Preparation, spectral studies, theoretical, electrochemical and antibacterial investigation of a new Schiff base and its some metal complexes

Article abstract of DOI:10.1016/j.molstruc.2014.06.062

A new Schiff base ligand, 1,6-Bis(2-(5-bromo-2-hydroxybenzylideneamino)-4- chlorophenoxy)hexane was synthesized. Some Schiff metal complexes of the new Schiff base were prepared by the reaction of some metal salts and the Schiff base. The complexes are non-electrolytes as shown by their molar conductivities (ΛM). The structures of metal complexes are proposed from elemental analysis, FT-IR, UV-vis, magnetic susceptibility measurements, molar conductivity measurements, mass spectra and thermal gravimetric analysis. In addition theoretical ¹H NMR, HOMO-LUMO studies of the ligand; antimicrobial and cyclic voltammetric studies of the compounds were also carried out. In this study antioxidant and antibacterial activities of the compounds were examined via in vitro methods.

Full text of DOI:10.1016/j.molstruc.2014.06.062

Journal of Molecular Structure 1075 (2014) 32–42
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Preparation, spectral studies, theoretical, electrochemical
and antibacterial investigation of a new Schiff base
and its some metal complexes
a,
a
b
c
d
a
S. Ilhan a, , H. Baykara , M.S. Seyitoglu , A. Levent , S. Özdemir , A. Dündar , A. Öztomsuk ,
⇑
⇑
M.H. Cornejo e
a Faculty of Science and Letters, Chemistry Department, Siirt University, Siirt, Turkey
b Batman University, Health Services Vocational College, 72100 Batman, Turkey
c Faculty of Science and Letters, Biology Department, Siirt University, Siirt, Turkey
d Mardin Artuklu University, Vocational Higher School of Health Services, Medical Promotion and Marketing Program, 47000 Mardin, Turkey
e Escuela Superior Politécnica del Litoral, ESPOL, CIDNA, Km 30.5 Via Perimetral, Guayaquil, Ecuador
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
1,6-Bis(2-(5-bromo-2-hydroxybenzylideneamino)-4-chlorophenoxy)hexane was synthesized.
[CuL], [NiL], [CoL], [ZnL], [VLCl] complexes of the ligand were synthesized.
In addition theoretical 1H- NMR, HOMO–LUMO studies of the ligand were done.
Antioxidant and antibacterial activities of the compounds were examined.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 May 2014
Received in revised form 19 June 2014
Accepted 20 June 2014
Available online 26 June 2014
A new Schiff base ligand, 1,6-Bis(2-(5-bromo-2-hydroxybenzylideneamino)-4-chlorophenoxy)hexane
was synthesized. Some Schiff metal complexes of the new Schiff base were prepared by the reaction of
some metal salts and the Schiff base. The complexes are non-electrolytes as shown by their molar
conductivities (KM). The structures of metal complexes are proposed from elemental analysis, FT-IR,
UV–vis, magnetic susceptibility measurements, molar conductivity measurements, mass spectra and
thermal gravimetric analysis. In addition theoretical 1H NMR, HOMO–LUMO studies of the ligand;
antimicrobial and cyclic voltammetric studies of the compounds were also carried out. In this study
antioxidant and antibacterial activities of the compounds were examined via in vitro methods.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Di-functional ligand
Cyclic voltammetry
Antioxidant
HOMO–LUMO
Mass spectra and Gaussian
Introduction
Yorke et al. synthesized 2,4,6-trimethyl-(di-2-pyridylmethyl-
ene)aniline and 2,6-di-isopropyl-(di-2-pyridylmethylene)aniline
Synthesis and characterization of Schiff bases and their
corresponding complexes are very important because of their
potential applications and properties such as anticancer, anticon-
vulsant, antitumor, antifungal, antibacterial, antitubercular,
antioxidant, antimalarias, antiflammatory, corrosion inhibitor, bio-
logical, anti-HIV and pesticidal properties [1–19]. Through the
years, Schiff bases played and developed a central role as chelating
ligands in the main group and transition metal coordination chem-
istry [20–24].
via condensation reactions between 2,20-dipyridyl ketone and
2,4,6-trimethylaniline and 2,6-di-isopropylaniline. They prepared
palladium (II) complexes of the new imine compounds and used
the complexes as catalysts in Suzuki coupling reaction of p-bromo-
anisole and phenylboronic acid [25].
Yang et al. described and explained the carbon–fluorine bond
activation and alkylation in polyfluoroaryl Schiff bases by in situ
19F NMR spectroscopy [26].
Pordel et al. synthesized and characterized novel 2-(5-hydrox-
yimino-1-alkyl-4,5-dihydro-1H-4-indazolyliden)-2-arylacetonitr-
iles. They found out and showed the intramolecular charge
transfer (ICT) due to their color intensities [27].
⇑
Corresponding authors. Tel.: +90 4842231224; fax: +90 4842232086.
E-mail addresses: hacibaykara@gmail.com, salihsiirt@gmail.com (H. Baykara).
http://dx.doi.org/10.1016/j.molstruc.2014.06.062
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
33
Suvitha et al. synthesized pycolinaldehyde oxime (PAO) by the
reaction of picolinaldehyde (2-formyl pyridine) with hydroxyl-
amine. They carried out some comparative studies too by means
of comparison of theoretical (B3LYP/6-311++G(d,p)) and experi-
mental FT-IR and FT-Raman spectra, and theoretical NMR values
to experimental values of PAO as supportive characterizations
studies. On the other hand, frontier orbitals’, HOMO–LUMO with
their energies (by time-dependent DFT (TD-DFT)), molecular
electrostatic potentials (MEP) were also visualized [28].
