ion is not in the same
molecular plane of the ligand.
3
.7.2 Natural charges and natural population
The natural population analysis performed on the electronic structures of the studied complexes
clearly describes the distribution of electrons in various sub-shells of their atomic orbits. The
natural charges on the coordinating sites in the core, valence and rydberg sub-shells and natural
electronic configuration of the metal in the studied complexes are presented in Tables 10 and 11 .
1
3

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The most electronegative centers are accumulated on N12, S19, O63, N43, S50 and O51. These
electronegative atoms have a tendency to donate electrons. Whereas, the most electropositive
atoms are Co, Cu, Zn, Cd and Hg have a tendency to accept electrons. The central metal ion in
the Co-complex received 1.6681e from the donating sites of the ligands with electronic
configuration 3d7.75. In case of Cu-, Zn-, Cd-, and Hg-complexes the central metal ions received
1
.1492e (3d9.37), 0.7358e (3d9.98), 0.76194e (4d9.99) and 0.9307e (5d9.97) from the active sites of
the ligands respectively. Total Lewis (effective core, core and valence Lewis) and total non-
Lewis (valence non-Lewis and rydberg non-Lewis) of the studied complexes are presented in
Table 12.
3
.7.3 Global reactivity descriptors
They include HOMO, LUMO, energy gap (Eg), chemical hardness (η), electronegativity (X),
chemical potential (V), electron affinity (A), ionization potential (I) and chemical softness(S).
The frontier molecular orbital (FMO) energies of the studied complexes were calculated using
B3LYP/LANL2DZ and presented in Fig. 8. HOMO energy characterizes the electron donating
ability, while LUMO energy characterizes the electron withdrawing ability. Energy gap (Eg)
between HOMO and LUMO characterizes the molecular chemical stability which is a critical
parameter in determining molecular electrical transport properties because it is a measure of
electron conductivity. The results in Fig. 8 and Tables 9 and 13 indicate that the smaller the
energy gap the easier the charge transfer and the polarization occurs within the molecule.
Furthermore, the order of increasing reactivity is: Co >> Zn > Cu > Cd > Hg. Using HOMO and
LUMO energies, ionization potential and electron affinity can be expressed as I~ -EHOMO, A~ -
ELUMO as shown in Tables 9 and 13. The variation of electronegativity (X) values is supported by
electrostatic potential, for any two molecules, where electron will be partially transferred from
one of low X to that of high X. The results show that the order of decreasing X (increasing CT
within the molecules) is: Cu > Hg > Zn > Cd > Co.
The chemical hardness (η) = (I-A)/2, electronegativity (X) = (I+A)/2, chemical potential (V)
=
- (I+E)/2 and chemical softness(S) =1/2η values are calculated and presented in Tables 9 and
1
3 The results of small η values for the studied compounds reflect the ability of charge transfer
inside the molecule. Therefore, the order of increasing the charge transfer within the molecule is:
Co > Hg > Zn > Cd > Cu. Considering η values, the higher the η values, the harder is the
molecule and vice versa.
1
4

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3
.7.4-Non-linear optical properties (NLO)
No experimental or theoretical investigations were found addressing NLO for the studied
complexes; therefore, this triggered our interest to undertake this study. NLO properties is the
key functions of frequency shifting, optical modulation, switching, laser, fiber, optical materials
logic and optical memory for the emerging technologies in areas such as telecommunications,
signal processing and optical inter connections [64-66]. In order to investigate the relationship
between molecular structure and NLO, the polarizibilities and hyperpolarizibilities of the studied
complexes are calculated using B3LYP/LANL2DZ. Total static dipole moment (µ), the mean
polarizibility α, the anisotropy of the polarizability ∆α and the mean first-order
hyperpolarizibility (β) of the studied complexes are listed in Table 14 The polarizibilities and
first-order hyperpolarizibilities are reported in atomic units (a.u.), the calculated values have
been converted into electrostatic units (esu) using conversion factor of 0.1482×10-24 esu for α
and 8.6393×10-33 esu for β. Urea is a standard prototype used in NLO studies. In this study,
Urea is chosen as a reference as there were no experimental values of NLO properties of the
studied complexes. The magnitude of the molecular hyperpolarizibility β is one of the key
factors in NLO system. The analysis of the β parameter show that Hg-complex is ~ 12 times
higher than (UREA), while Co(II), Cu(II), Zn(II), Cd(II) complexes are 7, 2, 8 and 5 higher than
the reference respectively. Therefore, the studied complexes are an efficacious candidate for
NLO materials.
4
. Conclusions
In this work, the reaction of some transition metal ions Co(II), Cu(II), Zn(II), Cd(II) and
1
Hg(II) with a Schiff base ligand has been studied. Elemental analyses, IR, H NMR, magnetic
measurement, molar conductance, and thermal analysis suggested the five complexes;
[
Co(HL) ]H O, [M(HL) ] (M= Cu, Zn and Cd) and [Hg(HL)Cl]. Kinetic and thermodynamic
2 2 2
parameters reflect the high stability of the complexes. The parent ligand (H L) and its metal
2
complexes were screened against some bacteria and fungi strains. Furthermore, DNA binding
interacation with calf-Thymus DNA (CT DNA) have been elucidated by electronic absorption
spectral titration and steady state fluorescence competition studies and the data revealed that, the
complexes bind to CT-DNA in the order Cu(II) complex > Co(II) complex > Zn(II) complex >
H L. The molecular geometry of the studied complexes in the ground state has been calculated
2
by using DFT-B3LYP/LANL2DZ level of theory. The optimized structure of the studied
1
5

