Mixed ligand complexes of Cu(II)-2-(2-pyridyl)-benzimidazole and aliphatic or aromatic dicarboxylic acids: Synthesis, characterization and biological activity

Article abstract of DOI:10.1016/j.saa.2006.12.073

The synthesis and structural characterization of mixed ligand complexes derived from 2-(2-pyridyl)-benzimidazole (PBI) (1ry ligand) and aliphatic or aromatic dicarboxylic acids (2ry ligand) are reported. Cu(II) complexes were characterized on the bases of their elemental analyses, IR, ESR and thermal analyses. The elemental analysis indicated the formation of mixed ligand complexes in a mole ratio 1:1:1 (Cu:L¹:L²), L¹ = PBI and L² = oxalic acid, phthalic acid or malonic acid. IR spectra showed that PBI acts as a neutral bidentate coordinated to the Cu(II) via the pyridyl and imidazolyl nitrogen atoms. The dicarboxylic acids are bidentate with monodentate carboxylate groups. Thermal decomposition study of complexes was monitored by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis in N₂ atmosphere. The decomposition course and steps were analysed and the activation parameters of the nonisothermal decomposition were calculated from the TG curves and discussed. The isolated metal chelates were screened for their antimicrobial activities and the results are reported, discussed and compared with some known antibiotics.

Full text of DOI:10.1016/j.saa.2006.12.073

Spectrochimica Acta Part A 68 (2007) 877–882
Mixed ligand complexes of Cu(II)-2-(2-pyridyl)-benzimidazole
and aliphatic or aromatic dicarboxylic acids:
Synthesis, characterization and biological activity
Ahmed A. El-Sherifa,∗, Bakir J.A. Jeraghb
a Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
b Department of Chemistry, Faculty of Science, Kuwait University,
P.O. Box 5969, Safat 13060, Kuwait
Received 27 June 2006; accepted 10 December 2006
Abstract
The synthesis and structural characterization of mixed ligand complexes derived from 2-(2-pyridyl)-benzimidazole (PBI) (1ry ligand) and
aliphatic or aromatic dicarboxylic acids (2ry ligand) are reported. Cu(II) complexes were characterized on the bases of their elemental analyses, IR,
ESR and thermal analyses. The elemental analysis indicated the formation of mixed ligand complexes in a mole ratio 1:1:1 (Cu:L1:L2), L1 = PBI and
L2 = oxalic acid, phthalic acid or malonic acid. IR spectra showed that PBI acts as a neutral bidentate coordinated to the Cu(II) via the pyridyl and
imidazolyl nitrogen atoms. The dicarboxylic acids are bidentate with monodentate carboxylate groups. Thermal decomposition study of complexes
was monitored by thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis in N2 atmosphere. The decomposition course and
steps were analysed and the activation parameters of the nonisothermal decomposition were calculated from the TG curves and discussed. The
isolated metal chelates were screened for their antimicrobial activities and the results are reported, discussed and compared with some known
antibiotics.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 2-(2-Pyridyl)-benzimidazole; Dicarboxylic acids; Complexes; IR; ESR; Thermal decomposition; Biological activity
1. Introduction
be related to their ability to chelate trace metal ions in biologi-
cal systems [6]. Furtheremore benzimidazole complexes exhibit
a broad spectrum of biological activities including antifungal
[7,8], antibacterial [9,10], antimicrobial [11,12], antiamobeic
[13,14], antiparasitic [15] and antitumor [16,17]. As a part of
researchonmetalcomplexeswithbenzimidazolederivativesand
carboxylate ligands [18,19], we aim to synthesize and charac-
terize copper(II) complexes with 2-(2-pyridyl)-benzimidazole
(I), and aliphatic or aromatic dicarboxylic acids. The study also
includes thermal analysis and biological activity of the synthe-
sized complexes.
