Synthesis, characterization, equilibrium study and biological activity of Cu(II), Ni(II) and Co(II) complexes of polydentate Schiff base ligand

Article abstract of DOI:10.1016/j.saa.2012.07.047

Schiff base ligand, 1,4-bis[(2-hydroxybenzaldehyde)propyl]piperazine (BHPP), and its Cu(II), Ni(II) and Co(II) metal complexes were synthesized and characterized by elemental analysis, magnetic susceptibility, molar conductance and spectral (IR and UV-vis

Full text of DOI:10.1016/j.saa.2012.07.047

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization, equilibrium study and biological activity of Cu(II),
Ni(II) and Co(II) complexes of polydentate Schiff base ligand
⇑
Ahmed A. El-Sherif , Mohamed R. Shehata, Mohamed M. Shoukry, Mohammad H. Barakat
Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
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
Schiff base ligand, 1,4-bis[(2-hydroxybenzaldehyde)propyl]piperazine (BHPP), and its Cu(II), Ni(II) and
Co(II) metal complexes were synthesized and characterized by elemental analysis, magnetic susceptibil-
ity, molar conductance and spectral (IR and UV–vis) studies. Metal complexes are formed in the 1:1 (M:L)
ratio and have the general formula [ML]ꢀnH2O, where M = Co(II), Ni(II) and Cu(II), L = BHPP. In all the stud-
ied complexes, the (BHPP) ligand behaves as a hexadentate divalent anion with coordination involving
the two azomethine nitrogens, the two nitrogen atoms of piperazine ring and the two deprotonated phe-
nolic OH-groups.
"
Synthesis of a new polydentate
Schiff base (BHPP).
Synthesis and characterization of
BHPP-Schiff base and its metal
complexes.
Investigation of the biological
activity of the free Schiff base and its
metal-chelates.
Protonation equilibria were
calculated for Schiff base (BHPP)
50% (v/v) DMSO–water solution.
Complex formation equilibria and
speciation of binary M(II)–BHPP
were investigated.
"
"
"
"
OH
CHO
EtOH
+
N
N
N
N
2
N
N
NH2
H2N
OH
HO
N
N
N
N
N
N
EtOH
pH = 8-9
N
M
N
+
n
+2
M
O
O
OH
HO
a r t i c l e i n f o
a b s t r a c t
Article history:
Schiff base ligand, 1,4-bis[(2-hydroxybenzaldehyde)propyl]piperazine (BHPP), and its Cu(II), Ni(II) and
Co(II) metal complexes were synthesized and characterized by elemental analysis, magnetic susceptibil-
ity, molar conductance and spectral (IR and UV–vis) studies. The ground state of BHPP ligand was inves-
tigated using the BUILDER module of MOE. Metal complexes are formed in the 1:1 (M:L) ratio as found
from the elemental analysis and found to have the general formula [ML]ꢀnH2O, where M = Co(II), Ni(II)
and Cu(II), L = BHPP. In all the studied complexes, the (BHPP) ligand behaves as a hexadentate divalent
anion with coordination involving the two azomethine nitrogen’s, the two nitrogen atoms of piperazine
ring and the two deprotonated phenolic OH-groups. The magnetic and spectral data indicates octahedral
geometry of metal(II) complexes. The ligand and their metal chelates have been screened for their anti-
microbial activities using the disc diffusion method against the selected bacteria and fungi. They were
found to be more active against Gram-positive than Gram-negative bacteria. Protonation constants of
(BHPP) ligand and stability constants of its Cu2+, Co2+ and Ni2+ complexes were determined by potentio-
metric titration method in 50% DMSO–water solution at ionic strength of 0.1 M sodium nitrate. It has
Received 26 April 2012
Received in revised form 3 July 2012
Accepted 11 July 2012
Available online 28 July 2012
Keywords:
Copper(II)
Electronic spectra
Stability constants
Potentiometric studies
Biological activity
⇑
Corresponding author. Tel.: +20 2 0235676562, mobile: +20 1060160168.
E-mail address: aelsherif72@yahoo.com (A.A. El-Sherif).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.07.047

890
A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
been observed that the protonated Schiff base ligand (BHPP) have four protonation constants. The diva-
lent metal ions Cu2+, Ni2+ and Co2+ form 1:1 complexes.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
0.05 Kcal/mol. Then, the obtained model was implemented to the
‘Systematic Conformational Search’ of the MOE.
Schiff bases played an important role in the development of
coordination chemistry as they readily form stable complexes with
most transition metals [1–5]. It is well known that some drugs
exhibit increased activity when administered as metal complexes
[6–8] and several metal chelates have been shown to inhibit tumor
growth [9]. Metal complexes of the Schiff base ligands have a vari-
ety of applications including clinical [10], analytical [11], industrial
[12] and biological [13], in addition to their important roles in
catalysis and organic synthesis [14]. Some research groups found
that the Schiff base metal complexes derived from salicylaldehyde
can specially cleave the DNA [15–17]. Some piperazine derivatives
were discovered to inhibit acute human immune deficiency HIV
virus from chronically and latently infected cells containing provi-
ral DNA [18]. Also the antimalarial activities of piperazine deriva-
tives are also known [19]. Therefore Schiff bases involving
piperazine group will be of biological significance [20]. In continu-
ation of our earlier work [21,22] and to wide the scope of investi-
gation on the coordination behavior of Schiff base ligand (BHPP)
derived from salicylaldehyde and 1,4-bis(3-aminopropyl)pipera-
zine, we report here the synthesis, characterization and biological
activity of (BHPP) and its complexes with a number of 3d divalent
metal ions (Co2+, Ni2+and Cu2+). We discuss also, the acid-dissocia-
tion of the free ligand and the stepwise stability constants for its
metal(II)-complexes. This was done through calculation of stability
constants for their complexes at different temperatures. This work
is also extended to present some correlations between the thermo-
dynamic functions and some of well-known properties of the
metal ions. Such work may help to explain the nature and driving
forces for the interactions occurring in biological systems, such as
metal-protein and metal-nucleic acid interactions.
