Synthesis, spectroscopic characterization, DNA interaction and biological activities of Mn(II), Co(II), Ni(II) and Cu(II) complexes with [(1H-1,2,4-triazole-3-ylimino)methyl]naphthalene-2-ol

Article abstract of DOI:10.1016/j.molstruc.2014.06.071

Manganese(II), cobalt(II), nickel(II) and copper(II) complexes of [(1H-1,2,4-triazole-3-ylimino)methyl]naphthalene-2-ol have been synthesized. The structure of complexes have been characterized by elemental analysis, molar conductance, magnetic moment mea

Full text of DOI:10.1016/j.molstruc.2014.06.071

Journal of Molecular Structure 1076 (2014) 251–261
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic characterization, DNA interaction
and biological activities of Mn(II), Co(II), Ni(II) and Cu(II) complexes
with [(1H-1,2,4-triazole-3-ylimino)methyl]naphthalene-2-ol
Mohamed Gaber a, Nadia A. El-Wakiel a, , Hoda El-Ghamry a,b, Shaimaa K. Fathalla a
⇑
a Chemistry Department, Faculty of Science, Tanta University, Tanta 31725, Egypt
b Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
Novel manganese (II), cobalt(II),
nickel(II) and copper(II) complexes of
[(1H-1,2,4-triazole-3-
ylimino)methyl]naphthalene-2-ol
were synthesized.
ꢀ
ꢀ
The structures of the prepared
complexes were elucidated.
The binding mode of the metal
complexes towards DNA were
studied.
ꢀ
The compounds were screened for
antibacterial and antifungal activity
indicating enhanced activity of ligand
upon chelation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2014
Received in revised form 21 June 2014
Accepted 23 June 2014
Available online 22 July 2014
Manganese(II), cobalt(II), nickel(II) and copper(II) complexes of [(1H-1,2,4-triazole-3-ylimino)
methyl]naphthalene-2-ol have been synthesized. The structure of complexes have been characterized
by elemental analysis, molar conductance, magnetic moment measurements and spectral (IR, 1H NMR,
EI-mass, UV–Vis and ESR), and thermal studies. The results showed that the chloro and nitrato Cu(II)
complexes have octahedral geometry while Ni(II), Co(II) and Mn(II) complexes in addition to acetato
Cu(II) complex have tetrahedral geometry. The possible structures of the metal complexes have been
computed using the molecular mechanic calculations using the hyper chem. 8.03 molecular modeling
program to confirm the proposed structures. The kinetic and thermodynamic parameters of the thermal
decomposition steps were calculated from the TG curves. The binding modes of the complexes with DNA
have been investigated by UV–Vis absorption titration. The results showed that the mode of binding of
the complexes to DNA is intercalative or non-intercalative binding modes. Schiff base and its metal
complexes have been screened for their in vitro antimicrobial activities against Gram positive bacteria
(Staphylococcus aureus), Gram negative bacteria (Escherichia coli and Pesudomonas aeruginosa), fungi
(Asperigllus flavus and Mucer) and yeast (Candida albicans and Malassezia furfur).
Keywords:
Triazole
Schiff base complexes
Spectral
Thermal
Biological studies
DNA interaction
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +20 40 3410037; fax: +20 40 3350804.
E-mail address: drnadia6@yahoo.com (N.A. El-Wakiel).
http://dx.doi.org/10.1016/j.molstruc.2014.06.071
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

252
M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
conductivity bridge. The Infrared spectra were recorded on a
Introduction
Perkin–Elmer 1430 IR spectrophotometer within the range 4000–
200 cmꢁ1 as KBr discs. Standard electron impact mass spectra
(E.I.) were determined using a Finnigan MAT 8222 Spectrometer
at 70 eV in micro analytical unit of Cairo University. The NMR
spectra were carried out using a Varian Mercury-300BB NMR spec-
trophotometer operating at 300 MHz after dissolving the samples
in d6-DMSO using tetramethylsilane as an internal standard. The
electronic absorption spectra were recorded using a Shimadzu
UV–Vis 240 spectrophotometer. The room temperature magnetic
susceptibility of the solid samples was measured using magnetic
susceptibility balance (Johnson Mtthey) 436 Devon Park Drive
employing the Gouy’s method. The thermogravimetric analysis
(TGA) of the solid samples were performed with the range 25–
800 °C using the Shimadzu TG-50 thermogravimetric analyzer
with different heating rate (5–20 °C/min.) under nitrogen atmo-
sphere. The X-band electron spin resonance spectra of powder
samples were recorded using Joel JES-FE2XG spectrometer model
equipped with an E101 micro wave bridge at room temperature.
The magnetic field was calibrated with diphenyl picryl hydrazyl
(DPPH). The antimicrobial spectra of the prepared compounds
were determined against the tested organisms on powdered sam-
ples using cut-plug method [32] in micro analytical unit, Botany
Department, Faculty of Science, Tanta University.
DNA is an important drug target and it regulates many bio-
chemical processes that occur in the cellular system. Many small
molecules exert their anticancer activities by binding with DNA,
thereby altering DNA replication and inhibiting the growth of
tumor cells. Binding studies of small molecules to DNA are very
important in the development of new therapeutic reagents and
DNA molecular probes [1].
