Synthesis, spectroscopic characterization, electrochemical behaviour, reactivity and antibacterial activity of some transition metal complexes with 2-(N-salicylideneamino)-3-carboxyethyl-4,5-dimethylthiophene

Article abstract of DOI:10.1016/j.saa.2007.11.003

Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with a potentially tridentate Schiff base, formed by condensation of 2-amino-3-carboxyethyl-4,5-dimethylthiophene with salicylaldehyde were synthesized and characterized on the basis of elemental analyses, molar conductance values, magnetic susceptibility measurements, UV-vis, IR, EPR and NMR spectral data, wherever possible and applicable. Spectral studies reveal that the free ligand exists in a bifunctionally hydrogen bonded manner and coordinates to the metal ion in a tridentate fashion through the deprotonated phenolate oxygen, azomethine nitrogen and ester carbonyl group. On the basis of electronic spectral data and magnetic susceptibility measurements, suitable geometry has been proposed for each complex. The EPR spectral data of the Cu(II) complex showed that the metal-ligand bonds have considerable covalent character. The Ni(II) complex has undergone facile transesterification reaction when refluxed in methanol for a lengthy period. X-ray diffraction studies of Cu(II) complex showed that the complex has an orthorhombic crystal lattice. In view of the biological activity of thiophene derivatives, the ligand and the complexes were subjected to antibacterial screening. It has been observed that the antibacterial activity of the ligand increased on chelation with metal ion.

Full text of DOI:10.1016/j.saa.2007.11.003

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 403–410
Synthesis, spectroscopic characterization, electrochemical behaviour,
reactivity and antibacterial activity of some transition metal complexes
with 2-(N-salicylideneamino)-3-carboxyethyl-4,5-dimethylthiophene
Varughese P. Daniel, B. Murukan, B. Sindhu Kumari, K. Mohanan∗
Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum 695581, Kerala, India
Received 23 January 2007; accepted 9 November 2007
Abstract
Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with a potentially tridentate Schiff base, formed by condensation of 2-amino-3-
carboxyethyl-4,5-dimethylthiophene withsalicylaldehydeweresynthesizedand characterizedon the basis ofelementalanalyses, molar conductance
values, magnetic susceptibility measurements, UV–vis, IR, EPR and NMR spectral data, wherever possible and applicable. Spectral studies reveal
that the free ligand exists in a bifunctionally hydrogen bonded manner and coordinates to the metal ion in a tridentate fashion through the
deprotonated phenolate oxygen, azomethine nitrogen and ester carbonyl group. On the basis of electronic spectral data and magnetic susceptibility
measurements, suitable geometry has been proposed for each complex. The EPR spectral data of the Cu(II) complex showed that the metal–ligand
bonds have considerable covalent character. The Ni(II) complex has undergone facile transesterification reaction when refluxed in methanol for a
lengthy period. X-ray diffraction studies of Cu(II) complex showed that the complex has an orthorhombic crystal lattice. In view of the biological
activity of thiophene derivatives, the ligand and the complexes were subjected to antibacterial screening. It has been observed that the antibacterial
activity of the ligand increased on chelation with metal ion.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Metal complexes; 2-(N-salicylideneamino)-3-carboxyethyl-4,5-dimethylthiophene; EPR spectrum; Cyclic voltammetry; Reactivity; XRD; Antibacterial
activity
1. Introduction
and two methyl groups [8]. The resulting compound, namely
2-amino-3-carboxyethyl-4,5-dimethylthiophene has been con-
densed with salicylaldehyde to form a potentially tridentate
Schiff base, viz., 2-(N-salicylideneamino)-3-carboxyethyl-4,5-
dimethylthiophene (Hsat) containing an ONO donor sequence.
Apart from providing stability to 2-aminothiophene, introduc-
tion of a carboxyethyl group at 3-position of the thiophene ring
has provided further scope for reactivity and a new coordination
site. Inadditiontothis, highlysubstitutedthiopheneshaveshown
extensive potential in pharmaceutical industry. The Schiff base
obtained has been versatile in forming a series of complexes with
Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) ions under well
defined conditions and these complexes have been investigated
with particular reference to the structural aspects of the ligand
moiety in the metal complexes. Besides the structural diversities
and bonding interactions, bioisosteric relationship of thiophene
to benzene has led to several structures of drug analogs in which
benzene rings have been replaced by thiophene rings and the
vivid applications of thiophene derivatives as important thera-
peutic agents have been well documented in literature [7]. It is
Schiff bases form an interesting class of chelating ligands
that has enjoyed popular use in the coordination chemistry
of transition, inner-transition and main group elements [1–5].