Metal complexes of biologically important ligands are some-
times more effective than free ligands [1]. Mixed ligand
complexes have a key role in biological chemistry [2] because
the mixed chelation occurs commonly in biological fluids as
millions of potential ligands are likely to compete for metal
ions in vivo [3]. These create specific structures [2–4] and
have been implicated in the storage and transport of active
substances through membranes. Among these ligands are benz-
imidazole and its derivatives where transition metal complexes
with benzimidazoles are progressively used to model impor-
tant bioinorganic systems [5]. For example, the antiviral activity
of some 2-substituted benzimidazole derivatives considered to
Abbreviations: PBI, 2-(2-pyridyl)-benzimidazole; ox, oxalic acid; mal, mal-
onic acid; phth, phthalic acid
∗
Corresponding author. Tel.: +2025676562/+20106086652.
E-mail address: aelsherif72@yahoo.com (A.A. El-Sherif).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.12.073

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A.A. El-Sherif, B.J.A. Jeragh / Spectrochimica Acta Part A 68 (2007) 877–882
Table 1
2. Experimental
Analytical data Cu(PBI)–La complexes
2.1. Materials and physical measurements
Compound
Mwt
Found (calculated) (%)
C
H
N
CuCl2·2H2O was supplied from BDH. 2-(2-Pyridyl)-
benzimidazole (PBI) was obtained from Aldrich Chem. Co. The
dicarboxylic acids: oxalic acid·2H2O (ox), malonic acid (mal)
and phthalic acid (phth) were provided by Sigma Chem. Co. The
microchemical analysis of the separated solid complexes for C,
H and N were performed in the Centeral Microanalytical Lab-
oratory, Chemistry Department, Cairo University, Egypt. The
analyses were performed twice to check the accuracy of the anal-
yses data. Infrared spectra were recoreded on a 8001-PC FTIR
Shimadzu spectrophotometer using KBr pellets. Measurement
was performed by using a dynamic nitrogen atmosphere with a
TGA-50 Shimadzu thermogravimetric analyzer at a flow rate of
20 mL min−1. The heating rate was 10 ◦C min−1 in the tempera-
ture range 30–1000 ◦C using platinum crucibles and the sample
sizes ranged in mass from 6 to 8 mg. The activation parameters
of the thermal events were calculated by Coats and Redfern’s
method [20].
[Cu(PBI)(ox)].H2O
364.5 45.88(46.09)
[Cu(PBI)(mal)].H2O 378.5 47.39(47.55)
2.96(3.01)
3.35(3.43)
3.11(3.07)
11.41(11.52)
11.01(11.09)
9.84(9.94)
[Cu(PBI)(Phth)]
422.5 56.23(56.80)
a
L: oxalic acid (ox); malonic acid (mal); phthalic acid (phth).
ism and by subtracting the diameter of inhibition zone resulting
with DMSO from the obtained in each case, both antibacterial
activities can be calculated as a mean of three replicates.
Three different concentrations (1, 2.5 and 5 mg mL−1) of the
test compounds were screened against Gram positive and Gram
negative bacteria in DMSO to evaluate the effect of concentra-
tion of the compounds on the activity of the bacteria.
3. Results and discussion
3.1. IR spectra and mode of bonding
2.2. Synthesis
In absence of a powerful technique such as X-ray crystallog-
raphy, IR spectra has proven to be the most suitable technique
to give enough information to elucidate the way of bonding of
the ligands to the metal ions. The main IR bands with their
tentative assignments are summarized in Table 2. The spec-
tra of all complexes exhibit two bands in the 3092–3061 cm−1
region in the complexes can be attributed to the primary lig-
and NH stretching vibration. The new bands at 1648–1635 and
1416–1385 cm−1 in the spectra of all complexes are assigned to
asymmetric and symmetric stretching vibrations of coordinated
carboxylate group [21]. This assignment is based on the fact that
the unionized and uncoordinated COO− stretching band occurs
at 1750–1700 cm−1 [21]. Decon showed that the magnitude
of the ꢀν = νasy(COO−)–νsym(COO−) can be used to distin-
guish between the different coordination modes of the ligands
either monodentate or bidentate[22]. This difference is found
to be 289–313 cm−1 reflecting the monodentate coordination of
each carboxylate group in the synthesized complexes. There-
fore, the dicarboxylic acids act as bidentate coordinated to the
copper(II) centers via two monodentate carboxylate groups. In
some complexes a band at 1573–1551 cm−1 can be attributed to
the νC N. The spectra of the complexes display the in-plane ring
deformation bands of the pyridine moiety, Table 2, at 760–753,
633–630 and 583–580 cm−1 which are higher than the free pyri-
dine ring suggesting the coordination of the pyridyl nitrogen to
the copper(II) ion.
