Synthesis, spectral characterization, structural investigation and antimicrobial studies of mononuclear Cu(II), Ni(II), Co(II), Zn(II) and Cd(II) complexes of a new potentially hexadentate N2O4 Schiff base ligand derived from salicylaldehyde

Article abstract of DOI:10.1016/j.molstruc.2012.07.056

A new potentially hexadentate N₂O₄ Schiff base ligand, H₂L derived from condensation reaction of an aromatic diamine and salicylaldehyde, and its metal complexes were characterized by elemental analyses, IR, UV-Vis, EI-MS, ¹H and ¹³C NMR spectra, as well as conductance measurements. It has been originated that the Schiff base ligand with Cu(II), Ni(II), Co(II), Cd(II) and Zn(II) ions form mononuclear complexes on 1:1 (metal:ligand) stoichiometry. The conductivity data confirm the non-electrolytic nature of the complexes. Also the crystal structures of the complexes [ZnL] and [CoL] have also been determined by using X-ray crystallographic technique. The Zn(II) and Co(II) complexes show a tetrahedral configuration. Electronic absorption spectra of the Cu(II) and Ni(II) complexes suggest a square-planar geometry around the central metal ion. The synthesized compounds have antibacterial activity against the three Gram-positive bacteria: Bacillus cereus, Enterococcus faecalis and Listeria monocytogenes and also against the three Gram-negative bacteria: Salmonella paraB, Citrobacter freundii and Enterobacter aerogenes. The results showed that in some cases the antibacterial activity of complexes were more than nalidixic acid and amoxicillin as standards.

Full text of DOI:10.1016/j.molstruc.2012.07.056

Journal of Molecular Structure 1032 (2013) 62–68
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectral characterization, structural investigation and antimicrobial
studies of mononuclear Cu(II), Ni(II), Co(II), Zn(II) and Cd(II) complexes of a
new potentially hexadentate N2O4 Schiff base ligand derived from salicylaldehyde
a
b
c
d
Hassan Keypour a, , Maryam Shayesteh , Majid Rezaeivala , Firoozeh Chalabian , Yalcin Elerman ,
⇑
Orhan Buyukgungor e
a Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran
b Department of Chemical Engineering, Hamedan University of Technology, Hamedan 65155, Iran
c Department of Biology, Islamic Azad University, Tehran North Campus, Tehran, Iran
d Department of Engineering Physics, Faculty of Engineering, University of Ankara, Besevler 06100, Ankara, Turkey
e Ondokuz Mayıs University, Faculty of Arts and Sciences, Department of Physics, Samsun 55139, Turkey
h i g h l i g h t s
"
"
"
Synthesis of five new symmetrical macroacyclic Schiff base complexes.
X-ray crystal structure determination of two new complexes.
Antibacterial studies of the prepared ligand and related complexes.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2012
Received in revised form 30 July 2012
Accepted 31 July 2012
Available online 11 August 2012
A new potentially hexadentate N2O4 Schiff base ligand, H2L derived from condensation reaction of an aro-
matic diamine and salicylaldehyde, and its metal complexes were characterized by elemental analyses,
IR, UV–Vis, EI–MS, 1H and 13C NMR spectra, as well as conductance measurements. It has been originated
that the Schiff base ligand with Cu(II), Ni(II), Co(II), Cd(II) and Zn(II) ions form mononuclear complexes on
1:1 (metal:ligand) stoichiometry. The conductivity data confirm the non-electrolytic nature of the com-
plexes. Also the crystal structures of the complexes [ZnL] and [CoL] have also been determined by using
X-ray crystallographic technique. The Zn(II) and Co(II) complexes show a tetrahedral configuration. Elec-
tronic absorption spectra of the Cu(II) and Ni(II) complexes suggest a square-planar geometry around the
central metal ion. The synthesized compounds have antibacterial activity against the three Gram-positive
bacteria: Bacillus cereus, Enterococcus faecalis and Listeria monocytogenes and also against the three Gram-
negative bacteria: Salmonella paraB, Citrobacter freundii and Enterobacter aerogenes. The results showed
that in some cases the antibacterial activity of complexes were more than nalidixic acid and amoxicillin
as standards.
