Synthesis, solvatochromaticity and bioactivities of some transition metal complexes with 2-(R-benzylideneamino)-pyridin-3-ol Schiff base derivatives

Article abstract of DOI:10.1016/j.saa.2010.03.026

New four Schiff bases are prepared by condensation of 2-amino-pyridin-3-ol with 3, 4-dihydroxy-benzaldehyde (I), 2-hydroxybenzaldehyde (II), 5-bromo-2-hydroxybenzaldehyde (III), and 4-dimethylaminobenzaldehyde (IV). The structures of these compounds are characterized based on elemental analyses (C. H. N), IR and ¹H NMR. Also, the electronic absorption spectra are recorded in organic solvents of different polarity. The solvents are selected to be covered a wide range of parameters (refractive index, dielectric constant and hydrogen bonding capacity). The UV-vis absorption spectra of Schiff base compounds are investigated in aqueous buffer solutions of varying pH and utilized for the determination of ionization constant, pK_a and activation free energy, ΔG^* of the ionization process. The biological activity against bacterial species and fungi as microorganisms representing different microbial categories such as (two Gram-negative bacteria, Eschericha coli and Agrobacterium sp.),three Gram-positive bacteria (Staphylococcus aureus, Bacillus subtlus and Bacillus megatherium), yeast (Candida albicans), and fungi (Aspergillus niger) were studied.

Full text of DOI:10.1016/j.saa.2010.03.026

Spectrochimica Acta Part A 77 (2010) 359–366
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, solvatochromaticity and bioactivities of some transition metal
complexes with 2-(R-benzylideneamino)-pyridin-3-ol Schiff base derivatives
I.S. Ahmed∗, M.A. Kassem
Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
New four Schiff bases are prepared by condensation of 2-amino-pyridin-3-ol with 3, 4-
dihydroxy-benzaldehyde (I), 2-hydroxybenzaldehyde (II), 5-bromo-2-hydroxybenzaldehyde (III), and
4-dimethylaminobenzaldehyde (IV). The structures of these compounds are characterized based on ele-
mental analyses (C. H. N), IR and 1H NMR. Also, the electronic absorption spectra are recorded in organic
solvents of different polarity. The solvents are selected to be covered a wide range of parameters (refrac-
tive index, dielectric constant and hydrogen bonding capacity). The UV–vis absorption spectra of Schiff
base compounds are investigated in aqueous buffer solutions of varying pH and utilized for the determi-
nation of ionization constant, pKa and activation free energy, ꢀG* of the ionization process. The biological
activity against bacterial species and fungi as microorganisms representing different microbial categories
such as (two Gram-negative bacteria, Eschericha coli and Agrobacterium sp.),three Gram-positive bacte-
ria (Staphylococcus aureus, Bacillus subtlus and Bacillus megatherium), yeast (Candida albicans), and fungi
(Aspergillus niger) were studied.
Received 28 December 2009
Received in revised form 10 March 2010
Accepted 16 March 2010
Keywords:
Schiff base compounds
Elemental analysis
Electronic absorption spectra
Biological activity
© 2010 Published by Elsevier B.V.
1. Introduction
2. Experimental
Many Schiff bases have received great attention due to their
uses as chemical intermediate and perfume bases in dyes and
rubber accelerators and in liquid crystals for electronic devices.
Some Schiff bases were tested for fungicidal activity, which is
related to their chemical structure [1]. A considerable number of
articles have been published on the spectral behavior of Schiff
bases and their application in different fields [2–7]. Recently, there
has been a growing interest in new application using Schiff base
and their transition metal complexes in the field of agriculture,
medicine and in catalysis [8–13]. Although Schiff bases contain-
ing a heterocyclic nucleus have efficient biological activity, yet
spectral studies of these compounds are comparatively minor
[14,15].
2.1. Materials and reagents
All chemicals used were of the analytical reagent grade (AR) and
of highest purity available used without further purification.
2.2. Synthesis of Schiff base compounds (I–IV)
The Schiff base compounds were synthesized according to the
recommended method for Schiff base compounds [16]. This was
achieved by the following.