Kharadi synthesized octahedral copper complexes of clioquinol
(CQ) and substituted terpyridine. Antimycobacterial properties of
the complexes and the ligands were investigated against Mycobac-
terium tuberculosis. Antioxidant properties of the compounds were
carried out by ferric-reducing method, and antimicrobial activities
of the compounds were also investigated against five different
microorganisms [29].
voltammetric (CV) experiments are recorded at room temperature
in extra pure dimethyl formamide (DMF), and ionic strength is
maintained at 0.1 mol/L by using electrochemical grade tetrabutyl-
ammonium perchlorate (TBAP) as the supporting electrolyte.
Solutions were deoxygenated by a stream of high purity nitrogen
for 15 min prior to the experiments, and during the experiments
nitrogen flow was maintained over the solution. 2,2-diphenyl-1-
picrylhydrazyl radical (DPPH), ferrous chloride,
a-tocopherol,
3-(2-pyridyl)-5,6-bis (4-phenyl-sulfonic acid)- 1,2,4-triazine (Fer-
rozine), ascorbic acid, ethylenediaminetetraacetic acid (EDTA),
Trolox and dimethylformamide (DMF) were purchased from Sigma
(Sigma–Aldrich GmbH, Sternheim, Germany). Methanol was
purchased from E. Merck. Blank and standard antimicrobial sus-
ceptibility test discs were purchased from Oxoid. All other chemi-
cals were analytical grade and obtained from either Sigma–Aldrich
or Merck.
Dhanaraj and Johnson synthesized a Schiff base by the reaction
of quinoxaline-2,3-(1,4H)-dione and 4-aminoantipyrine (QDAAP)
then prepared its new metal complexes and investigated antimi-
crobial properties of the compounds synthesized [30].
The 1,6-Bis(4-chloro-2-aminophenoxy)hexane used in the
synthesis was prepared from 4-chloro-2-nitrophenole, 1,6-dibro-
mohexane and K2CO3 as shown in Fig. 1 [33,34].
Upadhyay et al. synthesized and characterized monomeric
metal complexes, [Cu(L1)] and [Ni(L1)], by the reaction of Schiff
(L1 = C20H24NO2) base ligands with metal salts [31].
In this study, a new Schiff base, 1,6-Bis(2-(5-bromo-2-hydrox-
ybenzylideneamino)-4-chlorophenoxy)hexane and its metal
complexes were prepared and characterized. All of the compounds
were characterized by several characterization techniques
including FT-IR, 13C NMR, 1H NMR, UV–vis, magnetic susceptibility
studies, theoretical studies, electrochemical, antioxidant and anti-
microbial studies.
Synthesis of Schiff base
The 1,6-Bis(2-(5-bromo-2-hydroxybenzylideneamino)-4-chlo-
rophenoxy)hexane was synthesized by the reaction of 5-bromosal-
icylaldehyde (40 mmol) with equivalent amount of 1,6-Bis
(4-chloro-2-aminophenoxy) hexane (20 mmol) in ethanol med-
ium. The reaction was continued for 2 h and the mixture cooled
to room temperature, placed in a fridge for 24 h. Then the solid
product was filtered, washed with cold ethanol and dried at
25 °C in a vacuum oven (Fig. 2).
Characterization of ligand (H2L)
Experimental
Molecular Weight: 735. m.p.: 168–170 °C. Color: Yellow. Yield:
Material and methods
12.28 g (83.6%). Anal Calcd. for C32H28N2O4Cl2Br2 : C, 52.17, H, 3.81,
N, 3.81. Found: C, 53.91, H, 4.17, N, 3.85. IR (cmꢁ1): 3071
t(ArACH),
All the chemicals and solvents were used as received, without a
further purification. Elemental analyses were carried out on a LECO
CHNS model 932 elemental analyze device. FT-IR spectra were
recorded on a PERKIN ELMER SPECTRUM 100 FTIR spectrometer
on a universal ATR arm. Ultraviolet–visible spectroscopic studies
were done on a PERKIN ELMER LAMBDA 750 model UV Visible
spectrophotometer in the wavelength. Molar conductivities were
measured by using a Mettler Toledo Seven Multi pH Conductivity
Meter.
2915, 2871
t
(Alf.ACH), 1617
(ArAO), 1173,
= 3500), k2 = 275(e = 2900) nm k3 = 382 nm(e =
t
(C@N), 1492, 1469
t
(ArAC@C),
1286,
1257
t
1162 (RAO).
t
UV–vis:
k1 = 264 nm(
e
3200). 13C NMR (ppm, in DMSO-d6): OCH2CH2: 26.72, OCH2:
64.29, OHAC: 161.38, HC@N: 160.68, (ArAC): 108.92, 113.56,
119.42, 120.06, 121.47, 121.52, 129.15, 134.12, 136.03 1H NMR
(ppm, in DMSO-d6): d = 4.06 (t, OCH2), d = 1.75 (OCH2CH2, m),
1.52 (OCH2CH2CH2, s) d = 6.45–7.80 (ArAH), d = 8.95 (HC@N),
d = 13.64 (OH). Mass Spectra : m/z:735 [H2L]+.
1H and 13C NMR spectra were recorded using a BRUKER
AVANCE DPX-400 NMR spectrometer. Magnetic Susceptibilities
were measured by using a Sherwood Scientific Magnetic Suscepti-
bility Balance (Model MK1) at room temperature (20 °C) using
Hg[Co(SCN)4] as a calibrant; diamagnetic corrections were calcu-
lated from Pascal’s constants [32]. Mass spectral analyses were
determined on an Agilent 1100 Series LC/MSD mass spectrometer.