Among the prodigious number and variety of Schiff base com-
plexes, those derived from salicylaldimines form the major
part, mainly because of their synthetic proclivity and struc-
tural diversities. However, a deep survey of literature on
salicylaldimine complexes reveal that metal chelates of sal-
icylaldimines derived from heterocyclic systems particularly
those from aminothiophenes have been largely ignored [6].
Most probably the instability of aminothiophenes is mainly
responsible for this omission [7]. But in this investigation, 2-
aminothiophene has been made stable by suitable substitution at
the remaining positions of the thiophene ring by an ester group
∗
Corresponding author. Tel.: +91 471 241 8782.
E-mail address: drkmohanan@rediffmail.com (K. Mohanan).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.11.003

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V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
expected that metal chelates containing thiophene ring system
wouldcarrybioisostericrelationshipandtherapeuticvalues, thus
introducing another class of metal-based thiophene derivatives.
In this investigation we report the synthesis, spectroscopic char-
acterization, electrochemical behaviour, X-ray diffraction study,
reactivity and antibacterial activity of some 3d metal complexes
with a host of the title ligand.
2.4. Antibacterial screening
Antibacterial activity of the ligand and the metal complexes
were examined by agar diffusion method using streptomycin as
standard [9].
The minimum inhibitory concentration of the ligand and
complexes were ascertained using different bacteria. The con-
centration of the drug solution was maintained to be 200 ␮g/ml
in DMSO. One day prior to the test, the bacteria was inoculated
in a nutrient broth (inoculation medium) and kept in an incuba-
tor at 37 ◦C for 24 h. The hot nutrient agar solution (20 ml) was
transferred into sterilized petri dishes and allowed to attain room
temperature. The seed layer medium was melted and cooled to
∼45 ◦C with gentle shaking. The previously grown subculture
was added to the seed layer medium aseptically and mixed well.
It was immediately raked into the petri dishes and allowed to
attain room temperature. Using a sterile cork borer, small wells
were made in the agar medium. Then the drug solution (0.05 ml)
was added and the petri dishes were allowed to cool in air to facil-
itate diffusion. The petri dishes were then kept in an incubator
at 37 ◦C for about 48 h. On completion of the incubation period,
the zones of inhibition around the wells were measured.
2. Experimental
2.1. Materials and methods
All the chemicals used were of Analytical grade. Commercial
solvents were distilled and used for synthesis. For physico-
chemical measurements, the solvents were purified by standard
methods. Carbon, hydrogen, nitrogen and sulphur analyses were
performed using Elementar Systeme, Vario EL III CHN anal-
yser and the metal contents in the complexes were determined
using a Nulab GBC 902 atomic absorption spectrophotometer in
an acetylene/air flame. Molar conductance measurements were
conducted using 10−3 M solutions of the complexes in appro-
priate solvents at room temperature with a Systronic model 304
digital conductivity meter. Infrared spectral studies were car-
ried out using KBr discs on a Thermo-Nicolet Avatar 370 FT-IR
spectrophotometer and electronic spectra were recorded on a
Varian Cary 5000 UV–vis–NIR spectrophotometer. Far IR spec-
tra were recorded on a polytec FIR 30 Fourier spectrometer
using CsI discs. The proton NMR spectra of the ligand and the
zinc(II) complex were recorded on a JEOL GSX 400 MHz FT
NMR spectrometer employing TMS as internal reference and
CDCl3 as solvent. The EPR spectrum of the copper(II) complex
was recorded using a Varian-112 EPR spectrometer employing
DPPH as reference material. X-ray diffraction experiments were
carried out on a Siemens D 5005 model spectrometer. Cyclic
voltammetric study of copper(II) complex was performed with
a BAS CV-50 analyser.
2.5. Transesterification
Transesterification was carried out using a reported method
[10]. About 0.2 g of [Ni(sat)Cl] was dissolved in methanol
(100 ml) and refluxed for 72 h on a water-bath. The result-
ing solution was evaporated to dryness and the solid product
obtained was washed repeatedly with ether and dried over P2O5
in vacuum.
