Unsymmetrical Schiff bases functionalize as bibasic tetradentate (ONNO) and monobasic tridentate (NNO) ligands on complexation with some transition metal ions

Article abstract of DOI:10.1016/j.molstruc.2007.02.023

Three dissymmetrical Schiff bases have been prepared by the condensation of 2-hydroxyacetophenone, ethylenediamine and several aldehydes. The electronic transitions within these Schiff bases molecules and the effect of solvents of different polarities on

Full text of DOI:10.1016/j.molstruc.2007.02.023

Available online at www.sciencedirect.com
Journal of Molecular Structure 872 (2008) 113–122
www.elsevier.com/locate/molstruc
Unsymmetrical Schiff bases functionalize as bibasic
tetradentate (ONNO) and monobasic tridentate (NNO) ligands
on complexation with some transition metal ions
*
U.M. Rabie , A.S.A. Assran, M.H.M. Abou-El-Wafa
Chemistry Department, Faculty of Science at Qena, South Valley University, Qena 83523, Egypt
Received 23 August 2006; received in revised form 8 January 2007; accepted 19 February 2007
Available online 24 February 2007
Abstract
Three dissymmetrical Schiff bases have been prepared by the condensation of 2-hydroxyacetophenone, ethylenediamine and several
aldehydes. The electronic transitions within these Schiff bases molecules and the effect of solvents of different polarities on these transi-
tions have been investigated by UV/vis spectroscopy. Schiff bases complexes, binary 1:1 (metal:ligand) and ternary 1:1:1 (metal:ligan-
d:Lewis base, where Lewis base = imidazole or pyridine), with transition metals, Co(II), Ni(II), Cu(II), and Zn(II) have been
synthesized and characterized by elemental analysis, molar conductivity and electronic absorption and IR spectra. Further, the stoichi-
ometric ratios of the complexes in solutions and the formation constants of the interaction of Schiff base ligands with metal ions have
been determined.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Schiff bases; Binary and ternary complexes; Metal complexes; Spectral studies; Stoichiometric determination; Formation constants
1. Introduction
binding sites in the metallo-proteins and -enzymes, and
selectivity of natural systems with synthetic materials
[12,13]. Further, recent years have witnessed a great deal
of interest in the synthesis and characterization of transi-
tion metals complexes containing Schiff bases as ligands
due to their applications as catalysts for many reactions
[14–17] and relation to synthetic and natural oxygen carri-
ers [18].
In view of these finding and in continuation to our
previous work on the Schiff bases [19–22], this piece
of work has devoted with the aim to synthesize some
transition metals complexes with unsymmetrical Schiff
Schiff bases complexes have remained an important and
popular area of research due to their simple synthesis, ver-
satility, and diverse range of applications [1,2]. Thus, Schiff
bases have played a marvelous role in the development of
coordination chemistry as they readily form stable com-
plexes with most of the transition metals [1], which has sus-
tained their wide-spread interest over the past 150 years [2].
In the area of bioinorganic chemistry interest in Schiff
bases complexes with transition and inner-transition metals
has centered on the role of such complexes in providing
synthetic interesting models for the metal-containing sites
in metallo-proteins and -enzymes [3–11], whereas, unsym-
metrical Schiff bases ligands have clearly offered many
advantages over their symmetrical counterparts in the elu-
cidation of the composition and geometry of the metal ion
bases
ligands,
(R-benzaldehyde)(2-hydroxyacetophe-
none)ethylenediamine, and to examine their physical
properties involving spectral behaviors, the inter- or
intra-molecular charge transfer, the electrical conduc-
tance values and to trace complexes formation in solu-
tions and to determine the stoichiometric ratios of the
formed complexes either spectrophotometrically or by
conductometric titration.
*
Corresponding author.
E-mail address: umrabie@yahoo.com (U.M. Rabie).
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.02.023

114
U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
2. Experimental
stant. On the other hand, for purpose of the molar ratio
method [25], a series of solutions has been prepared in
which ligand concentration has kept constant, while that
of metal ion has been varied for covering the molar ratios
[M(II)]/[ligand] = 3.00–0.2. The spectra of all solutions
prepared have been recorded within the wavelength range
300–600 nm, where a solution containing the same concen-
tration of the ligand as that of the measured solution has
been used in the reference cell to eliminate any absorption
of this ligand with the absorption of the corresponding
complex.
2.1. Materials and solutions
All chemicals used, ethylenediamine, 2-hydroxyacetoph-
enone, aromatic aldehydes (o-hydroxybenzaldehyde, benz-
aldehyde, and p-hydroxybenzaldehyde), spectral grade
organic solvents (ethanol, methanol, dimethylformamide
(DMF), acetone, acetonitril, chloroform, carbontetrachlo-
ride, dichloromethane and petroleum ether), metal salts
(CoCl2Æ6H2O, Ni(CH3COO)2Æ4H2O, Cu(CH3COO)2Æ2H2O
and Zn(CH3COO)2Æ2H2O) and Lewis bases (Imidazole
and pyridine) were of extra pure products (Merck, Aldrich,
Sigma or B.D.H) and have been used without further
purification.
2.6. Conductance measurements
For this purpose, the relationship between the specific
conductance (k) and the molar ratio [L]/[M(II)] for the dif-
ferent complexes formed in solution is plotted, whereas,
when the concentration of the solution is C (mol dmꢀ3),
the specific conductance (k) is clearly due to C moles,
and the molar conductivity (K) will be: K = k/C
(ohmꢀ1 cm2 molꢀ1), the conductivity per mole of electro-
lyte. However, correction for the dilution effect is made
by multiplying the values of specific conductance by the
factor (V1 + V2)/V1, in which V1 is the original volume
of the metal ion solution and V2 is that of the ligand solu-
tion added. The graphs obtained have showed distinct
breaks, whereas, the locations of which have given an
information about the presence and stoichiometry of the
different complexes liable to exist in solutions.
2.2. Ligand solutions
Stock solutions of the ligands L1–L3 (1 · 10ꢀ2
mol dmꢀ3) have been prepared by dissolving the accurate
weight of each ligand in the appropriate volume of the sol-
vent. Solutions of the required concentrations have been
prepared by accurate dilution.
2.3. Metal salt solutions
Solutions (1 · 10ꢀ2 mol dmꢀ3) of each of the metal salts
investigated have been obtained by dissolving the accurate
weight of each in the appropriate volume of the solvent.