Keywords:
Potentially hexadentate Schiff base ligand
Metal complexes
X-ray crystal structures
Intermolecular p–p interactions
Antibacterial effects
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
tion as active parts of metalloenzymes [15,16]. Transition metal
complexes of Schiff base compounds are used as growth inhibiting
Schiff bases play an important role in inorganic chemistry as
they easily form stable complexes with most transition metal ions
[1–5]. The high stability potential of Schiff base complexes with
different oxidation states extended the application of these com-
pounds in a wide range. They can be used as biological models in
understanding the structure of bimolecular [6–9] or metallopro-
tein models [10–12]. This is due to their resemblance to natural
systems [13,14], also these compounds have received much atten-
agents for most of bacteria and fungi [17–21] also they are widely
used as potential therapeutics [22], they are useful in health and
skin care [23]. The complexes of this type are also used as catalysts
in organic redox and electrochemical reduction reactions [24–27]
for various chemical reactions. Bidentate and tetradentate Schiff
base ligands involving N,O donor sites possess many advantages
such as facile approach for synthesis, relative tolerance, readily
adjusted ancillary ligands, and tunable steric and electronic coordi-
nation environments on the metal centre. In view of above, tetra-
dentate N2O2 ligands and their transition metal complexes, act as
catalyst [28]. Various transition and inner transition metals
⇑
Corresponding author. Tel.: +98 811 8242044; fax: +98 811 8272404.
E-mail address: haskey1@yahoo.com (H. Keypour).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.07.056

H. Keypour et al. / Journal of Molecular Structure 1032 (2013) 62–68
63
complexes with bi-, tri- and tetradentate Schiff bases containing
2.4.2. [CuL]
nitrogen and oxygen donor atoms play important role in biological
systems and represent interesting models for metalloenzymes,
which efficiently catalyze the reduction of dinitrogen and dioxygen
[29]. Complexes of salen type ligands have been used as synthetic
oxygen carriers [30,31], catalysts for asymmetric epoxidation
[11,32–35], for the synthesis of optical and magnetic materials
[36,37], as bio-mimetic catalysts (e.g., models for SOD, catalase)
[38–41] etc. The famous Jacobsen and Katsuki catalysts belong to
the class of complexes with salen type ligands [33–35]. Recently,
we have reported synthesis, characterization and antibacterial
study of some new mononuclear and binuclear Schiff base com-
plexes [42–44]. In the present work, we have prepared new Schiff
base ligand, H2L. The structures of this compound and its related
Ni(II), Co(II), Cu(II), Cd(II) and Zn(II) complexes were characterized
by means of physical and spectral analyses.
A
methanol solution (30 ml) of Cu(NO3)2Á6H2O (0.2956 g,
1 mmol) was added to a warm solution of [H2L] (0.550 g, 1 mmol)
in methanol (50 ml). The mixture was stirred and heated to reflux
for 12 h. The resultant green solid product was collected by filtra-
tion and washed with cold methanol and dried in vacuo. Yield:
0.5 g (81.7%). M.p. 298 °C. Anal. Calc. for C36H24CuN2O4 (MW:
612.13): C, 70.6; H, 3.9; N, 4.6. Found: C, 70.9; H, 3.7; N, 4.5%. IR
(cmÀ1, KBr): 1608 (s,
mC@N). UV–Vis [k (nm), e
(MÀ1 cmÀ1)]:
266(37430), 319(sh), 385(13830), 688(57). Km = 1.9 cm2 XÀ1
molÀ1. MS (EI): (m/z) = 611 [CuL]+.
2.4.3. [CoL]
The preparation of the red microcrystalline complex followed
the same procedure described for [CuL] by using a solution of
CoCl2.6H2O (0.2379 g, 1 mmol) in 30 mL of methanol. Yield: 0.4 g
(65.8%). M.p. 335 °C. Anal. Calc. for C36H24CoN2O4 (MW:607.52):
C, 71.2; H, 4.0; N, 4.6. Found: C, 71.5; H, 4.3; N, 4.2%. IR (cmÀ1
,
2. Experimental
KBr): 1606 (s,
mC@N). UV–Vis [k (nm), e
(MÀ1 cmÀ1)]:
274(33596), 322(18581), 402(14809), 544 (207), 865 (23).
2.1. Starting materials
Km = 3.5 cm2
X
À1 molÀ1. MS (EI): (m/z) = 607 [CoL]+.