ACCEPTED MANUSCRIPT
complexes are non-linear with the metal ion is not in the same plane as the donating sites. The
HOMO-LUMO energy gap helped in analyzing the chemical reactivity, hardness, softness,
chemical potential and electro negativity. Natural charge distribution of the studied complexes
was studied which indicated the electronic charge distribution in the complexes. The calculated
dipole moment and first order hyperpolarizability results indicate that the complexes have a
reasonable good non-linear optical behavior.
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Figures Captions
Fig. 1. The mass spectrum of the ligand H L.
2
Fig. 2. IR spectrum of the ligand H L.
2
1
Fig. 3. The H NMR spectrum of the Schiff base ligand H L: (a) in absence of D O(b)in
2
2
presence of D O.
2
1
Fig. 4. The expanded H NMR spectrum of H L in the range 6.9-7.1 ppm (a) in absence of D O
2
2
(
b)in presence of D O.
2
Fig. 5. Electronic absorption spectra in DMSO solution of H L and its Co(II), Cu(II) and Zn(II)
2
complexes (100 µM) in the absence (……) and presence (____) of increasing amounts of CT-DNA
(
0-10 µM). Arrows show the changes in absorbance with respect to an increase in the DNA
concentration. Inset: linear plot for the calculation of the intrinsic CT DNA binding constant
Kb).
(
Fig. 6. Emission spectra of CT DNA-bound EB in Tris buffer [EB] = 30 µM, [CT DNA] = 200
µMin the absence (……) and presence (____) of increasing amounts of H L and its Co(II), Cu(II)
2
and Zn(II) complexes (0-100 µM). Arrows show the changes in emission intensity with respect
to an increase complex concentration (Inset: plot of Stern-Volmer plot of I /I Vs [H L]/[CT
o
2
DNA].and [M(II) complex]/[CT DNA].
Fig. 7. The optimized geometry, numbering system, vector of the dipole moment of the studied
complexes using B3LYP/LANL2DZ.
Fig. 8. HOMO-LUMO charge density maps of Co(II), Zn(II) and Hg(II) complexes using
B3LYP/LANL2DZ.
2
0

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Fig. 1.
2
1

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Fig. 2.
2
2

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Fig. 3.
2
3

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Fig. 4.
2
4

Fig. 5.
0
0
0
0
0
0
0
0
.35
.30
.25
.20
.15
.10
.05
.00
2.0
0
.4
H L
[
Co(HL) ].H O
-0.4
2
0.2
2
2
-0.5
0.0
-0.6
-0.7
1.5
-
0.2
0.4
-0.8
-0.9
-
-
1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
-0.6
0
.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
[DNA]x10 , (M)
5
5
[
DNA]x10 , (M)
1.0
0.5
0.0
350
2
50
300
350
400
450
500
550
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 6.
2
5

8000
6000
4000
2000
0
8000
6000
4000
2000
0
4
4
3
3
2
2
1
1
.5
.0
.5
.0
.5
.0
.5
.0
[
Co(HL) ]H O
4.0
3.5
3.0
2.5
2
2
H L
2
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
12
4
[
Ni(II) Copmplex]x10 , (M)
0
.0
0.5
1.0
1.5
2.0
2.5
4
[
Zn(II) Coplexx10 , (M)
560
580
600
620
640
660
680
700
560
580
600
620
640
660
680
700
Wave length (nm)
Wave length (nm)
8000
6000
4000
2000
0
8000
6000
4000
2000
0
4
.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
[
Zn(HL) ]
[
Cu(HL)2]
2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
.0
0.2
0.4
0.6
0.8
0
2
4
6
8
10
12
4
[Ni(II) Copmplex]x10 , (M)
4
[
Co(II) Coplex]x10 , (M)
560
580
600
620
640
660
680
700
560
580
600
620
640
660
680
700
Wave length (nm)
Wave length (nm)
2
6

ACCEPTED MANUSCRIPT
Fig. 7.
2
7

ACCEPTED MANUSCRIPT
Fig. 8.
Co(II) Complex
Zn(II) Complex
Hg(II) Complex
HOMO
LUMO
2
8