Synthesis and characterization of a series of transition metal complexes with a new symmetrical polyoxaaza macroacyclic Schiff base ligand: X-ray crystal structure of cobalt(II) and nickel(II) complexes and their antibacterial properties

Article abstract of DOI:10.1016/j.saa.2012.09.048

A new symmetrical [N₄O₂] hexadentate Schiff base ligand, (E)-N-(pyridin-2-ylmethylene)-2-(3-(2-((E)-pyridin-2-lmethyleneamino) phenoxy)naphthalen-2-yloxy)benzenamine, abbreviated to L, and its complexes of Ni(II), Cu(II), Zn(II), Co(II), Cd(II) and Mn(II) have been synthesized in the presence of metal ions. The complexes were structurally characterized by elemental analyses, IR, UV-Vis, NMR and molar conductivity. The crystal structures of two complexes, [NiL(ONO₂)₂]·2H ₂O and [CoLCl₂]CH₃OH·0.5H₂O, have been determined by a single crystal X-ray diffraction study. In these complexes, the ligand is coordinated in a neutral form via pyridine and azomethine nitrogen atoms. The metal ions complete their six coordination with two coordinated nitrate or chloride ions, forming a distorted octahedral geometry. The synthesized compounds have antibacterial activity against the three Gram-positive bacteria: Enterococcus faecalis, Bacillus cereus and Staphylococcus epid and also against the three Gram-negative bacteria: Citrobacter freundii, Enterobacter aerogenes and Salmonella typhi. The activity data show that the complexes are more potent antibacterials than the parent Schiff base.

Full text of DOI:10.1016/j.saa.2012.09.048

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of a series of transition metal complexes with
a new symmetrical polyoxaaza macroacyclic Schiff base ligand: X-ray crystal
structure of cobalt(II) and nickel(II) complexes and their antibacterial properties
a
b
c
d
Hassan Keypour a, , Maryam Shayesteh , Majid Rezaeivala , Firoozeh Chalabian , Laura Valencia
⇑
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 Departamento de Quimica Inorganica, Facultad de Quimica, Universidade de Vigo, 36310 Vigo, Pontevedra, Spain
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
The condensation of 2-(3-(2-aminophenoxy)naphthalen-2-yloxy)benzenamine with 2-pyridinecarbalde-
hyde gives a new aromatic ligand (L). From the direct reaction of this ligand with related metal salts in
methanolic solution, the neutral [ZnL(NO3)2], [CdL(NO3)2], [NiL(NO3)2], [CuL(NO3)2], [CoLCl2] and
[MnLCl2] complexes were prepared. Analytical, structural, spectral and molar conductivity measure-
ments were used to investigate the chemical structure of this ligand and its complexes. Also the synthe-
sized ligand and its complexes were screened for their antibacterial activity against six bacterial strains
and results showed that such complexes have higher antibacterial activity than that of the respective free
ligand against the same microorganism under identical experimental conditions.
"
Synthesis of six 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 10 May 2012
Received in revised form 11 September 2012
Accepted 20 September 2012
Available online 28 September 2012
A new symmetrical [N4O2] hexadentate Schiff base ligand, (E)-N-(pyridin-2-ylmethylene)-2-(3-(2-((E)-
pyridin-2-lmethyleneamino)phenoxy)naphthalen-2-yloxy)benzenamine, abbreviated to L, and its com-
plexes of Ni(II), Cu(II), Zn(II), Co(II), Cd(II) and Mn(II) have been synthesized in the presence of metal ions.
The complexes were structurally characterized by elemental analyses, IR, UV–Vis, NMR and molar con-
ductivity. The crystal structures of two complexes, [NiL(ONO2)2]ꢀ2H2O and [CoLCl2]CH3OHꢀ0.5H2O, have
been determined by a single crystal X-ray diffraction study. In these complexes, the ligand is coordinated
in a neutral form via pyridine and azomethine nitrogen atoms. The metal ions complete their six coordi-
nation with two coordinated nitrate or chloride ions, forming a distorted octahedral geometry. The syn-
thesized compounds have antibacterial activity against the three Gram-positive bacteria: Enterococcus
faecalis, Bacillus cereus and Staphylococcus epid and also against the three Gram-negative bacteria:
Keywords:
Symmetrical hexadentate Schiff base ligand
Metal complexes
X-ray structures
2-Pyridinecarbaldehyde
Antibacterial effects
⇑
Corresponding author. Tel.: +98 811 8242044; fax: +98 811 8272404.