Preparation of Schiff base complexes
The Schiff base complexes under investigation were prepared by
mixing 25 ml ethanolic solution of the Schiff base (0.01 mol), with
25 ml of the ethanolic solution of the metal salts (0.01 mol),
CuCl2ꢀ2H2O (1.70 g), CoCl2ꢀ6H2O (2.38 g), NiCl2ꢀ6H2O (2.37 g). Few
drops of NH4OH solution were added to adjust the pH 8–9. The ob-
tained mixture was refluxed with stirring for 2 h, and then kept
overnight to insure the complete reactions. Thus, the formed com-
plexes were filtered, collected and then washed several times with
hot ethanol until the filtrate becomes colorless. The complexes were
dried in a desiccator over anhydrous calcium chloride under vac-
uum. The yield ranged from 60–70%. The complexes are insoluble
in ethanol and methanol but soluble in DMF and DMSO. The dried
complexes were subjected to elemental and spectroscopic analysis.
Biological activity
Antimicrobial activity of the tested samples was determined
using a modified Kirby–Bauer disc diffusion method [25]. The anti-
bacterial activities were done by using gram +ve organisms (Staph-
ylococcus aureus and Bacillus subtillis) and gram ꢁve organisms
(Escherichia coli and Pseudomonas aeruginosa). These bacterial
strains were chosen as they are known human pathogens. Briefly,
100 ll of the test bacteria were grown in 10 ml of fresh media until
they reached a count of approximately 108 cells/ml or 105 cells/ml
for fungi [26]. Hundred microliters of microbial suspension was
spread onto agar plates corresponding to the broth in which they
were maintained. Isolated colonies of each organism that might
be playing a pathogenic role should be selected from primary agar
plates and tested for susceptibility by disc diffusion method of the
National Committee for Clinical Laboratory Standards (NCCLS)
[27]. Among the available media available, NCCLS recommends
Müller-Hinton agar due to: it results in good batch-to-batch repro-
ducibility. Plates inoculated with Gram (+) bacteria as S. aureus, B.
subtilis; Gram (ꢁ) bacteria as E. coli, P. aeuroginosa, they were incu-
bated at 35–37 °C for 24–48 h and fungi as Aspergillus flavus and
Candida albicans incubated at 30 °C for 24–48 h and then the diam-
eters of inhibition zones were measured in millimeters [25,26].
Standard discs of Ampicillin (Antibacterial agent), Amphotericin B
(Antifungal agent) served as positive controls for antimicrobial
Experimental
Materials
All chemicals used in this investigation were laboratory pure
including CuCl2ꢀ2H2O, CoCl2ꢀ6H2O, NiCl2ꢀ6H2O, Salicylaldehyde
and 1,4-bis-(3-aminopropyl)piperazine. All reagents were pro-
vided from Aldrich and Sigma Chemical Co.
Preparation of Schiff base
activity but filter discs impregnated with 10 ll of solvent were
The Schiff base, 1,4-bis[(2-hydroxybenzaldehyde)propyl]piper-
azine (BHPP), used was prepared by mixing an ethanolic solution
(20 ml) of 2.24 g (0.02 mol) of salicylaldehyde with 2.0 g
(0.01 mol) of 1,4-bis(3-aminopropyl) piperazine. The mixture then
refluxed with stirring for 2 h. The precipitate was collected by fil-
teration through Büchner funnel, recrystallized from ethanol and
dried at room temperature with 80% yield.
used as a negative control. The solution in different concentrations
(mg/ml) of each compound (free ligand, metal complexes and stan-
dard drug) in DMSO was prepared for testing against spore germi-
nation. The agar used is Müller-Hinton agar that is rigorously
tested for composition and pH. Further the depth of the agar in
the plate is a factor to be considered in the disc diffusion method.
This method is well documented and standard zones of inhibition
have been determined for susceptible and resistant values. When a
filter paper disc impregnated with a tested chemical is placed on
agar, the chemical will diffuse from the disc into the agar. This dif-
fusion will place the chemical in the agar only around the disc. The
solubility of the chemical and its molecular size will determine the
size of the area of chemical infiltration around the disc. If an organ-
ism is placed on the agar, it will not grow in the area around the
disc if it is susceptible to the chemical. This area of no growth
Molecular modeling
The structural model was built using the BUILDER module of
MOE [23]. Optimization Conformational analyses were performed
in a two-step procedure. First, the compound is submitted to en-
ergy minimization tool using the included MOPAC 7.0 [24]. The
geometry was optimized using the semiemperical PM3 Hamilto-
nian with Restricted Hartree-Fock (RHF) and RMS gradient of

A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
891
around the disc is known as a ‘‘Zone of inhibition’’ or ‘‘Clear Zone’’.
For the disc diffusion, the zone diameters were measured with
slipping calipers of the (NCCLS) [27]. Agar-based methods such
as E-test and disk diffusion can be good alternatives because they
are simpler and faster the broth-based methods [28,29].
stoichiometry and stability constants of the complexes formed
were determined by trying various possible composition models.
The model selected gave the best statistical fit and was chemically
consistent with the titration data without giving any systematic
drifts in the magnitudes of various residuals, as described else-
where [32]. The fitted model was tested by comparing the experi-
mental titration data points and the theoretical curve calculated
from the values of the acid dissociation constant of the ligand
and the formation constants of the corresponding complexes. The
species distribution diagrams were obtained using the program
SPECIES [33] under the experimental condition employed.