The metal content of the solutions has been determined
by the recommended methods of analysis [23] and the solu-
tions of the required concentrations have been prepared by
accurate dilution.
In the present study, the conductometric titration has
been performed by titrating 25 mL of 1 · 10ꢀ3 mol dmꢀ3
of the metal ion solution with 5 · 10ꢀ3 mol dmꢀ3 ligand
solution. The solvent used was DMF. The conductance
has been measured after stirring the solution for about
2 min.
2.4. Physical measurements
The electronic absorption spectra have been recorded on
a Perkin-Elmer Lambda 40 spectrophotometer equipped
with a Julabo FP 40 thermostat ( 0.1 ꢁC) using 1.0 cm
matched quartz cells. IR spectra of the solid compounds
have been recorded on a Shimadzu IR-408 spectrophotom-
eter. Conductance measurements have been carried out in
DMF at room temperature using Jenway 4320 conductivity
meter. 1H NMR measurements have been carried out in the
microanalytical laboratory, Cairo University. HPLC mea-
surements have been preformed at Qena Central labora-
tory, South Valley University. Elemental analyses for C,
H, N and Cl of the ligands and solid complexes have been
carried out in the microanalytical laboratories at Cairo and
Assiut Universities.
2.7. Synthesis of the (R-benzaldehyde)(2-hydroxyacetophenone)-
ethylenediamine compounds (L1–L3)
The compounds under investigation are: (o-hydroxy-
benzaldehyde)(2-hydroxyacetophenone)ethylenediamine
(L1), (benzaldehyde)(2-hydroxyacetophenone)ethylenedia-
mine (L2), and (p-hydroxybenzaldehyde)(2-hydroxyace-
tophenone)ethylenediamine (L3). These Schiff bases have
been prepared according to the following procedure:
0.61 g (0.01 mol) of ethylenediamine in a few amount of
hot ethanol has been added dropwisely with continuous
stirring to a hot ethanolic solution of 1.36 g (0.01 mol) of
2-hydroxyacetophenone followed by 1.22 g (0.01 mol) of
o-hydroxybenzaldehyde in case of (L1), 1.06 g (0.01 mol)
of benzaldehyde in case of (L2) and 1.22 g (0.01 mol) of
p-hydroxybenzaldehyde in case of (L3). The mixtures have
been refluxed on water bath for about 1 h with constant
stirring and then allowed to cool at room temperature.
The separated solid products have been filtered off and
twice crystallized from ethanol; (L1) 1.86 g, yield 66%;
(L2) 0.77 g, yield 73%; (L3) 1.64 g, yield 58%. The purity
2.5. Continuous variation and molar ratio methods
For purpose of continuous variation method [24], a ser-
ies of solutions has been prepared by mixing ligand and
metal ion solutions in varying proportions while keeping
the total molar concentration of the mixed solutions con-

U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
115
of the prepared compounds (L1–L3) has been checked
using HPLC measurements. Moreover, and for the com-
parison purpose, the well known symmetrical compounds
bis(salicylaldehyde)ethylenediamine and bis(2-hydroxyace-
tophenone)ethylenediamine have been prepared too and
their HPLC measurements have been compared with that
of (o-hydroxybenzaldehyde)(2-hydroxyacetophenone)ethy-
lenediamine (L1).
the CH of the group ACH@NA indicate the unsymmetri-
cal structure of the compound (L1).
2.8. Synthesis of the binary and ternary solid complexes
The binary solid complexes have been synthesized by
mixing a hot methanolic solution of each of the ligands
L1–L3 (0.01 mol) with the appropriate amount of each
metal salt (0.01 mol) of CoCl2Æ6H2O, Ni(CH3COO)2Æ4H2O,
Cu(CH3COO)2Æ2H2O and Zn(CH3COO)2Æ2H2O, and has
dissolved in ꢁ30 ml methanol. The mixtures have been
refluxed on a water bath for about 3 h and then evaporated
to small volumes and left overnight at room temperature.
The solid complexes have been filtered off on a water pump
and washed several times with cold methanol and kept in
desiccator over silica gel. The yields of these synthesized
binary complexes were in the range 32–48%.
On the other hand, the ternary solid complexes have
been synthesized by both direct and indirect methods.
For the direct method; 0.01 mol of the previously pre-
pared binary complex in 50 ml hot methanol has been
added to an equimolar amount (0.01 mol) of Lewis base
imidazole or pyridine. But, in the indirect method;
0.01 mol of any ligand (L1–L3) has been added to
0.01 mol of metal salt solution followed by 0.01 mol of
Lewis base. In each case, the mixture has been refluxed
for 3–4 h, left to cool at room temperature, and the
resulting precipitate has been filtered off and washed sev-
eral times with cold methanol, dried and kept in desicca-
tor over silica gel. The yields of these synthesized ternary
complexes were in the range 25–33%.
The prepared compounds (L1–L3) have been analyzed
for their C, H and N contents, and the results obtained
are in accordance with the structure postulated of each of
these compounds.
The structure of these compounds can be represented
schematically as follow:
CH3
C
CH2 CH2
N N
C
H
R
where
R = o-OH; (o-hydroxybenzaldehyde)(2-hydroxyace-
tophenone)ethylenediamine (L1)
= H; (benzaldehyde)(2-hydroxyacetophenone)ethy-
lenediamine (L2)
=
p-OH; (p-hydroxybenzaldehyde)(2-hydroxyace-
tophenone)ethylenediamine (L3)
The binary and ternary solid complexes have been ana-
lyzed for their carbon, hydrogen and nitrogen contents and
the found elemental analyses were in satisfactory agree-
ment with the calculated values.