E-mail address: haskey1@yahoo.com (H. Keypour).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.048

60
H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
Citrobacter freundii, Enterobacter aerogenes and Salmonella typhi. The activity data show that the com-
plexes are more potent antibacterials than the parent Schiff base.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
monochromated MoKa X-ray radiation (k = 0.71073 nm). All
the structures were refined on F2 by a full-matrix least-squares
procedure using anisotropic displacement parameters for all non-
hydrogen atoms. Crystal data and structure refinement for both
complexes are shown in Table 1.
Polydentate Schiff base ligands are of great interest because
their binding selectivity can be altered subtly by having the ligand
enforce a specific spatial arrangement of donor atoms (i.e. preorga-
nization of the donor atoms) or incorporate a set of donor atoms
tailored to the metal of interest. These types of ligands are able
to form varieties of complexes with different stoichiometry, struc-
ture, magnetic and spectral properties. These complexes attracted
the attention of many authors due to their dioxygen uptake [1],
oxidative catalysis [2], magnetic behavior and biological activities.
In addition, polydentate ligands have the potential to form com-
plexes of high stability and slow ligand exchange kinetics. This is
important in preventing rapid demetallation or complex decompo-
sition if the ligand is to direct the radionuclide to the target organ
or if the ligand is to be used as an effective chelating agent for the
treatment of metal overload [3]. Moreover, the metal complexes
derived from such ligand systems have immense importance in
different fields like biochemistry, catalysis, magnetochemistry,
materials sciences, etc. These ligands are generally consisting of
N3O3, N4O3, N2O4, N4O2, etc. donor atoms [4–14]. In this context,
we report the synthesis and characterization of a new hexadentate
N4O2 donor symmetrical Schiff base ligand and its metal
complexes, paying special attention to their X-ray crystal struc-
tures, electrical conductivity and antibacterial effects. In all these
complexes prepared, both etheric oxygen groups remain non-
coordinated to the metal ions. The antibacterial activity of Schiff
base ligand and its metal chelates are reported against the three
Gram-positive bacteria: Entrococcus faecalis (RITCC 2121), Bacillus
cereus (RITCC 1040) and Staphylococcus epidermidis (RITCC 1898)
and also against the three Gram-negative bacteria: Citrobacter
freundii (RITCC 1101), Enterobacter aerogene (RITCC 1145) and
Salmonella typhi (RITCC 1715).
Antibacterial activity test
In vitro activity test was carried out using the growth inhibitory
zone (well method) [43,44]. The antibacterial effects of compo-
nents was determined against the three Gram-positive bacteria
(Table 4): Enterococcus faecalis (RITCC 2121), Staphylococcus
epidermidis (RITCC 1898), and Bacillus cereus (RITCC 1040), and
against the three Gram-negative bacteria: Citrobacter freundii
(RITCC 1101), Enterobacter aerogenes (RITCC 1145), and Salmo-
nella typhi (RITCC 1715). Microorganisms (obtained from enrich-
ment 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 inhibitory activity was compared with that of stan-
dard antibiotics, such as cyprofloxacin (10
lg). After drilling wells
on the medium using a 6 mm cork borer, 100
lL of solution from
different compounds were poured into each well. The plates were
incubated at 37 °C overnight. The diameter of the inhibition zone
was measured 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 macrodilution 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 adding 1 ml extract with concentration
100 mg/ml in the first tube, we made a serial dilution of this ex-
tract 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 provided bacterial inoculums
concentration of 1.5 ꢂ 108 cfu/ml. Then 1 ml of bacterial suspen-
sion 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.
Experimental
Starting materials
2-(3-(2-Aminophenoxy)naphthalen-2-yloxy)benzenamine was
synthesized according to the literature procedure [15–19].
Solvents, naphthalene-2,3-diol, 2-pyridinecarbaldehyde, 1-fluoro-
2-nitrobenzene and metal salts were purchased from Merck and
were used without further purification.
