Spectroscopic characterization and biological activity of Zn(II), Cd(II), Sn(II) and Pb(II) complexes with Schiff base derived from pyrrole-2-carboxaldehyde and 2-amino phenol

Article abstract of DOI:10.1016/j.saa.2010.03.031

A new Schiff base 2-aminophenol-pyrrole-2-carboxaldehyde and its Zn(II), Cd(II), Sn(II) and Pb(II) complexes have been synthesized and characterized by various physicochemical studies. Spectral studies (IR and ¹H NMR) indicate deprotonation and

Full text of DOI:10.1016/j.saa.2010.03.031

Spectrochimica Acta Part A 76 (2010) 376–383
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic characterization and biological activity of Zn(II), Cd(II), Sn(II) and
Pb(II) complexes with Schiff base derived from pyrrole-2-carboxaldehyde and
2-amino phenol
Bibhesh K. Singha,∗, Anant Prakasha, Hemant K. Rajourb,
Narendar Bhojakc, Devjani Adhikaria
a Inorganic & Bioinorganic Research Lab, Department of Chemistry, Govt. Post Graduate College, Ranikhet 263645, Uttarakhand, India
b Department of Chemistry, Ramjas College, University of Delhi, Delhi 110007, India
c Department of Chemistry, Govt. Dunger College, MGS University, Bikaner, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new Schiff base 2-aminophenol-pyrrole-2-carboxaldehyde and its Zn(II), Cd(II), Sn(II) and Pb(II) com-
plexes have been synthesized and characterized by various physicochemical studies. Spectral studies (IR
and 1H NMR) indicate deprotonation and coordination of phenolic oxygen along with binding of pyr-
role nitrogen, azomethine nitrogen and anion with metal ions. The presence of lattice water molecule(s)
has also been confirmed by TG/DTA studies. Mass spectrum explains the successive degradation of the
molecular species in solution and justifies ML complexes. Kinetic and thermodynamic parameters were
computed from the thermal data using Coats and Redfern method, which confirm first order kinetics. The
bio-efficacy of the ligand and their complexes has been examined against the growth of bacteria in vitro
to evaluate their antimicrobial potential. Molecular structures of the complexes have been optimized by
MM2 calculations and suggest a tetrahedral geometry around metal ions.
Received 8 January 2010
Received in revised form 15 March 2010
Accepted 17 March 2010
Keywords:
Biological activities
Spectra
Schiff base
Metal complexes
Molecular modeling
Thermal studies
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
[4]. Furthermore, macrocyclic derivatives of these Schiff bases have
many fundamental biological functions, such as photosynthesis and
Schiff base ligands are considered “privileged ligands” because
they are easily prepared by the condensation between aldehydes
and amines. Stereogenic centers or other elements of chirality can
be introduced in the synthetic design. Schiff base ligand is able to
coordinate many different metals [1], and to stabilize them in var-
ious oxidation states. Structure–activity relationship of Schiff base
compounds are studied due to their antitumor, antimicrobial and
antiviral activities [2]. In recent years, because of new interesting
applications found in the field of pesticides and medicine, the metal
complexes with tridentate O, N, N types of alternative structures
have attracted the attention of chemist. Various metal complexes
with bi- and tridentate Schiff bases containing nitrogen and oxygen
donor atoms play important role in biological system and represent
interesting models for metalloenzymes, which efficiently catalyze
the reduction of dinitrogen and dioxygen [3]. Schiff base com-
plexes incorporating phenolic group as chelating moieties in the
ligand are considered as models for executing important biologi-
cal reactions and mimic the catalytic activities of metalloenzymes
transport of oxygen in mammalian and other respiratory system
[5].
In recent years metal compounds, which have a stable d10 elec-
tronic configuration, have received a lot of attention in the fields of
inorganic chemistry, biochemistry and environmental chemistry.