It is worthy to mention that, the asymmetric structures
of the prepared L1–L3 ligands have been confirmed by the
H1 NMR spectra measurements. Thus, H1 NMR spectra
of L1(o-OH), as a representative ligand, in CDCl3 has per-
formed a singlet at r 2.61 (3H, CH3); two triplets at d
2.87 and 3.95 ppm (4H, 2CH2); a multiplet at r 6.75–
7.29 (10H, 8H–AR–H and 2OH) and a singlets at r
8.33 (1H, CH). The signals of the OH protons of the
ligand L1 do not appear at the normal positions around
13 ppm (in CDCl3), but the spectra have showed the sig-
nals of the OH protons at lower d value in the form of
multiplets at d (6.75–7.29) for the 10H (8H of Ar–H
and 2H of 2OH). Appearance of the OH protons at lower
d value is perhaps due to the shielding of these protons
through six-membered ring hydrogen bonds. It is well
known [26–28] that the hydrogen bonding causes shift
to a lower field. In addition, the protons of the methylene
bridge do interact and have showed two triplets at d 2.87
and 3.95 ppm. Thus, protons of a methylene bridge would
show a singlets if the bridge represents an axis of symme-
try which this is not representing our case. The asymmet-
ric structure of the ligand has been confirmed as shown
(L1; o-OH) – Found: C, 72.12; H, 6.07; N, 9.84%. Calcd
for C17H18N2O2: C, 72.32; H, 6.43; N, 9.92%
(L2; R = H) – Found: C, 76.58; H, 6.79; N, 10.26%.
Calcd for C17H18N2O: C, 76.66; H, 6.81; N, 10.52%
(L1; p-OH) – Found: C, 72.55; H, 6.79; N, 9.20%. Calcd
for C17H18N2O2: C, 72.32; H, 6.43; N, 9.92%
[Co(L1–2H)(H2O)2] – Found: C, 53.96; H, 5.60; N,
7.25%. Calcd for C17H20N2O4Co: C, 54.41; H, 5.37;
N, 7.46%; molar conductance, 39.29 ohmꢀ1 molꢀ1 cm2
[Ni(L1–2H)(H2O)2] – Found: C, 54.68; H, 5.34; N,
7.67%. Calcd for C17H20N2O4Ni: C, 54.44; H, 5.37; N,
7.47%; molar conductance, 8.14 ohmꢀ1 molꢀ1 cm2
[Cu(L1–2H)(H2O)2] – Found: C, 53.97; H, 5.16; N,
7.48%. Calcd for C17H20N2O4Cu: C, 53.75; H, 5.31;
N, 7.37%; molar conductance, 7.51 ohmꢀ1 molꢀ1 cm2
[Zn(L1–2H)(H2O)2] – Found: C, 53.93; H, 4.92; N,
7.98%. Calcd for C17H20N2O4Zn: C, 53.49; H, 5.28; N,
7.34%; molar conductance, 4.98 ohmꢀ1 molꢀ1 cm2
[Co(L1–2H)Im(H2O)] – Found: C, 56.39; H, 5.70; N,
13.73%. Calcd for C20H22N4O3Co: C, 56.48; H, 5.21;
N, 13.17%; molar conductance, 42.62 ohmꢀ1 molꢀ1 cm2
before. In addition, appearance of
a singlet at d
2.33 ppm for the CH3 group of AC(CH3)@NA and, at
the same time, appearance of a singlet at d 8.33 ppm for

116
U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
[Co(L1–2H)Py(H2O)] – Found: C, 60.76; H, 5.82; N,
H, 5.14; Cl, 8.61; N, 6.80%; molar conductance,
9.29%. Calcd for C22H23N3O3Co: C, 60.55; H, 5.31;
N, 9.63%; molar conductance, 41.61 ohmꢀ1 molꢀ1 cm2
[Ni(L1–2H)Im(H2O)] – Found: C, 65.00; H, 5.13; N,
13.19%. Calcd for C20H22N4O3Ni: C, 56.51; H, 5.22;
N, 13.18%; molar conductance, 8.44 ohmꢀ1 molꢀ1 cm2
[Ni(L1–2H)Py(H2O)] – Found: C, 60.41; H, 5.67; N,
9.93%. Calcd for C22H23N3O3Ni: C, 60.59; H, 5.32; N,
9.64%; molar conductance, 5.53 ohmꢀ1 molꢀ1 cm2
[Cu(L1–2H)Im(H2O)] – Found: C, 56.13; H, 5.21; N,
12.92%. Calcd for C20H22N4O3Cu: C, 55.87; H, 5.16;
N, 13.03%; molar conductance, 7.11 ohmꢀ1 molꢀ1 cm2
[Cu(L1–2H)Py(H2O)] – Found: C, 59.37; H, 5.21; N,
9.90%. Calcd for C22H23N3O3Cu: C, 59.92; H, 5.26;
N, 9.53%; molar conductance, 7.82 ohmꢀ1 molꢀ1 cm2
[Zn(L1–2H)Im(H2O)] – Found: C, 55.84; H, 5.41; N,
12.24%. Calcd for C20H22N4O3Zn: C, 55.63; H, 5.14;
N, 12.98%; molar conductance, 13.49 ohmꢀ1 molꢀ1 cm2
[Zn(L1–2H)Py(H2O)] – Found: C, 59.37; H, 4.94; N,
9.28%. Calcd for C22H23N3O3Zn: C, 59.67; H, 5.25; N,
9.59%; molar conductance, 8.49 ohmꢀ1 molꢀ1 cm2
[Co(L2–H)Cl(H2O)2] – Found: C, 52.30; H, 5.33; Cl,
8.66; N, 7.09%. Calcd for C17H21ClN2O3Co: C, 51.73;
H, 5.11; Cl, 8.96; N, 7.10%; molar conductance,
47.67 ohmꢀ1 molꢀ1 cm2
37.17 ohmꢀ1 molꢀ1 cm2
[Ni(L3–H)Ac(H2O)2] – Found: C, 52.52; H, 5.41; N,
6.68%. Calcd for C19H24N2O6Ni: C, 52.45; H, 5.56; N,
6.44%; molar conductance, 6.02 ohmꢀ1 molꢀ1 cm2
[Cu(L3–H)Ac(H2O)2] – Found: C, 52.39; H, 5.84; N,
6.38%. Calcd for C19H24N2O6Cu: C, 51.87; H, 5.50;
N, 6.37%; molar conductance, 3.24 ohmꢀ1 molꢀ1 cm2
[Zn(L3–H)Ac(H2O)2] – Found: C, 51.92; H, 5.47; N,
6.86%. Calcd for C19H24N2O6Zn: C, 51.66; H, 5.48; N,
6.34%; molar conductance, 4.06 ohmꢀ1 molꢀ1 cm2
[Co(L3–H)ClÆIm(H2O)] – Found: C, 51.90; H, 5.16; Cl,
7.56; N, 12.97%. Calcd for C20H23ClN4O3Co: C,
52.00; H, 5.02; Cl, 7.68; N, 12.13%; molar conductance,
46.16 ohmꢀ1 molꢀ1 cm2
[Co(L3–H)ClÆPy(H2O)] – Found: C, 56.21; H, 5.38; Cl,
7.67; N, 8.64%. Calcd for C22H24ClN3O3Co: C, 55.88;
H, 5.12; Cl, 7.50; N, 8.89%; molar conductance,
49.89 ohmꢀ1 molꢀ1 cm2
[Ni(L3–H)AcÆIm(H2O)] – Found: C, 54.64; H, 5.43; N,
11.61%. Calcd for C22H26N4O5Ni: C, 54.46; H, 4.50;