The copper(II) complexes were synthesized according
to the general method viz.: 2-(2-pyridyl)-benzimidazole
(0.001 mol; 0.1952 g), dicarboxylic acid (0.001 mol; 0.126,
0.104, 0.166 g of oxalic, malonic and phthalic acid, respec-
tively) and CuCl2·2H2O (0.001 mol; 0.1705 g) were dissolved
in ethanol–water mixture, 1:1 (v/v) (50 mL). To this mixture,
a solid NaHCO3 (0.002 mol; 0.168 g) was added portionwise.
After complete addition of the bicarbonate, the reaction mixture
was stirred for 2 h at room temperature. The solid precipitated
was collected by filtration, washed several times with EtOH,
followed by Et2O and dried under vacuum over silica gel. The
analytical data were given in Table 1.
2.3. Biological activity
A 0.5 mL spore suspension (10−6 to 10−7 spore mL−1) of
each of the investigated micro-organisms was added to a sterile
agar medium just before solidification, then poured into sterile
petri dishes (9 cm in diameter) and left to solidify. By using a
sterilized cork borer (6 mm in diameter), three holes (wells) were
made in each dish, then 0.1 ml of the tested compounds dissolved
in DMSO (100 ␮g mL−1) were poured into these holes. Finally,
the dishes were incubated at 37 ◦C for 48 h where clear or inhibi-
tion zones were detected around each hole. 0.1 mL DMSO alone
was used as a control under the same conditions for each organ-
Table 2
Tentative assignment of the important infrared bands of the synthesized ternary complexes Cu(PBI)–L complexes
Complex
υNH
υC
υC
υCOO (symmetry)
υCOO (asymmetry)
ꢀν
δ(py)
N
C
PBI
3061, 3080
3061, 3087
3062, 3090
3067, 3092
1600
1573
1560
1551
1445
1459
1452
1456
–
–
–
620, 590, 547
760, 633, 583
740, 630, 582
753, 632, 580
[Cu(PBI)(ox)]·H2O
[Cu(PBI)(mal)]·H2O
[Cu(PBI)(Phth)]
1344
1331
1359
1635
1644
1648
291
313
289

A.A. El-Sherif, B.J.A. Jeragh / Spectrochimica Acta Part A 68 (2007) 877–882
879
Table 3
Electron spin resonance data of Cu(PBI)–L complexes
Compound
g
||
g
⊥
gavg
G
[Cu(PBI)(ox)]·H2O
[Cu(PBI)(mal)]·H2O
[Cu(PBI)(Phth)]
2.208
2.238
2.254
2.062
2.091
2.075
2.111
2.140
2.135
3.41
2.65
3.42
3.2. Electron spin resonance study
The X-band ESR spectra of the isolated solid complexes
recorded at room temperature are anisotropic and exhibit
two “g” values. The g and g values are computed from
||
⊥
the spectrum using DPPH free radical as “g” marker. The
average g values were calculated according to the equation
gav = (1/3)[g + 2g ] and the results are given in Table 3. Kivel-
||
⊥
son and Neiman [23] have reported that g < 2.3 and g > 2.3 is
||
||
characteristic for covalent and ionic characters, respectively. By
applying this criterion, the complexes under study have mainly
covalent metal–ligand bonding. The trend g > g > ge(2.0023)
observed for these complexes shows that the unpaired elec-
tron is localized in dx2−y2 orbital of the Cu(II) ion [24,25]. In
addition the exchange coupling interaction between two cop-
per centers in solid state given by Hathway [26,27] expression,
||
⊥
G = (g − 2.0023)/(g − 2.0023), led to the values of 2.65–3.42,
||
⊥
Table 3. According to Hathway, if the value of G is greater than
four, the exchange interaction is negligible, whereas when the
value of G is less than four a considerable interaction is indi-
cated in solid complexes. Accordingly there is a considerable
exchange interaction between the copper centers in the solid
state. Based on elemental analysis, IR and ESR data, a tetrago-
nally distorted copper(II) complexes is assumed for all isolated
complexes.