About twenty zinc enzymes are known in which zinc is generally
tetrahedrally four coordinate and bonded to hard donor atoms such
as nitrogen [6]. Previously, it has been reported that zinc(II) and
cadmium(II) complexes with Schiff bases type chelating ligand can
be used as an effective emitting layer and showed photo physical
properties [7]. Zinc complexes have been shown to be active as
antitumor, anti-HIV and antimicrobial agents [8]. So, our interest
is to establish spectroscopic bioactive model complexes of newly
synthesized Schiff base with metal ions having d10 configuration.
The present report deals with the synthesis, spectroscopic
and thermal characterization of zinc(II), cadmium(II), tin(II)
and lead(II) complexes with Schiff base derived from pyrrole-
2-carboxaldehyde and 2-amino phenol and to examine their
bio-efficacy of ligand as well as metal complexes. The free ligand
and its complexes have been tested in vitro against Escherichia coli
and Staphylococcus aureus bacteria with different concentrations,
in order to assess their antimicrobial potential.
∗
Corresponding author. Tel.: +91 9760014796; fax: +91 5966220372.
E-mail address: bibheshksingh@yahoo.co.in (B.K. Singh).
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.03.031

B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
377
2. Experimental
2.1. Materials and methodology
All the chemicals used were of analytical grade and used as pro-
cured. Solvents used were of analytical grade and were purified by
standard procedures. The stoichiometric analysis (C, H and N) of
the complexes was performed using Elementar vario EL III (Ger-
many) model. Metal contents were estimated on an AA-640-13
Shimadzu flame atomic absorption spectrophotometer in solu-
tions prepared by decomposing the complex in hot concentrated
HNO3. The molar conductance at 10−3 M dilution was measured
by Elico-Conductometer Bridge. The IR spectra were recorded on
Perkin-Elmer FTIR spectrophotometer in KBr and polyethylene pel-
lets. The UV–vis spectra were recorded in DMSO on Beckman
DU-64 spectrophotometer with quartz cells of 1 cm path length
and mass spectra (TOF-MS) were recorded on Waters (USA) KC-455
model with ES+ mode in DMSO. 1H NMR spectra were recorded
in DMSO solvent (solvent peak 3.8 ppm) on a Bruker Advance
400 instrument. Rigaku model 8150 thermoanalyser was used for
simultaneous recording of TG–DTA curves at a heating rate of
10 min−1. For TG, the instrument was calibrated using calcium
oxalate, while for DTA, calibration was done using indium metal,
both of which were supplied along with the instrument. A flat bed
type aluminum crucible was used with ␣-alumina (99% pure) as the
reference material for DTA. The number of decomposition steps was
identified using TG. The activation energy and Arrhenius constant
of the degradation process were obtained by Coats and Redfern
method.
Fig. 1. (a) Structure of the ligand and (b) structure of the complex.
structures. It enables the calculation of the total static energy
of a molecule in terms of deviations from reference unstrained
bond lengths, angles and torsions plus non-bonded interactions. On
account of non-bonded interactions and also the chemical sense of
each atom, treat the force field as a set of constants that have to
be fixed by appeal to experiment or more rigorous calculations. It
has been found that off-diagonal terms are usually largest when
neighboring atoms are involved, and so we have to take account
of non-bonded interactions, but only between next-nearest neigh-
bors.
2.2. Biological activity: antibacterial screening
In vitro antibacterial activity of the compounds against E. coli
and S. aureus were carried out using Muller Hinton Agar media (Hi
media). The activity was carried out using paper disc method. Base
plates were prepared by pouring 10 ml of autoclaved Muller Hinton
agar into sterilized Petri dishes (9 mm diameter) and allowing them
to settle. Molten autoclaved Muller Hinton that had been kept at
48 ◦C was incubated with a broth culture of the E. coli and S. aureus
bacteria and then poured over the base plate. The discs were air
dried and placed on the top of agar layer. The plates were incubated
for 24–30 h and the inhibition zones (mm) were measured around
each disc. As the organism grows, it forms a turbid layer, except in
the region where the concentration of antibacterial agent is above
the minimum inhibitory concentration, and a zone of inhibition
is seen. The size of the inhibition zone depends upon the culture
medium, incubation conditions, rate of diffusion and the concen-
tration of the antibacterial agent. The solutions of all compounds
were prepared in double distilled water and chloramphenicol was
used as a reference.