To
a
stirred and heated (60 ◦C) ethanolic solution of
2-amino-pyridin-3-ol (Aldrich) (110.11 g), solution of 5-bromo-
2-hydroxy-benzaldehyde (Aldrich) (201.02 g), 3,4-dihydroxy
benzaldehyde (Merck) (138.12 g), 4-dimethylaminobenzaldehyde
(Merck) (149.19 g) and 2-hydroxybenzaldehyde (Aldrich)
(122.12 g) in ethyl alcohol were added immediately in 1:1 mole
ratio. The reaction mixture was heated at reflux on a water-bath
for 4 h. By time the color of reaction mixture was charged to obtain
the Schiff base compounds. The volume of the reaction mixture
was evaporated to 30 ml and after cooling, the resulting Schiff
bases were filtered off, washed with ethanol 70% and crystallized
from the proper solvent. The purity and chemical structures were
detected by sharp melting point, the elemental analysis (C, H, N),
IR and 1H NMR spectra (Table 1). The Schiff base derivative under
study are shown as the following scheme:
In the present work, new Schiff base derivative and their com-
plexes are prepared and investigated using the UV–vis, IR, 1H NMR
spectra and the biological activity also are studied.
∗
Corresponding author. Tel.: +20 130122408034; fax: +20 133222875.
E-mail address: isahmed61@gmail.com (I.S. Ahmed).
1386-1425/$ – see front matter © 2010 Published by Elsevier B.V.
doi:10.1016/j.saa.2010.03.026

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I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
2.4. Instruments and measurements
2.4.1. pH measurements
The pH value of all solutions was adjusted to the required value
using pH-meter type HI 8014 HANA instruments.
2.4.2. Spectrophotometric measurements
All the absorption spectral measurements are made using
Jasco V-530 (UV–vis) spectrophotometer (Japan) with scanning
speed 400 nm/min, bandwidth 2.0 nm and equipped with 10.0 mm
matched quartz cells. The transition energies of the visible absorp-
tion peaks are calculated [17] using Eq. (1):
119.625
ꢁ(nm)
EJ (kJ mol−1) =
(1)
2.4.3. Instruments
The IR spectra were recorded as KBr discs using Matson FT-
IR spectrophotometer in the 4000–200 cm−1 range. The 1H NMR
spectrum of the ligands were performed using a varian EM 390-90
NMR spectrometer in d6-DMSO as solvent using tetramethylsilane
(TME) as an internal standard. The microanalysis of C, H and N were
performed in the Microanalytical Center of Cairo University.
2.5. Antibacterial and antifungal activities of the Schiff base
compounds and their complexes
The well-plate method (diffusion methods) was followed with
some modification [18]:
1. The spore suspension of the test organisms (Gram-negative
bacteria, such as Escherichia coli, and Agrobacterium sp., and
Gram-positive bacteria, Staphylococcus aureus, Bacillus sub-
tlus and Bacillus megatherium), yeast (Candida albicans), and
fungi (Aspergillus niger), was prepared in previously sterile and
checked inoculating flasks, each contained 50 ml of previously
and checked medium of the following composition for the target
test organisms as following.
For bacteria, nutrient broth medium: (beef extract 3.0 g, peptone
10.0 g, H2O 1000 ml). For yeast and fungi, Czapeks Dox broth
medium: (sucrose 30.0 g, MgSO4 0.5 g, KCl 0.5 g, FeSO4 0.01 g,
NaNO3 3.0 g, K2HPO4 1.0 g, H2O 1000 ml).
2. A solid medium containing the following ingredients for bac-
teria (g/l) (beef extract 3.0 g, peptone 10.0 g, agar 20.0 g, H2O
1000 ml), and for yeast and fungi (sucrose 30.0 g, MgSO4 0.5 g,
KCl 0.5 g, FeSO4 0.01 g, NaNO3 3.0 g, K2HPO4 1.0 g, agar 20.0 g,
H2O 1000 ml), was sterilized and divided while warm (50:55 ◦C)
in 15 ml portions among sterile Petri-dishes of 9 cm diameters.
3. 0.2 ml of the spore suspension was surface placed on the surface
of the solid medium in the Petri dished and spread all over the
surface.
4. 0.1 g of the test substances was weighted.
5. In Petri dishes containing the culture of one of the chosen
microorganisms, one well of 10 mm diameter was made in the
center of the dish. It was marked on the bottom of the dish, and
filled with the 0.10 g of the test substances.