Synthesis
L
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 2-pyridinecarbaldehyde (0.214 g, 2 mmol) in
methanol (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
(76.8%). M.p. 75 °C. Anal. Calc. For C34H24N4O2: C, 78.4; H, 4.7; N,
10.8. Found: C, 78.9; H, 4.2; N, 10.6%. IR (cmꢁ1, KBr): 1630 (s,
Physical measurements
Infrared spectra were collected using KBr pellets on a BIO-RAD
FTS-40A spectrophotometer (4000–400 cmꢁ1). 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 DMSO on a Bruker
Avance 300 MHz and Jeol 90 MHz spectrometer using Si(CH3)4 as
an internal standard.
m
C@N). UV–Vis [k (nm), e
(Mꢁ1 cmꢁ1)]: 242(72600), 256(30700),
307(6800).
[CuL(NO3)2]
A
methanol solution (30 ml) of Cu(NO3)2.6H2O (0.2956 g,
X-ray crystallography
1 mmol) was added to a warm solution of [L] (0.520 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:
The single crystal X-ray diffraction analyses were performed on
a Bruker SMART CCD diffractometer at 293(2) K, using graphite

H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
61
Table 1
Crystal data and structure refinement for [CoLCl2]CH3OHꢀ0.5H2O and [NiL(ONO2)2]ꢀ2H2O complexes.
Empirical formula
[CoLCl2]CH3OHꢀ0.5H2O
[NiL(ONO2)2]ꢀ2H2O
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
738.42
293(2)
0.71073
Monoclinic
C2/c
735.30
293(2)
0.71073
Monoclinic
C2/c
Unit cell dimensions
a (Å)
15.885(2)
16.105(4)
b (Å)
15.818(2)
15.816(4)
c (Å)
13.9430(17)
15.177(4)
b (°)
112.843(2)
3228.6(7)
4
1.412
0.740
114.290(3)
3523.7(14)
4
1.401
0.617
Volume (Å3)
Z
Calculated density (Mg/m3)
Absorption coefficient (mmꢁ1
Crystal size (mm3)
Theta range for data collection (°)
Limiting indices
Reflections collected/unique
Completeness to theta = 25.00
Absorption correction
)
0.36 ꢂ 0.29 ꢂ 0.27
0.60 ꢂ 0.47 ꢂ 0.25
1.90–25.01
1.89–25.02
ꢁ17 6 h 6 18, ꢁ18 6 k 6 18, ꢁ16 6 l 6 16
8349/2822 [R(int) = 0.0235]
99.1%
ꢁ18 6 h 6 19, ꢁ18 6 k 6 18, ꢁ18 6 l 6 12
8932/3098 [R(int) = 0.0332]
99.2%
Empirical
Empirical
Max. and min. transmission
Refinement method
0.8251 and 0.7764
Full-matrix least-squares on F2
2822/0/226
0.8610 and 0.7085
Full-matrix least-squares on F2
3098/0/253
Data/restraints/parameters
Goodness-of-fit on F2
1.158
1.125
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0311, wR2 = 0.0901
R1 = 0.0410, wR2 = 0.1071
0.426 and ꢁ0.177
R1 = 0.0952, wR2 = 0.2779
R1 = 0.1185, wR2 = 0.3012
1.060 and ꢁ0.612
Largest diff. peak and hole (e.Aꢁ3
)
0.6 g (84.7%). M.p. 228 °C. Anal. Calc. for C34H24CuN6O8 (MW:
[ZnL(NO3)2]
708.13): C, 57.7; H, 3.4; N, 11.9. Found: C, 57.3; H, 3.7; N, 11.6%.
The preparation of the yellow complex followed the same proce-
dure described for [CuL(NO3)2] by using a solution of Zn(NO3)2ꢀ6H2O
(0.2975 g, 1 mmol) in 30 ml of methanol. Yield: 0.6 g (84.5%). M.p.
344 °C. Anal. Calc. for C34H24ZnN6O8(MW: 709.98): C, 57.5; H, 3.4;
N, 11.8. Found: C, 57.3; H, 3.2; N, 11.6%. IR (cmꢁ1, KBr): 1626 (s,
IR (cmꢁ1, KBr): 1626 (s,
m
C@N). UV–Vis [k (nm),
242(60000), 269(31500), 317(sh), 370(10800), 612 (sh). Km = 2
cm2 ꢁ1 molꢁ1
e
(Mꢁ1 cmꢁ1)]:
X
.
m
C@N). UV–Vis [k (nm), e
(Mꢁ1 cmꢁ1)]: 270(41,400), 305(sh),
324(21,200), 353(13,400). KM = 16 Oꢁ1 cm2 molꢁ1
.