N, 11.55%; molar conductance, 7.51 ohmꢀ1 molꢀ1 cm2
[Ni(L3–H)AcÆPy(H2O)] – Found: C, 58.29; H, 5.18; N,
8.57%. Calcd for C24H27N3O5Ni: C, 58.10; H, 5.48; N,
8.47%; molar conductance, 6.27 ohmꢀ1 molꢀ1 cm2
[Cu(L3–H)AcÆIm(H2O)] – Found: C, 53.22; H, 4.98; N,
10.99%. Calcd for C22H26N4O5Cu: C, 53.93; H, 5.35;
N, 11.43%; molar conductance, 2.39 ohmꢀ1 molꢀ1 cm2
[Cu(L3–H)AcÆPy(H2O)] – Found: C, 57.64; H, 5.06; N,
8.31%. Calcd for C24H27N3O5Cu: C, 57.54; H, 5.43;
N, 8.39%; molar conductance, 3.43 ohmꢀ1 molꢀ1 cm2
[Ni(L2–H)Ac(H2O)2] – Found: C, 54.89; H, 5.63; N,
7.06%. Calcd for C19H24N2O5Ni: C, 54.58; H, 5.54; N,
6.70%; molar conductance, 5.48 ohmꢀ1 molꢀ1 cm2
[Cu(L2–H)Ac(H2O)2] – Found: C, 53.34; H, 5.68; N,
6.63%. Calcd for C19H24N2O5Cu: C, 53.96; H, 5.48;
N, 6.62%; molar conductance, 9.37 ohmꢀ1 molꢀ1 cm2
[Zn(L2–H)Ac(H2O)2] – Found: C, 54.01; H, 5.82; N,
6.13%. Calcd for C19H24N2O5Zn: C, 53.72; H, 5.46; N,
6.60%; molar conductance, 4.93 ohmꢀ1 molꢀ1 cm2
[Co(L2–H)ClÆIm(H2O)] – Found: C, 54.92; H, 4.29; Cl,
7.72; N, 12.79%. Calcd for C20H23ClN4O2Co: C,
54.00; H, 4.99; Cl, 7.95; N, 12.60%; molar conductance,
40.30 ohmꢀ1 molꢀ1 cm2
3. Results and discussion
3.1. Electronic absorption spectra of L1–L3 solutions in
organic solvents of different polarities
[Co(L2–H)ClÆPy(H2O)] – Found: C, 58.50; H, 4.19; Cl,
7.89; N, 9.45%. Calcd for C22H24ClN3O2Co: C, 57.97;
H, 5.08; Cl, 7.76; N, 9.22%; molar conductance,
47.23 ohmꢀ1 molꢀ1 cm2
With the aim to attain information about the types of
the electronic transitions and interaction liable to exist in
solutions, the electronic spectra have been measured of
the subject compounds (L1–L3) in several organic solvents
of different polarities, ethanol, methanol, chloroform, car-
bon tetrachloride, dichloromethane, acetonitril and
dimethylformamide (DMF). Generally, the recorded spec-
tra of L1–L3 in different organic solvents have displayed
three main absorption bands (cf Table 1). The UV band
appeared in the range 280–291 nm is assignable to the
localized electronic transition of the benzenoid system of
the compounds. Such an assignment is achieved from its
higher energy, lower wavelength, and higher molar
absorptivity.
[Ni(L2–H)AcÆIm(H2O)] – Found: C, 56.50; H, 5.00; N,
11.55%. Calcd for C22H26N4O4Ni: C, 56.44; H, 5.38; N,
11.97%; molar conductance, 5.36 ohmꢀ1 molꢀ1 cm2
[Ni(L2–H)AcÆPy(H2O)] – Found: C, 60.31; H, 5.02; N,
8.47%. Calcd for C24H27N3O4Ni: C, 60.16; H, 5.47; N,
8.77%; molar conductance, 5.05 ohmꢀ1 molꢀ1 cm2
[Cu(L2–H)AcÆIm(H2O)] – Found: C, 56.40; H, 4.96; N,
12.17%. Calcd for C22H26N4O4Cu: C, 55.91; H, 5.13;
N, 11.96%; molar conductance, 11.80 ohmꢀ1 molꢀ1 cm2
[Cu(L2–H)AcÆPy(H2O)] – Found: C, 58.89; H, 5.26; N,
8.84%. Calcd for C24H27N3O4Cu: C, 59.56; H, 5.41;
N, 8.68%; molar conductance, 11.79 ohmꢀ1 molꢀ1
cm2
The second band has been observed in the range 315–
330 nm as a broad and/or a shoulder band. This band
can be ascribed to the p–p* transitions of the benzenoid
system of the compounds strongly overlapped with n–p*
electronic transition that involves the nonbonding electrons
[Co(L3–H)Cl(H2O)2] – Found: C, 49.78; H, 4.88; Cl,
8.52; N, 6.55%. Calcd for C17H21ClN2O4Co: C, 49.59;

U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
117
Table 1
Electronic absorption spectral bands of the Schiff bases L1–L3 solutions, 1 · 10ꢀ3 mol dmꢀ3, in pure organic solvents at 25 ꢁC
Solvents
L1(R = o-OH)
L2(R = H)
L3(R = p-OH)
Assignment
kmax
emax
emax
kmax
kmax
emax
Ethanol
281
320 b
–
1.15
2.53
–
–
–
–
323
379 sh
390 b
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
323
379 sh
390
5.40
2.30
2.39
6.79
2.30
2.40
394 b
0.52
Methanol
CHCl3
CCl4
285 sh
317 b
380 sh
395
2.04
7.86
2.07
2.26
–
–
–
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
320 b
384 b
–
6.00
1.33
–
321 b
376 b
403 b
6.20
3.02
2.97
–
–
5.88
–
–
–
–
–
–
–
0.98
–
–
2.47
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
321 b
–
399 b
330 b
–
394 b
0.28
390 sh
1.25
–
–
5.88
–
–
–
–
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
321 b
–
399 b
326 b
–
409 sh
10.57
–
0.33
323 b
–
–
11.30
–
–
0.28
CH2Cl2
CH3CN
DMF
–
319
–
–
–
–
9.04
–
–
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
10.70
–
0.35
319 b
–
399 sh
320 b
–
394 b
10.05
–
0.83
399 b
0.73
–
317
–
–
–
–
–
–
6.64
–
p–p*, within aromatic system
p–p*, within C@N gps
Intramolecular CT transition
Intramolecular CT transition
10.05
–
0.50
319 b
388 sh
–
7.12
1.03
–
316 b
–
394 b
391 b
1.07
291 sh
317 b
–
8.70
6.79
–
–
–
8.53
–
–
–
7.73
–
p–p*, within aromatic system
p–p*, within C@N gps
317 b
–
315 b
–
Intramolecular CT transition
403 sh
0.40
390 sh
0.93
394 sh
0.83
Intramolecular CT transition
k = nm; e = 103 dm3 molꢀ1 cmꢀ1; sh, shoulder; b, broad.