Transition metal complexes have been widely exploited for this
purpose because by changing the ligand environment one can tune
the DNA binding and cleaving ability of the metal complexes. The
binding mechanisms between DNA and various redox substances,
such as metal complexes [2–10], anticancer or antivirus drugs
[11] and organic dyes [12] have been investigated. The importance
criteria for the development of metallodrugs as chemotherapeutic
agents are the ability of the metallodrug to bring about DNA
cleavage. The DNA binding mechanism and behavior of the metal
complexes are closely related to the size, shape and planarity of
the intercalative ligands. It has been found that the coordination
geometries and donor atoms of the ligands play key roles in deter-
mining the binding mode between metal complexes and DNA [13].
On the other hand, the metal ions and their flexible valences,
which are responsible for the geometry of complexes, affect the
intercalating ability of the metal complexes to DNA [14].
Schiff base complexes of transition metals are of particular
interest to inorganic chemists because of their structural, spectral
and chemical properties which are dependent on the nature of
the ligand structure. Depending on their interesting structural
properties and wide ranging uses, studies of Schiff base complexes
have attracted the attention of many investigators. 1,2,4-Triazoles
and their derivatives represent an interesting class of compounds
possessing a wide spectrum of biological activities such as anti-
fungal, anticancer, anti-inflammatory, antibacterial properties
and antitumor [15–23] activities. Moreover, the metal complexes
of 1,2,4-triazole derivatives have been extensively investigated
[24–31].
Molecular modeling studies
An attempt to gain a better insight on the molecular structure of
Metal complexes, geometric optimization and conformational
analysis has been performed by the use of MM+ force field as
implemented in hyperchem 8.0 [33]. Semi empirical method PM3
is then used for optimizing the full geometry of the system using
Polak–Ribiere (conjugate gradient) algorithm and Unrestricted
Hartee–Fock (UHF) is employed keeping RMS gradient of
0.01 kcal/mol. All the calculations refer to isolated molecules in
vacuum.
Based on these facts and as part of our going studies on the
synthesis, structural investigation and biological activity of Schiff
bases metal complexes, we report her the synthesize and charac-
terization of Mn(II), Co(II), Ni(II) and Cu(II) complexes of the titled
Schiff base. All newly synthesized compounds have been charac-
terized by microanalysis and spectroscopic methods (IR, 1H NMR,
EI-mass, Uv–Vis and ESR), as well as thermal studies. The synthe-
sized complexes were investigated for their DNA interaction using
absorption titration. The Schiff base and its metal complexes have
been screened for their in vitro antimicrobial activities against
Gram positive bacteria (Staphylococcus aureus), Gram negative
bacteria (Escherichia coli and Pesudomonas aeruginosa), fungi (Aspe-
rigllus flavus and Mucer) and yeast (Candida albicans and Malassezia
furfur).
DNA binding studies
The experiments were carried out in Tris–HCl buffer (5.0 mM of
tris(hydroxymethyl)-aminomethane and 50 mM NaCl) at pH 7.2.
Tris–HCl buffer was prepared using deionized and triple distilled
water. Solutions of CT-DNA in Tris–HCl gave a ratio of UV absor-
bance at 260 and 280 nm (A260/A280) of 1.8–1.9, indicating that
the DNA was sufficiently free from protein [34]. The stock solution
of DNA was prepared by dissolving DNA in Tris–HCl buffer. Con-
centrated stock solutions of metal complexes were prepared by
dissolving each complex in DMF and diluted suitably to the
required concentration. Absorption titration experiments were
performed with a fixed concentration of each complex (20 ppm),
while gradually increasing the concentration of DNA. When the
absorption spectra were measured, an equal amount of DNA was
added to both the complex solutions and the reference solution
to eliminate the absorbance of DNA itself.
Experimental
Materials and methods
For viscosity measurements, a viscometer was thermostated in
water-bath maintained at 25 °C. The flow time for each sample was
measured three times using digital stopwatch and an average flow
time was calculated. The rate of flow of the DNA (3 ꢂ 10ꢁ5 M) and
DNA with various concentrations of each complex were measured.
The relative specific viscosity was calculated using the equation
All compounds used in the present study were of pure grade
available from BDH, Aldrich or Sigma. The solvents used for the
spectral studies were spectroscopic grade from Aldrich.
The elemental microanalysis of the solid compounds were
performed at the microanalytical center, Cairo University using
Perkin–Elmer 2400 CHN Elemental analyzer. Metal content was
estimated complexometrically using standard EDTA titration.
Molar conductivities in DMF (10ꢁ3 M) at room temperature
(25 °C) were measured using conductance bridge of the type 523
g
= (t ꢁ to)/to, where to is the flow time for the Tris–HCl buffer
alone and t is the observed flow time for DNA in the absence and
1/3
presence of the complex. Data are presented as (
(where = [complex]/[DNA]), go and are the viscosity of DNA in
the absence and presence of the complex [35,36].
g/go
)
vs. r
g

M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
253
11
12
13
HN
N
10
9
H
C
3
2
1
N
N
4
8
7
5
HO
6
Fig. 1. Structure of the prepared Schiff base ligand (SB).