[CoLCl2]CH3OHꢀ0.5H2O
The preparation of the red microcrystalline complex followed
the same procedure described for [CuL(NO3)2] by using a solution
of CoCl2ꢀ6H2O (0.2378 g, 1 mmol) in 30 ml of methanol. Yield:
0.5 g (76.9%). M.p. 350 °C. Anal. Calc. for C35H29CoCl2N4O3.5(MW:
691.47): C, 60.8; H, 4.2; N, 8.1. Found: C, 59.5; H, 3.8; N, 7.9%. IR
[CdL(NO3)2]
The preparation of the yellow complex followed the same proce-
dure described for [CuL(NO3)2] by using a solution of Cd(NO3)2ꢀ6H2O
(0.3445 g, 1 mmol) in 30 ml of methanol. Yield: 0.6 g (79.3%). M.p.
270 °C. Anal. Calc. for C34H24CdN6O8 (MW: 756.99): C, 53.9; H, 3.2;
N, 11.1. Found: C, 53.7; H, 3.5; N, 10.9%. IR (cmꢁ1, KBr): 1633 (s,
(cmꢁ1, KBr): 1629 (s,
mC@N). UV–Vis [k (nm), e
(Mꢁ1 cmꢁ1)]:
242(37,100), 277(19,300), 309(14,200), 328 (12,800), 351 (8500),
384(2200). Km = 0.3 cm2
X .
ꢁ1 molꢁ1
m
C@N). UV–Vis [k (nm), e
(Mꢁ1 cmꢁ1)]: 270(37,700), 303(19,600),
317(17,100), 348(10,200). KM = 13 Oꢁ1 cm2 molꢁ1
.
[NiL(ONO2)2]ꢀ2H2O
The preparation of the brown microcrystalline complex fol-
lowed the same procedure described for [CuL(NO3)2] by using a
solution of Ni(NO3)2ꢀ6H2O (0.2908 g, 1 mmol) in 30 ml of
methanol. Yield: 0.6 g (85.3%). M.p. 305 °C. Anal. Calc. for
Results and discussion
One new potentially hexadentate N4O2 Schiff base ligand (L) has
been easily prepared from the reaction of 2-(3-(2-aminophen-
oxy)naphthalen-2-yloxy)benzenamine and 2-pyridinecarbalde-
hyde. Complexes of this ligand with copper(II), nickel(II), zinc(II)
and cadmium(II) nitrate and also cobalt(II) and manganese(II)
chloride were synthesized by template reactions starting from
metal salts, 2-pyridinecarbaldehyde and 2-(3-(2-aminophen-
oxy)naphthalen-2-yloxy)benzenamine (mole ratio 1:1:1), respec-
tively. The same products were obtained also by direct reactions
of metal salts and the ligand L in equimolar ratio (1:1). The compo-
sition of the products was not affected by the change of the mole
ratio of reacting species. The complexes are soluble in common or-
ganic solvents such as CHCl3, EtOH, MeOH, and THF. In both cases
studied, they turned out to be of non-electrolyte nature, as deter-
mined by molar conductivity measurements. Complexes were
characterized by IR spectra, elemental analysis, molar conductance
C
34H28NiN6O10(MW: 739.30): C, 55.2; H, 3.8; N, 11.4. Found: C,
55.5; H, 3.3; N, 11.4%. IR (cmꢁ1, KBr): 1634 (s,
m
C@N). UV–Vis [k
(nm),
e
(Mꢁ1 cmꢁ1)]: 242(30,600), 282(16,800), 309(15,300),
355(sh), 398(3600), 452(1500), 512(1200). Km = 4 cm2
X .
ꢁ1 molꢁ1
[MnLCl2]
The preparation of the orange complex followed the same
procedure described for [CuL(NO3)2] by using solution of
MnCl2ꢀ4H2O (0.1979 g, 1 mmol) in 30 ml of methanol. Yield: 0.5 g
(77.4%). M.p. 297 °C. Anal. Calc. for 34H24MnCl2N4O2 (MW:
646.40): C, 63.2; H, 3.7; N, 8.7. Found: C, 63.5; H, 3.3; N,
8.5%. IR (cmꢁ1
KBr): 1629 (s, C@N). UV–Vis [k (nm),
(Mꢁ1 cmꢁ1)]: 271(31,600), 287(26,400), 308(19,700), 321(10,600).