of the azomethine nitrogen atom. A similar assignment was
described by Jaffe et al. [29]. This assignment is substanti-
ated by the fact that the position of this band suffers small
solvent shifts where changing the polarity of the medium
(cf Table 1).
molecule essentially as intramolecular charge transfer
(CT) interaction. The CT seems to originate from the aryl
moiety, whereas, the positive charge azomethine carbon
atom is the acceptor center. Table 1 reveals that, the posi-
tion of the CT band is quite sensitive to the nature of the
constituent, R, attached to the aldehyde moiety. Generally,
for compounds L1–L3, the excitation energy of this band
decreases as the electron donor power of the constituent,
R, is increased according to the following sequence:
Table 1 also indicates that the kmax of L1(R = o-OH)
observed at 320 nm has the lower molar absorptivity value
relative to that of the other compounds in ethanol. This
comes in confirmation with n–p* nature where the n-elec-
trons of the two o-OH groups (L1) are blocked by the strong
H-bonding established between the azomethine nitrogen and
the o-OH groups as represented schematically below.
L1ðR ¼ o-OHÞ > L3ðR ¼ p-OHÞ ꢁ L2ðR ¼ HÞ
From the previous order, it could be emphasized that
the kmax of CT band of the L1 (R = o-OH) compound is
longer than that of other compounds (L2 and L3) due to
the established H-bonding between the o-OH groups and
the azomethine nitrogen which, in turn, results in high elec-
tron density on the oxygen atom of the o-OH groups. This
will, of course, leads to a higher contribution of electrons
on the azomethine double bond, i.e. the p-electron cloud
is more delocalized and consequently is easier to be excited.
This behavior substantiates the CT character of this band.
In DMF as a solvent, the spectra of L1–L3 have dis-
played a broad or a shoulder band ranged between 390
and 410 nm. This band can be attributed to an intermolec-
CH3
C
CH2 CH2
C
N
N
H
OH
HO
The longer wavelength band(s) in the recorded spectra
of the compounds L1–L3 observed in the range 380–
410 nm can be ascribed to the transition within the whole

118
U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
ular CT transition [30] taking place thorough the interac-
tion between the solute and solvent molecules via H-bond-
ing [31].
molar ratio (MR) of Yoe and Jones [25] methods, and con-
ductometric titration method have been applied. Thus, the
curves plotted for CV method are characterized either by
one clear maximum located at ligand mole fraction equals
to ꢁ0.50 and/or ꢁ0.70, for most cases, or by a shoulder
accompanied by a maximum located also at the same
ligand mole fraction values; i.e. ꢁ0.50 and/or 0.70. This
behavior clearly indicates that the stoichiometry of the dif-
ferent complexes formed in solution from the reaction of
the ligands (L1–L3) with the metal ions Co(II), Ni(II),
Cu(II) and Zn(II) is 1:1 and/or 1:2 (M:L).
It is to be noted that for the complexes of Ni(II) with
L1(R = o-OH), Cu(II) and Ni(II) with L2(R = H) the CV
plots have possessed a well-defined shoulder at a mole frac-
tion of the ligand near 0.55 and a well characterized peak at
ꢁ0.70. This behavior can be explained on the basis that 1:1
complexes species are formed in solution but, they dispro-
portionate to form 1:2 (metal to ligand) complexes species
where the latter are more stable. Such a disproportionate
reaction can be represented by the following equation:
Further, data given in Table 1 indicate the absence of
any band in the range 320–330 nm in the spectra of
L2(R = H) in CHCl3, (in contrast to the spectra of the
other compounds, L1 and L3, in CHCl3). Instead, a shoul-
der band has been observed around 390 nm. This may be
due to the overlapping of these bands with other intramo-
lecular CT band supposed to appear ꢁ390 nm.
On the other hand, the data in Table 1 reveal the
absence of any absorption band due to intramolecular
CT interaction in case of the recorded spectra of the com-
pounds L1(R = o-OH) and L3(R = p-OH) in CCl4 as dem-
onstrated. This can be interpreted on the principle of the
difficult CT interaction in the solute molecules of these
compounds in CCl4 as a solvent.
3.2. Electronic absorption spectra and stoichiometry of the
complexes formed in solution
2MLðsolutionÞ ꢀ ML2ðsolutionÞ þ MðsolutionÞ
Electronic absorption spectra of the free methanolic L1–
L3 solutions and the spectra of the complexes solutions
resulting from adding the metals salts solutions to each
ligand solution have been measured. Generally, it is evident
that addition of metal ion to the ligand solution causes dis-
tinguishable changes in the visible absorption spectra of the
ligand. This behavior suggests an instantaneous complex
formation in solution from the reaction of each of the
ligands under investigation with each of the metal ions
used.