Antimicrobial activities
The antimicrobial spectrum of the prepared compounds was
determined against the tested organisms on powdered samples
using cut-plug method [37]. The prepared media heated in water
bath with stirring, autoclaved, and poured in sterile petri dishes
avoiding air bubbles, then left to solidify at room temperature. A
regular well of 8 mm diameter on the center of each petri dish
were cut using cork borer. plates were seeded with different micro-
organisms and central wells were filled with 20 mg powder of the
prepared ligands and some of their complexes for the tested bacte-
ria, fungi and yeast, then incubated 24 h for bacteria and 48 h for
fungi and yeast, after which the diameter of the inhibition zones
were measured and the compounds which produced the highest
inhibition zone was further assayed at different concentration in
order to quantify its inhibitory effect using the turbidity method
[38]. In this experiment, different concentrations of each complex
was prepared in dimethylsulfoxide (DMSO) separately. The used
concentrations were (30–1000 lg/ml), zero concentration was
consider as a control. 0.5 ml of each standard organism suspension
was mixed with 9.5 ml of each corresponding media in sterile test
tube contained the tested complexes to give (30–1000 lg/ml). The
results were recorded after incubation [39] in shaker incubator for
48 h. The turbidity of each mixture was measured then the
turbidity of each mixture was measured and surviving ratio was
calculated as a percentage of surviving cell number for each
concentration to that of the original spore suspension at zero con-
centration (used as control) and finally the data are represented
graphically.
Fig. 2. Infrared spectra of ligand SB and its metal complexes.
8 h. This was achieved by refluxing 0.01 mol (1.72 g) of 2-
hydroxy-1-naphthaldehyde with 0.01 mol (0.84 g) of 3-amino-
1,2,4-triazole in methanol (100 ml) as a solvent. The condensation
product separated on cooling was filtered off, washed several times
with methanol till constant melting point and finally dried in a vac-
uum desiccator over anhydrous calcium chloride.
SB: 1H NMR (300 MHz, DMSO-d6) d (ppm): 14.96 (s, 1H, NH),
10.79 (s, 1H, OH), 10.08 (s, 1H, CH@N), 8.42 (s, 1H, C1AH), 8.13
(d, J = 8.1 Hz, 1H, C12AH), 8.05 (d, 1H, J = 9 Hz, C7AH), 7.88 (d,
1H, J = 8.1 Hz, C9AH), 7.61 (dd, J = 8.1/7 Hz, C11AH), 7.43 (dd,
1H, J = 8.1/7 Hz, C10AH), 7.22 (d, 1H, J = 9 Hz, C6AH).
Synthesis of Schiff-base ligand (SB)
The Schiff base ligand SB, (Fig. 1), was prepared by the conden-
sation of 3-amino-1,2,4-triazole with 2-hydroxy-1-naphthalde-
hyde. A 1:1 mixture of them in hot methanol was refluxed for
Table 1
Elemental analysis, molecular weight and physical properties of the prepared compounds.
Compounds
Molecular formula (empirical formula) (M.wt.)
m.p. (°C)
Elemental analysis
K(mb)
%C(a)
%H(a)
%N(a)
%M(a)
–
SB
1
C13H10N4O
(238.24)
[Cu2L Cl3(H2O)5]ꢃH2O
240
65.54
4.2
23.52
–
(65.85)
26.98
(27.29)
48.37
(47.48)
27.43
(27.69)
44.03
(43.84)
44.37
(44.86)
49.48
(4.8)
3.66
(3.28)
3.93
(3.71)
3.88
(4.48)
4.01
(3.99)
4.43
(4.21)
4.02
(23.83)
9.68
(9.95)
14.56
(14.81)
14.00
(14.45)
11.74
(11.91)
11.19
(11.54)
14.89
236
21.96
(21.38)
16.51
(16.17)
18.14
(18.87)
18.44
(18.44)
17.65
(18.19)
14.60
3.85
62.8
17
(C13H21Cl3Cu2N4O7) (578.78)
[Cu2(L)2(OAc)(MeOH)]ꢃOAcꢃH2O
(C31H30Cu2N8O8) (769.71)
[Cu2L(NO3)3(H2O)3(MeOH)2]ꢃMeOH
(C16H27Cu2N7O16) (700.51)
[Ni3(L)2(OAc)4(H2O)2]ꢃMeOH
(C35H38Ni3N8O13) (954.8)
[Co3(L)2(OAc)4(MeOH)2]ꢃH2OꢃMeOH
(C37H44Co3N8O14) (1001.59)
[Mn2(L)2(OAc)(MeOH)]ꢃOAcꢃH2O
(C31H30Mn2N8O8) (752.49)
2
>300
>300
>300
>300
>300
3
4
2.99
4.54
69.46
5
6
(49.71)
(3.91)
(15.21)
(15.12)

254
M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
are collected in Table 1. The results of elemental analysis obtained
are in good agreement with those calculated for the suggested
molecular formulas. The low molar conductance values of
complexes 1, 3, 4 and 5 revealed that these complexes are non-
electrolytes in DMF [40]. Complexes 2 and 6 have molar conduc-
tance values of 62.8 and 69.4 ohmꢁ1 molꢁ1 cm2, respectively, in
DMF, which is in accordance with 1:1 electrolytes. The electrolytic
nature of these complexes was further confirmed by the addition
of FeCl3 solution to the DMF solution of these complexes which
result in the formation of red brown coloration confirming the
non coordination of one acetate ion.