2.3. Synthesis of metal complexes
The solid chelates of stoichiometric ratios (1:1) (M:L) for some
selected metal chelates were prepared by mixing the metal ions
solutions with a hot alcoholic solution of ligands. The mixture was
then refluxed on a water bath for about 8.0 h and then allowed to
cool where by the solid chelates were separated and recrystallized
from absolute ethanol, dried and persevered in a desiccator over
dried silica gel.
6. The Petri dishes were incubated at 5 ◦C for 2 h to permit good
diffusion and then transferred to an incubator at 37 ◦C for 48 h
for bacterial test organisms, and at 30 ◦C for 72 h for yeast and
fungi test organisms.
7. Antimicrobial activities were observed after 48 h of incubation
time for bacteria at 37 ◦C, and after 72 h of incubation time for
yeast and fungi at 30 ◦C.

I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
361
Table 1
Physical data of prepared ligands (I–IV).
Compd.
Empirical formula (F. Wt.)
Microanalysis, calc. (found)
M.P (◦C)
Yield (%)
Color
Gray
% C
% H
% N
I
C12H10N2O3 (230.22)
C12H10N2O2 (214.22)
62.60
4.38
12.17
(12.03)
280
155
173
186
85
88
73
70
(62.90)
(4.23)
II
67.28
4.71
13.08
(11.99)
Yellow
Orange
Blue
(65.14)
(4.81)
III
IV
C
12H9BrN2O2 (293.12)
49.17
3.09
9.56 (5.14)
(45.90)
(2.88)
C14H15N3O (241.29)
69.69
3.27
17.41
(68.40)
(7.31)
(12.03)
Fig. 2. Absorption spectra of (1 × 10−4 M) ligand II in different organic solvents.
Fig. 1. Absorption spectra of (1 × 10−4 M) ligands I–IV in ethanol.
The longer wavelength band (D) at (400–469 nm) can be
assigned to an intramolecular charge transfer interaction from aro-
matic group to the azo-methene group (–CH N–) at 450, 460,
469 and 400 nm for ligands I, II, III and IV respectively which
originate from the substitutents phyenyl ring which increases the
dipole moment of the solute during the electronic transition, the
Franck–Condon excited state is formed in a strained solvent cage
of the oriented dipoles, thus the energy of the ground state is
increased and this produces a bathochromic shift [19]. However,
the hydrogen bond formation between the C N group and the o-
OH groups on the phenyl-ring shifts the charge transfer band to
longer wavelength.
3. Results and discussion
3.1. The electronic absorption spectra
3.1.1. The electronic absorption spectra of the Schiff bases in
ethanol
The electronic absorption spectra of Schiff base compounds
(I–V) in ethanol display bands as shown in Fig. 1 and Table 2. The
first two bands A and B are observed at range (225–240 nm) and
*
(272–299 nm) are referred to the ␲–␲ transitions of the aromatic
moieties with εmax 0.67 × 104 and 2.33 × 104 L mol−1 cm−1 for I and
II respectively. The donor substitute (hydroxyl group) causes a red
shift for the pyridine moieties ␲–␲* band. The third band (C) ranged
3.1.2. The solvatochromaticity of Schiff base derivatives in
different organic solvent
The electronic absorption spectra of the investigated Schiff base
ligands are collected in Table 3. For the spectra of Schiff base com-
pounds. The spectrum of each compound exhibits mainly four
bands. The lowest transition at (355–469 nm) and is assigned to
the intramolecular charge transfer. This band shows positive sol-
vatochromism (bathochromic shift) upon increasing the solvent
polarity (Table 4). This means that a pronounced change in posi-
tion of an electronic absorption band is accompanying a charge in
*
(314–343 nm) is due to ␲–␲ transition in the azomethine system
(–CH N–). Thisbandisblueshift in ligands IandIVrelativetoligand
II. This can be explained by the inductive effect of the two methyl
groups which increases the possibility for the blocking of electron
density in CH N group with ethanol molecules. For ligand II there
is a shoulder at 272 nm which may be due to the delocalization of
hydrogen atom from (OH) groups to give keto enol form which be
represented as follows (Fig. 2):

362
I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
Table 2
UV–vis spectra data of ligands (I–IV) in ethanol.
Comp.