Km = 18 cm2 ꢁ1 molꢁ1
a
C
,
m
e
X
.
(Km), UV–Vis spectra and in the case of [NiL(ONO2)2]ꢀ2H2O and

62
H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
[CoLCl2]CH3OHꢀ0.5H2O with X-ray diffraction. The analytical and
spectral data are completely consistent with the proposed formu-
lation. The metals are also bound to ligands through the azome-
thine nitrogens, deduced from the observed decrease or increase
in C@N stretching frequency upon complexation of ligand to metal
ions.
protons appear as doublets in this complex indicating the
non-equivalence of the two imino groups of the molecule which
would result from distorted octahedral geometry around the metal
ion [20]. Finally, the doublet assigned to H(17) of the pyridine ring
(8.26 and 8.29 ppm, for [ZnL(NO3)2] and 8.55 and 8.63 ppm, for
[CdL(NO3)2] is shifted to lower field in these complexes consistent
with coordination through the pyridine nitrogen. Slightly lower
frequency shifts are also observed for the aromatic protons when
compared with the free ligand. The 13C NMR spectra of the zinc
and cadmium complexes show a peak at 163.72 and 162.91 ppm,
respectively corresponding to the imine carbon atoms. In 13C
NMR spectra of these two complexes should be exhibited sixteen
main peaks, however, for [CdL(NO3)2 15 main signals (118.635–
150.642 ppm) appeared, it seems that two carbons in this region
are overlapping to each other due to high similarity of carbons
and low resolution of instrument. However, we suppose that these
two complexes are similar and carbon atoms are chemically equiv-
alent but the signals of the 13C NMR spectrum of the [ZnL(NO3)2] in
the aromatic region, (112.545–150.956 ppm), are more than
[CdL(NO3)2 supporting the non-equivalence of the aromatic ring
carbons environments in this complex.
IR spectra
The IR spectra of the complexes show a sharp band in the range
1626–1634 cmꢁ1, attributed to
m
(C@N), which is shifted to higher
or lower frequency on going from the free ligand (at 1630 cmꢁ1
)
to the complexes. This is indicative of the coordination of the imine
nitrogen to the metal and is consistent with X-ray diffraction re-
sults. Furthermore, the absence of C@O and NAH stretching vibra-
tions in the spectra of the ligands, related to aldehyde and diamine,
respectively, indicate occurrence of Schiff base condensation.
Appearance of a strong sharp band at 1384 cmꢁ1 demonstrates
the presence of ionic nitrate (NOꢁ3 ) in [NiL(ONO2)2]ꢀ2H2O,
[CuL(NO3)2], [ZnL(NO3)2] and [CdL(NO3)2] complexes.
NMR spectra
Electronic absorption spectroscopy
The 1H NMR spectra of the ligand L and its related diamagnetic
complexes ([ZnL(NO3)2] and [CdL(NO3)2]) have been recorded in
DMSO-d6, 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 at 8.575 ppm is assigned to the azomethine protons.
Multiplets of aromatic protons appear in the region 6.950–
7.558 ppm. 13C NMR spectrum of this ligand shows that in the
region corresponding to the signals of aromatic rings carbons
(115.908–155.037 ppm) fourteen peaks instead of sixteen
expected peaks are observed, due to high similarity of carbons
and low resolution of instrument. The imine carbon appear at
161.497 ppm. These data support the formulation of the Schiff
base, L.
The electronic spectral data of the Schiff base and its metal com-
plexes is given in Table 2. The ligand shows three strong peaks at
242, 256 and 307 nm. The two strong bands at 242 and 256 nm
are attributed to benzene p– p–
pꢃ and imino pꢃ transitions respec-
tively [21]. The third band in the spectra of the Schiff base
(307 nm) is assigned to n–pꢃ transition. In the metal complexes
this band is shifted to a longer wavelength with increasing inten-
sity. This shift may be attributed to the donation of the lone pairs
of electron of the nitrogen atoms of the Schiff base to the metal ion
(M N).