2.4. Synthesis of ligand and complexes
2.4.1. Synthesis of 2-aminophenol-pyrrole-2-carboxaldehyde
Pyrrole-2-carboxaldehyde (30 mmol) was dissolved in absolute
ethanol (20 ml) added dropwise to a solution of 2-aminophenol
(30 mmol) in absolute ethanol (20 ml) with constant stirring. Stir-
ring was continued with heating at 80 ◦C for 3 h. A brown colored
powder was collected by vacuum filtration and dried overnight in
vacuum. The yield and melting point of the product were deter-
mined.
2.4.2. Synthesis of metal complexes
2-Aminophenol-pyrrole-2-carboxaldehyde (5 mmol) in 20 ml of
absolute ethanol was added dropwise to a solution containing
metal salts (5 mmol) in absolute alcohol (20 ml). The mixture was
heated while being stirred. The precipitate was filtered, washed
with cold alcohol and dried under vacuum over silica gel. The yield
and melting point of each product were determined. The metal salts
used were zinc acetate, cadmium nitrate, stannous chloride and
lead acetate.
2.3. Molecular modeling
3D molecular modeling of the proposed structure of the com-
plexes was performed using HyperChem version 7.1 program
package. The correct stereochemistry was assured through the
manipulation and modification of the molecular coordinates to
obtain reasonable low energy molecular geometries. The poten-
tial energy of the molecule was the sum of the following terms
(E) = Estr + Eang + Etor + Evdw + Eoop + Eele, where all Es represent the
energy values corresponding to the given types of interaction
(kcal mol−1). The subscripts str, ang, tor, vdw, oop and ele denote
bond stretching, angle bonding, torsion, deformation, vander-
waals interactions, out of plain bending and electronic interaction,
respectively. The molecular mechanics describe the application
of classical mechanics to determination of molecular equilibrium
3. Results and discussion
The synthesized compounds (Fig. 1a and b) are crystalline and
non-hygroscopic in nature. It is insoluble in water, partially solu-
ble in ethanol but soluble in acetone, DMF and DMSO. Composition
and identity of the assembled compounds were deduced from ele-
mental analyses, spectroscopic techniques (IR, UV–vis, 1H NMR,
TOF-MS) and thermal studies. The analytical data of the complexes
indicated 1:1 metal to ligand stoichiometry. Possible composition

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B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
Table 1
Analytical and physical data of ligands and their complexes.
S. no
Compounds (empirical formula)
Colors
MP (◦C)
Yield (%)
Elemental analysis (found/calc.)
C
H
N
M
1
2
3
4
5
L (C11H10N2)
Brown
130
190
200
175
360
84
83
81
86
83
70.93 (70.96)
47.63 (47.65)
34.95 (34.97)
35.19 (35.20)
33.20 (33.24)
5.39 (5.37)
4.26 (4.27)
2.90 (2.91)
3.45 (3.46)
2.97 (2.98)
15.09 (15.05)
8.53 (8.55)
11.07 (11.12)
7.44 (7.46)
5.92 (5.96)
–
I (C13H14O4N2Zn)
II (C11H11O5N3Cd)
III (C11H13O3N2ClSn)
IV (C13H14O4N2Pb)
Light yellow
Light brown
Cream
19.98 (19.96)
29.82 (29.78)
31.63 (31.65)
44.16 (44.15)
Light yellow
Table 2
Spectroscopic data (IR, 1H NMR, UV) of ligand and complexes.