2-(3-(2-Aminophenoxy)naphthalen-2-yloxy)benzenamine was
synthesized according to the literature procedure [45–49]. Sol-
vents, naphthalene-2,3-diol, Salicylaldehyde, 1-fluoro-2-nitroben-
zene and metal salts were purchased from Merck and were used
without further purification.
2.4.4. [NiL]
The preparation of the green complex followed the same proce-
dure described for [CuL] by using a solution of Ni(NO3)2Á6H2O
(0.2908 g, 1 mmol) in 30 mL of methanol. Yield: 0.5 g (82.3%).
M.p. 315 °C. Anal. Calc. for C36H24NiN2O4 (MW: 607.28): C, 71.2;
H, 4.0; N, 4.6. Found: C, 71.5; H, 3.8; N, 4.9%. IR (cmÀ1, KBr):
1610 (s,
m
C@N). UV–Vis [k (nm),
412(3378), 531(42),
À1 molÀ1. MS (EI): (m/z) = 607([NiL+H]+).
e
(MÀ1 cmÀ1)]: 262(34723),
599(38), 713(30(.
2.2. Physical measurements
318(17896),
Km = 4 cm2
X
Infrared spectra were collected using KBr pellets on a BIO-RAD
FTS-40A spectrophotometer (4000–400 cmÀ1).
A Perkin-Elmer,
Lambda 45 (UV–Vis) spectrophotometer was used to record the
electronic spectra. CHN analyses were carried out using a Perkin-
Elmer, CHNS/O elemental analyzer model 2400. Conductance mea-
surements were performed using a Hanna HI 8820 conductivity
meter. 1H and 13C NMR spectra were taken in CDCl3 on a Bruker
Avance 300 MHz and Jeol 90 MHz spectrometer using Si(CH3)4 as
an internal standard.
2.4.5. [ZnL]
The preparation of the yellow microcrystalline complex fol-
lowed the same procedure described for [CuL] by using a solution
of Zn(NO3)2.6H2O (0.2975 g, 1 mmol) in 30 mL of methanol. Yield:
0.5 g (81.4%). M.p. 287 °C. Anal. Calc. for C36H24ZnN2O4 (MW:
612.0): C, 70.4; H, 3.9; N, 4.6. Found: C, 70.5; H, 3.7; N, 4.2%. IR
(cmÀ1, KBr): 1609 (s,
mC@N). UV–Vis [k (nm), e
(MÀ1 cmÀ1)]:
275(39924), 322(25329), 338(sh), 407(16041). Km = 5.3 cm2 XÀ1
molÀ1. MS (EI): (m/z) = 612 [ZnL]+.
2.3. X-ray crystallography
Vapour diffusion of diethyl ether into a solution of [CoL] and
[ZnL] in chloroform, afforded green and yellow prismatic crystals
suitable for study by X-ray crystallography respectively. Details
of the X-ray experiments and crystal data are summarized in Ta-
ble 1. Measurements were made on a STOE IPDS-2 two circle dif-
2.4.6. [CdL]
The preparation of the yellow complex followed the same pro-
cedure described for [CuL] by using a solution of Cd(NO3)2Á4H2O
(0.3085 g, 1 mmol) in 30 mL of methanol. Yield: 0.6 g (90.8%).
M.p. 340 °C. Anal. Calc. for C36H24CdN2O4: (MW: 661.0): C, 65.4;
H, 3.7; N, 4.2. Found: C, 65.7; H, 3.3; N, 3.9%. IR (cmÀ1, KBr):
fractometer at 296 K, using graphite monochromated Mo K
a
1613 (s,
mC@N). UV–Vis [k (nm), e
(MÀ1 cmÀ1)]: 262(39999),
X
À1 cm2 molÀ1. MS (EI): (m/
radiation (k = 0.71073 Å). The structure was solved by direct meth-
ods using SHELXS-97 and refined by full-matrix least-squares and
Fourier techniques using SHELXL-97 [50].
324(23112), 398(13014). Km = 4.9
z) = 661 ([CdL]+).