Fig. 3. 1H NMR spectra of ligand SB.
Synthesis of complexes
Infrared absorption spectra
All the complexes were prepared by refluxing 1 mmol of the
Schiff base ligand dissolved in methanol with 1 mmol of each
metal salt dissolved in methanol, where either copper salts
(acetate, chloride, or nitrate), manganese acetate, cobalt acetate,
or nickel acetate was used as a metal source. The mixtures were
then refluxed under stirring for 4 h. The solid complexes which
separated out on hot were filtered off, washed several times with
methanol and finally dried in a vacuum desiccator over anhydrous
CaCl2.
The infrared spectra exhibited by the Schiff-base ligand (SB) and
its complexes are given in Fig. 2 whereas the important IR spectral
bands and their assignments are given in Table S1. The IR spectra of
the free ligand showed two bands at 3426 cmꢁ1 and 3102 cmꢁ1
which assigned to phenolic OH and NH groups, respectively. The
strong and intense bands appearing at 1613 cmꢁ1 and 1572 cmꢁ1
are due to tC@N of triazole moiety and azomethine group, respec-
tively. These bands were shifted to higher or lower frequencies
on complex formation. This shift is due to coordination of triazole
and azomethine nitrogen to the metal center. IR spectra of all com-
plexes showed broad bands at 3370–3425 cmꢁ1 which can be
Results and discussion
assigned to tOH of water or methanol molecules solvates. The pres-
The elemental analysis and molar conductance results
ence of coordinated or lattice water or methanol molecules renders
it difficult to follow the behavior of the OH groups in complex for-
mation from the stretching vibration region. The complexes 2, 4, 5
and 6 have two bands at 1581–1593 cmꢁ1 and 1386–1393 cmꢁ1
assigned to masy and msym of the acetate ion, respectively. The
The stoichiometry and formulation of the metal complexes are
based on their elemental analyses, molar conductivity and IR spec-
tral data. The results of elemental analysis and molar conductance
HN
N
HN
N
H
C
H
C
N
N
N
N
HO
221(221.08)
100 %
238 (238.09)
35.7 %
N
HC
N
N
N
H
127 (127.05)
21.3 %
140 (140.06)
(1.1 %)
82 (82.06)
2.3%
77 (77.04)
11.9 %
51 (51.02)
11.01%
Scheme 1. The proposed fragments of Schiff base ligand (SB).

M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
255
HN
N
Ac
N
H
C
O
O
H
C
Cu
N
Ac. H2O
N
N
N
C
C
Cu
O
H
H
Cu
N
N
O
N
HN
N
MeOH
HN
469 (469.06)
28.02 %
769 (768.08)
44.77 %
O
O
N
N
C
C
Cu
H
H
Cu
N
O
N
N
N
HN
HN
300 (300.01)
24.76 %
393 (393.04)
30.99 %
O
N
C
N
C
H
Cu
N
H
O
N
HN
N
N
HN
237 (237.08)
48.41 %
265 (265.02)
100 %
O
77 (77.04)
12.82 %
144 (143.05)
43.96 %
Scheme 2. The proposed fragments of Cu complex 2.
Table 2
ESR spectral data of Cu(II) complexes.
1H NMR spectra
Complex
g||
g\
geff
A||
g||/A||
G
The 1H NMR spectrum of the Schiff base ligand (SB) is shown in
Fig. 3 in which signals corresponding to the aromatic hydrogen
appeared within the range 7.22–8.13 ppm. The spectrum of the
ligand showed a singlet at 10.08, 10.79 and 14.96 ppm correspond-
ing to CH@N, OH and NH protons, respectively. The heteroaromatic
proton appeared as a singlet at 8.42 ppm.
1
2
3
2.257
2.281
2.231
2.076
2.093
2.06
2.156
2.136
2.085
94.28
–
3.35
3.021
3.85
92.86 ꢂ 10ꢁ4
128.57
245.26
–
difference between those two bands is 192–200 cmꢁ1 which
suggests that the acetate group is monodentate [41].
EI-mass spectra
The IR spectrum of complex 3 showed a strong band at
1382 cmꢁ1 which assigned to monodentate nitrato coordination.
For complex 1, a weak band appeared at 279 cmꢁ1 due to MACl
mode. The participation of oxygen and nitrogen atoms in coordina-
tion to the metal ion is further supported by the appearance of
non-ligand bands within the range 572–628 and 499–514 cmꢁ1
due to tMAO and tMAN respectively.
The EI-mass spectrum of Schiff base ligand (SB) showed a
molecular ion peak at m/z 238 that is equivalent to its molecular
weight [L]+. The peaks due to the various fragments appeared at
m/z 221(100% base peak), 169 (39.9%), 139 (1.1%), 127 (21.3%),
77 (11.9%), 63 (22.5%), 51 (11.1%) which correspond to the
fragments shown in Scheme 1.

256
M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
The molecular ion peaks of complexes have been used to con-
4A2 ? 4T1(P) transition due to tetrahedral arrangement. The Mn(II)
complex 6 showed, two broad bands at 15,151 and 22,727 cmꢁ1
corresponding to 6A1 ? 4T2(G) and 6A1 ? 4E(G) transition, respec-
tively, which corresponds to a tetrahedral structure.
firm the proposed formula. The multipeak patterns of the mass
spectra give an impression of the successive degradation of the
compounds with the series of peaks corresponding to the various
fragments. All these fragments led to the formation of the species
[ML]+ which underwent demetallation to form the species [L]+.