S. No
Compound
Infrared (cm−1
)
1HNMR ı ppm
UV (nm)
ꢀ(C N)
ꢀ(M–N)
Others
(CH N)
Others
1
2
L
I
1630(s)
1620(s)
–
–
8.75
8.11
252, 318
274, 320
527(s)
[ꢀas(CO2)1632(s)], [ꢀs(CO2) 1412(s)]
2.99 (acetate gr)
3
4
5
II
III
IV
1613(s)
1625(s)
1629(s)
536(s)
490(s)
530(s)
ꢀ(NO3) [1384,1272,731,965]
ꢀ(Sn–Cl) 365(s)
[ꢀas(CO2):1620(s)], [ꢀs(CO2):1418(s)]
8.27
8.52
7.75
276, 332
278, 345
275, 340
2.49–8.08 (acetate gr)
band between 3200 and 3450 cm−1, which can be attributed to
phenolic OH group. This band disappears in all complexes, which
can be attributed to the involvement of phenolic OH group in
coordination. The involvement of deprotonated phenolic moiety
in complexes is confirmed by the shift of ꢀ(C–O) stretching band
observed at 1283 cm−1 in the free ligand to a lower frequency to
the extent 10–20 cm−1 [10]. The shift of ꢀ(C–O) band at 1283 cm−1
to a lower frequency suggests the weakening of ꢀ(C–O) and for-
mation of stronger M–O bond. The free Schiff base ligand showed
a strong band at 1630 cm−1, which is characteristic of the azome-
thine (–HC N) group [11]. Coordination of the Schiff base to the
metal through the nitrogen atom is expected to reduce electron
of the complexes (Table 1) was calculated and compared with
the experimental values and the molar conductivities of com-
plexes have been studied in DMSO of 10−3 M of their solutions
at room temperature. It is concluded from the results that all
complexes are found to have molar conductance values in the
range of 20–25 ohm−1 mol−1 cm2. These low values of conduc-
tance are indicating the non-electrolytic nature of these complexes
[9].
3.1. Spectroscopic studies
3.1.1. Infrared spectra and mode of bonding
density in the azomethine link and lower the ꢁC
absorption
N
In the absence of a powerful technique such as X-ray crystal-
lography, infrared spectra have proven to be the most suitable
technique to give enough information’s to elucidate the nature of
bonding of the ligand to the metal ions. The significant infrared
bands of the Schiff base and their metal complexes are given in
Table 2. The observed bands may be classified into those origi-
nating from the ligand and those arising from the bonds formed
between metal ions and the coordinating sites (Fig. 2: IR spec-
trum of complex II). Infrared spectrum of the ligand, shows a broad
frequency. The band due to ꢁC
is shifted to lower frequencies
N
and appears around 1613–1625 cm−1, indicating coordination of
the azomethine nitrogen to metal ions [12]. The coordination of
the azomethine nitrogen is further supported by the appearance
of bands in the range of 490–540 cm−1 due to ꢁM–N. The N–H
stretching frequency at 3135–2900 cm−1 in the free ligand showed
considerable shift in all the complexes, indicating participation of
this N–H group in complexes [13]. All complexes showed broad
Fig. 2. IR spectrum of complex II.

B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
379
band around 3400 cm−1 due to ꢀ(OH) from water molecules. This
band is absent in the ligand. This has been confirmed with thermal
studies.
The complex I has bands at 1632(s) cm−1 and 1412(s) cm−1
which can be assigned to ꢀas(CO2) and ꢀs(CO2) fundamental
stretching bands respectively, which are in agreement with the
acetate groups being monodentate [14] because the difference
ꢂ [ꢂ = ꢀas(CO2) − ꢀs(CO2)] is 1632–1412 = 220 cm−1. The band at
1412 cm−1 is due to ꢀs(CO2) mode of bonding of acetate. Simi-
larly in complex IV bands at 1620(s) cm−1 and 1418(s) cm−1 have
been assigned to ꢀas(CO2) and ꢀs(CO2) fundamental stretching
bands respectively indicating monodentate coordination of acetate
group.
In addition to the modified slightly on account of coordination,
infrared spectra of the complex II show absorption bands at ca.
1272, 1094, 731, 1384, 965, 1038 cm−1 due to coordinated nitrato
group. These bands are assigned as ꢀ1, ꢀ2, ꢀ3, ꢀ4, ꢀ5, ꢀ6 modes
respectively, and their frequencies are consistent with those asso-
ciated with terminally bonded monodentate nitrato group [15]. In
addition to these two weak bands with a separation of ca. 12 cm−1
appear in the 1800–1700 cm−1 region indicating clearly the exclu-
sive presence of terminal monodentate nitrato group [16].