Thermal stability properties were investigated on a EXSTAR S II TG/
DTA 6300 Model thermal analyzer. A standard office computer
(with 4 GB RAM) was used to carry out the theoretical calculations.
Gaussian 09 and Gauss View 5 software packages were used for the
optimization, theoretical 1H NMR, frontier orbitals and visualiza-
tion of the entire input and output files. Electrochemical experi-
ments were carried out by using an Autolab PGSTAT 128 N
potentiostat, with a three electrode system, glassy carbon working
Synthesis of Schiff base complexes
A solution of a metal salt such as Cu(II), Zn(II), Ni(II), Co(II) and
V(III) in DMF (40 mL) was reacted with the Schiff base ligand
(2 mmol) in DMF (60 mL) with 1:1 M ratio (Fig. 3). The content
was refluxed for two hours on a hot plate-magnetic stirrer. The
content was cooled to the room temperature, and solid product
isolated by filtration, washed with cold ethyl alcohol and dried at
25 °C in a vacuum oven. The complexes yielded were not soluble
in organic solvents besides dimethyl sulfoxide and dimethyl
formamide.
Characterization of [CuL]
electrode (
U
: 3 mm, BAS), platinum wire as auxiliary electrode and
Molecular Weight: 797. m.p.: 202 °C decompose. Color: light
brown. Yield: 1.23 g (77.4%). Anal Calcd. for CuC32H26N2O4Cl2Br2:
C, 48.18, H, 3.26, N, 3.51 . Found: C, 49.02, H, 3.83, N, 3.48. Selected
Ag/AgCl (NaCl 3 M, Model RE-1, BAS, USA) as reference electrode.
The reference electrode is separated from the bulk solution by a
fritted-glass bridge filled with the solvent/supporting electrolyte
mixture. Before starting each experiment, the glassy carbon
IR data (KBr,
t
cmꢁ1): 3063
(C@N), 1490, 1473 (ArAC@C), 1233, 1197
(RAO). k1 = 266 nm( = 12,100), k2 = 286 nm(e
t
(ArACH), 2913, 2873
(ArAO), 1161,
= 9200),
t(Alf.ACH),
1607
1113
t
t
t
electrode is polished manually with alumina (
U
: 0.01
lm). Cyclic
t
e

34
S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
Cl
NO2
O
CH2
(CH2)4
CH2
O
Cl
NO2
OH
+
+
2
K2CO3
Br CH2(CH2)4CH2 Br
-H2O, -CO2, -KBr
NO2
Cl
NO2
O2N
NH2
H2N
Pd/C
+
Cl
O
(CH2)6
O
Cl
N2H4.H2O
Cl
O
(CH2)6
O
Cl
Fig. 1. Synthesis of the 1,6-Bis(4-chloro-2-aminophenoxy)hexane.
Cl
Cl
O
Br
N
CH
NH2
O
O
HO
Br
H
+
2
-2H2O
OH
HO
O
O
N
CH
NH2
Br
Cl
Cl
Fig. 2. Synthesis of the ligand (H2L).
k3 = 394 nm(
e
= 5600)
leff = 1.89 Bohr Magneton (B.M.) Mass Spec-
t
(RAO). UV–vis (kmax, nm) in DMF: k1 = 266 nm(
e
= 3500),
tra : m/z:797 [CuL]+.
k2 = 352 nm(e = 3200), k3 = 436 nm(e = 960). leff = 3.69 B.M. Mass
Spectra : m/z:793 [CoL + H]+.
Characterization of [NiL]
Characterization of [ZnL]
Molecular Weight: 791. m.p.: 110 °C decompose. Color: light
brown. Yield: 1.14 g (72.2%). Anal Calcd. for NiC32H26N2O4Cl2Br2:
C, 48.55, H, 3.28, N, 3.54. Found: C, 47.08, H, 3.30, N, 4.06. Selected
Molecular Weight: 798. m.p.: 152–154 °C. Color: Orange. Yield:
1.05 g (66 %). Anal Calcd. for ZnC32H26N2O4Cl2Br2: C, 48.12, H, 3.26,
IR data (KBr,
1609, (C@N), 1488, 1450
1159, 1187 (RAO). UV–vis (kmax, nm) in DMF: k1 = 260 nm
t
cmꢁ1)): 3066
t
(ArACH), 2929, 2857
t(Alf.ACH),
N, 3.51. Found: C, 48.90, H, 4.22, N, 3.88. Selected IR data (KBr, t
t
t(ArAC@C), 1286, 1240
t(ArAO),
cmꢁ1): 3070
1491, 1459
t
(ArACH), 2947, 2857 (Alf.ACH), 1611,
t
t
t
(C@N),
(RAO).
t
t
(ArAC@C), 1286, 1257 t(ArAO), 1174, 1160
(e
= 5000), k2 = 334 nm(
e
= 1700), k3 = 430 nm(
e
= 1500).
leff = 2.91
1H NMR (ppm): d = 4.29 (OCH2), d = 6.92–7.96 (ArAH), d = 9.02
B.M. Mass Spectra : m/z:790 [NiL–H]+.
(HC@N). UV–vis (kmax, nm) in DMF: k1 = 259 nm(
k2 = 274 nm( = 3200), k3 = 361 nm( = 9600) k4 = 431 nm(
1100).
eff = Diamagnetic. Mass Spectra : m/z:797 [ZnL-H]+.
e = 4300),
e
e
e =
l
Characterization of [CoL]
Molecular Weight: 792. m.p.: 157–160 °C. Color: Brown. Yield:
1.09 g (68.8%). Anal Calcd. for CoC32H26N2O4Cl2Br2: C, 48.48, H,
3.28, N, 3.54. Found: C, 50.15, H, 4.21, N, 3.97. Selected IR data
Characterization of [VLCl]
Molecular Weight: 820. m.p.: 167–170 °C. Color: Dark Brown.