Spectra bands
A
B
C
D
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
I
II
III
IV
225
232
228
240
0.67
2.33
2.2
299
272
–
0.44
1.34
–
–
–
450
366
469
400
0.12
1.42
0.11
0.11
314
343
340
1.35
0.87
2.59
1.03
–
–
the polarity of the medium. This observed behaviour is accounted as
those molecules in the ground state and in the excited state indicate
different polarities. This interpretation involves that highly sim-
plifying assumption that Schiff bases with non-polarized ground
state are more strongly polarized in protic solvents, because the
high-energy polar structure of the excited state is stabilized. The
energy difference between ground states and excited states is
decreased and the excitation energy is decreased. The approxima-
tion of the energy levels expresses itself in a bathochromic shift
of the spectrum with increasing polarity of solvents. The other
absorption bands corresponding to the highest energy centered at
(204–287 nm) are localized in the aromatic rings and are due to a
(␲–␲*) transition. These bands show a negative solvatochromism
in solvent like diethyl ether, benzene and toluene. Those aprotic
solvents destabilize the polarized electronic state. This leads to a
hyposochromic shifting of the spectrum with decreasing solvent
polarity. The UV–vis spectrum of Schiff base compounds II and III
exhibit mainly two absorption bands. A positive solvatochromism
is observed for (n–␲*) absorption in case of II whereas a negative
solvatochromism is observed for this band in case of III where there
is a bromide atom. The hyposochromic shift in the band follow the
order Br ꢀ H in an agreement with the order of their electronega-
tivities.
The UV–vis spectra of ligands II and III in toluene exhibit two
absorption bands, A band is formed at about 285 and 288 nm are
assigned to a (␲–␲*) transition which are blue shifted in all other
solvents. Also, the second band at 358 and 345 nm for II and III
respectively are blue shifted in the other solvents. A negative sol-
vatochromism is noticed in all other solvents. The spectral shifts
decrease with gradual introduction of the more polar solvent. In
these solvents the hydroxyl groups in the pyridine moiety will be
the proton acceptor center in the process of hydrogen bonds for-
mation whereas the solvents that have the ability to form hydrogen
bonds are the proton donors.
3.1.3. The electronic absorption spectra of the ligands in buffer
solutions
The absorption spectra of 3 × 10−5 M of ligands I–V in universal
buffer [20] solutions of varying pH in UV–vis region (200–600 nm)
were done.
The spectra obtained indicate that the absorbance values and
position of the absorbance bands change with pH of the medium
due to the following:
1. In case of ligand I, the absorption spectra shows an absorp-
tion band with ꢁmax = 230 nm corresponding to unionized H3L
Table 3
UV–vis spectra data of ligands (I–IV) in organic solvents.
Solvents
A
B
C
D
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
ꢁmax (nm)
εmax ×104
Ligand I
DMF
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
–
236
–
240
–
–
1.31
–
0.11
–
–
291
320
295
–
–
–
1.16
1.05
0.06
–
–
–
–
–
370
332
317
307
–
–
0.05
0.08
0.12
0.11
466
–
–
–
–
0.21
–
–
–
–
–
–
–
Ligand II
DMF
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
–
213
–
238
–
–
2.29
–
1.52
–
0.17
269
273
285
269
280
260
0.79
0.88
0.33
1.06
0.75
0.10
321
318
–
356
357
–
0.617
0.94
–
1.77
1.67
–
355
358
369
372
357
–
0.77
0.94
1.03
1.57
1.53
0.21
213
Ligand III
DMF
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
–
222
–
237
–
–
2.52
–
1.06
–
1.99
268
–
288
–
281
251
0.95
–
0.23
–
0.52
0.86
344
342
343
342
344
344
1.21
0.77
0.84
0.55
1.50
0.50
374
462
–
372
–
1.13
0.12
–
0.51
–
226
–
–
Ligand IV
DMF
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
268
207
–
239
–
0.31
0.49
–
0.714
–
–
240
280
–
–
234
–
330
343
330
320
330
312
2.78
1.96
1.36
2.34
1.18
2.33
342
–
–
–
–
2.75
–
–
–
–
0.51
0.29
–
–
1.09
211
0.622
–
–

I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
363
Table 4
Prominent visible absorbance of ligands (I–IV) in different solvents and transition energy.