Instruments
Potentiometric measurements were made using a Metrohm 686
titroprocessor equipped with a 665 Dosimat (Switzerland-Heri-
sau). A thermostatted glass-cell was used equipped with a mag-
netic stirring system, a Metrohm glass electrode, a thermometric
probe, a microburet delivery tube and a salt bridge connected with
the reference cell filled with 0.1 M KCl solution in which saturated
calomel electrode was dipped. Temperature was maintained con-
stant inside the cell at 25.0 0.1 °C, by the circulating water by a
thermostated bath. All potentiometric measurements in this study
were carried out in water–DMSO mixtures containing 50% DMSO
because of low solubility of Schiff base ligand and possible hydro-
lysis in aqueous solution. The microchemical analysis of the sepa-
rated solid chelates for C, H and N were performed in the
microanalytical Center, Cairo University. The analyses were per-
formed twice to check the accuracy of the analysis data. Infrared
spectra were recorded on a 8001-PC FTIR Shimadzu spectropho-
tometer using KBr pellets. The solid reflectance spectra were mea-
sured on a Schimadzu UV-3101pc spectrophotometer. The molar
Results and discussion
The condensation of salicylaldehyde with 1,4-bis(3-aminopro-
pyl)piperazine in boiling ethanol yields 1,4-bis[(2-hydroxybenzal-
dehyde)propyl]piperazine Schiff base compound (BHPP). The
ligand is soluble in solvents such as DMF and DMSO. The Schiff
base and complexes are stable in air. The melting points of the
complexes are higher than that of the ligand revealing that the
complexes are much more stable than the ligand. The chemical
equations concerning the formation of the Schiff base and the com-
plexes represented in Scheme 1. The interaction of Schiff base li-
gand with copper(II), cobalt(II) or nickel(II) chloride in EtOH in a
molar ratio 1:1 under reflux conditions gave the products pre-
sented in Table 1. The elemental analysis data of the Schiff base
and its complexes are given in Table 1. The data show the forma-
tion of 1:1 Complex of the formula [ML]ꢀnH2O, where M represents
Co(II), Ni(II) and Cu(II) ions, while L represents the deprotonated
Schiff base and n = 1 or 3.
conductance of the complexes was measured for 1.00 ꢂ 10ꢁ3
M
DMSO solutions at 25 1 °C using a systronic conductivity bridge
type 305. The magnetic susceptibility measurements for the com-
plexes were determined at 25 °C by the Gouy balance using
Hg[Co(SCN)4] as a calibrant.
NMR Spectra
Potentiometric titrations
NMR spectra data of the symmetric Schiff base: 1,4-bis[(2-hy-
droxy benzaldehyde)propyl]piprazine (BHBPP) are given in table
2. The HNMR spectra, (BHBPP) shows one singlet at d 13.4 ppm
corresponding to two phenolic –OH protons and one singlet at
9.01 ppm corresponding to two azomethine protons (–CH@N–).
The pH meter readings were converted into hydrogen ion con-
centration as reported in literature [30]. The ionic product
(Kw = [H+][OHꢁ]) was calculated at a constant ionic strength of
0.10 mol dmꢁ3 with NaNO3 in 50% aqueous DMSO solutions based
on measurements of [OHꢁ] and pH in several series of experiments.
We calculated the reproducible values of pKw for the examined 50%
aqueous dimethyl sulfoxide solution [31]. The used pKw value is
15.50 [21].
Potentiometric titrations were carried out at constant tempera-
ture and in an inert atmosphere of nitrogen with CO2-free standard-
ized 0.05 mol dmꢁ3 NaOH as titrant in a 40.0 ml solution at
constant ionic strength 0.1 mol dmꢁ3, (adjusted with NaNO3). The
protonation constants of the 1,4-bis[(2-hydroxybenzaldehyde)pro-
pyl]piperazine Schiff base ligand (BHPP), were determined poten-
tiometrically by titrating (1.25 ꢂ 10ꢁ3 M) of the ligand solution
(40 cm3). The stability constants of the complexes were determined
using potentiometric data obtained from (40 cm3) mixture contain-
ing the metal ion and (BHPP) (1.25 ꢂ 10ꢁ3 M). The equilibrium con-
stants evaluated from the titration data are defined by Eqs. (1) and
(2), where M, L and H stand for the metal(II) ion (Cu2+, Co2+and
Ni2+), Schiff base ligand (BHPP) and proton, respectively.
1
Molecular modeling studies
In trying to achieve better insight into the molecular structure
of ligand, conformational analysis has been performed using the
OH
CHO
EtOH
+
N
N
N
N
2
N
N
NH2
H2N
OH
HO
pM þ qL þ rH ꢀ MpLqHr
ð1Þ
ð2Þ
N
N
N
N
N
N
EtOH
½MpLqHrꢃ
N
M
N
+
+2
bpqr
¼
nH2O
M
p
q
r
½Mꢃ ½Lꢃ ½Hꢃ
O
O
OH
HO
Data processing
The calculations were obtained from ca. 100 data points in each
titration using the computer program MINIQUAD-75 [30]. The
Scheme 1. Structure of Schiff base ligand and its metal(II) complexes where
M = Cu(II), Ni(II) and Co(II).

892
A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
Table 1
Table 4
Analytical and physical data of compounds.
Molar conductance, magnetic moment and electronic spectral data of the Schiff base
ligand and its metal(II)-complexes.
Compound Yield
(%)
Mwt
m.p
(°C)
% Found(Calc.)