In complex III, the ꢀ(Sn–Cl) band is observed at 365(s) cm−1
which showed terminal rather than bridging chlorine [14].
3.1.2. 1H NMR spectra and electronic spectra
The 1H NMR spectrum of the ligand and the complexes were
recorded to confirm the binding of the Schiff base to the metal
ions (Fig. 3a and b). The spectra of the complexes showed a sin-
glet in the region ı 7.75–8.52 ppm, which has been assigned to the
azomethine proton (–HC N) (Table 2). The position of the azome-
thine signal in the complexes is downfield in comparison with
that of the free ligand, suggesting deshielding of the azomethine
proton due to its coordination to metal ions through the azome-
thine nitrogen. In the region 6.70–7.90 ppm were assigned chemical
shifts for hydrogen of symmetrical aromatic ring of ligand and
peaks in the region 6.2–6.7 ppm were assigned chemical shift of
pyrrole hydrogen [17]. A new peak at ı 11.76 ppm, characteristic
of intramolecular hydrogen bonded phenolic OH group is disap-
peared in the spectra of the complexes indicating deprotonation
of phenolic proton and confirming coordination through phenolic
oxygen. In complexes the peak in the region of 3.0–3.5 ppm were
assigned for coordinated water and another peak at 4.5 ppm espe-
cially in DMSO solvent (Fig. 3b: 1H NMR spectrum of complex IV)
confirms the hydrogen bonded water molecule [17]. A new peak
in the region ı 2.5–3.0 ppm, characteristic of acetate groups (6H)
present in the spectrum of complexes I and IV is absent in the
spectrum of the ligand. A weak peak in the region ı 3.1–3.3 ppm
in complexes I and IV may be explained with the intramolecu-
lar hydrogen bonding acetate group with the coordinated water
molecule [17].
Fig. 3. (a) 1HNMR spectrum of ligand and (b) 1HNMR spectrum of complex IV.
of molecular ion peaks have been used to confirm the proposed
formula (Fig. 4a–d). The mass spectrum of ligand having peaks at
186{100% m/z} and 145{34% m/z}, respectively, to the molecular
ion peak [M]++1 and [C9H8NO]+, confirming purity of the ligand.
The mass spectra of complex I have a prominent peaks like 329{50%
m/z} showing molecular ion peak, peak at 228/229{74% m/z} is
{[M]+−H2O−ZnO}−1, peak at 187{90% m/z} is [L]++2 and peak at
146{63% m/z} is [C9H8NO]+ respectively, showing usual degrada-
tion pattern. In complex II the degradation pattern again showing
similar trends like peak at 379 {42% m/z} is molecular ion peak
and subsequent peaks are 250{40% m/z} of [M]+−[CdO], 186{36%
m/z} of [LH]+ and 146 {40% m/z} of [C9H8NO]+ respectively. In com-
plex III, the degradation pattern have prominent peaks at 186{100%
m/z} and 145{47% m/z} showing [LH]+ and [C9H8NO]+ respectively.
The complex IV shows degradation peak at 469/470 {35% m/z}
of molecular ion peak, 450 {100% m/z} is {[M]+−H2O}, 388{37%
m/z} is {[M]+−H2O−CH3CO2} and 183{20% m/z} is [LH]+ respec-
tively. Likewise peaks attributable to monomeric species as [M(L)]+
and [HL]+ are usually present in the mass spectra of these sys-
tems [20]. Last two fragments at 185/186 and 146/145 are [LH]+
and [C9H8NO]+ of ligand peaks that appear in all these complexes
showing similar pattern of degradation.
The electronic spectra (Table 2) of the DMSO solutions of the
free ligand, recorded in the 250–800 nm exhibit bands in the range
250–280 and 300–340 nm assigned to the ␲ → ␲* and n → ␲* tran-
sitions respectively of the azomethine group and it is shifted to
longer wavelength on coordination through azomethine nitrogen
in the complexes [18].