No.
Solvents
ꢁmax (nm)
εmax (L mol−1 cm−1) ×104
Transition energy (kJ mol−1
)
Ligand I
1
2
3
4
5
6
7
Ethanol
DMF
450
466
320
370
332
317
307
0.12
0.21
1.05
0.05
0.08
0.12
0.11
265.83
256.71
373.83
323.31
360.32
377.37
323.11
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
Ligand II
1
2
3
4
5
6
7
Ethanol
DMF
366
355
358
369
372
357
365
1.42
0.77
0.94
1.03
1.57
1.53
0.21
256.71
336.97
334.15
324.19
321.57
335.08
327.74
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
Ligand III
1
2
3
4
5
6
7
Ethanol
DMF
469
374
462
343
342
344
344
0.11
1.13
0.12
0.84
0.55
1.50
0.50
255.06
319.85
258.93
348.76
349.78
347.74
347.74
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
Ligand IV
1
2
3
4
5
6
7
Ethanol
DMF
400
342
343
330
320
330
312
0.11
2.75
1.69
1.36
2.34
1.18
2.33
299.06
349.78
348.76
362.50
373.83
362.50
383.41
Methanol
Toluene
Diethylether
Benzene
Cyclohexane
form, the extinction of this band decreases with increasing
pH values of solution while another absorption band with
ꢁmax = 310 nm corresponding to the ionized H2L− form. The
absorbance of this band decreases with increasing pH val-
ues of solution until pH 10.2. This behaviour indicates that at
varying hydrogen ion concentration the equilibrium between
undissociated and dissociated form are present in different
ratios.
increasing pH of the solution due to the change from the union-
ized form to the ionzied form.
The pH–absorbance curves of the ligands at different wave-
length corresponding to ꢁmax are of typical Z or S shaped. This
behaviour is utilized for the determination of the acid dissocia-
tion constant (pKa) of these compounds by applying the half-height
method (HHM) [21], the modified limiting absorbance method
(LAM) [22], the colleter method (CM) [23] and the modified isos-
bestic point method (IPM) [24] (Table 5).
2. The electronic spectra of ligands II and III show form different
bands with two clear isosbestic points. These bands at ꢁmax = 225,
251, 310 and 378 nm for ligand II and at ꢁmax = 220, 250, 313 and
390 nm for ligand III. The first two band in both ligand decrease
in absorbance with increase pH values of solution which indicate
that these bands for undissociated form H2L. The bands at higher
wavelength increases in absorbance with increase pH values of
solution and these bands corresponding to dissociated forms of
two ligands. The isosbestic points at ꢁmax = 285 and 347 nm for
II and at ꢁ = 355 and 278 nm for III indicating the existence of
an acid base equilibrium between two different form of these
compounds as shown in the following:
The values of pKa are used in calculating the free energy change
(ꢀG*) of the ionization process at 298 K from the relationship.
ꢀG∗ = −2.303RT logK
The increase of ꢀG* leads to the thermodynamically favored.
3.2. Electronic absorption spectra of the chelates
Based on the fact that, electronic absorption spectra are very
diagnostic of the stereochemistry of metal chelates, the electronic
absorption spectra of the solid chelates prepared in the present
work are measured as nujol mull and in solution using DMF as
solvent within the range 200–900 nm. The spectra of the free lig-
ands (I–IV) in DMF indicate that ꢂ(cm−1) of CT band are 26,666,
25,641, 24,330 and 27,322 cm−1 for I, II, III and IV respectively
whereas that taken in nujol mull are 27,777, 27,855, 2770 and
26,737 cm−1 respectively. The ꢂ(cm−1) of the CT band of all metal
chelates of these ligands displays a red or blue shift in nujol mull
and DMF compared to that of the free ligands. This generally con-
firms that such ligands interact with the metal ion, and the metal
ion environments are different leading to the formation of different
geometrical types of complexes. The difference in ꢂ¯(cm)−1 values
of the bands in DMF than in nujol mull indicates that the crystal
3. The spectra of ligand IV show two different bands with ꢁmax
equals 244 and 352 nm corresponding to HL and L− forms,
respectively. The extinctions of the two bands increase with

364
I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
Table 5
The values of pKa (dissociation constant) for Schiff base ligands at 298 K.