3.3. Thermal analysis
Fig. 1. TG and DTG plots of Cu-PBI-dicarboxylic acid complexes. Curves: (a)
[Cu(PBI)(ox)]; (b) [Cu(PBI)(mal)]; (c) [Cu(PBI)(phth)].
The thermogravimetric (TG) and the derivative thermogravi-
metric (DTG) plots of complexes are given in Fig. 1. The
correlations between the different decomposition steps of the
complexes with the corresponding weight losses are discussed
in terms of the proposed formulae of the complexes. The tem-
perature range, the predicted intermediates and final products
are given in Table 4.
in agreement with the formation of CuO as the final residue (TG
22.2%, calculated 21.8%).
3.3.2. [Cu(PBI)(mal)]·H2O
The Thermogram given in Fig. 1b of [Cu(PBI)(mal)]·H2O
exhibits three significant thermal events within the tempera-
ture range (37–366 ◦C). The first step of decomposition within
the temperature range (37–100 ◦C) corresponds to the loss of
water molecule of hydration with a mass loss 4.5% (calculated
4.7%). The second step (125–224 ◦C) corresponds to the loss of
malonic anhydride (C3H2O3) with mass loss 22.5% (calculated
22.7%). The third step (291–366 ◦C) corresponds to the loss PBI
molecule with mass loss 50.9% (calculated 51.5%). The ener-
gies of activation were 31.6, 16.7 and 64.4 kJ mol−1 for the first,
second and third steps, respectively. The total mass loss up to
366 ◦C is in agreement with the formation of CuO as the final
residue (TG 22.1%, calculated 21.1%).
3.3.1. [Cu(PBI)(ox)]·H2O
The TG and DTG Curves of [Cu(PBI)(ox)].H2O are shown in
Fig. 1a. The thermogram exhibits three main distinct decomposi-
tion steps. The first step of decomposition within the temperature
range (50–140 ◦C) corresponds to the loss of water molecule of
hydration with a mass loss 4.7% (calculated 4.9%). The second
step (209–323 ◦C) corresponds to the loss of oxalic anhydride
molecule(C2O3) with mass loss 18.6% (calculated 19.75%). The
third step (407–551 ◦C) corresponds to the loss of PBI molecule
with mass loss 54.5% (calculated 53.5%). The energies of acti-
vation were 22.4, 35.0 and 47.1 kJ mol−1 for the first, second
and third steps, respectively. The total mass loss up to 551 ◦C is

880
A.A. El-Sherif, B.J.A. Jeragh / Spectrochimica Acta Part A 68 (2007) 877–882
Table 4
Stepwise thermal degradation data obtained from TGA curves for Cu(PBI)–L complexes
Complex
Molar mass
TG range (◦C)
DTGmax
Weight loss
Found
Assignment
Metallic residue
found (calculated %)
Calculated
[Cu(PBI)(ox)]·H2O
[Cu(PBI)(mal)]·H2O
[Cu(PBI)(Phth)]
364.5
378.5
422.5
50–140
209–323
407–551
91
274
474
4.7
18.60
54.5
4.9
19.75
53.5
Loss of H2O
Loss of oxalic anhydride
Loss of PBI
CuO 22.2 (21.8)
CuO 22.1 (21.1)
CuO 19.5(18.8)
37–100
125–224
291–366
68
224
325
4.5
22.5
50.9
4.7
22.7
51.5
Loss of H2O
Loss of malonic anhydride
Loss of PBI
226–325
351–526
284
484
34.8
45.7
35.0
46.1
Loss of phthalic anhydride
Loss of PBI
3.3.3. [Cu(PBI)(Phth)]
activation was calculated using Eq. (15) [29].
ꢂ
ꢃ
ꢄꢅ
The TG and DTG curves of [Cu(PBI)(phth)] are shown in
Fig. 1c. The TGA curve of the Cu(PBI)(phthalate) complex
shows two stages of decomposition within the temperature range
(226–526 ◦C). The first step of decomposition within the tem-
perature range (226–325 ◦C) corresponds to the loss phthalic
anhydride molecule with a mass loss 34.8% (calculated 35.0%).