3.1.3. TOF-mass spectra
Mass spectrometry has been successfully used to investigate
molecular species in solution [19]. The pattern of mass spectrum
gives an impression of the successive degradation of the target
compound with the series of peaks corresponding to the various
fragments. Their intensity gives an idea of stability of fragments.
The recorded mass spectra of the ligand and their metal complexes
3.2. Thermal decomposition kinetics
Recently, there has been increasing interest in determining the
rate-dependent parameters of solid-state non-isothermal decom-
position reactions by analysis of TG curves [21]. Thermogravimetric

380
B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
(TG) and differential thermogravimetric analysis (DTA) were car-
ried out for Zn(II), Cd(II), Sn(II) and Pb(II)–Schiff base complexes
in ambient condition (Fig. 5a–d). The correlations between the
different decomposition steps of the complexes with the corre-
sponding weight losses are reported in Table 3.The final product
of decomposition at 810 K corresponds to the formation of metal
oxide as end product, which was confirmed by comparing the
observed/estimated and the calculated mass of the pyrolysis prod-
uct.
The kinetic analysis parameters such as activation energy (ꢃE*),
enthalpy of activation (ꢃH*), entropy of activation (ꢃS*), free
energy change of decomposition (ꢃG*) were evaluated graphically
Fig. 4. (a) Mass spectrum of ligand; (b) mass spectrum of complex I; (c) mass spectrum of complex II and (d) mass spectrum of complex IV.

B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
381
Fig. 5. (a) TG/DTA curve of complex I; (b) TG/DTA curve of complex II; (c) TG/DTA curve of complex III and (d) TG/DTA curve of complex IV.
by employing the Coats–Redfern relation [22].
The entropy of activation (ꢃS*) and the free energy change of
activation (ꢃG*) were calculated using Eqs. (2) and (3):
ꢀ
ꢁ
ꢂꢃ
ꢀ
ꢃ
ꢀ
ꢁ
ꢂꢃ
1 − ˛
AR
E∗
Ah
kT
ꢃS∗ (JK−1 mol−1) = 2.303R log
(2)
(3)
log − log
= log
−
(1)
ꢄE∗(1 − 2RT/E∗)
T2
2.303RT
ꢃG∗(J mol−1) = ꢃH∗ − TꢃS∗
where ˛ is the mass loss up to the temperature T, R the gas constant,
E* the activation energy in J mol−1, ꢄ the linear heating rate and
(1 − 2RT/E*) ≈ 1. A plot of left hand side of Eq. (1) against 1/T gives a
slope from which E* was calculated and A (Arrhenius constant) was
determined from the intercept. From relevant data, linearization
plots confirm first order kinetics. It has been found that E* values
for complexes > ligand.
where k and h are the Boltzman and Plank constants, respectively.
The calculated values of ꢃE*, A, ꢃS* and ꢃG* for the decomposi-
tion steps of the complexes are given in Table 4. According to the
kinetic data obtained from the TG curves, all the complexes have
negative entropy, which indicates that the complexes are formed
spontaneously. The negative entropy also indicates a more ordered
Table 3
Thermo analytical data.
Complex
Step
TGrange/K
DTAmax/K thermal effect
Massloss cal (obs) (%)
Assignment
Metallic residue
I
I
II
III
318–348
413–448
553–683
333 Endo
428 Endo
623 Exo
5.48 (5.47)
17.97 (17.95)
41.18 (41.20)
H2O
CH4 + CO2
Organic moiety
ZnO
CdO
II
I
II
315–363
473–698
338 Endo
568 Exo
21.16 (21.19)
44.98 (44.95)
H2O + [NO2 + O]
Organic moiety
III
I
II
III
303–343
423–453
493–633
328 Endo
445 Endo
538 Exo
9.57 (9.60)
9.70 (9.72)
39.61 (39.60)
2H2O
HCl
Organic moiety
SnO
PbO
IV
I
II
III
303–343
413–449
553–673
328 Endo
433 Endo
603 Exo
3.83 (3.80)
12.55 (12.59)
36.15 (36.18)
H2O + [NO2 + O]
CH4 + CO2
Organic moiety

382
B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
Table 4
Thermodynamic activation parameters.