Electronic absorption spectra of the Co(II), Ni(II) and Cu(II)
complexes were recorded in CDCl3 and in the case of Cd(II), Zn(II)
and Mn(II) complexes in DMF solutions. The bonds below 317 nm
The 1H NMR spectra of the complexes are almost similar to that
of the ligand L, with slight shift to lower fields. The signal for the
imine proton in the zinc and cadmium complexes appears at
8.92 (for [ZnL(NO3)2]) and 8.77 and 8.84 ppm (for [CdL(NO3)2])
and these are shifted downfield with respect to the corresponding
signal in the free ligand, indicating that the metal–nitrogen bond is
retained in solution. In the case of the cadmium complex the imino
are attributed to the p–
pꢃ and n–pꢃ transitions, within the ligand.
In the electronic spectra of these complexes, there is a band around
321–370 nm. This band is assigned as a charge transfer (CT)
transition [22,23]. The electronic spectrum of Cu(II) complex dis-
plays a broad band at 612 nm assignable to 2Eg ? 2T2g transition,
characteristic of octahedral geometry around Cu(II) ion [24].
The Ni(II) complex exhibited three bands at 512, 452 and 398 nm
attributed to the 3A2g ? 3T2g
(m (m2) and
1), 3A2g ? 3T1g (F)
3A2g ? 3T1g(P) (
m3) transitions respectively, which indicate an
octahedral geometry around Ni(II) ion [25]. The electronic spec-
trum of Co (II) complex exhibits two absorption bands at
4
384 and 351 nm that are assigned to the T1g(F) ? 4A2g(F) and
1T1g(F) ? 4T1g(P) transitions, respectively, being characteristic of
an octahedral geometry [26]. The complexes of Zn(II) and Cd(II)
are diamagnetic without any d–d transition. Mn(II) complex does
not show any d–d transition [27].
Molar conductivity
The molar conductivity (KM)
data (0.3–18 Oꢁ1 molꢁ1 cm2),
measured at 25 °C using 10ꢁ3 M solutions of the complexes in
CHCl3 ([MLX2], M@Co, Ni, Cu) or DMF ([MLX2], M@Zn, Mn, Cd) sol-
vents. These data suggest that two nitrate ions are present inside of
the coordination sphere. It also indicates that the chloride anions
bind to the metal ions as ligands and do not ionize. The results of
elemental analysis indicate that the composition of the complexes
is [ML(NO3)2] (M@Cu, Zn, Ni and Cd) or [MLCl2] (M@Co and Mn)
and the ratio of M/L in all complexes are 1:1 [28–33].
Fig. 1. Structure of ligand, L along with NMR numbering.

H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
63
Table 2
Electronic spectroscopy data (nm) for ligand L and related complexes.
a
Compound
kmax (nm) (e)
Intraligand (LL)
CT
d–d
[L]
242 (72600)
242 (30600)
242 (60000)
242 (37100)
271 (31600)
270 (41400)
270 (37700)
256 (30700)
282 (16800)
269 (31500)
277 (19300)
287 (26400)
305 (sh)
307 (6800)
309 (15300)
317 (sh)
309 (14200)
308(19700)
324(21200)
317 (17100)
[NiL(NO3)2]
[CuL(NO3)2]
[CoLCl2]
[MnLCl2]
[ZnL(NO3)2]
[CdL(NO3)2]
355 (sh)
398 (3600)
612 (sh)
351 (8500)
452 (1500)
384 (2200)
512 (1200)
370 (10800)
328 (12800)
321 (10600)
353(13400)
348 (10200)
30 (19600)
a
Molꢁ1 cmꢁ1
.
X-ray structures
positions are occupied by N(1) and N(1)#1 of the pyridine nitro-
gens in both complexes. In this way [NiL(ONO2)2]ꢀ2H2O has two
trans OAMAN(iminic) bond angles in the range 168.1(3) and [CoL-
Cl2]CH3OHꢀ0.5H2O has also two trans ClAMAN(iminic) bond angles
in the range 163.50(5), and each complex has a trans N(pyridinic)-
AMAN(pyridinic) bond angle in the range 175.1(3) and 168.51(10)
for [NiL(ONO2)2]ꢀ2H2O and [CoLCl2]CH3OHꢀ0.5H2O complexes
respectively, which involves donor atoms from the pyridine rings
in each ligand. All of the values are significantly different to the
theoretical value of 180° expected for a regular geometry. Three
different bond distances can be found in the complexes which cor-
respond to the three types of donor atom that exist in the ligands
(Table 3). In general, all of the bond distances to the metal ions can
be considered normal and are similar to those found in other com-
pounds that contain similar neutral hexadentate aromatic ligands
with the metals in an octahedral environment. For example, NiAO,
NiAN(iminic) and NiAN(pyridinic) bond distances in the [NiL(ONO2)2]-
ꢀ2H2O complex are 2.159(8) Å, 2.140(5) Å and 2.087(6) Å,
respectively [34–38]. Values for these three distances for
analogous cobalt complex [CoLCl2ꢀCH3OHꢀ0.5H2O are 2.4143(7) Å
(Cl(1)ACo(1)), 2.2355(19) Å and 2.1497(19) Å, respectively [39–
42], which are comparable to corresponding distances reported
in the literature.