3. Results and discussion
Analytical data indicated that salicylaldehyde condensed
with 2-amino-3-carboxyethyl-4,5-dimethylthiophene in 1:1
molar ratio and the product formed well defined complexes with
the metal salts. Formation of the complexes can be symbolized
as follows
2.2. Synthesis of ligand
The starting material for the synthesis of the ligand, 2-
amino-3-carboxyethyl-4,5-dimethylthiophene, was prepared by
Gewald synthesis [8]. The ligand was prepared by condensing
salicylaldehyde with an ethanolic solution of 2-amino-3-
carboxyethyl-4,5-dimethylthiophene in 1:1 molar ratio (78%
yield, m.p. 130 ◦C).
MX2 + Hsat → [M(sat)X] + HX
M = Ni(II), Cu(II) or Zn(II); X = CI or NO3.
MX2 + 2Hsat → [M(sat)2] + 2HX
2.3. Preparation of metal complexes
M = Mn(II), Fe(II) or Co(II); X = CI or NO3; Hsat = 2-(N-
salicylideneamino)-3-carboxyethyl-4,5-dimethylthiophene.
Formulation of the complexes has been based on their ele-
mental analytical data, molar conductance values and magnetic
susceptibility data. Mn(II), Fe(II) and Co(II) complexes showed
1:2 metal–ligand stoichiometry while the other complexes
exhibited 1:1 metal–ligand ratio (Table 1). All the complexes are
non-hygroscopic, decomposed above 250 ◦C and possess good
keeping qualities. The molar conductance values adequately
support the non-electrolytic nature of the metal complexes [11].
The metal complexes were prepared by the following gen-
eral procedure. To a hot magnetically stirred ethanolic solution
(60 ml) of the ligand, added aqueous/ethanolic solution of
metal(II) salts in appropriate ratios. The pH of the solution was
adjusted to 6.5–7.0 and refluxed on a water-bath for 4–6 h. Then
the volume of the solution was reduced to half the initial volume
by evaporating on a water-bath. On cooling, the complex sepa-
rated was filtered, dried and further purified by recrystallization
from hot ethanol.

V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
405
Table 1
Analytical data and other details of the metal complexes
Complex
Molecular mass
Yield (%)
Analytical dataa
M
Molar conductance
(ꢀ−1 cm2 mol−1) DMSO
C
H
N
S
[Mn(sat)2]
[Fe(sat)2]
[Co(sat)2]
[Ni(sat)Cl]
[Cu(sat)Cl]
[Zn(sat)Cl]
659
660
663
396
401
403
74
70
75
72
76
73
8.5 (8.3)
8.7 (8.5)
8.7 (8.9)
14.7 (14.8)
15.7 (15.8)
16.0 (16.2)
58.1 (58.3)
58.4 (58.1)
57.7 (57.9)
48.6 (48.4)
47.7 (47.9)
47.5 (47.6)
4.8 (4.9)
4.9 (4.8)
4.7 (4.8)
4.1 (4.0)
3.8 (3.9)
4.0 (3.9)
4.3 (4.2)
4.4 (4.2)
4.1 (4.2)
3.6 (3.5)
3.3 (3.5)
3.7 (3.5)
9.8 (9.7)
9.8 (9.7)
9.6 (9.7)
8.2 (8.1)
7.8 (7.9)
7.9 (8.0)
9.8
9.6
9.8
8.6
9.4
8.8
a
Calculated values are given in bracket.
*
*
3.1. Structure of the ligand
phenolimine form (␲ → ␲ and n → ␲ transitions). On the
basis of the above spectral data, a phenolimine structural form
of the ligand has been established (Fig. 1a). It has been reported
that some amino acid Schiff bases derived from 2-hydroxy-1-
naphthaldehyde exists predominantly in the quinoneamine form
and the ligand is coordinated to the metal ion through the azome-
thine nitrogen, one of the carboxylate oxygen atoms and the
quinone oxygen [14].
On the basis of UV, IR and proton NMR spectral studies, a
phenolimine structure has been established for the ligand. Intro-
duction of a hydroxy group at ortho position to the azomethine
linkage raises the possibility of phenolimine–quinoneamine tau-
tomerism. A bulk of information concerning this could be
obtained from ultraviolet spectral studies, a method well suited
for such investigation. It has been reported that the tautomeric
equilibrium depends on the extent of conjugation, nature and
position of the substituent, polarity of the solvent and this phe-
nomenon has drawn considerable attention both from theoretical
and experimental point of view [12]. Ultraviolet spectral studies
on N-salicylidenebutylamine gave some important conclusion
relating to this. Ultraviolet spectrum of the above compound in
ethanol exhibited strong bands at 253 and 312 nm for the phe-
nolimine form (assignable for ␲ → ␲ and n → ␲ transitions
respectively) and much weaker bands at 278 and 401 nm for the
quinoneamine form [13]. Spectra of compounds which cannot
acquire the quinonoid form, for example, 3-hydroxybenzilidene
bases lack the ca. 400 nm band altogether and systems which
assume the quinonoid form show a prominent band at ca.