The second step (351–526 ◦C) corresponds to the loss of PBI
with mass loss 45.7% (calculated 46.1%). The energies of acti-
vation were 102.3 and 9.8 kJ mol−1 for the first and second
steps, respectively. The total mass loss up to 526 ◦C is in agree-
ment with the formation of CuO as a final residue (TG 19.5%,
calculated18.8%).
Ah
kT
ꢀS∗ = 2.303R log
(15)
where h and k stand for the Planck and Boltzmann constants,
respectively and T is the Peak temperature from the DTG curve.
The free energy of activation ꢀG* and the enthalpy of activation
ꢀH*were calculated using Eqs. (3) and (4), respectively.
ꢀH∗ = Ea − RT
ꢀG∗ = ꢀH∗ − T ꢀS∗
(3)
(4)
The left-hand side of Eqs. (1) and (2) is plotted and in test-
ing the thermal decomposition data of complexes, Eqs. (1) and
(2) expressing the integer and non-integer orders were plotted.
The first order equation is found to be better. The kinetic data
obtained from the nonisothermal decomposition of the com-
plexes are cited in Table 5. According to the kinetic data obtained
from DTG curves, all the complexes have negative entropy, indi-
cating that the activated complexes are more ordered than the
reactants.
3.4. Kinetic data of the decomposition of complexes
The thermodynamic activation parameters of decomposition
processes of complexes namely activation energy (Ea), enthalpy
(ꢀH*), entropy (ꢀS*) and Gibbs free energy change of the
decomposition (ꢀG*) were evaluated graphically by employ-
ing the Coats–Redfern relation method [20]. This method, as
reviewed by Johnson and Gallagher [28] is an integral method
that assumes various orders of reaction and compares the lin-
earity in each case to select the correct order. The equations are
given below:
3.5. Antibacterial activity
The biological activity of the isolated mixed ligand com-
plexes and Chloroamphenicol (as a standard compound) were
tested against bacteria because bacteriums can achieve resis-
tance to antibiotics through biochemical and morphological
modifications [30]. In testing the antibacterial activity of these
compounds, we used more than one test organism to increase the
chance of detecting antibiotic principles in tested materials. The
organisms used in the present investigations included two Gram
positive bacteria (Staphylococcus aureus and Bacillus subtillis
and two Gram negative bacteria (Pseudomonas aereuguinosa
and Escherichia coli). The diffusion agar technique was used
for screening the antibacterial activity of the synthesized mixed
ligand complexes [31–34]. The medium used for growing the
culture was nutrient agar. The results of the bactericidal screen-
ing of the synthesized compounds are recorded in Table 6. The
data obtained reflect the following results:
ꢀ
ꢁ
ꢂ
ꢃ
ꢄꢅ
1 − (1 − α)1−n
T2(1 − n)
AR
θEa
2RT
Ea
Ea
log
=log
1 −
−
,
2.303RT
for n = 1
(1)
ꢆ
ꢂ
ꢃ
ꢄꢅ
−log(1 − α)
T2
AR
θEa
2RT
Ea
Ea
log
= log
1 −
−
,
2.303RT
for n = 1
(2)
where α is the fraction of sample decomposed at time t, T the
derivative peak temperature, A the frequency factor, Ea the acti-
vation energy, R the gas constant, θ is the heating rate and
2
∼
(1 − (2RT/E )) 1. A plot of log{−log(1 − α)/T }versus 1/T
=
a
gives a slope from which the Ea is calculated and A (Arrhe-
1. Antibacterial activity of all mixed ligand complexes towards
nius factor) is determined from the intercept. The entropy of
E. coli are detected; except [Cu(PBI)(ox)] complex.

A.A. El-Sherif, B.J.A. Jeragh / Spectrochimica Acta Part A 68 (2007) 877–882
881
Table 5
The kinetic parameters for the non-isothermal decomposition of the Cu(PBI)–L complexes
Range (◦C)
Ta
Ea (kJ mol−1
)
A (S−1
)
ꢀH* (kJ mol−1
)
ꢀS* (kJ mol−1
)
ꢀG* (kJ mol−1
)
[Cu(PBI)(ox)]·H2O
50–140
209–323
91
274
474
22.45
35.01
47.06
1.43
19.42
30.45
40.85
−243.9
−257.9
−268.1
108.2
171.5
241.1
4.0E−01
407–551
1.6E−01
[Cu(PBI)(mal)]·H2O
32–77
171–250
48
234
375
31.59
16.67
64.35
4.34E+01
1.9E−03
1.4E+02
28.76
12.54
59.38
−215.08
−301.6
102.1
162.4
184.9
321–408
−210.01
[Cu(PBI)(phth)]
226–325
351–526
284
484
102.3
9.79
2.27E+06
7.12E−05
97.71
3.50
−128.8
−332.4
169.5
255.1
a
The peak temperature from the DTG curve.