2.4. Synthesis
3. Results and discussion
2.4.1. H2L
A new potentially hexadentate N2O4 Schiff base ligand, (H2L)
has been easily prepared from the reaction of 2-(3-(2-aminophen-
oxy)naphthalen-2-yloxy)benzenamine and Salicylaldehyde. Com-
plexes of this ligand with copper(II), nickel(II), zinc(II) and
cadmium(II) nitrate and also cobalt(II) chloride were synthesized
by template reactions starting from metal salts, Salicylaldehyde
2-(3-(2-Aminophenoxy)naphthalen-2-yloxy)benzenamine
(0.342 g, 1 mmol) in methanol (20 ml) was added dropwise with
stirring to a solution of Salicylaldehyde (0.244 g, 2 mmol) in meth-
anol (30 ml). The mixture was stirred and heated to reflux for 12 h.
A yellow precipitate was obtained that was filtered off and washed
with cold methanol and dried in vacuo. Yield: 0.4 g (72.7%). M.p.
95 °C. Anal. Calc. For C36H26N2O4: C, 78.5; H, 4.8; N, 5.1. Found:
and
2-(3-(2-aminophenoxy)naphthalen-2-yloxy)benzenamine
(mole ratio 1:1:1), respectively. The same products were obtained
also by direct reactions of metal salts and the ligand H2L in equi-
molar ratio (1:1). The compositions of the products were not af-
fected by the change of the mole ratio of reacting species. The
C, 78.9; H, 4.7; N, 5.3%. IR (cmÀ1, KBr): 1618 (s,
(nm),
(MÀ1 cmÀ1)]: 265(37962), 320(21326). MS (EI): m/z = 550
[H2L]+.
mC@N). UV–Vis [k
e

64
H. Keypour et al. / Journal of Molecular Structure 1032 (2013) 62–68
Table 1
Crystal data and structure refinement for [ZnL] and [CoL] complexes.
Empirical formula
C36H24N2O4Zn
C36H24CoN2O4
Formula weight
Temperature
613.94
296 K
607.50
296 K
Wavelength (Å)
Crystal system
Space group
0.71073
Triclinic
PÀ1
0.71073
Triclinic
PÀ1
Unit cell dimensions
a (Å)
b (Å)
c (Å)
10.0051(9)
11.1982(10)
13.5544(12)
9.9815(8)
11.3765(8)
13.4111(11)
a
(°)
97.102(7)
98.067(6)
b (°)
103.135(7)
102.999(6)
c
(°)
102.617(7)
1419.0(2)
102.118(6)
1422.41(19)
Volume (Å3)
Z
2
2
Calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
Reflections collected/unique
Completeness to theta = 25.00
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1.437 Mg/m3
1.418 Mg/m3
0.910 mmÀ1
0.65 mmÀ1
632
626
0.28 Â 0.24 Â 0.19 mm3
0.66 Â 0.33 Â 0.17 mm3
2.2°–27.3°
2.2°–26.5°
À12 6 h 6 12, À14 6 k 6 13, À17 6 l 6 15
5762/ 3757 [R(int) = 0.059]
98%
À12 6 h 6 12, À14 6 k 6 14, À16 6 l 6 16
5889/4156 [R(int) = 0.050]
99.8%
Empirical
Empirical
0.888 and 0.781
Full-matrix least-squares on F2
5762/0/388
0.933 and 0.836
Full-matrix least-squares on F2
5889/0/388
0.91
1.01
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0445, wR2 = 0.0821
R1 = 0.0450, wR2 = 0.0816
0.41 and À0.31 eAÀ3
R1 = 0.0568, wR2 = 0.090
R1 = 0.0495, wR2 = 0.0824
0.34 and À0.21 eAÀ3
complexes are soluble in common organic solvents such as CHCl3,
EtOH, MeOH, THF and etc. In both cases studied, they turned out to
be of non-electrolyte nature, as determined by molar conductivity
measurements. Complexes were characterized by IR spectra, ele-
mental analysis, molar conductance (Km), Mass spectroscopy,
UV–Vis spectra and in the case of [ZnL] and [CoL] with X-ray dif-
fraction. The analytical and spectral data are completely consistent
with the proposed formulation. The metals are also bound to
ligands through the azomethine nitrogens, deduced from the ob-
served decrease in C@N stretching frequency upon complexation
of ligand to metal ions.
peak at 8.560 ppm is assigned to the azomethine protons. Multi-
plets of aromatic protons appear in the region 6.865–7.658 ppm.