400 nm. Recently this phenomenon was examined in detail by
Antonov et al., who explained this type of tautomerism exhib-
ited by 2-hydroxy-1-naphthaldehyde Schiff base and arrived at
the conclusion that the compound exists in the quinonoid form
[12]. The ligand under investigation is also capable of exhibit-
ing phenolimine–quinoneamine tautomerism (Fig. 1a and b).
However, the ultraviolet spectrum of the ligand (recorded in
ethanol) gave characteristic bands at 255 and 315 nm for the
In agreement with UV spectral data, infrared spectrum
of the ligand exhibited a broad band spreading well over
the region 3300–3000 cm−1 and centred at 3200 cm−1, which
can be attributed to hydrogen bonded phenolic ν(O H) and
ν(C O) has been observed at 1287 cm−1. A strong band appear-
ing at 1702 cm−1 is due to the ester carbonyl group and
another medium intensity band appearing at 1630 cm−1 can be
assignable to ν(C N) of the azomethine group. However vibra-
tions characteristic of the substituted thiophene ring have been
observed at 1530, 1425 and 1360 cm−1 [15]. The ester car-
bonyl group is also involved in weak hydrogen bonding with
the phenolic OH of the salicylaldehyde moiety forming a sort
of bifunctional hydrogen bonding [16]. However, in competi-
tion with azomethine group, for phenolic OH, ester carbonyl
can manage only a meagre share. This explains why the ester
carbonyl frequency in the ligand is relatively higher than that
in the free amine (1660 cm−1). Above spectral data adequately
support the formulation of the ligand as in Fig. 1a. The proton
magnetic resonance spectral data of the ligand adequately sup-
port the facts drawn on the basis of UV and IR spectral data. The
proton NMR spectrum of the ligand recorded in CDCl3 (Fig. 2)
exhibited a signal at 12.81δ. The downward shift of the proton is
*
*
Fig. 1. (a and b) Tautomeric structures of the ligand.

406
V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
Fig. 2. NMR spectrum of the ligand.
presumably due to strong internal hydrogen bonding [17]. Signal
for the hydrogen of the azomethine group has been observed at
8.89δ. Signals appearing at 1.50δ and 2.60δ can be attributed to
methyl protons and methylene protons respectively of the ester
group. On the basis of the spectral values, an internally hydrogen
bonded phenolimine structure has been proposed for the ligand.
the azomethine nitrogen in coordination with metal ion. The
decrease in ester carbonyl frequency ν(C O) by ca. 40–45 cm−1
in the metal complexes give adequate evidence for chelation by
the ester carbonyl group. This type of coordination by ester car-
bonyl group has been already reported by several investigators
[18,19]. The vibrational characteristics of substituted thiophene
ring of the ligand remains almost unaffected in the metal com-
plexes. This observation rules out the possibility of coordination
by the ring sulphur atom. Thus the ligand acted as monobasic tri-
dentate, bonding to the metal ion through the phenolate oxygen,
azomethine nitrogen and ester carbonyl group (Figs. 3 and 4).
Apart from these bands, the non-ligand bands of low intensity
appearing in the region 540–550, 440–450 and 350–360 cm−1
can be assigned to ν(M O), ν(M N) and ν(M Cl) vibrations
respectively [20,21]. Absence of ν(M S) bands in the far IR
spectra gives added evidence for non-participation of ring sul-
phur atom in bond formation. The above mode of bonding
suggested by infrared spectral studies is reinforced by the pro-
ton NMR spectral study of the zinc(II) complex. In the spectrum
of Zn(II) complex, the absence of OH proton signal is a clear
indication that phenolic oxygen is bonded to the metal ion after
deprotonation. A careful comparison of the position of other
3.2. Structure of the metal complexes
The characteristic ultraviolet spectral bands for the pheno-
limine form of the ligand is only marginally red shifted in the
relevant regions indicating that the phenolimine structural form
of the ligand persists in the metal complexes also.