C
Complex
BHPP
KM
leff. (B.M.)
kmax (cmꢁ1
)
Assignment
a
H
N
–
–
29851
34722
15015
27174
10660
15151
25316
10204
14084
22727
n ? p⁄
BHPP
80
70
62
60
408.5 62
559.5 188
70.49(70.55) 7.84(7.89) 13.69(13.74)
51.42(51.47) 6.39(6.43) 10.01(10.02)
p
?
p⁄
CuLꢀ3H2O
NiLꢀH2O
CoLꢀ3H2O
CuLꢀ3H2O
NiLꢀH2O
3.41
7.50
1.87
3.17
2B1g
?
2Eg
518.7 >300 55.48(55.52) 6.12(6.17) 10.75(10.79)
554.9 170 51.85(51.90) 6.45(6.48) 10.06(10.09)
L-M CT
3A2g(F) ? 3T2g(F)
3A2g(F) ? 3T1g(F)
3A2g(F) ? 3T1g(P)
4T1g(F) ? 4T2g(F)
4T1g(F) ? 4A2g(F)
4T1g(F) ? 4T1g(P)
CoLꢀ3H2O
6.22
5.05
Table 2
1H NMR data for the free Schiff base ligand.
a
Molar conductance measured for 10ꢁ3 M DMSO solution,
X .
ꢁ1 cm2 molꢁ1
Compound
1H NMR assignment
d (ppm)
BHPP
(s; 2H; OH)
(s; 2H; azomethine-H)
(m; 4H, –CH2–)
(t; 8H; CH2-piperazine)
(t; 4H; –CH2–N@)
(t; 4H; CH2–N)
13.40
9.01
1.75
3.10
4.01
3.50
7.8
of 3.41–7.5
X
ꢁ1 cm2 molꢁ1. These values indicate that, all synthe-
sized complexes are nonelectrolytes. This is in accordance with
the fact that conductivity values for a nonelectrolytes are below
50
X
ꢁ1 cm2 molꢁ1 in DMSO solution [35,36].
(m; 8H; aromatic protons)
IR Spectral data
The IR spectrum of 1,4-bis[(2-hydroxybenzaldehyde)pro-
pyl]piperazine Schiff base ligand (BHPP) reveals
a band at
3429 cmꢁ1 due to
t(OH). This band is absent in the Cu(II), Ni(II)
and Co(II) complexes, indicating coordination through the deproto-
nated phenolic OH group [21,37]. The IR-spectra of BHPP-Schiff
base ligand and its Cu(II) complex were represented in Figs. 2
and 3 respectively. The bands in the 2930–2221 cmꢁ1 region (med-
ium to weak) can be assigned to CꢁH stretching vibrations in both
the primary and secondary ligands [38]. The spectra of solid com-
plexes exhibit a broad band at 3410 cmꢁ1 which is attributed to
t(OH) of the water molecule of crystallization [39,40] and a sharp
medium peak at ca. 1535 cmꢁ1 (Table 5) due to H–O–H bending
vibrations [41]. A large number of Schiff bases was structurally
determined so far, revealing that crystal structures of compounds
of this class were dominated by the O–Hꢀ ꢀ ꢀN hydrogen bond
[42]. For the free ligand, the broad band centered at 2770 cmꢁ1 is
assigned to the intramolecular H-bonding vibrations (O–Hꢀ ꢀ ꢀN).
This situation is common for aromatic azomethine compounds
containing ortho-OH group [43]. These bands disappear in the
complexes, as a result of proton substitution by metal ion coordi-
nation to oxygen. The medium intensity bands observed for Schiff
base ligand at 1150 cmꢁ1 can be attributed to phenolic (C–O)
vibration modes. These bands are observed for the complexes at
lower wave numbers by 20 cmꢁ1 relative to the free ligand sug-
gesting the involvement of the two phenolic oxygen atoms in coor-
dination [44,45]. The IR spectra of the BHPP-Schiff base shows
strong intensity absorption band at 1639 cmꢁ1 assigned to C@N
stretching mode. This band is slightly shifted towards lower fre-
quencies in the complexes by 15–19 cmꢁ1, and this change in the
frequencies shows that the imine nitrogen atom coordinated to
Metal(II) ions. The presence of aromatic rings has been identified
by their characteristic ring vibrations at 1450–1493, 10180–1107
and 752 cmꢁ1 region. The absence of bands characteristic of free
Fig. 1. Optimized structure of ligand by Semiempirical PM3, hydrogen atoms are
removed for clarity.
Table 3
Calculated energies of ligand at PM3.
Ea
HFb
HOMOc
LUMOd
Dipolee
2.2482
ꢁ105449.870
ꢁ17.1465
ꢁ8.7953
ꢁ0.0852
a
b
c
E: the total energy (kcal/mol).
HF: heat of formation (kcal/mol).
HOMO: highest occupied molecular orbital (eV).
LUMO: lowest occupied molecular orbital (eV).
Dipole: dipole moment calculate.
d
e
MMFF94 force-field [34] implemented in MOE. The most stable
conformer was fully optimized with molecular orbital functions
PM3, as represented in Fig. 1. The computed molecular parameters,
total energy, electronic energy, heat of formation, the highest occu-
pied molecular orbital (HOMO) energies, the lowest unoccupied
molecular orbital (LUMO) energies and the dipole moment for
the studied compound were calculated (Table 3).
C@O and primary amine t(NH) indicates that complete condensa-
tion has occurred and confirms the formation of the proposed
Schiff base framework. In all of the present complexes a medium
and/or weak bands observed in the 462–474 and at 412–
Molar conductance measurements
418 cmꢁ1, are attributed to the
t(M–O) and t(M–N) vibrations
By using the relation: KM = K/C, the molar conductance values
of the prepared complexes with the mentioned metal ions under
investigation were determined using 1 ꢂ 10ꢁ3 M DMSO solution.