Yield: 0.95 g (58 %). Anal Calcd. for VC32H26N2O4Cl3Br2: C, 46.83,
H, 3.17 , N, 3.41 . Found: C, 47.22, H, 4.10, N, 3.97. Selected IR data
(KBr,
t
cmꢁ1): 3056
t
(ArACH), 2932, 2856
t(Alf.ACH), 1610,
t(C@N), 1489, 1452
t
(ArAC@C), 1285, 1248 (ArAO), 1156, 1122
t

S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
35
Cl
O
Cl
Cl
O
Br
Br
Br
N
CH
V
N
CH
M
N
CH
O
O
O
X
HO
or
Cl
+
M(Salt)x.nH2O
O
O
HO
O
O
O
N
CH
N
CH
N
CH
Br
Br
Br
Cl
Cl
Cl
M= Zn(II), Cu(II), Co(II), Ni(II),
M= Cu, Ni, Co, Zn, V
x= 2-3
Fig. 3. Synthesis of the complexes.
(KBr,
t
t
t
cmꢁ1): 3069
(C@N), 1491, 1471
(RAO). UV–vis (kmax, nm) in DMF: k1 = 262 nm(
t
(ArACH), 2940, 2859
(ArAC@C), 1281, 1253 (ArAO), 1126, 1173
= 3300),
eff = 2.84 B.M. Mass Spectra : m/z:822
t
(Alf.ACH), 1611,
Antimicrobial analyses
t
t
In vitro antibacterial studies were done for determining the
antimicrobial potentials of the compounds used in this study. For
this purpose disc diffusion method was used [37]. Escherichia coli
(ATCC 10536), Staphylococcus aureus (ATCC 6538), Bacillus subtilis
(6051) Legionella pneumophila subsp. pneumophiia (ATCC
33152), Enterococcus hirae (ATCC 10541), Micrococcus luteus (ATCC
9341), and Pseudomonas aeruginosa (ATCC 9027) were used as test
microorganisms. Bacterial cultures were incubated at 37 °C for
24 h in Nutrient Broth. Each plate was filled with 25 mL of Nutrient
Agar. 100 lL of the bacteria culture suspension inoculated to petri
plates and dealt with a drigalski spatula. The blank discs (6.0 mm,
Oxoid Ltd, Wade Road, Basingstoke, Hants, RG24 8PW, UK.) were
impregnated with 15
e
k2 = 362 nm(
e
= 3320).
l
[VLCl + 2H]+.
Antioxidant analyses of the compounds
DPPH scavenging activity
Free radical scavenging capacities of the complexes were deter-
mined by previously reported procedure using the stable 2,2-
diphenyl-1-picrylhydrazyl radical (DPPH) [35]. The solutions of
the compounds (0.1 mL) at different concentrations (5, 10, 25, 50
and 100 mg/L) were added to 3.9 mL of freshly prepared DPPH in
methanol. The mixture was shaken and stands for 30 min in the
dark at room temperature. Finally the absorbance were measured
at 517 nm against a blank solution which consist 4 mL methanol.
Ascorbic acid and Trolox were used as positive control to compare
the activities. The formula of DPPH scavenging activity is;
lL compound solutions. Same volume
(15 L) of DMF was used as control. The inoculated plates were
l
incubated for 24 h. The diameters of the inhibition zones were
measured with calipers.
Results and discussion
I% ¼ ðAcontrol ꢁ AsampleÞ=Acontrol ꢂ 100;
Schiff base and complexes
where Acontrol is the absorbance of the control reaction (containing
all reagents except the test compound), and Asample is the absor-
bance of the test compound. Tests were carried out in triplicate.
Trolox and ascorbic acid were used as positive control.
In this work, we have synthesized a new Schiff-base ligand and
its some metal complexes. The ligand and metal complexes were
characterized by elemental analysis, IR, UV–vis, TGA-DTA, 1H
NMR and 13C NMR, conductivity measurements, cyclic voltamme-
try and magnetic susceptibility studies. Some theoretical computa-
tions such as optimization, 1H NMR, frontier orbitals (Highest
Occupied Molecular Orbital (HOMO) and Lowest Unoccupied
Molecular Orbital (LUMO)) visualization studies were also carried
out as supportive characterizations for the ligand. Additionally a
linear regression analysis was also done to determine the compat-
ibility between theoretical and experimental 1H NMR shifts.
Chelating effects on ferrous ions
Metal chelating effects on ferrous ions exhibited by the com-
pounds was carried out as described by Hsu et al. [36] 1 mL of
the synthesized compounds solutions (DMF) at varied concentra-
tions, 0.1 mL 2 mM FeCl2.H2O, 0.2 mL 5 mM ferrozine-1,2,4-tri-
azine (Sigma) and 3.7 mL methanol were mixed in a test tube
and reacted for 10 min. The absorbance of reactants was measured
at 562 nm. The mixture without extract was used as control. A
lower absorbance indicates a higher ferrous ion chelating capacity
and was check against EDTA (Sigma) as a positive control using for-
mula below;
FTIR spectra
Some characteristic IR bands/peaks of the compounds can be
seen in Table 1. Due to the strong intramolecular interactions
between hydroxyl groups and imine groups in the ligand, hydroxyl
(OAH) stretching band/peak could not be observed in the region
expected (ꢃ3300 cmꢁ1). In the literature there are many papers
that explains the intramolecular hydrogen bonding [38,39]. The
Metal chelating effect : ðAcontrol ꢁ AsampleÞ=Acontrol ꢂ 100
where Acontrol is the absorbance of the control reaction (containing
only FeCl2 and ferrozine), and Asample is the absorbance of the com-
pounds/reference. EDTA was used as a positive control.