The diagnostic infrared spectral data of the complexes
are presented in Table 2. The broad band due to internally
hydrogen-bonded phenolic OH group disappear from the
region 3300–3000 cm−1 indicating deprotonation and formation
of metal–oxygen bond. Consequently the band due to ν(C O)
is increased by ca. 35–40 cm−1 in the metal complexes and this
adequately supported the bond formation by phenolate oxygen.
The azomethine stretching frequency ν(C N) is shifted to lower
frequency by ca. 20–30 cm−1 that indicates the involvement of
Table 2
Infrared and far infrared spectral data of the ligand and metal complexes
Hsat
[Mn(sat)2]
[Fe(sat)2]
[Co(sat)2]
[Ni(sat)Cl]
[Cu(sat)Cl]
[Zn(sat)Cl]
Tentative assignments
3300–3000b
1702s
1630m
1530m
1425m
1360m
1287s
–
–
1662s
1606m
1532m
1427m
1362m
1322s
542w
448w
–
–
1660s
1608m
1528m
1424m
1361m
1327s
549w
442w
–
–
1658s
1610m
1530m
1426m
1358m
1325s
548w
449w
–
–
1660s
1605m
1532m
1427m
1362m
1323s
546w
–
1657s
1607m
1531m
1426m
1359m
1327s
547w
–
1659s
1608m
1529m
1425m
1361m
1326s
550w
Hydrogen bonded ␯(OH)
␯(C O)
␯(C N)
Substituted thiophene ring
Substituted thiophene ring
Substituted thiophene ring
␯(C O)
␯(M O)
␯(M N)
␯(M Cl)
–
–
450w
351w
446w
358w
444w
360w

V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
407
Fig. 3. Structure of the 1:1 metal complexes.
signals in this complex with those of the ligand indicates a
downward shift of the other protons by about 0.10–0.25δ in the
metal complexes. Thus the azomethine proton signal observed
at 8.89δ in the free ligand is shifted down field by 0.23δ. The
positions of the other proton signals have been also observed
in the expected regions and have been shifted only slightly as a
result of metallation.
Fig. 4. Structure of the 1:2 metal complexes.
adequate support to an octahedral environment around the Co(II)
ion [24]. The nickel(II) and zinc(II) complexes were found to
be diamagnetic. The electronic spectral data of the nickel(II)
complex are consistent with a square planar geometry around
the metal ion [25]. However, for the zinc(II) complex an tetra-
hedral geometry has been presumed. It has been reported that
for a tetracoordinated zinc(II) complexes, tetrahedral geometry
is the most preferred one [26]. The copper(II) complex having
a magnetic moment value of 1.88 BM, a broad d–d band cen-
tred at 13,850 cm−1, which supports a distorted square planar
geometry around the metal ion [27].
3.3. Electronic spectra
Visible spectral data along with magnetic susceptibility mea-
surements gave adequate support in establishing the geometry
of the metal complexes. These data along with the tentative
assignments are presented in Table 3. Electronic spectrum of
manganese(II) complex exhibited three peaks in the visible
region. These data along with the magnetic moment values are
compatible with an octahedral geometry around the metal ion
[22]. However, spectrum of the iron(II) complex showed an
3.4. EPR spectral study
5
5
absorption band at 11,050 cm−1 characteristic of T2g → Eg
transition in an octahedral environment [23]. Cobalt(II) complex
exhibited two low energy bands at 7350 and 17,200 cm−1 and a
high energy band at 20,500 cm−1. These observations together
with the magnetic moment value of the cobalt(II) complex give
The X-band EPR spectrum of copper(II) complex has been
recorded in the solid state at room temperature and also in
DMSO at 77 K using DPPH as the ‘g’ marker (Fig. 5). The
solution spectrum of the complex possessed well-resolved g
||
Table 3
Electronic spectral data and magnetic moment values of the metal complexes
Complex
Absorption bands (cm−1
)
Tentative assignments
Magnetic moment (BM)
5.94
[Mn(sat)2]
14,000
16,500
19,500
6A1g
6A1g
6A1g
→
→
→
→
4T1g
4T2g
4Eg, 4A1g
[Fe(sat)2]
[Co(sat)2]
11,050
5T2g
5Eg
5.20
4.84
4
7,350
17,200
20,500
4T1g(F) → T2g(F)
4T1g(F) → A2g(F)
4
4T1g(F) → T1g(P)
4
[Ni(sat)Cl]
[Cu(sat)Cl]
13,500
18,800
1A1g
1A1g
→
1A2g
1B1g
D
→
13,850
2B1g
→
2A1g
1.88

408
V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
dination. Based on this observation, a distorted square planar
geometry is proposed for the complex. The EPR study of the
copper(II) complex has provided supportive evidence to the
conclusion obtained on the basis of electronic spectrum and
magnetic moment value.