The proposed fragmentation pattern of complex 2, as a representa-
tive example, is shown in Scheme 2. The first peak appearing at m/z
769 (44.77%) represents the molecular ion peak of the complex. For
other metal complexes, the peaks observed at m/z 578, 700, 954,
998 and 754 for complexes 1, 3, 4, 5 and 6, respectively, correspond
to the molecular ion peaks of these compounds. The results of both
elemental analyses and mass spectra of the prepared complexes
are in satisfactory agreement with each other, which confirmed
the proposed molecular formulas.
Electron Spin Resonance (ESR)
The X-band ESR spectra of Cu(II) complexes have been recorded
in the solid state at room temperature and the ESR spectral
parameters (geff, g||, g\) are calculated and listed in Table 2. As
set of g values satisfying g|| > g\ indicates the unpaired electron
2
most likely resides in the dXꢁY2 (or less likely dXY) ground state orbi-
tal [44]. The g|| is moderately sensitive function for indicating cova-
lency of the metal–ligand bonding, in which g|| is normally 2.3 or
larger for ionic bonding and less than this value for covalent bonding
[45]. The g|| values of complexes 1, 2 and 3 are less than 2.3 indica-
tive of a significant degree of covalent bonding. The positive contri-
bution in geff values of the complexes under investigation (1, 2 and 3)
than the value of free electron (2.0023) indicated an increase in the
covalent nature of bonding between the metal ion and ligand mole-
cules. The g||/A|| may be considered as an empirical index of tetrahe-
dral distortion and square planar geometry [46]. The range reported
for square planar complexes is 105–135 cmꢁ1 and for tetrahedral
distorted complexes 150–250 cmꢁ1. The g||/A|| value of complex 2
is 245.26 cm which lies in the range expected for tetrahedral rather
than square planar complexes. The g values are related to the axial
symmetry parameter, G, by the expression G = (g|| ꢁ 2)/(g\ ꢁ 2),
which indicates the extent of exchange interaction between cupper
centers in polycrystalline solids. If the G value is greater than four,
the exchange interaction is negligible, while a value less than four
indicates a considerable exchange interaction in the complex [47].
The G value of complexes 1, 2 and 3 were found to be less than four
(3.021, 3.38 and 3.85, respectively) suggesting considerable
interactions in the solid state.
Magnetic susceptibility studies
The subnormal leff values of Cu(II) complexes (1.42, 1.44 and
1.4 B.M. for complexes 1, 2 and 3, respectively) reveals the CuACu
interaction. The magnetic moment value of Ni(II) complex 4
(3.26 B.M) lies in the range reported for a tetrahedral structure.
The observed magnetic moment of Co(II) complex 5 was found to
be 4.18 B.M [42] indicating the presence of three unpaired elec-
trons in the d-orbital in tetrahedral structure. Mn(II) complex 6
has low magnetic moment (5.27 B.M) due to metal–metal
interaction.
The electronic absorption spectra
The Nujoll mull electronic spectra of the Cu(II) complexes 1 and
3 displayed two bands at 14,793 and 24,390 cmꢁ1 for complex 1
and at 14,706 and 23,809 cmꢁ1 for complex 3 which can be
assigned to 2B1g ? 2B2g and 2B1g ? 2Eg transition, respectively
indicating distorted octahedral geometry around the metal center.
The Cu(II) complex 2 displayed two absorption bands at 14,925
and 23,256 cmꢁ1 which are attributed to 2T2 ? 2E2 and charge
transfer transitions, respectively, in tetrahedral Cu(II) complexes
[43]. The spectrum of Ni(II) complex 4 showed a medium intensity
band at 16,949 cmꢁ1 assignable to 3T1 ? 3T1(P) m3 transition in a
tetrahedral structure. The electronic spectrum of Co(II) complex
Thermogravimetric analysis of the complexes
The thermal behavior of the metal complexes was studied using
TG technique. The stages of decomposition, temperature range,
weight loss percentages as well as decomposition products are
given in Table 3. The thermal decomposition of complex 1, as an
example, takes place in three steps. The first decomposition step
5
exhibited a
band at 14,925 cmꢁ1 which is assigned to
Table 3
Thermogravimetric analysis data (TGA) of the metal complexes.
No
Molecular formula
Temp. range
Mass loss%
Assignment
(°C)
Found
Calc.