36
S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
FT-IR spectra of the aldehyde and amine showed the IR bands at
1735 and 3420 cmꢁ1 due to
(C@O) and (NH2) stretching vibra-
of the Zn(II) complex. Those signals were observed in the 1H NMR
spectra of the ligand. The elimination of the phenolic protons or
the absence of phenolic proton signal proofs the formation of the
[ZnL] complex. In the 1H NMR spectra of [ZnL] complex imine
group’s proton (ACH@NA) showed a higher value than the ligand’s
imine group. This is another proof for the formation of [ZnL]
complex [51].
t
t
tions disappeared and instead, in the IR spectra of the ligand a
new band was observed 1617 cmꢁ1 attributed to azomethine
t(C@N) group. This finding/change suggests that the amino and
aldehyde groups of the starting reactants have been converted into
the corresponding Schiff base. The peak at ꢃ1285 cmꢁ1 in the IR
spectrum of the ligand is assigned to the phenolic CAO stretching
vibration. This peak is found in a region lower than ꢃ1285 cmꢁ1 in
the spectra of the metal complexes. The peaks around 1610 cmꢁ1
in FT-IR of the complexes can be assigned to stretching vibrations
TGA studies
The thermal stabilities of the compounds were confirmed by
thermal gravimetric analyses. The termogravimetric analyses
(TGA) are given in Table 2 and the thermograms were obtained
at a heating rate of 10 °C/min in a nitrogen atmosphere over a tem-
perature range of 50–900 °C. [29,31]. The detailed information
about thermal stabilities, decompositions can be found in Table 2.
of azomethine
of azomethine
t
t
(AC@N) groups [40–42]. The stretching vibrations
(AC@N) groups in the complexes were shifted to
lower values. It is assumed that these shifts to the lower values
are the proofs for the coordination between C@N groups and
metals [43]. In the spectra of the compounds the bands at 2965–
2855 cmꢁ1 are assigned to
t(Aliphatic CAH) groups and 3050–
3100 cmꢁ1 t(Aromatic CAH) stretching vibrations [44].
Mass spectra
Electronic spectra
The mass spectra of the complexes peaks attributable to the
molecular ions m/z: 735 [H2L]+, m/z: 797 [CuL]+, m/z: 790 [NiL-
H]+, m/z: 793 [CoL + H]+, m/z: 797 [ZnL-H]+, m/z: 822 [VLCl + 2H]+.
The ultraviolet–visible (UV–vis) spectral data (in DMF) of the
compounds at room temperature can be found in experimental
section. The electronic spectra of the compounds showed few
bands in the UV–vis region (200–900 nm). The absorption bands
below at ca.300 nm are practically identical and can be assigned
Theoretical studies
Structure optimization [52] carried out by applying B3LYP level
of Density Functional Theory (DFT) [53] at the 6-311G basis set.
Optimized 3D structure of the ligand can be seen in Fig. 4. NMR
calculations [54] carried out by applying DFT and Hartree Fock
(HF) [55] theories with different levels and basis sets (Fig. 5 and
Table 3). In this study, Gauge-Independent Atomic Orbital (GIAO)
method was applied to compute theoretical NMR shifts of the
ligand, at gas phase [56–60].
to
p ?
p* transitions in the benzene ring and azomethine (AC@N)
groups. The absorption bands observed within the ca. 300–400 nm
range are due to the n ? p* transitions in imine and phenoxy
groups of the ligand and its metal complexes [45].The transitions
such as n ? p* and
p ?
p* in the ligand were shifted to higher
wavelengths in the complexes [46]. These changes are the signs
the formation of the coordination between metal and ligand
[45,46].
Frontier orbitals (Highest Occupied Molecular Orbital (HOMO)
and Lowest Unoccupied Molecular Orbital (LUMO)) and transition
energy levels are shown in Fig. 6. Frontier molecular orbitals ener-
gies were computed at B3LYP/6-311G level of the DFT method [61].
The difference of the energy level between HOMO and LUMO orbi-
tals can be also seen in Fig. 6. The energy levels of HOMO and
LUMO structures were found ꢁ6.1918 eV and ꢁ2.5752 eV respec-
tively. The excitation energy value for the HOMO–LUMO transition
was found as 3.6166 eV and, this energy value can be assigned to
Magnetic measurements and molar conductivity
The magnetic moments of the complexes were measured room
temperature. The measurements give additional information for
characterization of the complexes. The magnetic moment values
of the complexes can be found in the characterization section. It
was determined that all the complexes are non-electrolytes. The
molar conductivity values of the complexes (in DMF) are in the
range reported for non-electrolytes [47–49].
the
p
?
p* transition in the aromatic rings and C@N groups
bonds can be clearly seen in Fig. 6 by the
(Fig. 6). Formation of
p
interaction of p orbitals, in the HOMO view.
1H NMR and 13C NMR
On the other hand, a linear regression analysis was also done to
control the compatibility between experimental and theoretical 1H
NMR shifts of the ligand, as a supportive characterization tech-
nique to the experimental characterization techniques. Regression
analysis data of the theoretical and experimental NMR shifts can be
seen in Table 4. The table shows the comparison of experimental
data to theoretical Hartree–Fock and B3LYP methods’ data.
Although outlier, being the most extreme observations, can have
many anomalous causes, its exclusion can make the model
The 1H NMR and 13C NMR spectra of the ligand and its Zn(II)
complex were recorded (solvent, DMSO-d6). The interpretations
of those spectra are given in the experimental section. Structural
information about the ligand was provided by 13C NMR spectra
in details. All of the aliphatic, aromatic and azomethine carbon
(CH@N) shifts were confirmed in the regions expected [50]. The
phenolic protons’ signals were not observed in the 1H NMR spectra
Table 1
Physical characterization, analytical, molar conductance and spectral data of the compounds.