3.5. X-ray diffraction
X-ray diffraction study on [Cu(sat)Cl] has been carried out
and the data obtained are presented in Table 4. The X-ray
diffraction pattern of the complex indicates high crystallinity
of the complex. The diffraction study recorded 15 reflections
between2θ rangingfrom15◦ to60◦ withmaximaat2θ = 16.1535
˚
which corresponds to interplanar distance d = 5.4824 A. The
main peaks have been indexed and the sin2θ and 2θ values
obtained have been compared with calculated values [32,33]. A
comparison of these values revealed good agreement between
calculated and observed values of sin2θ and 2θ. The complex
was successfully indexed to orthorhombic crystal systems with
Fig. 5. EPR spectrum of [Cu(sat)Cl].
and a broad g regions. The various Hamiltonian parameters
⊥
have been calculated for this complex (g = 2.2350, g = 2.1140,
||
⊥
A = 170). It has been reported that g value of copper(II)
||
||
˚
˚
˚
the lattice constants; a = 10.3461 A, b = 6.9821 A, c = 4.2850 A
complex can be used as a measure of the covalent character
of the metal–ligand bond. If the value is more than 2.3, the
metal–ligand bond is essentially ionic and the value less than
2.3 is indicative of covalent character [28]. Apart from this, the
covalency parameter (α2) has been calculated using Kivelson
and Neiman equation [29]. The covalency parameter (α2 = 0.79)
indicates considerable covalent character for the metal–ligand
bond [30]. Also the trend g > g > ge observed for this com-
3
˚
and unit cell volume 309.5377 A . Although single crystal X-
ray crystallographic investigation is the most precise source of
information regarding the structure of a complex, the difficulty
of obtaining crystalline complexes in proper symmetric form
has rendered this method unsuitable for such a study. However,
a variety of other spectroscopic techniques could be used with
good effect for characterizing the metal complexes.
||
⊥
plex indicated that the unpaired electron is most likely in the
dx2−y2 orbital. The empirical ratio g /A is frequently used to
3.6. Cyclic voltammetry
|| ||
evaluate distortions in tetracoordinated copper(II) complexes
[31]. The ratio close to 100, indicates a roughly square pla-
nar structure around the copper(II) ion and the values from
170 to 250 are indicative of a distorted tetrahedral geometry.
The four peaks in the spectrum are evidently due to the cou-
pling of the electron spin of the 63Cu nucleus (I = 3/2). The
peaks are broad and have the appearance of ill-resolved triplets.
The breadth and triplet appearance can be attributed to hyper-
fine splitting by the nitrogen atom (I = 1) of the ligand. The
triplet appearance is adduced as an evidence for nitrogen coor-
The copper(II) complex, [Cu(sat)Cl] has been subjected to
cyclic voltammetric studies with a view to examine its elec-
trochemical behaviour. A glassy carbon was used as working
electrode, Ag/AgCl as reference electrode and platinum wire as
auxiliary electrode. All the measurements were carried out using
a 2 mM solution at room temperature in the potential range −1.5
to +1.5 V with a scan rate 80 mV s−1. The solution was degassed
with argon and kept under argon atmosphere throughout the
experiment.
Table 4
X-ray diffraction of [Cu(sat)Cl]
Observed sin2θ
Calculated sin2θ
h k l
Observed 2θ
Calculated 2θ
˚
Peak no.