The complexes derived from 2-(3-(2-aminophenoxy)naphtha-
lene-2-yloxy)benzenamine are easily prepared by reacting the li-
gand
L directly with the relevant nitrate of nickel, copper,
cadmium and zinc and also chloride of cobalt and manganese.
Crystals suitable for X-ray analysis were obtained for nickel and co-
balt complexes. The structures of the mononuclear [CoLCl2]CH3-
OHꢀ0.5H2O and [NiL(ONO2)2]ꢀ2H2O complexes are depicted in
Figs. 2 and 3 which show that the hexadentate aromatic ligand
adopt a distorted octahedral arrangement at the metal in both
complexes. In each complex, the ligand L binds the metal center
through the pyridine and the imine N-atoms, forming two five-
membered chelate rings. The nitrate and chloride groups are ar-
ranged in a cis manner affording an overall octahedral motif. The
dihedral angle between the two five membered chelate rings in
[CoLCl2]CH3OHꢀ5H2O (74.06(1)°) is significantly smaller than that
in [NiL(ONO2)2]ꢀ2H2O (79.01(1)°). The bond parameters associated
with the metal ions in [CoLCl2]CH3OHꢀ0.5H2O and [NiL(ONO2)2]-
ꢀ2H2O are listed in Table 3. In order to make easier direct compar-
ison, the numbering schemes are the same for each compound.
Given the great similarities between the complexes they will be
discussed together in a comparative manner.
Coordination around the nickel(II) and cobalt(II) ions is defined
by the N4 donor set of the ligand, L, and also by two oxygen atoms
from two nitrate groups or two chloride atoms in [NiL(ONO2)2]-
ꢀ2H2O and [CoLCl2]CH3OHꢀ0.5H2O complexes respectively. In
[NiL(ONO2)2]ꢀ2H2O the basal plane is defined by N(2), N(2)#1,
O(1N) and O(1N)#1 from the two iminic nitrogens and two nitrate
oxygens whilst in the case of [CoLCl2]CH3OHꢀ0.5H2O, the basal
plane is defined by N(2), N(2)#1, Cl(1) and Cl(1)#1. The apical
Antibacterial activities
The antibacterial activity results evidently showed that the
most of prepared chemicals have relatively good antibacterial ef-
fects against the studied bacterial strains Especially among the
compounds, [ZnL(NO3)2] and [CuL(NO3)2] are more stronger than
others even though [CoLCl2], [NiL(NO3)2] and [MnLCl2] are good
Fig. 2. Molecular structure of [NiL(ONO2)2]ꢀ2H2O showing 50% probability thermal ellipsoids using ORTEP.

64
H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
Fig. 3. Molecular structure of [CoLCl2]CH3OHꢀ0.5H2O showing 50% probability thermal ellipsoids using ORTEP.
Table 3
Selected bond lengths (A) and bond angles (°) for complexes [CoLCl2]CH3OH. 0.5H2O and [NiL(ONO2)2]2H2O.