In addition, results of the MR method have supported
the results of the CV method. It is evident that, the rela-
tionships obtained for the MR method are characterized
by one or two breaks located at molar ratios [M(II)]/[L]
equals to 1.0 and/or 2.0. This indicates that the stoichiom-
etric ratio of the complexes, which are liable to form in
solution from the reaction of each of the ligands L1
(R = o-OH) with Co(II), Ni(II), Cu(II) and Zn(II), and
L2(R = H) with Co(II), Cu(II), Zn(II) and L3(R = p-OH)
with Co(II) is 1:1 (M:L), while the stoichiometry of react-
ing L2, L3 with Ni(II) is 1:1 and 1:2 (M:L) depending on
the nature of both the metal ion and the ligand. Thus,
the plots of absorbance vs [M]/[L] molar ratio for Ni(II)–
L2, and Ni(II)–L3, indicate the formation of 1:1 complex
species. After complete formation of 1:1 complexes, new
complex species are being formed, but these are unstable.
Therefore, the absorbance decreases again beyond a ratio
of [M]/[L] equals to 1.6 which reveals a decomposition of
the complexes formed in solution with higher ratio of metal
to ligand.
Further evidence has been given by the conductometric
titration method. Thus, on plotting the specific conduc-
tance values, as a function of [L]/[M(II)] molar ratio, it is
evident that, the titration curves obtained are straight
lines for the majority of the points recorded, and these lines
are accompanied with breaks corresponding to the
stoichiometric ratios of the complexes formed in solutions.
Moreover, it is apparent that, the breaks take place at
the molar ratios 1:1 and/or 1:2 metal to ligand, which
are in a good agreement with those determined spectro-
phometrically.
The longer wavelength band(s) of each free ligand (L1–
L3) at 317–403 nm has acquired a red shift on complex for-
mation with the metal ion. A new shoulder has been
observed at a longer wavelength (in range of 390–
435 nm) in case of the complex solution of L1 with Co(II),
Ni(II) and Cu(II), L2 and L3 with Co(II) and Cu(II). This
behavior can be explained as follows.
The longer wavelength band observed in the spectrum of
the free ligands L1–L3 can be assigned to an intramolecular
CT transition liable to take place within the solute mole-
cule. Thus the red shift observed in the kmax of this band
on complex formation with the divalent transition metal
ions Co(II), Ni(II), and Cu(II) can be interpreted on the
principle of an expected easier CT transition within the
complexed Schiff bases (L1–L3) rather than within the free
ones. This is due to the positive charge of the coordinating
metal ions. On the other hand, the observed new shoulder
around 440 nm in the spectra of the complexes solutions
can be likely ascribed to an intermolecular transition from
the ligands molecules to the vacant orbitals localized on the
coordinated metal ions, i.e. L fi MCT.
Aiming to determine the stoichiometry of the different
complexes liable to form in solution from the interaction
of Co(II), Ni(II), Cu(II) and Zn(II) ions with the com-
pounds under investigation, L1–L3, two different spectro-
photometric methods, continuous variation (CV) [24] and
On the other hand, it has been found that the conduc-
tance has changed from (7.9–39.6), (6.1–15.9), (39.5–59.1)
and (1.0–8.8) · 10ꢀ6 ohmꢀ1 cm2 molꢀ1 during the titration
of each Co(II), Ni(II), Cu(II) and Zn(II) metal ion solu-

U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
119
tions, respectively, with all the ligands (L1–L3) used. With
respect to the same ligand, the specific conductance value
generally runs according to the following sequence:
CuðIIÞ > NiðIIÞ > CoðIIÞ > ZnðIIÞ
This is in agreement with the general order of the stabil-
ity of the complexes of these metal ions which is established
before [29]. Meanwhile, the stability of each metal com-
plexed with the different ligands used decreases according
to the sequence:
CuðIIÞ > NiðIIÞ > CoðIIÞ > ZnðIIÞ
This is in agreement with the general order of stability of
the complexes of these metal ions which has been estab-
lished before by Grinberg and Yatsimirsk [32] and by
Irving and Williams [33]. This behavior clearly indicates
that the anions of the ligands L1–L3 are coordinated to
the metal ions Co(II), Ni(II), Cu(II) and Zn(II). This fact
is in accordance with the results of the elemental analyses
of the synthesized solid complexes. Thus, the chemical
reactions liable to take place in solution between the metal
ions Co(II), Ni(II), Cu(II) and Zn(II) and the ligands L1–L3
to form 1:1 and/or 1:2 (M:L) complexes can be represented
by the following chemical equations:
L3ðR ¼ p-OHÞ > L1ðR ¼ o-OHÞ > L2ðR ¼ HÞ
This is in somewhat in agreement with decreasing elec-
tron releasing character of the substituents in the same
direction which results in a decrease in the basicity of the
azomethine nitrogen of the ligand and, consequently, the
tendency toward complex formation is expected to
decrease. For example the ligand L3(R = p-OH) has a ten-
dency to act as a good r-donor owing to the expected high
basicity of the two bonding sites (the two azomethine nitro-
gen atoms) of L3 as the result of the high electron releasing
power of the OH group in the para position in L3(p-OH)
relative to that of the nonsubstituted L2 (R = H) or even
of the OH hydroxyl in the ortho position in L1(o-OH).
H2L þ MðIIÞ ꢀ LMðIIÞ þ 2Hþ ð1:1 complexÞ
2
2H2L þ MðIIÞ ꢀ ðLÞ MðIIÞ þ 4Hþ ð1:2 complexÞ
where H2L = (o-OH) compound (L1)
HL þ MðIIÞ ꢀ LMðIIÞ þ Hþ ð1:1 complexÞ
2HL þ MðIIÞ ꢀ L2MðIIÞ þ 2Hþ ð1:2 complexÞ
where HL = L2 and L3.
3.4. Characterization of the solid complexes isolated in
presence and absence of imidazole or pyridine as a Lewis
base
The results of the chemical analyses of the different syn-
thesized solid products of the reaction of each of the
ligands L1–L3 with the metal ions Co(II), Ni(II), Cu(II)
and Zn(II) have suggested the formation of the 1:1 (ligand:-
metal) and 1:1:1 (ligand:metal:Lewis base) chelates. The
suggested general formulae of the obtained chelates are
[M(L1–2H)(H2O)2], [M(L1–2H)(Lewis)(H2O)], [M(L–
3.3. Evaluation of the apparent formation constants of the
different metal chelates formed
The data obtained from the two spectrophotometric
methods, CV and MR, which were used in this work to
establish the stoichiometry of the metal complexes, were
also utilized for the determination of the formation con-
stants (Kf) of such complexes making use the following
equations [34]:
H)X(H2O)2]
and
[M(L–H)(Lewis)X(H2O)]
where
L1(R = o-OH), L (L2(R = H) or L3(R = p-OH)), X = Clꢀ
or CH3COOꢀ and Lewis base = imidazole (Im) or pyridine
(Py). This indicates that the deprotonated ligands L1–L3
have coordinated to the metal ion.