d (A)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
5.4824
5.0430
4.3111
3.9253
3.7687
3.5481
3.2689
3.0391
2.8310
2.5878
2.3472
2.2635
2.2270
2.0012
1.6184
0.0197
0.0233
0.0319
0.0385
0.0417
0.0471
0.0552
0.0642
0.0740
0.0885
0.1076
0.1157
0.1196
0.1481
0.2265
0.0177
0.0221
0.0343
0.0378
0.0444
0.0486
0.0544
0.0620
0.0809
0.0886
0.1095
0.1150
0.1210
0.1473
0.2270
1 1 0
2 0 0
2 1 0
1 0 1
0 1 1
0 2 0
2 0 1
3 1 0
0 2 1
4 0 0
0 3 0
1 3 0
4 0 1
1 3 1
0 4 1
16.1535
17.5715
20.5848
22.6336
23.5871
25.0766
27.2581
29.3635
31.5765
34.6330
38.3149
39.7893
40.4698
45.2759
56.8415
15.2968
17.1266
21.3594
22.4387
24.3516
25.4937
26.9970
28.8502
33.0698
34.6516
38.6549
39.6608
40.7119
45.1534
56.9136

V.P. Daniel et al. / Spectrochimica Acta Part A 70 (2008) 403–410
409
Table 5
The CV profile displayed two waves at Epc values −0.11 and
Antibacterial study of the ligand and metal complexes
+0.13 V corresponding to the reversible reduction of the metal
complex and irreversible reduction of the ligand respectively.
Also the copper(II) complex has undergone oxidation process
at Epa = −0.18 V. The separation between the peak potential
(ꢁEp) is nearly close to 70 mV indicate one electron transfer
in the electrode reaction and the observed reaction voltage of
the complex is lower than that of the ligand and the ratio of the
anodic to cathodic peak current (ipa/ipc) nearly equal to one.
From these observations it is concluded that the redox process
is diffusion controlled and demetallation of copper(II) complex
on the electrode is not involved [34].
Compound
Zone of inhibition (mm)
S. aureus
Alpha-H. S. cocci
Hsat
14
21
23
22
25
27
24
30
12
20
22
19
21
23
24
27
[Mn(sat)2]
[Fe(sat)2]
[Co(sat)2]
[Ni(sat)Cl]
[Cu(sat)Cl]
[Zn(sat)Cl]
Standard
3.7. Transesterification
electron delocalization over the whole chelate ring. Lipids and
polysaccharides are some important constituents of cell walls
and membranes, which are preferred for metal ion interaction.
Reduction in polarity in turn increases the lipophilic character of
thechelateandtheinteractionbetweenthemetalionandthelipid
isfavoured. Thismayresultinbreakingdownofthepermeability
barrier of the cell and interference with the normal cell processes
[36]. Ifthegeometryandchargedistributionaroundthemolecule
are incompatible with those around the pores of the bacterial cell
wall, penetration through the wall by the toxic agent cannot takes
place and this in turn prevent the toxic reaction within the pores
[36]. The presence of co-ligand also plays a decisive role in
determining the antibacterial property of metal complexes [37].
Nature of the metal ion, nature of the ligand, coordinating sites,
geometry of the complex, hydrophilicity, lipophilicity, presence
of co-ligand, pharmacokinetic factors etc also play decisive roles
in determining the antibacterial activity of Schiff bases and their
metal complexes. The mode of action of the complexes also
indulge in the formation of hydrogen bonded interaction through
the coordinated anion, azomethine group etc with the active cen-
ters of the cell constituents resulting in interference with the
normal cell processes [20].
Transesterification reaction effect interchange of ester frag-
ment on the substituent group attached to the coordinated
azomethine group. There are several reports that metal chelates
of carboxylic esters undergo facile transesterification on
refluxing with alcohol for lengthy periods. Two types of trans-
esterification reactions are reported; those in which the ester
group is coordinated to the metal ion and the other type involves
interexchange reactions of non-coordinated ester group. The
complex [Ni(sat)Cl] has been subjected to transesterification
reaction in methanol medium using a reported method [10].
The crystallinity, appearance and the solubility behaviour of the
product obtained after transesterification have been distinctly
different from those of [Ni(sat)Cl]. Apart from these observa-
tions, the ester carbonyl stretching frequency observed for the
methyl derivative at 1652 cm−1 is a direct indication for the
occurrence of transesterification. Substitution of ethyl group by
methyl group is further confirmed by proton NMR spectrum
of the nickel(II) complex. Several mechanisms have been pro-
posed for transesterification reactions. However it appears that
increased nucleophilicity of the acyl carbon atom induced by
the azomethine group is of prime importance. It has been also
reported that alkoxycarbonyl group immediately attached the
␣-carbon atom can be transesterified easily [10]. In view of
the difficulty in preparing metal chelates of esters, this general
method of synthesis by transesterification should prove benefi-
cial.
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