The molar conductance values, given in Table 4, are in the range
[46] respectively, supporting the participation of the nitrogen atom
of the azomethine group and phenolate oxygen of the ligand in the
complexation with metal ions [47]. These bands are absent in the
free ligand spectra.

A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
893
Fig. 2. IR spectrum of BHPP-Schiff-Base.
Fig. 3. IR spectrum of Cu(II)–BHPP-Schiff-Base.
Table 5
Tentative assignment of the important infrared bands of the synthesized complexes.
Ligand/complex
t )
(cmꢁ1
tOH
tC=N
t(O–Hꢀ ꢀ ꢀN)
t(C–O)
d(O–H)
d (ring)
tM–N
tM–O
BHPP
3429
3410
3410
3410
1639
1624
1620
1620
2808
1150
1130
1130
1130
1393
1381
1381
1377
752
760
756
756
–
–
CuLꢀ3H2O
NiLꢀH2O
CoLꢀ3H2O
–
–
–
462
459
474
412
418
413

894
A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
at different wavelengths, each one is corresponding to certain tran-
sition which suggests the geometry of the complex compounds.
Electronic absorption spectra arise from the electronic transitions
within a molecule or ion from a lower to a higher electronic energy
level. The transition metal ions generally show a number of d–d
transition bands depending on their electronic configuration from
d1 to d9 in UV–vis regions. The copper(II) complexes generally
show a broad band in the 13,000–18,000 cmꢁ1 region assigned
to the envelope of 2B1g ? 2Eg + 2B2g + 9A1g transitions [52]. In the
present study, Cu(II) complex show an intense broad band
(Fig. 4) at 15015 cmꢁ1 (Table 4), which may be assigned to
2B1g ? 2Eg suggesting a distorted octahedral geometry for this
complex in the solid state. The band observed at 27174 cmꢁ1 refers
to ligand-metal charge transfer transition (L ? Cu(II) CT).
330.00 380.00 430.00 480.00 530.00 580.00 630.00 680.00 730.00 780.00
The Co(II) complexes generally give rise to three absorption
bands in the visible region under the influence of the octahedral
wavelength, nm
4
Fig. 4. Electronic spectra of Cu(II)–BHPP complex.
field by the excitation of the electron from the ground state T1g
4
4
4
(F) to the excited states T2g (F), A2g (F) and T1g (P). In the
Co(II)-Schiff base complex, three bands are observed at
Electronic spectra and magnetic properties
10204 cmꢁ1
(m
1), 14084 and 22727 cm–1
(m
3) corresponding to
9T1g(F)?4T2g (F) (
t
1), T1g(F) ? 4A2g(F) (
t
2) and T1g(F) ? 4T1 g(P)
4
4
The magnetic susceptibility measurements provide information
to characterize the structure of the complexes. The magnetic mo-
ments of the complexes were measured at room temperature
and the data are listed in Table 4.
(
t
3) transitions, respectively, characteristic of octahedral of com-
plex [53–55].
The electronic spectra of Ni(II)-Schiff base complex displayed
three bands at 10660, 15151 and 25316 cmꢁ1 corresponding to
3
3
The room temperature magnetic moment for cobalt (II) com-
plex, found to be 5.05 B.M. (normal range for octahedral Co(II)
complexes is 4.3–5.2 B.M.), is indicative of octahedral geometry
[48,49]. The room temperature magnetic moment of Ni(II) Schiff
base ligand complex (3.17 B.M.) is corresponding to two unpaired
electrons (paramagnetic) and in the range for an octahedral envi-
ronment [48,49]. Also the magnetic moment of Cu(II)-Schiff base
(1.87 B.M.) falls within the range normally observed for hexa-
coordinate Cu(II) complexes [50], but slightly higher than the
spin-only value 1.73 B.M. expected for one unpaired electron,
which offers possibility of an octahedral geometry [51]. In the elec-
tronic spectra of free Schiff base ligands, the bands at 34722 and
3A2g(F) ? 3T2g(F) ( 1), A2g(F) ? 3T1g(F) ( 2) and A2g(F) ? 3T1g(P)
t t
(t3) transitions, respectively, suggestive of octahedral of geometry
[51,56].
Antimicrobial activity
The Schiff base ligand is biologically active and its activity may
arise from the piprazine-N and the hydroxyl groups which may
play an important role in the antibacterial activity [57], as well
as the presence of two imine groups which imports in elucidating
the mechanism of transformation reaction in biological systems
[58]. To assess the biological potential of the synthesized com-
pounds, the Schiff base ligand and its metal complexes were tested
against the selected bacteria and fungi (Fig. 5). The antimicrobial
data were collected in Tables 6 and 7.
29850 cmꢁ1 can be attributed to
p
–
p⁄ and n ꢁ p⁄ transitions. A
comparison of the electronic spectra of the free ligands with those
of the corresponding metal complexes show some shift. This can be
considered as evidence for the complex formation. Additionally the
solid reflectance spectra of metal complexes show different bands
The synthesized compounds were found to be more toxic
compared with their parent Schiff base ligand against the same
25
16
14
12
10
8
Pseudomonas Aeuroginosa
Bacillus subtillis
20
15
10
5
6
4
2
0
0
L
CuL.3H2O NiL.H2O CoL.3H2O Ampicillin
Staphylococcus aureus
25
25
20
15
10
5
E. Coli
20
15
10
5
0
0
L
CuL.3H2O NiL.H2O CoL.3H2O Ampicillin
L
CuL.3H2O NiL.H2O CoL.3H2O Ampicillin
Fig. 5. Biological activity of metal complexes towards different types of bacterial strains.