Compound
Yield (%)
Color
Melting point (°C)
Formula weight
(Calcd.) Found %C
%H
%N
m
(C@N) (cmꢁ1
)
leff (BM)
Ligand (H2L)
[CuL]
[NiL]
[CoL]
[ZnL]
12.28 (83.6)
1.23 (77.4)
1.14 (72.2)
1.09 (68.8)
1.05 (66)
Yellow
168–170
735
797
791
792
798
820
(52.17) 53.91
(48.18) 49.02
(48.55) 47.08
(48.48) 50.15
(48.12) 48.90
(46.83) 47.22
(3.81) 4.17
(3.26) 3.83
(3.28) 3.30
(3.28) 4.21
(3.26) 4.22
(3.17) 4.10
(3.81) 3.85
(3.51) 3.48
(3.54) 4.06
(3.54) 3.97
(3.51) 3.88
(3.41) 3.97
1617 m
1607 m
1609 m
1610 m
1611 m
1611 m
–
Light Brown
Light Brown
Brown
Orange
Dark Brown
202 Decom.
110 Decom.
157–160
152–154
167–170
1.89
2.91
3.69
–
[VLCl]
0.95 (58)
2.84
m : medium.

S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
37
Table 2
TGA data of the complexes.
Compounds
First Step, °C (DTG °C)
Weight loss %,
Second Step, °C(DTG °C)
Weight loss %,
Third Step, °C (DTG °C)
Weight loss %,
Calculated (Found)
Decomposition group
Calculated (Found)
Decomposition group
Calculated (Found)
Decomposition group
Ligand (H2L)
[CuL]
290–365 (324)
42.2 (41.3)
C12H6Br2
365–615 (531)
67.6 (69.1)
C6H12O2 and Cl2
260–380 (323)
53.4 (54.0)
380–756 (462)
90.1 (89.5)
C
12H6Br2
Residue CuO
[NiL]
287–376 (326)
19.6 (20.5)
C6H3Br
376–608 (566)
53.9 (54.1)
C6H3Br and C6H12O2
608–805 (674)
90.6 (90.0)
Residue NiO
[CoL]
298–332 (330)
19.6 (20.0)
C6H3Br
332–481 (453)
53.8 (53.1)
C6H3Br and C6H12O2
481–684 (616)
90.7 (90.0)
Residue CoO
[ZnL]
316–355 (350)
19.5 (20.1)
C6H3Br
355–491 (420)
53.4 (55.2)
C6H3Br and C6H12O2
491–808 (695)
89.7 (89.8)
Residue ZnO
[VLCl]
264–311 (305)
18.9 (21.3)
C6H3Br
311–452 (420)
51.9 (53.2)
C6H3Br and C6H12O2
Fig. 4. Optimized 3D structure of the ligand (H2L) (B3LYP/6-311G).
methodologically sounds. The R-Squared statistic indicates that the
models as fitted explain 70.7% included outlier, but 93.2% without
outlier of the variability in experimental data. Moreover the corre-
lation coefficient equals 0.841 and 0.965 for the model with outlier
change and difference between theoretical and experimental NMR
shifts can be assigned to acidic character of phenolic protons and
executing theoretical calculations in the gas phase [62–64].
and without outlier respectively, indicating
a
shift from
a
Electrochemical analysis
moderately strong relationship to strong relationship between
the variables. Finally, the estimated standard error shows the stan-
dard deviation of the residuals to be 1.947 for the model with out-
lier, and 0.860 for model without outlier. This value can be used to
construct prediction limits for new observations.
According to the regression analysis, there is a good conformity
between experimental and theoretical 1H NMR shifts (Fig. 5 and
Table 3) but only the values for phenolic protons’ (the proton num-
bered 70) experimental NMR shifts (13.6437) are very different
(Table 3) than theoretical (HF = 4.0769 and B3LYP = 3.3614)
because of this difference, estimated standard error is higher than
expected. If the values for the 70 numbered protons neglected then
a very high accuracy regression results are obtained (Table 4). The
The CV technique (on a glassy carbon electrode in DMF contain-
ing 0.1 M TBAP, in the concentration of 1 ꢂ 10ꢁ3 M vs. Ag/AgCl)
was applied to investigate the electrochemical properties of the
compounds. The electrochemical data is presented in Table 5.
The oxidation potentials can be measured by cyclic voltammetry
and then the HOMO and LUMO values are calculable [65]. The cor-
responding HOMO and LUMO levels were calculated using IVc and
IIa waves for the L ligand. The estimations were done with the
empirical relation ELUMO = [(EIVc–E1/2(ferrocene)) + 4.8] eV or
EHOMO = [(EIIa–E1/2(ferrocene)) + 4.8] eV. Ferrocene was used as
external standard. It shows two peaks at 0.34 and 0.40 V hence
the E1/2 (ferrocence) is equal to 0.37 V which can be used in

38
S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
and +0.48 V).
DEp for Ia/Ic for this redox couple was also found
16
14
12
10
8
Experimental 1H-NMR vs HF/6-31G 1H-NMR
Experimental 1H-NMR vs B3LYP/6-311G 1H-NMR
as 230 mV. This is the sign of a quasi-reversible electron transfer
in the electrode reaction. It should be taken in consideration [66–
68] that after occurring the one-electron reduction process corre-
sponding to [CuIIL]/[CuIL], part of [CuIL] species are chemically
decomposed to copper metal (reaction 1). So, it can be said that
the corresponding anodic waves (IIa and IIIa) are related to the
re-oxidation of electrodeposited copper metal to free Cu+/Cu2+
according to the reaction 2:
6
½CuIILꢄ $ Cuþ2 þ L þ eꢁ $ Cuþ þ L
Cuþ $ Cuþ2 þ eꢁ
ð1Þ
ð2Þ
4
2
0
The effect of scan rates on CuL complex was investigated between
the 50–1000 mV/s (Fig. 8(B)). At high scan rate (200 mV/s>), The
IIa peak was not observed (Fig. 9(B)).