13C NMR spectrum of the [H2L] ligand in the region corresponding
to the signals of aromatic rings carbons (116.331–149.634 ppm)
are rather than eighteen expected peaks supporting the non-equiv-
alence of the aromatic ring carbons environments in this ligand.
The imine carbons appear at 161.174 ppm and 163.371 ppm. These
data support the formulation of the Schiff base ligand, H2L.
The 1H NMR spectra of the complexes are almost similar to that
of the ligand with slight shift to lower fields. The signal for the
imine proton in the zinc and cadmium complexes appears at
8.588 (for [ZnL] complex) and 8.594 and 8.700 ppm (for [CdL] com-
3.1. IR spectra
16
The IR spectra of the complexes show a sharp band in the range
1606–1613 cmÀ1, attributed to
m(C@N), which is shifted to lower
17
15
frequency on going from the free ligand (at 1618 cmÀ1) to the com-
plexes. This is indicative of the coordination of the imine nitrogen
to the metal and is consistent with X-ray diffraction results. Fur-
thermore, the absence of C@O and NAH stretching vibrations in
the spectra of the ligands, related to aldehyde and diamine, respec-
tively, indicate occurrence of Schiff base condensation. Deprotona-
tion of all phenolic functions is confirmed by the lack of OAH
stretching bands in the IR region 3400–3300 cmÀ1 for all com-
plexes [51,52]. The band at 1179 cmÀ1 for [H2L] is ascribed to the
phenolic CAO stretching vibrations. This band is shifted to lower
frequencies due to O-metal coordination. Representative spectra
of the ligand H2L and its related complexes are presented in Fig. S1.
18
14
OH
HO
O
13
12
11
N
N
10
9
6
O
5
7
4
2
8
3
3.2. NMR spectra
The 1H NMR spectra of the ligand, H2L and it is related diamag-
netic complexes ([ZnL] and [CdL]) were measured in CDCl3, with
TMS as internal standard. The data is depicted in Table S1. The
NMR numbering of the atoms is shown in Fig. 1. A sharp singlet
1
Fig. 1. Structure of H2L ligand, along with NMR numbering.

H. Keypour et al. / Journal of Molecular Structure 1032 (2013) 62–68
65
Table 2
Electronic spectroscopy data (nm) for ligand L and related complexes.
a
Compound
kmax (nm) (e)
Intraligand (LL)
CT
d–d
[H2L]
[NiL]
[CuL]
[CoL]
[ZnL]
[CdL]
265(37962)
262(34723)
266(37430)
274(33596)
275(39924)
262(39999)
320(21236)
318(17896)
319(sh)
322(18581)
322(25329)
324(23112)
412(3378)
385(13830)
402(14809)
338(sh)
531(42)b
688(57)b
544(207)
599(38)
865(23)
713(30)
407(16041)
398(13014)
a
MolÀ1 cmÀ1
Shoulder.
.
b
Fig. 3. Molecular structure of [CoL] showing 30% probability thermal ellipsoids
using ORTEP.
Fig. 2. Molecular structure of [ZnL] showing 30% probability thermal ellipsoids
using ORTEP.
Table 3
Selected bond lengths (Å) and bond angles (°) for complexes [CoL] and [ZnL].
plex) and these are shifted downfield with respect to the corre-
sponding signal in the free ligand, indicating that the metal-nitro-
gen bond is retained in solution. In the case of the cadmium
complex for the imino protons, two signals appear in this complex
indicating the non-equivalence of the two imino groups of the mol-
ecule. Slightly lower frequency shifts are also observed for the aro-
matic protons when compared with the free ligand.