Complex
Order
1
Step
E* (J mol−1
)
A × 105 (s−1
)
ꢃS* (J K−1 mol−1
)
ꢃH* (J mol−1
)
ꢃG*(kJ mol−1
)
I
I
41.10
39.96
41.55
55.74
53.24
60.06
79.00
60.52
50.96
47.78
47.78
1.35
0.99
0.73
3.61
1.93
3.51
4.34
2.14
3.08
2.05
1.47
−147.37
−152.02
−157.67
−139.32
−148.83
−139.31
−140.13
−147.52
−140.39
−146.08
−151.59
72.02
98.64
243.89
143.32
476.89
98.22
182.67
317.81
142.52
159.06
572.34
49.14
65.16
98.47
47.23
85.01
45.79
62.96
79.68
46.18
63.41
91.98
II
III
I
II
I
II
II
I
II
III
II
1
1
III
IV
1
activated state that may be possible through the chemisorptions of
oxygen and other decomposition products. The negative values of
the entropies of activation are compensated by the values of the
enthalpies of activation, leading to almost the same values for the
free energies of activation [23].
phenol moiety and its metal complexes were determined against
two bacterium under different concentrations, as described in
Section 2. The agar well-diffusion method was employed for the
bacteria with respect to chloramphenicol as standard drug. The
results showed that some compounds are very effective on some
microorganisms (Fig. 7a and b). The ligand and complexes I and
II exhibited high activity against E. coli (100 ppm) and others
show very effective against S. aureus (100 ppm) bacteria. The zinc
complex showed better activity than other metal complexes for
both microorganisms. The activity of any compound is a complex
combination of steric, electronic and pharmacokinetic factors. A
possible explanation for the toxicity of the complexes has been
postulated in the light of chelation theory. The presence of N and
O donor groups in the ligand and its metal complexes inhibited
enzyme production because enzymes that require free hydroxyl
group for their activity appear to be especially susceptible to
deactivation by the metal ion of complexes. Chelation reduces the
polarity of the central ion from partial sharing of its positive charge
with the donor groups; ␲-electron delocalization in this chelating
3.3. Molecular structure and analysis of bonding modes
Molecular mechanics attempt to reproduce molecular geome-
tries, energies and other features by adjusting bond length, bond
angles and torsion angles to equilibrium values that are depen-
dent on the hybridization of an atom and its bonding scheme. In
order to obtain estimates of structural details of these complexes,
we have optimized the molecular structure of complexes. Energy
minimization was repeated several times to find the global mini-
mum. The energy minimization value for tetrahedral and without
restricting the structure for the complex II is almost same i.e.,
32.674 kcal mol−1. This supports tetrahedral geometry of the metal
complex [24]. The optimized molecular structure and bond length
of complex II are represented in Fig. 6.
3.4. Biological studies
The bacteriological effect of 2-aminophenol-pyrrole-2-
carboxaldehyde containing pyrrole-2-carboxaldehyde and 2-amino
Fig. 7. (a) Effect of different concentrations of ligand and complexes with Escherichia
coli and (b) effect of concentration of ligand and complexes with Staphylococcus
aureus.
Fig. 6. Optimized structure of complex II.

B.K. Singh et al. / Spectrochimica Acta Part A 76 (2010) 376–383
383
ring also increases the lipophilic nature of the central atom,
favoring permeation through the lipid layer of the membrane [25].
All the metal complexes are more toxic than the ligand.
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A new bioactive 2-aminophenol-pyrrole-2-carboxaldehyde Schiff
base as deprotonation and coordination of phenolic oxygen with
metal ions along with binding with pyrrole nitrogen, azomethine
nitrogen and anion with tetrahedral geometry were prepared and
their structures were determined with spectroscopic and ther-
mal studies. Molecular modeling has been used to optimize the
structure of the metal complexes and its bond length has been
determined.
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industrial Research, New Delhi, India for financial assistance under
research grant scheme.
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