0
2
4
6
8
10
12
14
16
Theoretical 1H-NMR shifts (ppm)
The voltammograms of NiL complex investigated in the same
experimental conditions (from +1.0 V to ꢁ2.3 V at 100 mV sꢁ1 in
(Fig. 7(C)), two waves (Ic and IIc at about ꢁ0.78 and ꢁ1.81 V,
respectively) and two anodic waves (Ia and IIa at about ꢁ1.71,
and +0.45 V, respectively). The effect of scan rates on NiL complex
was investigated between 50 and 100 mV/s (Fig. 8(C)).
Fig. 5. The scatter plot for the compatibility of experimental and theoretical 1H
NMR shifts (for the ligand).
equation to calculate the EHOMO and ELUMO. Based on cyclic voltam-
metry results, Ligand (L) shows EHOMO = 5.11 eV, Egap = 1.96 eV
and ELUMO = 2.48 eV.
In the potential range of ꢁ2.3 V to +1.0 V at scan rate of
100 mV sꢁ1
, on the cathodic side, the CV of CoL complex
In the cathodic direction from +1.0 V to ꢁ2.3 V at scan rate of
100 mV sꢁ1, the CV of L is characterized by four cathodic waves
(Ic, IIc, IIIc and IVc at about ꢁ0.25 V, ꢁ0.48, ꢁ1.68 and ꢁ1.95 V
respectively) and two waves were observed (Ia and IIa at about
+0.12 V and +0.68 V respectively) as depicted by cyclic voltammo-
grams given in Fig. 7(A). At high scanning rates (200 mV/s>), IIa
peak was not observed (Fig. 8(A)).
In Fig. 7(B), the voltammogram taken at in the reduction direc-
tion of CuL complex in 1 ꢂ 10ꢁ3 concentration, at the potential
range of ꢁ2.3 V and +1.0 V, at 100 mV/s scan rate is seen. The CV
of CuL was characterized by a cathodic peak (Ic at about ꢁ1.28 V)
and three anodic waves (Ia, IIa and IIIa t about ꢁ1.05 V, ꢁ0.30 V
(Fig. 7(D)) shows two cathodic peaks (Ic and IIc at about ꢁ0.73
and ꢁ1.12 V, respectively) and one anodic peak ((Ia at about
+0.39 V). Of these, Ic oxidation step seems to be weak.
From the CVs (Fig. 7), it was found that the initial oxidation
peak current of L, CuL, NiL and CoL progressively increased and a
negative shift in the peak potentials existed with increasing scan
rate. From the results obtained between 50 and 1000 mV sꢁ1, a plot
of logarithm of peak current significantly correlated with the loga-
rithm of scan rate for all L, CuL, NiL and CoL with slopes between
0.43, 0,47, 0.49 and 0.50, respectively (correlation coefficient
between 0.986, 0.957, 0.969 and 0.949). These results show that
the redox processes were predominantly diffusion controlled in
Table 3
Comparison of the experimental and theoretical 1H NMR shifts.
Nuclei
1H NMR Experimental (ppm)
1H NMR HF/6-31G (d) (ppm)
Error
0.258
1H NMR B3LYP/6-311G (2d,p) (ppm)
Error
0.973
46-H
47-H
66-H
67-H
43-H
50-H
63-H
45-H
68-H
64-H
49-H
44-H
62-H
65-H
48-H
69-H
70-H
52-H
51-H
61-H
53-H
54-H
60-H
59-H
57-H
56-H
58-H
55-H
13.6437
8.9501
8.9385
7.7944
7.7298
7.4977
7.2226
7.7561
7.5458
7.0540
9.9231
6.8604
4.0788
7.2655
7.1616
6.6756
13.6437
4.0393
4.0243
4.0636
1.7547
1.7547
1.7547
1.7547
1.5235
1.5235
1.5235
1.5235
13.3856
8.5169
8.1988
6.9815
6.8610
6.7919
6.7911
6.7697
6.7511
6.7108
6.6984
6.6630
6.5456
6.2158
6.1233
5.9982
4.0769
3.7051
3.6833
3.3950
1.9551
1.8423
1.7227
1.6431
1.144
12.6701
7.8014
7.4833
6.2660
6.1455
6.0764
6.0756
6.0542
6.0356
5.9953
5.9829
5.9475
5.8301
5.5003
5.4078
5.2827
3.3614
2.9896
2.9678
2.6795
1.2396
1.1268
1.0072
0.9276
0.4285
0.3974
0.3956
0.3686
0.433
0.739
0.812
0.868
0.705
0.431
0.986
0.794
0.343
3.224
0.197
ꢁ2.466
1.049
1.038
0.677
9.566
0.334
0.341
0.668
ꢁ0.200
ꢁ0.087
0.032
0.111
0.379
0.410
0.412
0.439
1.148
1.455
1.528
1.584
1.421
1.147
1.701
1.510
1.058
3.940
0.912
ꢁ1.751
1.765
1.753
1.392
10.282
1.049
1.056
1.384
0.515
0.627
0.747
0.827
1.095
1.126
1.127
1.154
1.1129
1.1111
1.0841

S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
39
Fig. 6. Mesh view of the frontier orbitals (HOMO–LUMO) of the ligand and the energy levels (for the ligand).