Complex [ZnL]
Bond
Complex [CoL]
Bond
Distance
2.017(2)
2.019(2)
1.934(2)
1.9539(18)
2.546(4)
3.404(3)
Distance
2.027(2)
2.0153(19)
1.927(2)
1.9274(19)
2.521(3)
3.462(3)
N(1)AZn(1)
N(2)AZn(1)
O(1)AZn(1)
O(2)AZn(1)
O(4)AZn(1)
O(3)AZn(1)
N(1)ACo(1)
N(2)ACo(1)
O(3)ACo(1)
O(4)ACo(1)
O(2)ACo(1)
O(1)ACo(1)
The 13C NMR spectra of the zinc and cadmium complexes show
two peaks at 161.324 and 163.696 ppm (for [ZnL] complex) and
161.298 and 163.712 ppm (for [CdL] complex), corresponding to
the imine carbon atoms. In 13C NMR spectra of these two com-
plexes should be exhibited eighteen main peaks; however, for
[CdL] complex sixteen main signals (117.121–149.712 ppm) ap-
peared, it seems that two carbons in this region are overlapping
to other carbons due to high similarity of carbons and low resolu-
tion of instrument. However, these two complexes are same to
each other and these carbon atoms are chemically equivalent but
the 13C NMR spectrum of the [ZnL] complex in the region corre-
sponding to the signals of aromatic rings carbons (114.259–
150.728 ppm) are rather than [CdL] complex supporting the non-
equivalence of the aromatic ring carbons environments in this
complex. Representative spectra of [ZnL] and [CdL] complexes
are presented in Fig. S2.
Bond
Angle
Bond
Angle
O(1)AZn(1)AO(2)
O(1)AZn(1)AN(1)
O(2)AZn(1)AN(1)
O(1)AZn(1)AN(2)
O(2)AZn(1)AN(2)
N(1)AZn(1)AN(2)
107.65(10)
95.58(9)
101.18(8)
117.51(9)
94.70(8)
O(3)ACo(1)AO(4)
O(3)ACo(1)AN(2)
O(4)ACo(1)AN(2)
O(3)ACo(1)AN(1)
O(4)ACo(1)AN(1)
N(2)ACo(1)AN(1)
108.39(9)
101.75(8)
94.66(8)
94.64(8)
120.64(8)
133.77(8)
136.74(10)
to intraligand
p ?
pà and n ? pà transitions. In the electronic spec-
tra of the complexes, the intraligand transitions are slightly shifted
as a result of coordination. The complexes of the Co(II), Ni(II) and
Cu(II) show the low intensity bands at ca. 531–713 nm, which
are assigned as d-d transition of the metal ions. The electronic
spectrum of Ni(II) complex exhibited three absorption bands at
531, 599 and 713 nm which may be assigned to three spin allowed
transitions, 1A1g ? 1A2g 1A1g ? 1B2g and 1A1g ? 1Eg, respectively
,
3.3. Electronic absorption spectroscopy
characteristic of square planar geometry around Ni(II) ion [53].
The former band at 544 nm is probably due to the 4A2(F) ?
4T1(P) for [CoL] complex transition of tetrahedral geometry [54].
The electronic spectra of the ligands and their metal complexes
were recorded in CHCl3. The bands below 338 nm are attributable

66
H. Keypour et al. / Journal of Molecular Structure 1032 (2013) 62–68
Fig. 4. Intermolecular p–p-stacking interaction in [ZnL] complex.
Table 4
Quantitative antimicrobial assay results (zone of growth inhibition) of the free Schiff base ligand and related complexes.
Microorganism
Standards
Minimum inhibitory concentration (mg/ml)
Main compounds
Nalidixic acid
Amoxycilin
[L]
[CdL]
[ZnL]
[CoL]
[NiL]
[CuL]
Bacillus cereus (+)
20
–
–
25
25
25
35
20
30
20
–
–
–
–
–
–
–
35
–
25
30
–
–
–
–
30
–
–
–
20
30
25
20
35
–
45
40
–
15
–
–
–
–
Enterococcus faecalis (+)
Listeria monocytogenes (+)
Salmonella paraB (À)
Citrobacter freundii (À)
Enterobacter aerogenes (À)
15
25
–
50
–
The electronic spectra of the Cu(II) complex show an absorption
band at 688 nm attributed to the 2Eg ? 2T2g transition, character-
istic for square planar geometry [55,56]. The complexes of Zn(II)
and Cd(II) are diamagnetic without any d–d transition. All spectra
of all complexes show an intense band at ca. 385–412 nm, due to
charge transfer transition [57–60]. The electronic spectral details
of the complexes are given in Table 2.
given in Table 1 and selected bond distances and angles are given
in Table 3.