Complex [NiL(ONO2)2]
Bond
Complex [CoLCl2]
Distance
Bond
Distance
N(1)ANi(1)
N(2)ANi(1)
O(1N)ANi(1)
Ni(1)AN(1)#1
Ni(1)AN(2)#1
Ni(1)AO(1N)#1
2.087(6)
2.140(5)
2.159(8)
2.087(6)
2.140(5)
2.159(8)
N(1)ACo(1)
N(2)ACo(1)
Cl(1)ACo(1)
Co(1)AN(1)#1
Co(1)AN(2)#1
Co(1)ACl(1)#1
2.1497(19)
2.2355(19)
2.4143(7)
2.1497(19)
2.2354(19)
2.4143(7)
Bond
Angle
Bond
Angle
N(1)ANi(1)AN(1)#1
N(1)ANi(1)AN(2)
175.1(3)
78.5(2)
97.8(2)
97.8(2)
78.5(2)
83.8(3)
89.6(3)
94.0(3)
168.1(3)
97.3(2)
94.0(3)
89.6(3)
97.3(2)
168.1(3)
84.0(4)
N(1)ACo(1)AN(1)#1
N(1)ACo(1)AN(2)
168.51(10)
74.82(7)
96.20(7)
96.20(7)
74.82(7)
79.55(9)
92.96(5)
94.33(5)
163.50(5)
91.05(5)
94.33(5)
92.96(5)
91.05(5)
163.50(5)
101.06(4)
N(1)#1ANi(1)AN(2)
N(1)ANi(1)AN(2)#1
N(l)#lANi(l)AN(2)#l
N(2)ANi(1)AN(2)#1
N(l)ANi(l)AO(1N)#1
N(l)#lANi(l)AO(1N)#1
N(2)ANi(1)AO(1N)#1
N(2)#lANi(l)AO(1N)#1
N(1)ANi(1)AO(1N)
N(l)#lANi(l)AO(1N)
N(2)ANi(1)AO(1N)
N(2)#1ANi(1)AO(1N)
O(1N)#1ANi(l)AO(1N)
N(1)#1ACo(1)AN(2)
N(1)ACo(1)AN(2)#1
N(1)#1ACo(1)AN(2)#1
N(2)ACo(1)AN(2)#1
N(1)ACo(1)ACl(1)
N(1)#1ACo(1)ACl(1)
N(2)ACo(1)ACl(1)
N(2)#1ACo(1)ACl(1)
N(1)ACo(1)ACl(1)#1
N(1)#1ACo(1)ACl(1)#1
N(2)ACo(1)ACl(1)#1
N(2)#1ACo(1)ACl(1)#1
Cl(1)ACo(1)ACl(1)#1
Table 4
Quantitative antimicrobial assay results (zone of growth inhibition) of the free Schiff base ligand and related complexes.
Microorganism
Standard
Minimum inhibitory concentration (mg/ml)
Main compounds
Cyprofloxacin
[L]
[CdL(NO3)2]
[ZnL(NO3)2]
[CoLCl2]
[NiL(NO3)2]
[MnLCl2]
[CuL(NO3)2]
Enterococcus faecalis(+)
Bacillus cereus(+)
Staphylococcus epid(+)
Citrobacter freundii(ꢁ)
Enterobacter aerogenes(ꢁ)
Salmonella typhi(-)
35
25
35
35
40
30
–
–
10
–
–
–
15
15
30
20
20
20
20
20
25
–
10
20
15
15
15
10
15
20
20
10
15
15
–
25
15
30
20
15
15
15
10
10
–
–
–
20
because they have good effects on Gram-negative bacteria and also
[CdL(NO3)2] has moderate activity against three of them. Free
Schiff base ligand shows weak activity just on Estaphylococcus
epidermidis and these results show that antibacterial activities of
this ligand is not comparable to its relative complexes. In general,
comparison of the microbial activity of N2O2, N4O2, etc. Schiff base

H. Keypour et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 59–66
65
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(NO3)2]
[ZnL(NO3)2]
[CoLCl2]
[NiL(NO3)2]
[MnLCl2]
[CuL(NO3)2]
Enterococcus faecalis(+)
Bacillus cereus(+)
Staphylococcus epid(+)
Citrobacter freundii(ꢁ)
Enterobacter aerogenes(ꢁ)
Salmonella typhi(ꢁ)
–
–
100
–
–
–
50
100
100
–
50
50
3.125
25
25
25
25
6.25
–
100
25
50
50
50
100
50
25
25
100
50
50
–
6.25
50
3.125
25
50
50
–
–
25
25
ligands and their related complexes show that such complexes
have higher antibacterial activity than that of the respective free li-
gand against the same microorganism under identical experimen-
tal conditions [45–52]. Such an increased activity for the metal
chelates as compared to the free ligands can be explained on the
basis of chelation theory [53]. Chelation considerably reduces the
polarity of the metal ion because of the partial sharing of its posi-
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