Table 6
Antibacterial activity of the isolated mixed ligand Cu(PBI)–L complexes
Complex
Gram positive
Gram negative
Pseudomonas aereuguinosa
Staphylococcus aureus
Bacillus subtillis
Escherichia coli
1a
2.5a
5a
1
2.5
5
1
2.5
5
1
2.5
5
1
2
3
++
+
−
++
++
++
++
+++
+++
+++
++
+
+
++
++
++
++
++
+++
+++
+++
+++
+++
+
++
+
++
+++
+++
++
+++
+++
+++
R
−
+
+
+
++
−
++
++
++
+++
+++
++
+
Chloroamphenicol (standard)
Amikain
Augmantin
Unasyn
++
++
R
+++
+++
R
R
R
Septrin
Cefobid
R
R
Ampicillin
Erythrpmycin
R
+++
++
R
The test was done using the diffusion agar technique. (−) Non-detected, inhibition values = 0.7–0.9 cm beyond control; (+) less active, inhibition values = 1–1.5 cm,
beyond control; (++) moderate active, inhibition values > 1.5 cm, beyond control; (+++) highly active. (1) [Cu(PBI)(ox)]; (2) [Cu(PBI)(mal)]; (3) [Cu(PBI)(phth)].
a
Concentration (mg ml−1).
2. The two complexes [Cu(PBI)(phth)] and [Cu(PBI)(ox)] have
moderate activity towards P. aereuguinosa and high activity
towards B. subtillis and S. aureus.
3. The activity of the isolated complexes increases as the con-
centration increases because it is a well-known fact that
concentration plays a vital role in increasing the degree of
inhibition [35].
effect toward S. aureus in contrast to the synthesized mixed
ligand complexes. However, Augmantin, Unasyn, Septrin,
Cefobid and Erythromycin, have resistance effect toward
P. aereuguinosa while all synthesized mixed ligand com-
plexes have a greater effect. [Cu(PBI)(mal)] show moderate
activity toward Pseudomonas aereuginosa similar to that of
Ampicillin antibiotic.
4. The data in Table 6 shows that E. coli was inhib-
ited by the isolated complexes according to this order
4. Conclusion
[Cu(PBI)(phth)] > [Cu(PBI)(mal)] > [Cu(PBI)(ox)]
com-
plexes. The importance of this lies in the fact that these
complexes could be applied fairly in the treatment of some
common diseases caused by E. coli, e.g. septicaemia,
gastroenteritis, urinary tract infections and hospital acquired
infections [36,37].
The synthetic procedure in this work resulted in the for-
mation of complexes in the molar ratio (1:1:1) (M:L1:L2),
respectively. In these complexes the 1ry ligand (PBI) coor-
dinated to Cu(II) metal ion through the two nitrogen atoms
of imidazole and pyridine rings while the dicarboxylic acids
are coordinated via the two oxygen atoms of carboxylate
groups. From elemental analysis, IR and ESR all com-
plexes having a tetragonal distorted structure with covalent
metal–ligand bonding. Also these complexes show antibacterial
activity, so synthesis of such complexes might be recom-
mended.
5. Furtheremore, some known antibiotics are tested toward S.
aureus (as Gram positive) and P. aereuguinosa (as Gram
negative). The selected antibiotics are Amikain, Augmantin,
Unasyn, Septrin, Cefobid, Ampicillin and Erythromycin.
The given data in Table 6 show that Amikain, Augmantin,
Unasyn, Septrin, Cefobid and Ampicillin have resistance

882
A.A. El-Sherif, B.J.A. Jeragh / Spectrochimica Acta Part A 68 (2007) 877–882
[18] (a) A.A. El-Sherif, M.M. Shoukry, J. Coord. Chem. 58 (2005) 1401;
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