[ZnL] and [CoL] complexes display the same coordination geom-
etry. The Co(II) and Zn(II) ions are four-coordinate, binding to the
phenoxide oxygen atoms O(3) and O(4) in [CoL] complex and
O(1) and O(2) in [ZnL] complex and also the azomethine nitrogen
atoms N(1) and N(2) in a geometry distorted tetrahedral. The
CoAN bond lengths are nearly identical to each other (Co(1)AN(1)
2.027(2) Å, Co(1)AN(2) 2.0153(19) Å) with a N(1)-Co(1)–N(2) bond
angle of 133.77(8)°. The Co(1)AO(3) (1.927(2) Å) and Co(1)AO(4)
(1.9274(19) Å) bond lengths are consistent with coordination of
phenoxide, rather than neutral phenol [64]. The ether O atoms,
O(1) and O(2), are not coordinated, lying 3.462(3) Å and
2.521(3) Å, respectively, from the Co ion.
The bond lengths involving the Zn(II) ion in [ZnL] are similar to
those found in [CoL], with the Zn(1)AO(1) and Zn(1)AO(2) dis-
tances (1.934 Å and 1.9539 Å, respectively) again consistent with
coordination of phenoxide [64]. The ether O atoms, O(3) and
O(4), are not again coordinated, lying 3.404(3) Å and 2.546(4) Å,
respectively, from the Zn ion.
3.4. Molar conductivity
The molar conductivity (KM) data (1.9–5.3
X
À1 molÀ1 cm2),
measured at 25 °C using 10À3 M solutions of the complexes in
CHCl3 solvent. These low values indicate that all their complexes
are non-electrolytes due to the absence of any counter ions in their
structures [61,62]. The molar conductance values of these com-
plexes indicate that the [H2L] Schiff base ligand is coordinated to
the copper(II), zinc(II), nickel(II), cadmium(II) and cobalt(II) ions
as a doubly negatively charged anions. Therefore, it seems that
two phenolic OH have been deprotonated and bonded to the metal
ions as oxygen anion [63].
Four-coordination in [CoL] and [ZnL] arises from the inability of
the ether oxygen atoms to be positioned at a suitable distance from
the Co(II) ion or Zn(II) ion for bonding.
3.5. X-ray structures
Analysis of the short intermolecular ring-ring interactions in
[CoL] and [ZnL] complexes shows face to face intermolecular
p,
Suitable crystals of [ZnL] and [CoL] were obtained from a chlo-
roform solution by slow diffusion of diethyl ether. The ORTEP
views of the complexes are shown in Figs. 2 and 3, respectively.
Crystallographic data and structure refinement parameters are
p-interactions between the molecules. The naphthalene groups
of adjacent molecules lie parallel in both complexes. In [CoL], the
interplanar distance of the planes containing the naphthalene

H. Keypour et al. / Journal of Molecular Structure 1032 (2013) 62–68
67
Table 5
Quantitative antimicrobial assay results (minimal inhibitory concentrations) of the free Schiff base ligand and related complexes.
Microorganism
Minimum inhibitory concentration (mg/ml)
Main compounds
[L]
[CdL]
[ZnL]
[CoL]
[NiL]
[CuL]
Bacillus cereus (+)
–
–
–
–
–
–
3.125
–
12.5
6.25
–
–
–
–
6.25
–
–
–
50
6.25
12.5
50
6.25
–
3.125
3.125
–
50
–
–
–
–
Enterococcus faecalis (+)
Listeria monocytogenes (+)
Salmonella paraB (À)
Citrobacter freundii (À)
Enterobacter aerogenes (À)
12.5
–
1.56
–
groups is 3.20 Å, whilst the distance between centroids of the near-
est benzene sub-unities of the naphthalene groups is 4.42 Å. In the
[ZnL] complex the interplanar distance is 3.23 Å, and the distance
between centroids of the nearest benzene sub-unities of the groups
is 4.36 Å (Fig. 4).