Ligand
I
pKa
ꢁmax (nm)
ꢀG* (kJ mol−1
)
HHM
LAM
CM
IPM
Av.V
pKa1
pKa2
pKa3
3.72
6.20
8.64
4.05
6.36
8.61
–
7.02
–
–
–
–
3.89
6.53
8.63
310
230
310
22.19
37.26
49.24
II
pKa1
pKa2
4.10
6.20
5.21
6.34
6.11
–
4.53
5.92
4.98
6.15
251
310
28.42
35.09
III
IV
pKa1
pKa2
6.82
7.28
6.43
6.97
–
6.35
5.83
6.71
6.36
6.82
313
250
36.29
38.91
pKa1
7.79
7.21
8.01
–
7.67
352
43.76
HHM: Half height method, LAM: modified limiting absorbance method, CM: Colleter method, IPM: modified isosbestic point method, Av.V: average value, ꢀG*: free energy
change.
Table 6
Some selected bands of diagnostic importance for the IR spectra of ligands (I–IV).
Ligand
I
Assignment
II
III
IV
3568 (w)
–
–
2730 (w)
11631 (w)
1590 (s)
–
1308 (m)
1388 (w)
–
3318 (w)
3056 (w)
2922 (w)
–
1617 (s)
1572 (m)
–
1292 (s)
1373 (w)
–
1096 (w)
1032 (w)
3232 (w)
3047 (w)
2921 (w)
2872 (w)
1624 (s)
1576 (m)
–
1308 (w)
1371 (w)
–
3439 (w)
3049 (w)
2916 (m)
2819 (w)
1603 (s)
1547 (m)
1437 (w)
1312 (w)
1372 (w)
1167 (s)
1065 (m)
–
␯(OH)
␯(CH) asym (Ar)
␯(CH) asym (CH3)
␯(CH) sym (CH3)
␯C
␯C
O
N
␯N–CH3
␯(C–N)
␦(OH)
␦ (CH3)
␯(C–OH)
␯(CH N)
1055 (w)
–
1071 (w)
1010 (m)
sym: Symmetric, asym: asymmetric, s: strong, m: medium, w: weak, b: broaded.
state of the complexes differ in the solid state than in solution,
and some kind of bonding was formed between the chelate and
DMF.
the Schiff base ligands showed charge transfer bands at 26,246,
25,974, 28,011 and 23,255 cm−1 for I–IV respectively. Also the vis-
ible d-d electronic spectra bands at 19,960, 15,337, 18,726 and
17,331 cm−1 for silver complexes with I–IV respectively. Finaly,
the nujol mull electronic absorption spectra of zinc complexes
with the Schiff base ligands showed charge transfer bands at
25,445, 23,474, 21,645 and 24,096 cm−1 for ligands I–IV, respec-
tively.
The nujol mull electronic absorption spectra of copper com-
plexes with Schiff base ligands gave charge transfer bands at
26,041, 22,371, 25,773 and 23,640 cm−1 for I–IV complexes,
respectively, with copper ion. The visible d–d electronic spec-
tral band at 13,550 and 20,661 cm−1 for complexes of Cu2+-I
and Cu2+-IV respectively. The nujol mull electronic absorption
spectra of cadmium complexes with the ligands under inves-
tigation gave charge transfer bands at 23,201, 23,041, 25,773
and 26,109 cm−1 for Cd2+ complexes with I–IV respectively. Also,
the nujol mull electronic absorption spectra of mercury com-
plexes with ligands under studies gave charge transfer bands
at 24,213, 24,213, 27,700 and 27,624 cm−1 for Hg2+ complexes
with Schiff base ligands I–IV, respectively. On the other hand, the
nujol mull electronic absorption spectra of silver complexes with
3.3. The infrared spectra
The IR-spectra of chelates, in comparison with those of the free
ligands display certain changes which give an idea about the type
of bonds and their structure.
All compounds exhibit two week intensity bands at 2916–2922
and 2819–2872 cm−1 corresponding to asymmetric and symmet-
ric stretching vibrations of the aromatic C–H groups: ␯
give
(C N)
Table 7
Assignment and chemical shift (ppm) of different types of protons of ligands under investigation.