A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
895
Table 6
The results indicate that, the three complexes exhibited moder-
ate activity against the fungal strains when compared with stan-
dard Amphotericin.
Antibacterial activity of the isolated complexes.
Complex
Gram positive
Gram negative
The tested complexes were more active against Gram-positive
than Gram-negative bacteria, it may be concluded that the antimi-
crobial activity of the compounds is related to cell wall structure of
the bacteria. It is possible because the cell wall is essential to the
survival of bacteria and some antibiotics are able to kill bacteria
by inhibiting a step in the synthesis of peptidoglycan. Gram-posi-
tive bacteria possess a thick cell wall containing many layers of
peptidoglycan and teichoic acids, but in contrast, Gram negative
bacteria have a relatively thin cell wall consisting of a few layers
of peptidoglycan surrounded by a second lipid membrane contain-
ing lipopolysaccharides and lipoproteins. These differences in cell
wall structure can produce differences in antibacterial susceptibil-
ity and some antibiotics can kill only Gram-positive bacteria and is
infective against Gram-negative pathogens [60].
Staphylococcus Bacillus
Pseudomonas Escherichia
aereuguinosa coli
aureus
subtillis
C(mg mlꢁ1
BHPP
CuLꢀ3H2O
NiLꢀH2O
CoLꢀ3H2O
Ampicillin
)
1
–
9
–
–
–
2.5
9
13
12
10
12
5
1
–
–
–
–
–
2.5
9
10 15
10 13
5
10
1
–
2.5
9
5
10
1
–
2.5
9
5
10
10
20
18
17
14
10 15 20 10 14 20
–
–
–
10 13 10 12 17
10 12 10 12 16
–
10
11 15
0
14
–
13 17
(standard)
Table 7
Antifungal activity of the Schiff base ligand and its metal complexes.
Ligand/complex
Fungi
Structure activity relationships evidence that the complexation
with copper enhances the antimicrobial activity of the ligands
against some of the tested organisms. Since copper chelates have
an enhanced antimicrobial activity, in comparison to their analo-
gous containing nickel(II) and cobalt(II) ions, the metal seems to
play a relevant role in the activity of these compounds. These re-
sults may be due to higher stability constant of the Cu(II) com-
plexes than the other complexes (Table 8). A similar behaviour of
the increased antibacterial activity was also exhibited by copper
derivatives of 2,6-diacetylpyridine-bis(2-thenoylhydrazone) in
comparison to the corresponding Co(II), Ni(II) and Zn(II) chelates
[61].
Aspergillus flavus
Candida albicans
BHPP
12
15
14
13
17
12
16
15
13
17
CuLꢀ3H2O
NiLꢀH2O
CoLꢀ3H2O
Amphotericin (standard)
Table 8
Formation constant of MpLqHr species in 50% DMSO–water mixture.
= 0.1 mol dmꢁ3 NaNO3, T = 25.0 0.1 °C).
(l
System
BHPP
pqra
Logbb
Sc
011
012
013
014
110
111
110
111
110
111
10.56 0.01
19.89 0.02
28.38 0.02
36.06 0.03
14.09 0.05
22.68 0.04
6.74 0.07
15.64 0.08
5.86 0.05
14.87 0.1
1.1Eꢁ07
Equilibrium studies
Protonation constants of Schiff base ligand
The study of complex formation by the studied Schiff base can
not be carried out in aqueous solution because of the nature of
the compound involved. These metal complexes as well as the li-
gands themselves are insoluble in water. The stoichiometric pro-
tonation constants of the investigated Schiff base ligand were
determined in 50% DMSO–water mixture at 25 °C and these con-
stants are tabulated in Table 8. The BHPP ligand studied here have
four protonation constants. The species distribution of the BHPP li-
gand is plotted in Fig. 6. By rising of pH, the ligand (H4L+4) loses its
protons forming H3L+3 anion with maximum formation degree of
95% at pH 7.9. As pH increases, the second and third protons begin
deprotonation reaching a maximum percent of 55% and 67% at pH
8 and 10 respectively. As conditions become more alkaline, the
Cu–BHPP
Ni–BHPP
Co–BHPP
2.1Eꢁ06
1.5Eꢁ06
1.6Eꢁ06
p, q and r are the corresponding to metal(II), (BHPP) and H+, respectively.
Standard deviations are given in parentheses.
Sum stoichiometric coefficients of square of residuals.
a
b
c
micro-organism and under the identical experimental conditions.
The increase in biological activity of the metal chelates may be
due to the effect of the metal ion on the normal cell process. A pos-
sible mode of toxicity increase may be considered in the light of
Tweedy’s chelation theory [59]. Chelation considerably reduce
the polarity of the metal ion because of partial sharing of its posi-
100
H4L
L
tive charge with the donor group and possible p-electron delocal-
ization within the whole chelate ring system that is formed during
coordination. Such chelation could enhance the lipophilic character
of the central metal atom and hence increasing the hydrophobic
character and liposolubility of the complex favoring its permeation
through the lipid layers of the cell membrane. This enhances the
rate of uptake/entrance and thus the antimicrobial activity of the
testing compounds. Accordingly, the antimicrobial activity of the
three complexes can be referred to the increase of their lipophilic
character which in turn deactivates enzymes responsible for respi-
ration processes and probably other cellular enzymes, which play a
vital role in various metabolic pathways of the tested micro-
organisms.
80
HL
H3L
H2L
60
40
20
0
5
6
7
8
9
10
11
12
The antibacterial activity can be ordered as [Cu(BHPP)] >
[Ni(BHPP)] > [Co(BHPP)], suggesting that the lipophilic behaviour
increases in the same order.
pH
Fig. 6. Concentration distribution diagram of BHPP ligand system.