Table 4
Regression analysis data of the theoretical and experimental 1H NMR shifts.
Compared methods
B
M
R-squared
Standard error
Correlation coefficient
Experimental vs. HF
0.773
1.493
0.247
0.995
1.005
1.005
1.045
1.045
0.707
0.707
0.932
0.932
1.947
1.947
0.860
0.860
0.841
0.841
0.965
0.965
Experimental vs. B3LYP
Experimental vs. HFa
Experimental vs. B3LYPa
a
Without outlier.
ascorbic acid and as 98.58 % by trolox. In this study almost the
same results with Agırtas et al. were obtained [69,70].
Table 5
Voltammetric results at scan rate of 100 mV sꢁ1 vs. Ag/AgCl. Ec: cathodic potential,
Ea: anodic potential.
Metal chelating and antibacterial activity
Compound Ec (V)
Ea (V)
In the study of metal chelating activity studied compounds
were executed quite high antioxidant activity as shown in
Fig. 10. Especially ligand showed high activities even at low con-
centrations. The chelating activities of compounds at 100 mg/L
concentration were 97.31% for ligand, 90.10% for V(III), 89.48%
for Zn(II), 88.65% for Ni(II), 84.53% for Cu(II) and 66.80% for Co(II).
EDTA showed slightly higher activity than ligand. From this point
of view ligand may be used alternatively at the applications that
EDTA was used. Some chemical modifications may make ligand
more powerful than EDTA. For this reason further laboratory stud-
ies should be done. The metal chelating activity findings are better
than the study of Agirtas et al. [69,70].
H2L
CuL
Ic: ꢁ0.25, IIc: ꢁ0.48, IIIc: ꢁ1.68
Ia: +0.12, IIa: +0.68
IVc: ꢁ1.95
Ic: ꢁ1.28
Ia: ꢁ1.05, IIa: ꢁ0.30, IIIa:
+0.48
NiL
CoL
Ic: ꢁ0.78, IIc: ꢁ1.81
Ic: ꢁ0.73, IIc: ꢁ1.12
Ia: ꢁ1.71, IIa: +0.45
Ia: +0.39
the whole scan rates range studied. Voltammetric results are
presented in Table 5.
Antioxidant and antimicrobial studies
Compounds were analyzed for their antibacterial activities as
seen in Table 6. According to the results obtained from the study
some results were not reliable; except Cu(II), all the compounds
showed slightly weak activity against B. subtilis. Ni(II) showed
the highest antibacterial activity as 24 mm against E. coli. This
activity is higher than the amikacin and equal to tetracycline.
Amongst the compounds V(III) was exhibited antibacterial activity
only against E. hirae as 13 mm. Only Ni(II) exhibited antibacterial
activity against to all bacteria. All the compounds tested in this
study showed antimicrobial activity against E. hirae. Amikacin
and tetracycline displayed excellent activity against to test bacteria
DPPH scavenging activity
In this study, the antioxidant activity of the DMF solutions of
the five compounds was studied using the DPPH scavenging assay.
In DPPH scavenging activity different concentrations of com-
pounds were studied (5, 10, 25, 50 and 100 mg/L). The antioxidant
activities of the compounds were increased with concentration as
seen in Fig. 9. The highest activity was exhibited by V(III) as
49.29% this is followed by Zn(II) as 38.41% , ligand as 32.15%, Ni(II)
as 30.37%, Co(II) as 25.53% and Cu(II) as 25.29%. The standard anti-
oxidants used in this study showed excellent activity as 99.05 % by

40
S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
Fig. 7. Cyclic voltammograms of 1 ꢂ 10ꢁ3 mol Lꢁ1 compound solutions in DMF at glassy carbon electrode; (A) L, (B) CuL, (C) NiL, (D) CoL scan rate, v: 100 mV sꢁ1
.
Fig. 8. Cyclic voltammograms of 1 ꢂ 10ꢁ3 mol Lꢁ1 compound solutions at different scan rates; (A) L, (B) CuL, (C) NiL, (D) CoL.

S. Ilhan et al. / Journal of Molecular Structure 1075 (2014) 32–42
41
120
100
80
60
40
20
0
100
Ligand
Cu(II)
50
EDTA
V(III)
Ni(II)
Co(II)
25
10
5
Zn(II)
Zn(II)
Co(II)
Ni(II)
V(III)
Ascorbic Acid
Trolox
Cu(II)
Ligand
5
10
25
50
100
Concentration (mg/L)
0
20
40
60
80
100
120
Fig. 9. DPPH scavenging activity of different concentrations of Schiff base and its
metal complexes.
% Ferrous Chelating Activity
Fig. 10. Metal chelating activity of Schiff base and its metal complexes.
Table 6
Antibacterial activity of compounds and standard antibiotics.
Bacteria
Compounds and antibiotic discsa
L
Cu(II)
Ni(II)
Co(II)
Zn(II)
V(III)
AK
TE
P. aeriginosa
S. aureus
L. pneumophila subsp pneumophiia
B. subtilis
M. luteus
E. hirae
E. coli
0
7
8
7
8
0
8
10
0
7
12
0
11
10
8
8
0
0
8
0
9
8
0
0
8
9
9
0
0
10
10
8
21
24
16
20
18
21
23
24
32
20
23
24
18
24
8
12
10
24
10
0
7
10
13
12
a
Inhibition diameter in millimeters. H2L = Ligand, AK = Amikacin (30
l
g), TE = Tetracycline (30
lg).
compared to the compounds. The highest antimicrobial activity
exhibited by tetracycline against S. aureus as 32 mm amongst the
compounds and standard antibiotics.
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