1
[Cu2L Cl3(H2O)5]ꢃH2O
26–93
3.03
3.11
15.59
53.72
2.34
Loss of hydrated H2O molecule
Loss of 5 coordinated H2O
93–295
295–580
32–109
109–442
442–604
604–718
40–100
100–252
252–630
42–100
100–643
14.79
54.62
2.82
19.75
25
33.06
3.97
16.79
54.94
3.35
Loss of Clꢁ anions and decomposition of the organic ligand with formation of CuO
Loss of 1 hydrated H2O molecule
2
[Cu2(L)2(OAc)(MeOH)]ꢃOAcꢃH2O
19.5
Loss of 1 MeOH molecule, and the two acetate anions
Dissociation of two triazole rings and 2(CH = N)
Further decomposition of the organic ligand with formation of CuO as final product
Loss of 1 MeOH molecule
Loss of 3 H2O and 2 MeOH
Decomposition of the organic ligand with formation of CuO as final product
Loss of 1 MeOH molecule
Loss of 2 coordinated H2O, acetate anions and dissociation of the organic ligand forming
NiO2 as final product
Loss of 1 H2O and 1 MeOH
Loss of 2 MeOH molecule, acetate anions and dissociation of the organic ligand with
formation of CoO as final product
Loss of1 hydrated H2O molecule
24.71
32.76
4.5
16.85
55.85
3.63
3
4
5
6
[Cu2L (NO3)3(H2O)3(MeOH)2]ꢃMeOH
[Ni3(L)2(OAc)4(H2O)2]ꢃMeOH
67.42
68.29
[Co3(L)2(OAc)4(MeOH)2]ꢃH2OꢃMeOH
[Mn2(L)2(OAc)(MeOH)]ꢃOAcꢃH2O
30–100
100–470
5.33
71.43
4.99
72.49
33–111
111–359
359–882
2.54
11.63
65.45
2.39
12.1
66.6
Loss of 1 MeOH and outer acetate molecule
Loss of coordinated acetate anion and dissociation of the organic ligand forming MnO as
final product

M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
257
D
D
Sꢄ ¼ R½lnðAh=kTÞ ꢁ 1ꢅ
appeared within the temperature range 26–93 °C with mass loss of
3.03% which corresponded to elimination of one hydrated water
molecule. The second step appeared at 93–295 °C with mass loss
of 14.79%, assigned to the elimination of five coordinated water
molecules. The third step above 295 °C represents the volatiliza-
tion of Clꢁ anions and degradation of the organic ligand leaving
CuO as the final product.
Gꢄ ¼
D
Hꢄ ꢁ T
D
Sꢄ
where k is Boltzmann constant and h is Plank’s constant. From the
above results, the following features can be deduced:
1. The negative values of the activation entropies
D
S⁄ indicate a
more ordered activated complex than the reactants and/or the
reactions are slower than normal.
Mechanistic and non-mechanistic methods were used to deter-
mine the kinetic data from TG curves. The kinetic parameters of
activation energy (E), reaction order (n) and frequency factor (A)
were obtained from TG data by non-isothermal integral method
proposed by Coats–Redfern [48] and by approximation method
proposed by Horowitz–Metzger [49]. The values obtained by
approximation method were different from those calculated by
integral method due to the different mathematical treatment of
the obtained data [50]. The kinetic parameters obtained from the
previously mentioned methods are listed in Tables S1 and S2.
The thermodynamic activation parameters namely, enthalpy
2. The positive values of the
processes are endothermic.
D
H⁄ mean that the decomposition
3. Kinetics of the thermal decomposition stages of all complexes
under study obeys, in most cases, the first order kinetics.
4. The values of activation energies increase as the maximum
temperature of decomposition increases reflecting higher
stability of the complexes formed under investigation.
5. The positive values of
neous process.
6. It was found that the order of the activation energy values of the
first decomposition step for the acetato complexes is complex
D
G⁄ suggested that this is a non-sponta-
(D
H⁄), entropy (
D D
S⁄) and free energy of decomposition ( G⁄) were
evaluated using the following relationships:
5 > complex 4 > complex 6 > complex
complexes of different anion the order is complex 3 > complex
2 while for Cu(II)
D
Hꢄ ¼ Eꢄ ꢁ RT
OH2
Cu
Cl
Cl
OH2
HN
N
Ac
O
N
H2O
H
C
M
N
H
C
N
Ac. H2O
N
H2O
N
C
H
M
N
HN
N
O
N
HN
O
Cl
H2O
MeOH
OH2
Complex 1
Complex 2 M=Cu; Complex 6 M=Mn
H2O
Me
O
MeOH
O2NO
C
O
C
Ni
Me
O
O
ONO2
MeOH
Cu
N
H
C
OH2
N
H
N
N
Ni
N
N
H
C
N
C
MeOH
MeOH
O
N
O
HN
N
O
C
H
N
H
O2NO
H2O
OH2
N
Me
Ni
Complex 3
O
O
O
O
C
H2O
Me
Complex 4
HOMe
O
C
Co
Me
Me
C
O
O
O
N
H
C
H
N
N
N
H2O.MeOH
O
Co
O
N
N
C
H
N
H
N
O
Co
O
C
O
C
O
Me
MeOH
Me
Complex 5
Scheme 3. Structures of the prepared complexes.

258
M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
1 > complex 2. This differences may be due to the structure of
the complexes and the electronic configuration of the metal(II)
ion.
Molecular modeling studies
Since single crystals could not be grown for the complexes
under investigation, it was thought worthwhile to obtain structural
information through molecular modeling. Based on the
stereochemistry of the metal complexes proposed on the basis of
analytical and spectroscopic studies, the possible structures of
some selected metal complexes have been computed using the
molecular mechanic calculations by the hyper chem. 8.03 molecu-
lar modeling program. The molecular modeling of the free ligand
and its complexes (Figs. S1–S7) showed the following:
(i) A large variation in C@N bond length on coordination via the
N-atom of group azomethine.
(ii) The absence of OAH as well as the appearance of MAO bond
indicated the participation of phenolic OAH in complex for-
mation [51].