896
100
A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
[Cu(BHPP)] > [Ni (BHPP)] > [Co(BHPP)] which is in accordance
Cu
CuL
CuHL
with the stability constants order log KCu–BHPP = 14.09 > log
KNi–BHPP = 6.74 > log KCo–BHPP = 5.86.
80
60
40
20
0
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.07.047.
References
[1] C. Sousa, C. Freire, B. de Castro, Molecules 8 (2003) 894.
[2] V.K. Singh, R. Sinha, R. Kant, B.K. Sinha, Asian, J. Chem. 10 (1998) 532.
[3] L.T. Yildirm, O. Atakol, Cryst. Res. Technol. 37 (2002) 1352.
[4] A. Pui, Croat. Chem. Acta 75 (2002) 165.
[5] A.D. Garnovski, A.L. Nivorozhkin, V.I. Minki, Coord. Chem. Rev. 126 (1993) 1.
[6] M.J.M. Campbell, Coord. Chem. Rev. 15 (1975) 279.
4
5
6
7
8
9
10
11
pH
Fig. 7. Concentration distribution of various species as a function of pH in the
Cu–BHPP system.
[7] D.R. Williams, Chem. Rev. 72 (1972) 203.
[8] A. Furst, R.T. Haro, Prog. Exp. Tumor Res. 12 (1969) 102.
[9] F.B. Dwyer, E. Mayhew, E.M.F. Roe, A. Shulman, Br. J. Cancer 19 (1965) 195.
[10] H. Sharghi, M.A. Nasser, Bull. Chem. Soc. (Jpn.) 76 (2003) 137.
[11] X.M. Ouyang, B.L. Fei, T.A. Okamuro, W.Y. Sun, W.X. Tang, N. Ueyama, Chem.
Lett. 362 (2002).
fourth proton begins deprotonation to the free ligand L4ꢁ anion
which is the predominant species at pH > 11.
[12] N. Raman, Y.P. Raja, A. Kulandaismy, Proc. Ind. Acad. Sci. (Chem. Sc.) 113
(2001) 183.
[13] C. Jayabalakrishnan, K. Natarajan, Trans. Met. Chem. 27 (2002) 75.
[14] W.T. Gao, Z. Zheng, Molecules 7 (2002) 511.
Stability constants of the Schiff base complexes
In the potentiometric titration curves of BHPP Schiff base with
metal(II) ions, it was observed that all metal ions depress the titra-
tion curve of the free ligand by the release of protons according to
the abilities of the metal ions to bind to the Schiff base ligand. In
order to investigate change in the concentration of the metal (II)
complexes with pH, the stability constant values are evaluated
and given in Table 8. The species distribution diagram for Cu–BHPP
system is examined (Fig. 7) as a representative example of meta-
l(II) complexes. In the Cu–BHPP distribution diagram, it will be
seen that the complex CuHL is predominated in the pH range
6.2–6.8 with formation percentage of 98%. CuL species starts to
form at pH 7 and its concentration increases with increase of pH.
By examining the values of stability constants, it was found that,
the magnitude of stability constants sequence is found to be Cu2+
> Ni2+ > Co2+, in agreement with the Irving–Williams order [62]
for divalent metal ions of the 3d series. It is clear from Table 8 that
the stability of Cu(II) complex is considerably larger as compared
to other metals of the 3d series. Under the influence of the ligand
field, Cu(II) (3d9) will receive some extra stabilization [60] due to
tetragonal distortion of octahedral symmetry in their complexes.
The Cu(II) complex will be further stabilized due to the Jahn-Teller
effect [63].
[15] R. Sylvain, J.L. Bernier, M.J. Waring, J. Org. Chem. 61 (1996) 2326.
[16] B. Santanu, S.M. Subbrangsu, J. Chem. Soc., Chem. Commun. (1995) 2489.
[17] D.J. Gravert, J.H. Griffin, J. Org. Chem. 58 (1993) 820.
[18] J.A. Turbin, R.W. Buckheit Jr., D. Derse, M. Hollingshead, K. Williamson, C.
Palamone, M.C. Osterling, S.A. Hill, L. Graham, C.A. Schaeffer, M. Bu, M. Huang,
W.M. Cholody, C.J. Michejda, W.G. Rice, Antimicrob. Agents Chemother. 42
(1998) 487.
[19] M. Medou, G. Priem, L. Rocheblave, G. Pepe, M. Meyer, J.C. Chermann, J.L. Kraus,
Eur. J. Med. Chem. 34 (1999) 625.
[20] S.S. Konstantinivi, B.C. Radovanovi, Z. Caki, V. Vasic, J. Ser, Chem. Soc. 68 (2003)
641.
[21] A.A. El-Sherif, T.M. Eldebss, Spectrochim. Acta A 79 (2011) 1803.
[22] A.A. El-Sherif, J. Solution Chem. 35 (2006) 1287–1301.
[23] Chemical Computing Group. Inc, MOE, 10 (2009).
[24] J.J.P. Stewart, MOPAC Manual, seventh ed., 1993.
[25] A.W. Bauer, W.M. Kirby, C. Sherris, M. Turck, Antibiotic susceptibility testing
by a standardized single disk method, J. Am. Clin. Path. 45 (1966) 493.
[26] M.A. Pfaller, L. Burmeister, M.A. Bartlett, M.G. Rinaldi, Multicenter evaluation
of four methods of yeast inoculum preparation, J. Clin. Microbiol. 26 (1988)
1437.
[27] National Committee for Clinical Laboratory Standards (1993), Methods for
dilution antimicrobial susceptibility tests for bacteria that grow aerobically.
Approved standard M7-A3, Villanova, Pa.