Ligand
Chemical shift
(ppm) of protons
(ppm) of aromatic rings
I
H(1)
8.21
H(2)
7.42
H(3)
6.83
II
I
6.37–6.405
6.44–6.57
6.83–6.868
II
10.27
10.22
9.67
13.81
13.74
3.05
9.48
9.48
7.68
7.24–7.26
7.356–7.436
6.35–6.87
6.93–6.95
III
IV
7.713–7.721
6.82–6.86
6.92–6.46
6.82
6.77

I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
365
medium intensity bands at 1547–1572 cm−1. This band being
shifted to lower wavenumber due to the contribution of the C
in an intramolecular hydrogen bond. The spectra of these com-
pounds exhibit the ␯
as week band at 3231–3439 cm−1. While
N
(OH)
␯
bands are observed as weak intensity at 1065–1096 cm−1
.
(C–OH)
The low values of ␯
reflects the existence of intramolecular
OH
hydrogen bond between OH and C N groups [25]. The exis-
tence of coordinated water or water of hderation in all chelates
cause difficulty in drawing conclusions from ␯
band for the
(OH)
hydroxyl group. The participation of the hydroxyl group in chela-
tion is confirmed by the appearance of new band at 420–540 cm−1
which is due to ␯
vibration. The bands at 370–400 cm−1
(M–O)
is attributed to ␯(M–N), while the bands appear at the range
340–350 cm−1 corresponding to ␯(M–Cl). The infrared spectra of
the free Schiff base compounds under investigation exhibited
medium intensity bands within 1292–1312 cm−1 are assigned to
␯
C–N
(Table 6).
Fig. 3. Biological activity on some prepared metal complexes.
3.4. 1H NMR spectra of the ligands (I–IV)
The 1H NMR spectra of schiff base compounds I–V are done
and chemical shifts of the different signals are record in Table 7.
The signals lying at very downfield side 8.21–10.27 ppm are due
to the hydrogen of OH group attached to the pyridine ring, while
signals lying in range 7.42–13.81 ppm are referred to the OH
group attached to phenyl ring. A new multiple signals at range
6.37–7.713 ppm are due to the protons of the phenyl group.
Also, in ligand IV the signal at 3.05 ppm is due to the two methyl
groups attached to the nitrogen atom. The signals lying in range
6.83–9.48 ppm are referred to the proton of azomethine group of
these Schiff base compounds. The signals of OH group lying at
downfield side 7.42 ppm for ligand can be attributed to the con-
tribution of the OH group in intramolecular and intermolecular
hydrogen bond as following:
3.5. Biological activity studies
In this study (Fig. 3), microorganisms representing different
microbial categories (two Gram-negative bacteria, E. coli and
Agrobacterium sp.), three Gram-positive bacteria (S. aureus, B. sub-
tlus and B. megatherium), yeast (C. albicans), and fungi (A. niger),
were used. The study included 18 compounds, 3 ligands (I, II and
III) and 15 complexes of different metal ions (Cu2+, Cd2+, Hg2+, Ag+
and Zn2+). The diffusion agar technique was used to evaluate the
antibacterial activity of the ligands and the chelates [18].
The results of the bacterial screening of the synthesized com-
pounds are recorded in Table 8. As about ligand I and III the data
reflect that these two Schiff base ligands have an activity in compar-
ison with I, whereas their complexes have moderate activity with
different degrees except in case of Zn2+ complex in case of I, II and
III, and Hg2+ and Ag+ complexes in case of III.
Against all the organisms, ligand I did not exhibit any remark-
able activity compared with its complexes whereas all complexes
showed moderate to high activities. It is suggested that the
Table 8
Responses of various microorganisms to the Schiff bases and their complexes in vitro culture (numbers in the table represent the extent of the inhibition zone diameters in
mm).
Compound
Escherichia coli
Bacillus subtlus
Candida albicans
Aspergillus niger
Staphylococcus aureus
Agrobacterium sp.