(iii) The bond angles of Schiff base moiety are altered somewhat
upon coordination.
Fig. 4. (left) Absorption spectra of complex 2 upon the titration with CT-DNA.
Arrows indicates the change upon increasing the concentration of CT-DNA from 0 to
40
lM. [Complex] = 25 lM, (right) plot of [DNA]/(ea ꢁ ef) vs. [DNA] for the
absorption titration of CT-DNA with the complex.
Selected calculated bond lengths and angles of metal complexes
are presented in Table S4.
Based on the information gained from the previous studies, the
structure of complexes formed between the ligand with the metal
ion [Mn(II), Co(II), Ni(II) and Cu(II)] can be represented in
Scheme 3.
DNA binding interaction
Electronic absorption titration
The DNA binding studies of metal complexes with CT DNA were
carried out by employing electronic absorption spectral titration. It
is a general observation that hypochromicity in the absorption
spectra accompanies the binding of the molecules to DNA [52].
The absorption titration was performed by keeping the concentra-
tion of the complex constant while varying the DNA concentration.
The extent of the spectral change is related to the strength of
binding.
Figs. 4 and 5 show the absorption spectra of complexes 2 and 4
at constant complex concentration in the absence and presence of
calf-thymus (CT) DNA.
Fig. 5. (left) Absorption spectra of complex 6 upon the titration with CT-DNA.
Arrow indicates the change upon increasing the concentration of CT-DNA from 0 to
40
lM. [Complex] = 25 lM, (right) Plot of [DNA]/(ea ꢁ ef) vs. [DNA] for the absorp-
The electronic absorption spectra of complexes 2 and 4 consist
of two bands. The absorption band centered at 425 nm is assigned
to metal-to-ligand charge transfer (MLCT) transition. The second
band at 329 and 320 nm for complexes 2 and 4, respectively, is
tion titration of CT-DNA with the complex.
concentration is increased, the
p–
p⁄ transition bands appearing
attributed to
p–
p⁄ transition. With increasing concentration of
at 318 and 340 nm, for complexes 1 and 6, respectively, yielding
an absorbance hyperchromism of 4.46% and 41.34% for complexes
1 and 6, respectively, without change in wavelength. This implies
that some interaction occurred between complex and the surface
of DNA molecule and the interaction is not an intercalation [56].
The strong hyperchromism of complex 6 suggests a mode of bind-
ing that involves a stacking interaction between the complex and
the base pairs of DNA [57].
CT-DNA, the electronic spectra of complexes 2 and 4 showed a
clear hypochromism of about 10.27% and 13.4% and bathochro-
mism of 4 nm and 3 nm for complexes 2 and 4, respectively. These
spectral characteristics suggested that both complexes interact
with CT DNA by the intercalative binding modes, because interca-
lation, in most cases, leads to hypochromism and bathochromism
in the electronic absorption spectra [53].
The intercalative mode involve a stacking interaction between
an aromatic chromophore and the base pairs of DNA [54] while
hypochromism is a spectral feature depicting non-covalent inter-
actions, particularly electrostatic, groove binding resulting from
the damage of secondary structure of the DNA double helix. The
extent of the hypochromism is commonly consisting with the
strength of intercalative interaction [55].
To quantify the extent of binding of the complexes with CT-
DNA, the intrinsic binding constant Kb was calculated by monitor-
ing the change in the absorbance of the metal to ligand transfer
band (MLCT), with increasing the concentration of DNA using the
following equation:
½DNAꢅ=ðea
ꢁ
ef Þ ¼ ½DNAꢅ=ðeb
ꢁ
ef Þ þ 1=Kbðeb
ꢁ
ef
Þ
On the other hand, the absorption spectra of complexes 1 and 6
presented hyperchromism upon titration with DNA. As the DNA
where [DNA] is the concentration of DNA in base pairs, the apparent
absorption coefficients ea ef and eb correspond to Aobsd/[Metal], the
,

M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
259
needed to be taken together to determine the DNA binding mode
of complexes. The viscosity studies provide a strong argument for
intercalation. The viscosity measurement is based on the flow rate
of a DNA solution through a capillary viscometer. A classical inter-
action mode results in lengthening the DNA helix as base pairs are
separated to accommodate the binding ligand leading to the
increase of DNA viscosity. On the other hand, a complex that binds
exclusively in the DNA grooves by partial and/or non-classical
intercalation typically causes less pounced (positive or negative)
or no change of viscosity in DNA solution.
The effect of increasing amount of complexes on the relative
viscosity of DNA is shown in Fig. 6. The data obtained reveals that:
(i) For complex 4, the relative viscosity of DNA is increased by
increasing the concentration of complex 4. This phenome-
non may explained by the insertion of the compound in
between the DNA base pairs, leading to an increase in the
separation of base pairs at intercalation sites and an increase
in overall DNA length. The increase in DNA viscosity in case
of complex 4 suggests that complex 4 combined to DNA via
intercalative binding mode.
Fig. 6. Effect of the increasing amount of complexes 1, 2, 4 and 6 on the relative
viscosity of CT-DNA at 25 °C, [DNA] is 3 ꢂ 10ꢁ5 M.