[28] L.D. Liebowitz, H.R. Ashbee, E.G.V. Evans, Y. Chong, N. Mallatova, M. Zaidi, D.
Gibbs, Global Antifungal Surveillance Group, A two year global evaluation of
the susceptibility of Candida species to fluconazole by disk diffusion design,
Microbiol. Infet. Dis. 24 (27) (2001).
[29] M.J. Matar, L. Ostrosky-Zeichner, V.L. Paetznick, J.R. Rodriguez, E. Chen, J.H.
Rex, Correlation between E-test, disk diffusion and microdilution methods for
antifungal susceptibility testing of fluconazole and voriconazole, Antimicrob.
Agents Chemother. 47 (2003) 1647.
Conclusions
[30] T. Gündüz, E. Kılıç, F. Köseog˘lu, E. Canel, Anal. Chim. Acta 282 (1993) 489.
[31] E.P. Serjant, Potentiometry and Potentiometric Titrations, Wiley, New York,
1984.
A series of three coordinated transition metal(II) complexes
have been synthesized by the reaction of metal chloride with the
hexadentate Schiff base (BHPP), which was prepared by the con-
densation of salicylaldehyde with 1,4-bis-(3-aminopropyl)pipera-
zine. The synthetic procedure in this work resulted in the
formation of complexes in the molar ratio (1:1) (M:BHPP). The
newly synthesized Schiff base participated in bonding to copper
as dibasic hexadentate ligand through the azomethine nitrogen,
piperazine nitrogen and phenolic oxygen atoms. The ionization
constants of the investigated BHPP–Schiff base-ligand have been
determined potentiometrically. The complex formation equilibria
were investigated to ascertain the composition and stability con-
stants of the complexes. The concentration distribution diagrams
of the complexes were evaluated. The antibacterial activity results
indicated that tested complexes were more active against the se-
lected types of bacteria than the free BHPP ligand. The antibacterial
activity of the isolated metal chelates obeyed this order
[32] P. Gans, A. Sabatini, A. Vacca, An improved computer program for the
computation of formation constants from potentiometric data, Inorg. Chim.
Acta 18 (1976) 237–239.
[33] L. Pettit, University of Leeds, Personal Communication.
[34] T.A. Halgren, J. Comput. Chem. 17 (1996) 490.
[35] W.J. Geary, Coord. Chem. Rev. 7 (1971) 81.
[36] L.K. Thompson, F.L. Lee, E.J. Gabe, Inorg. Chem. 27 (1988) 39.
[37] Z.M. Zaki, S.S. Haggag, A.A. Soayed, Spectrosc. Lett. 31 (4) (1983) 757–766.
[38] A.A. El-Sherif, J. Solution Chem. 39 (2010) 1562–1581.
[39] S.L. Stefan, B.A. EL-Shetary, W.G. Hanna, S.B. El-Maraphy, J. Microchem. 35
(1987) 51–65.
[40] M.D. Boghaci, M. Lashanizadagan, Synth. React. Inorg. Met.-Org. Chem. 30
(2000) 1393.
[41] K. Nakamato, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 3rd ed., Wiley, New York, 1978.
[42] H. Ünver, K. Polat, M. Ucar, D.M. Zengin, Spect. Lett. 36 (2003) 287.
[43] M. Dolaz, M. Tümer, Trans. Met. Chem. 29 (5) (2004) 528–536.
[44] A.L. Vance, N.W. Alcock, J.A. Heppert, D.H. Busch, Inorg. Chem. 37 (1998) 6912–
6920.
[45] J.J. Foil, A. Terron, D. Mullet, V. Moreno, Inorg. Chem. Acta 135 (1987) 197.
[46] J.R. Ferraro, Plenum press, New York, 1971.

A.A. El-Sherif et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 889–897
897
[47] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic
Chemistry, 6th ed., John Wiley & Sons Inc., New York, 2003.
[48] B.N. Figgis, J. Lewis, Modern Coordination Chem, Interscience, New York, 1967.
[49] N.R.S. Kumar, M. Nethiji, K.C. Patel, Polyhedron 10 (1991) 365.
[50] B.K. Patel, M.M. Patel, Indian J. Chem. 29 (1990) 90–92.
[51] A.B.P. Lever, ‘‘Inorganic Electronic Spectroscopy’’, 2nd ed., Elsevier, New York,
1984.
[55] B.S. Grag, P.K. Singh, S.K. Grag, Synth. React. Inorg. Met.-Org. Chem. 23 (1993)
17.
[56] H.B. Gray, C.J. Balhausen, J. Am. Chem. Soc. 85 (1963) 260.
[57] A.S.A. Zidan, Phosphorus Sulfur Silicon 178 (2003) 567.
[58] N. Sari, S. Arslan, E. Logoglu, I. Sakiyan, J. Sci. 16 (2003) 283.
[59] B.G. Tweedy, Phyto Pathol. 55 (1964) 910.
[60] A.L. Koch, Clin. Microbiol. Rev. 16 (2003) 673.
[61] M. Carcelli, P. Mazza, C. Pelizzi, G. Pelizzi, F. Zani, J. Inorg. Biochem. 57 (1995)
43–62.
[52] A.A. El-Asmy, T.Y. Al-Ansi, R.R. Amine, M.M. Mounir, Polyhedron 9 (1990)
2029.
[53] V.P. Singh, A. Katiyar, J. Pestic. Biochem. Physiol. 92 (2008) 8–14.
[54] S. Chandra, S. Sharma, Trans. Met. Chem. 27 (2002) 732.
[62] H. Irving, H.S. Rossotti, J. Chem. Soc. (1953) 3397.
[63] H. Irving, R.J.P. Williams, J. Chem. Soc. (1953) 3192.