A=Am
Kf ¼
Kf ¼
for ð1:1 L:MÞ complexes
for ð2:1 L:MÞ complexes
2
ð1 ꢀ A=AmÞ C
The molar conductance values of the DMF solutions
(1 · 10ꢀ3 mol dmꢀ3) of the different synthesized metal
complexes have been determined and have been shown
in the experimental part following the elemental analysis
data for each synthesized complex. It is evident that the
molar conductance values for all the synthesized com-
plexes of the ligands L1–L3 with Co(II) in absence and/
or presence of Lewis base are in the rang 37–
50 ohmꢀ1 molꢀ1 cm2, while with the other metals (Ni(II),
Cu(II) and Zn(II)) the values are in the range 2–
14 ohmꢀ1 molꢀ1 cm2. This suggests the nonelectrolyte nat-
A=Am
3
4C2ð1 ꢀ A=AmÞ
where Am is the limiting absorbance value corresponding to
the maximum formation of the complex and A is the absor-
bance at a given metal ion concentration C.
The mean Kf values obtained for the metal complexes
under investigation are given in Table 2.
Examination of the results shown in this Table 2 reveals
that the order of stability of the 1:1 or 1:2 complexes of a
given ligand runs according to the following sequence:
Table 2
Apparent formation constant values (Kf) of the L1–L3 complexes with different transition metals
Ligand
Co(II) complex
1:1
Ni(II) complex
1:1
Cu(II) complex
1:1
Zn(II) complex
1:2
1:2
1:2
1:1
1:2
L1(R = o-OH)
L2(R = H)
L3(R = p-OH)
1.98 · 105
–
–
–
18.10 · 108
4.31 · 108
–
2.50 · 105
–
–
–
–
5.48 · 108
2.25 · 108
22.06 · 108
–
3.24 · 108
–
4.36 · 108
–
32.90 · 108
3.30 · 105
–
37.50 · 108

120
U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
ure of these complexes as reported [35]. Accordingly, one
can deduce that in the different metal complexes under
investigation, the anions Clꢀ or CH3COOꢀ are coordi-
nated to the metal ion as displayed in the molecular for-
mulae of these complexes, shown hereafter. However, this
conclusion is reasonable since in 1:1 or 1:1:1 complexes
each of the metal ions studied is still having unoccupied
coordination sites. This is based on the expected tendency
of the studied metal ions to form complexes with high
coordination number, six, for Co(II), Ni(II), Cu(II) and
Zn(II).
The electronic absorption spectra of Zn(II)–L3 are
characterized by a shoulder ꢁ550 nm (having very low
molar absorption) with a tail into near infrared region
that could be assigned as d–d band and the complex
Zn(II)–L3 has a d–d transition. However, it is hard to
detect any d–d band in the spectra of Zn(II)–L1 and
(L2) complexes. Therefore, the structures of these com-
plexes are difficult to be assigned but for Zn(II)–L3 com-
plex, octahedral structure can be proposed. Furthermore,
the well-defined visible absorption band appeared in the
spectra of Zn(II)–(L1–L3) in the range 343–351 nm
(emax = 740–10,100 dm3 molꢀ1 cmꢀ1) can be assigned to
intramolecular CT transitions liable to take place within
the complexed Schiff base molecule.
In addition to the previous given discussion, it is neces-
sary to refer that the bands observed in the ranges 368–382,
396–399, 351–357 and 343–351 nm in the spectra of Co(II),
Ni(II), Cu(II) and Zn(II) complexes with L1–L3 are quite
sensitive to the change of the metal ion and the molecular
structure of the ligand. Comparing the position of this
band on transfer from metal ion to another with the same
ligand reveals that kmax of this band shifts (in general) to
lower energy in the following order:
On the other hand, the electronic absorption spectra of
the methanolic solutions of the different synthesized, binary
and, ternary complexes have been recorded in the wave-
length range 250–750 nm.
The electronic absorption spectra of Co(II)–(L1–L3) com-
plexes are characterized by a weak crystal field broad band or
a shoulder (552–597 nm) (emax = 13–183 dm3 molꢀ1 cmꢀ1
)
followed by a long tail into the low energy region. This
behavior suggests an octahedral structure for these com-
plexes, [36] and is assigned to 4T1g(F) fi 4T1g(P) transitions
[37]. The observed visible band in the recorded spectra of
Co(II)–(L1–L3) complexes in the range 368–382 nm
(emax = 2660–3490 dm3 molꢀ1 cmꢀ1) can be ascribed to an
intramolecular CT transition within the complexed Schiff
bases molecules.
CoðIIÞ > NiðIIÞ ꢁ CuðIIÞ > ZnðIIÞ
This indicates that this band is corresponding to ligand
to metal CT. This is in agreement with the reported trend
[36] where it is concluded that kmax of LMCT shifts to
longer wavelengths as the metal becomes more readily
reducible. Further confirmation of assigning this band as
LMCT transition, is attained by comparing the kmax on
changing the ligand with the same given metal ion. As a
general trend, kmax of this band acquires a shift to longer
wavelengths as the electron releasing power of the constit-
uent (R), attached to the phenyl moiety is increased. Such a
shift indicates an easier electronic transfer to the metal ion.
With respect to Co(II)–(L1–L3) complexes as an example,
the order was found to be
The electronic absorption spectra of Ni(II)–(L1–L3) are
characterized by a broad band or a shoulder covering the
long wavelength region 540–553 nm (emax = 13–
340 dm3 molꢀ1 cmꢀ1) where the spectra exhibit long tail
into the near infrared region. This behavior can be assigned
to an octahedral 3A2gfi 3T1g(P) transition [36]. Further,
the absorption band observed in the recorded spectra of
these complexes in the range 396–399 nm (emax = 2230–
7530 dm3 molꢀ1 cmꢀ1) can be ascribed to an intramolecu-
lar CT transition within the complexed Schiff bases
molecules.