(ii) The viscosity of DNA remains almost unchanged upon addi-
tion of complexes 1, 2 and 6 suggesting non-intercalative
and probably groove binding mode of complexes [64].
extinction coefficient for the free complex and the extinction
coefficient for the bound complex, respectively. In plots of [DNA]/
(
ea
ꢁ
ef) vs. [DNA], Kb is given by the ratio of the slope to the inter-
cept through a plot of [DNA]/(ea
ꢁ
e
f) vs. [DNA]/(eb
ꢁ
ef).
For complex 6, the Kb value 3.45 ꢂ 105 Mꢁ1 suggests a strong
binding of Mn-complex 6 with DNA close enough to the classical
intercalator Ethidium bromide binding affinity to DNA (Kb = 1.23 -
ꢂ 105 Mꢁ1) [57].
Antimicrobial activity
The biological activity of the synthesized Schiff base ligand (SB)
and its metal complexes have been studied for their antimicrobial
activity against Gram positive bacteria (S. aureus), Gram negative
bacteria (E. coli and P. aeruginosa), fungi (A. flavus and Mucer) and
yeast (C. albicans and M. furfur). These organisms were chosen as
they are known as human pathogens. Tetracycline, Amphotericin
B and Streptomycin were used as standard drugs. The capability
of the prepared compounds to inhibit the growth of microorgan-
isms was tested on solid media through of inhibition zone diame-
ter; the obtained data are shown in Table 4 It was observed that SB
is active Gram negative bacteria (E. coli), Gram positive bacteria,
fungi (A. flavus) and yeast (M. furfur) (diameters inhibition zone
ranged between 16 and 30 mm). The Cu(II) complex 1 showed
activity against all the tested organisms (diameters inhibition zone
ranged between 14 and 30 mm) except the yeast, M. furfur. The
results showed also that complex 3 showed activity against fungi,
one type of Gram negative bacteria (E. coli) and yeast (C. albicans).
The Ni(II) complex 4 showed the highest activity against A. flavus
with inhibition zone diameters 45 mm. It also showed activity
against E. coli and A. flavus. The Co(II) complex 5 was active against
Gram negative, fungi and C. albicans, and gave the highest activity
For Cu(II) complexes 1 and 2, the higher Kb value obtained for
complex 1 suggests that more base pairs are available for binding
than in complex 2. The lower Kb value of complex 2 indicates the
non-intercalative binding interaction with DNA and probable
groove binding or external binding is suggested [56].
Also, the Kb values for Cu-complexes are of:
(i) Lower magnitude than that of the Cu complexes reported in
the literature such as DNA intercalator [Cu(TAN)(O2CMe)],
(Kb = 9.8 ꢂ 105 Mꢁ1 [58]; [Cu(flmq)2 (py)2], (Kb = 8.12 ꢂ 105 -
Mꢁ1) [59] and [Cu(dthp)Cl2] (Kb = 6.9 ꢂ 105 Mꢁ1) [60].
(ii) Larger than that of [Cu(Itpy)2](ClO4) (Kb = 4.26 ꢂ 103 Mꢁ1
[61]; [Cu(bpy)(Gly)Cl] 2H2O, (Kb = 1.84 ꢂ 103 Mꢁ1) [10].
)
(iii) COMPARABLE to [(phen)Cu(l-bipp)Cu(phen)]ClO4 (Kb = 1.6 -
ꢂ 104 Mꢁ1 [62]) [Cu(imda) (dpq)], (Kb = 1.7 ꢂ 104 Mꢁ1 [63].
Viscosity measurements
The optical methods cannot be used exclusively to prove inter-
calation so other methods such as viscosity measurement is
Table 4
Inhibition zone diameter (mm) produced by 0.02 g ligands and its complexes against different organisms.
Compounds
Organisms
Bacteria
E. coli
Fungi
Yeast
P. aureus
St. aureus
A. flavus
Mucor
M. furfur
C. albicans
SB
1
3
4
16
25
20
19
28
33
–
ꢁvea
20
21
20
18.5
45
45
–
ꢁve
28
13
ꢁve
10
–
30
ꢁve
32
10
ꢁve
25
–
14
19.5
ꢁve
ꢁve
ꢁve
30
ꢁve
ꢁve
ꢁve
ꢁve
–
ꢁve
ꢁve
11
30
–
5
Tetracyclineb
Amphotericin Bc
Streptomycind
–
–
20
–
18
–
–
17
20
–
–
–
a
Inactive against the tested organism.
Standard drugs used.
b,c,d

260
M. Gaber et al. / Journal of Molecular Structure 1076 (2014) 251–261
Table 5
metal complexes enhanced, in most cases, the antimicrobial activ-
ity of the free ligand.
The ratio of surviving cell number of A. flavus in the presence of different
concentrations of complexes 4 and 5 using turbidity method.
Comp
Conc.(
0
l
g/ml)
Acknowledgment
30
60
120
250
500
1000
% Surviving ratio
The authors are thankful to Prof. Dr. Y.A.G. Mahmoud, Tanta
University, Faculty of Science, Botany Department, for his assis-
tance to obtain the biological results of the synthesized
compounds.
4
5
100
100
90
46.5
75
20
54
15.65
33.5
10
20
9.5
5
7.5
Appendix A. Supplementary material
100
Complex 4
Complex 5
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.06.
071.
80
60
40
20
0
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