Bacillus megatherium
0.1 g
0.1 g
0.1 g
0.1 g
0.1 g
0.1 g
0.1 g
I
25
20
30
40
50
25
20
20
15
20
30
0
35
10
15
0
0
0
0
25
25
30
35
20
20
25
20
20
20
0
30
10
20
0
0
0
0
30
10
20
30
0
22
15
10
25
15
0
30
0
10
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
15
20
20
25
0
20
10
0
10
15
0
40
0
0
0
0
0
0
0
0
20
15
15
0
19
10
0
15
10
0
30
0
0
0
0
Cu2+-I
Cd2+-I
Hg2+-I
Ag+-I
Zn2+-I
II
0
25
15
0
0
10
0
35
0
10
0
0
0
30
20
10
15
0
Cu2+-II
Cd2+-II
Hg2+-II
Ag+-II
Zn2+-II
III
Cu2+-III
Cd2+-III
Hg2+-III
Ag+-III
Zn2+-III
IV
15
0
0
0
0
15
10
25
0
0
0
30
20
15
30
25
0
25
10
20
25
20
0
20
15
20
10
20
25
25
20
10
15
10
0
35
15
15
10
20
0
Cu2+-IV
Cd2+-IV
Hg2+-IV
Ag+-IV
Zn2+-IV
0
0

366
I.S. Ahmed, M.A. Kassem / Spectrochimica Acta Part A 77 (2010) 359–366
complexes having antimicrobial activity may act either by killing
the microbe or by inhibiting multiplication of the microbe by block-
ing their active sites [26].
Such increased activity of the metal chelates can be explained on
the basis of chelation theory. On chelation, the polarity of the metal
ion will be reduced to a great extent due to the overlap with the
ligand orbital. Further, it increases the delocalization of ␲-electrons
over the whole chelate ring and enhances the lipophilicity of the
complexes [27].
[6] T. Ueno, M. Ohashi, M. Kono, K. Kono, A. Suzur, T. Yamane, Y. Watanabe, Inorg.
Chem. 43 (2004) 2852.
[7] A.A. Khandar, K. Nejati, Z. Rezvani, Molecules 10 (2005) 302.
[8] B.S. Holla, M. Mahalinga, M.S. Karthikeyan, B. Poojary, P.M. Akberali, N.S.
Kumari, Eur. J. Med. Chem. 40 (2005) 1173.
[9] I. Kaya, S. Cullhaglu, M. Gull, Synth. Met. 156 (2006) 1123.
[10] Z. Guo, R. Xing, S. Liv, Z. Zhong, X. Ji, L. Wang, P. Li, Carbohydr. Res. 342 (2007)
1329.
[11] L. Shi, H.M. Ge, S.H. Tan, H.Q. Li, Y.C. Song, H.L. Zhu, R.X. Ten, Eur. J. Med. Chem.
42 (2007) 1329.
[12] V.P. Daniel, B. Murukan, B.S. Kumori, K. Mohanan, Spectrochim. Acta Part A
(2008).
[13] K.K. Upedhay, A. Kumar, S. Upadhyay, P.C. Mishra, J. Mol. Struct. 873 (2008) 5.
[14] K. Kurzak, I.K. Biernacka, B. Zurowska, J. Solut. Chem. 28 (2) (1999).
[15] S.M. Ben-Saber, A.A. Maihub, S.S. Hudere, M.M. El-ajaily, Microchem. J. 81
(2005) 191.
[16] E. Tas, I. VCar, V.T. Kasumov, A. Kilic, A. Bulut, Spectrochim. Acta Part A 68 (2007)
463.
[17] G. Laus, H. Schottenberger, N. Schuler, K. Wurst, R.H. Herber, J. Chem. Soc. Perkin
Trans. 2 (2002) 1445.
[18] N.S. Egorov, Microbe-antogonists and Biological Assessment of their Antibiotics
Activity, Vysshaya Shkola Publishers, Moscow, Russia, 1965.
[19] C. Reichardt, Solvent and Solvent Effects in Organic Chemistry, 3rd ed., Wiley
VCH, Germany, 2004.
4. Conclusions
Schiff base compounds (I–IV) and their chelates are prepared
and characterized by using elemental analysis (C, H, N), IR, 1H NMR
and UV–vis spectra. The electronic absorption spectra of the ligands
are recorded in different solvents and in universal buffer solutions.
The pKa of the compounds I–IV are determined applying different
methods. The biological activity against bacterial species and fungi
are investigated sucessfuly.
[20] H.T.S. Britton, Hydrogen Ions, 4th ed., Champan and Hall, 1952, p. 364.
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[23] J.C. Colleter, Ann. Chim. 5 (1969) 415.
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