The electronic absorption spectra of Cu(II)–(L1–L3) com-
plexes are characterized by a broad band or a shoulder cov-
ering the region 550–574 nm (emax = 190–400 dm3
molꢀ1 cmꢀ1). In addition, such spectra exhibita long tail into
near infrared region. Since it has been described before that
octahedral Cu(II) complexes give rise to one absorption
band in the region ꢁ600 nm which often exhibit a tail into
the near infrared region [36]. Therefore octahedral structure
can be proposed for the studied Cu(II)–(L1–L3) complexes.
It is worthy to note that the relatively higher molar
absorption of the observed weak crystal field absorption
shoulders in the spectra of the different studied Cu(II)–
(L1–L3) complexes can be attributed to the expected centric
structure of these complexes, i.e. distorted octahedral struc-
ture [38]. Again one recalls that the well-defined visible
absorption band appeared in the spectra of all examined
Cu(II) complexes in the range 351–357 nm (emax = 2260–
8510 dm3 molꢀ1 cmꢀ1) can be assigned to an intramolecu-
lar CT transition liable to take place within the complexed
Schiff bases molecules.
L1 > L3 > L2
In the ultraviolet region, the bands observed in the
ranges 272–282, 268–270, 274–279 and 270–276 nm in spec-
tra of Co(II), Ni(II), Cu(II) and Zn(II)–(L1–L3), respec-
tively, can be ascribed to an intraligand electronic
transition, because bands in these regions are sensitive
enough to the type of metal ion used.
The electronic absorption spectra of the mixed-ligand
complexes showed some minor changes in the position
and intensity of the CT bands. This behavior confirms
coordination of Lewis bases (Im or Py) in the mixed-ligand
complexes. The substitution of H2O by a Lewis base may
result in a change of the type of orbital from which the elec-
tron is transferred to the metal ion. The d–d bands dis-
played by the mixed-ligand complexes are located almost
at the same positions as in the spectra of the binary che-
lates, indicating that the geometry around the metal ions
is not change [39].

U.M. Rabie et al. / Journal of Molecular Structure 872 (2008) 113–122
121
On the other hand, to reach a conclusive idea about the
structure of metal chelates with the subject ligands, infra-
red spectra of the free ligands (L1–L3) and their complexes
were recorded. In such cases, an insight view in the type of
bonds formed between the ligand and the central metal ion
can be gained from a careful investigation of the spectrum
of the complex essentially in comparison to that of the free
ligand, and accordingly, the tentative assignment of some
of the important bands has been made. With respect to
the ligands L1–L3, instead of an expected sharp strong
band at 3500 cmꢀ1 due to stretching vibration of OH
groups, a broad band appeared covering the range 2500–
3500 cmꢀ1. This band includes both C–H and hydrogen
bonded OH stretching frequencies. This assignment is rea-
sonable since some literature report [40–42] a frequency of
hydrogen bonded OH near 2800 cmꢀ1. Similar absorption
band was observed in the region 2600–2800 cmꢀ1 in the
spectra of bis(salicylidene)diamine [42]. This band disap-
pears in the IR spectra of the corresponding metal chelates
where instead a weak band appears near 3000 cmꢀ1 which
is assignable to the stretching of C–H. This also indicates
that the phenolic oxygen is coordinated to the metal ion.
Furthermore, the phenolic C–O stretching vibrations
appeared at 1280–1295 cmꢀ1 in the IR spectra of L1–L3
was found to have an appreciable shift to lower frequency
in the IR spectra of the corresponding metal chelates con-
firming its coordination to the metal ion [9,43,44]. The IR
spectra of all complexes show a broad band in the region
3050–3600 cmꢀ1. This indicates that water molecules exist
in the complexes prepared.
cates that coordination of the Lewis base nitrogen (Im, Py)
decreases the electron demand of the positive charge local-
ized on the metal ion.
Based on the foregoing discussion, it could be concluded
that the Schiff bases ligands L2 and L3 act as monobasic tri-
dentate ONN ligands in 1:1 and 1:1:1 complexes while the
ligand L1(R = o-OH) acts as bibasic tetradentate ONNO
ligand in 1:1 and 1:1:1 complexes where an expected five-
and six-membered chelate rings are formed. Hence, one
can draw the following structures for the chelates formed.
CH3
CH3
H2C
CH
CH2
N
H2C
CH
CH2
N
R
N
C
R
N
C
M
X
M
Lewis
H2O
O
O
(H2O)2
(I)
X
(1:1)
(II) (1:1:1)
CH3
CH3
CH2
H2C
CH2
H2C
CH
CH
N
N
C
N
N
C
M
M
O
O
O
O
(H2O)2
H2O
Lewis
(III) (1:1)
(IV) (1:1:1)
(I) = 1:1 complexes of Schiff bases L2 and L3
(II) = 1:1:1 complexes of Schiff bases L2 and L3
(III) = 1:1 complexes of Schiff base L1 (R=o-OH)
(IV) = 1:1:1 complexes of Schiff base L1 (R=o-OH)
R = H (L2) and p-OH (L3)
-
-
A strong band in the region 1595–1645 cmꢀ1 is observed
in the IR spectra of all ligands (L1–L3), which may be
assigned to the stretching vibration of the azomethine
group [45]. Generally this band is shifted to lower frequen-
cies on complexation. Thus, it suggests that coordination
of each of the ligand under investigation with the studied
metal ions takes place through the nitrogen azomethine.
However, the shift of this band to lower frequencies reveals
that the bond order of the AC@NA linkage is lowered
upon their coordination to the metal ion.
X= Cl or CH3COO
Lewis base = imidazole (Im) or pyridine (Py)
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.molstruc.2007.02.023.
The two bands appeared in the IR spectra of the free
ligands and in the IR spectra of the corresponding metal che-
lates in the regions 1530–1580 cmꢀ1 and 1430–1455 cmꢀ1 are
assigned to C@C stretching vibrations of the aromatic sys-
tem. The observed shifts to lower or higher frequencies
may be due to the involvement of C@C band in some meso-
meric interactions upon the formation of chelate rings.
In addition, the IR spectra of the complexes of ligands
L2(R = H) and L3(R = p-OH) have shown the Co–Cl
(stretch) bands in the range of 665–820 cmꢀ1 in case of
cobalt complexes and the C@O (vs or s) stretch bands,
belonging to the acetate group, in the range of 1690–
1720 cmꢀ1 in the Ni, Cu, and Zn complexes.
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