In general, comparison of the microbial activity of N2O2, N4O2 and
etc. Schiff base ligands and their related complexes show that such
complexes have higher antibacterial activity than that of the
respective free ligand against the same microorganism under iden-
tical experimental conditions [67–74]. Such an increased activity
for the metal chelates as compared to the free ligands can be ex-
plained on the basis of chelation theory [75]. Chelation consider-
ably reduces the polarity of the metal ion because of the partial
sharing of its positive charge with the donor groups and possible
3.6. Antibacterial activity test
In vitro activity test was carried out using the growth inhibitory
zone (well method) [65,66]. Antibacterial activities (zone of
growth inhibition and minimal inhibitory concentrations) of the
free Schiff base and its complexes and of nalidixic acid and amox-
ycillin (as a standard compounds) are shown in Table 4.The anti-
bacterial effects of components was determined against the three
Gram-positive bacteria (Table 4): E faecalis (PTCC 2121), Bacillus
cereus (PTCC 1040), and Listeria monocytogenes (PTCC 1293), and
against the three Gram-negative bacteria: Salmonella paraB (PTCC
1711), C. freundii (PTCC 1101), and Enterobacter aerogenes (PTCC
1145). Microorganisms (obtained from enrichment culture of the
microorganisms in 1 mL Muller–Hinton broth incubated at 37 °C
for 12 h) were cultured on Muller–Hinton agar medium. The inhib-
itory activity was compared with that of standard antibiotics, such
p
-electron delocalization over the chelate ring. Such chelation
could increase the lipophilic character of the central metal atom,
which subsequently favors the permeation through the lipid layer
of cell membrane [76–79].
4. Conclusion
The work described in this paper involved the synthesis and
spectroscopic characterization of a series of cobalt, nickel, copper,
zinc and cadmium complexes with a new Schiff base ligand de-
rived from 2-(3-(2-aminophenoxy)naphthalen-2-yloxy)benzen-
amine. These complexes were characterized by using different
physiochemical techniques. Molecular structures for complexes
[CoL] and [ZnL] have been revealed by single crystal X-ray analysis.
These complexes are all neutral and found to have a distorted tet-
rahedral geometry with the four donor atoms of the ligand. The
synthesized compounds have antibacterial activity against the
three Gram-positive bacteria: B. cereus, Enterococcus faecalis and
L. monocytogenes and also against the three Gram-negative bacte-
ria: S. paraB, Citrobacter freundii and E. aerogenes. Unlike the parent
Schiff base ligand, the complexes showed that good antibacterial
activity. The antibacterial activities for the [CdL] and [NiL] com-
plexes are more than those found for nalidixic acid and amoxicillin,
especially [NiL] complex showed that strong activity against two
gram-positive bacteria, B. cereus, L. monocytogenes, and two
gram-negative bacteria, S. paraB and E. aerogenes.
as nalidixic acid and amoxycilin (10
medium using a 6 mm cork borer, 100
l
g). After drilling wells on the
L of solution from different
l
compounds were poured into each well. The plates were incubated
at 37 °C overnight. The diameter of the inhibition zone was mea-
sured as precisely as possible. Each test was carried out in triplicate
and the average was calculated for inhibition zone diameters. A
blank containing only DMSO showed no inhibition in a preliminary
test. The macro-dilution broth susceptibility assay was used for the
evaluation of minimal inhibitory concentration (MIC) (Table 5).
The use of 12 test tubes is required by the macro-dilution method.
By including 1 mL Muller-Hinton broth in each test, and then add-
ing 1 mL extract with concentration 100 mg/ml in the first tube, we
made a serial dilution of this extract from the first tube to the last
tube. Bacterial suspensions were prepared to match the turbidity
of 0.5 McFarland turbidity standards. Matching this turbidity pro-
vided bacterial inoculums concentration of 1.5 Â 108 cfu/mL. Then
1 mL of bacterial suspension was added to each test tube. After
incubation at 37 °C for 24 h, the last tube was determined as the
minimal inhibitory concentration (MIC) without turbidity. The
antibacterial activity results evidently showed that most of pre-
pared chemicals have relatively good antibacterial effects against
the studied bacterial strains in comparison of standards. Especially,
among the compounds, [NiL] and [CdL] are more stronger than oth-
ers even though [CoL] is good because it has good effects on gram-
negative bacteria and also [CuL] and [ZnL] have moderate activity
against one of them so free Schiff base ligand did not has any activ-
ity on bacteria. The antibacterial activities for the [CdL] and [NiL]
complexes are more than those found for nalidixic acid and amox-
icillin, especially [NiL] complex showed that strong activity against
B. cereus (+), L. monocytogenes (+), S. paraB (À) and E. aerogenes (À).
Acknowledgments
We are grateful to the Faculty of chemistry of Bu-Ali Sina
University, National Foundation of elites (BMN) and Ministry of
science, Research & Technology of Iran, for financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2012.
07.056.
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