Synthesis, characterization, and antibacterial studies of oxovanadium(IV) complexes with thiazole-derived schiff bases

Article abstract of DOI:10.1080/10426500903403099

Oxovanadium(IV) complexes of Schiff bases, derived from 2-amino-4-phenyl thiazole/substituted 2-amino-4-phenyl thiazoles and thiophene-2-aldehyde have been synthesized and characterized on the basis of elemental analysis, molar conductance measurements, magnetic susceptibility data, and UV-visible, and IR spectral studies. All the complexes are monomeric possessing a 1:2 (metal:ligand) stoichiometry. On the basis of these data, a square pyramidal geometry has been assigned for the complexes. A few complexes have been subjected to thermal decomposition studies. The ligands and their metal complexes have been screened for their antibacterial activities. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. Copyright

Full text of DOI:10.1080/10426500903403099

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Synthesis, Characterization, and
Antibacterial Studies of Oxovanadium(IV)
Complexes with Thiazole-Derived Schiff
Bases
Y. Sindhu a , C. J. Athira a , M. S. Sujamol a & K. Mohanan a
a Department of Chemistry , University of Kerala, Kariavattom
Campus , Trivandrum, Kerala, India
Published online: 25 Aug 2010.
To cite this article: Y. Sindhu , C. J. Athira , M. S. Sujamol & K. Mohanan (2010) Synthesis,
Characterization, and Antibacterial Studies of Oxovanadium(IV) Complexes with Thiazole-Derived
Schiff Bases, Phosphorus, Sulfur, and Silicon and the Related Elements, 185:9, 1955-1963, DOI:
10.1080/10426500903403099
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Phosphorus, Sulfur, and Silicon, 185:1955–1963, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903403099
SYNTHESIS, CHARACTERIZATION, AND ANTIBACTERIAL
STUDIES OF OXOVANADIUM(IV) COMPLEXES
WITH THIAZOLE-DERIVED SCHIFF BASES
Y. Sindhu, C. J. Athira, M. S. Sujamol, and K. Mohanan
Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum,
Kerala, India
Oxovanadium(IV) complexes of Schiff bases, derived from 2-amino-4-phenyl thi-
azole/substituted 2-amino-4-phenyl thiazoles and thiophene-2-aldehyde have been
synthesized and characterized on the basis of elemental analysis, molar conductance
measurements, magnetic susceptibility data, and UV-visible, and IR spectral studies. All
the complexes are monomeric possessing a 1:2 (metal:ligand) stoichiometry. On the basis
of these data, a square pyramidal geometry has been assigned for the complexes. A few
complexes have been subjected to thermal decomposition studies. The ligands and their
metal complexes have been screened for their antibacterial activities.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental
file.
Keywords 2-Amino-4-phenylthiazole; antibacterial activity; oxovanadium(IV); square pyra-
midal geometry; thermal decomposition; thiophene-2-aldehyde
INTRODUCTION
Transition metal complexes of Schiff bases have received considerable attention,
mainly due to their preparative accessibility, structural diversity, and wide range of appli-
cations in various fields.1–3 Recently, metal complexes of ligands containing nitrogen and
other donor atoms have attained much interest due to their remarkable biological activi-
ties.4,5 Among these ligands, thiazole and its derivatives have been well studied owing to
their significant antifungal, antibacterial, and anticancer activities.6–9 Recognition of thia-
zole and its derivatives in vitamin B and coenzyme carboxylase has generated increasing
interest in their structural and functional properties and has created an exciting topic for
research.10,11
Among the transition elements, vanadium is versatile in forming complexes with rel-
evant biological properties and possesses a number of stable and accessible oxidation states.
It has been observed that there is a series of nitrogenase metalloenzymes that have vana-
dium at their active sites.12,13 Apart from this, insulinomimetic potency of vanadium(IV)
Received 11 March 2009; accepted 11 October 2009.
Address correspondence to K. Mohanan, Department of Chemistry, University of Kerala, Kariavattom
Campus, Trivandrum- 695 581, Kerala, India. E-mail: drkmohanan@rediffmail.com
1955

1956
Y. SINDHU ET AL.
Table I Analytical data and other details of the ligands
Analytical data calculated (found)
Compound/
molecular formula
Color
Yellow
Yield (%)
C
H
N
S
L1
76
78
76
75
77
74
78
62.22
(62.35)
58.74
(58.68)
60.00
(60.09)
53.35
(53.22)
55.15
(55.24)
48.13
(48.02)
63.37
3.72
(3.60)
3.52
(3.59)
4.02
(4.08)
2.87
(2.75)
2.97
(2.88)
2.59
(2.65)
4.25
10.36
(10.27)
9.78
(9.85)
9.36
(9.42)
20.35
(20.38)
9.18
(9.29)
8.01
23.71
(23.60)
22.40
(22.31)
21.35
(21.42)
20.35
(20.43)
21.03
(21.09)
18.35
(18.25)
22.56
C14H10N2S2
L2
C14H10N2S2O
L3
C15H12N2S2O
L4
C14H9N3S2O2
L5
C14H9N2S2Cl
L6
Brown
Yellowish brown
Yellowish brown
Yellow
Yellowish brown
Yellow
C14H9N2S2Br
L7
(8.10)
9.85
C15H12N2S2
(63.42)
(4.16)
(9.76)
(22.45)
compounds has stimulated further investigation into vanadium complexes.14,15 However
the chemistry of oxovanadium complexes with Schiff bases derived from heterocyclic
amines and heterocyclic aldehydes has received less attention so far, and most parts of the
biochemistry of oxovanadium complexes remain obscure. In this article, we describe the
synthesis, spectroscopic characterization, thermal decomposition, and antibacterial stud-
ies of some oxovanadium(IV) complexes of Schiff bases formed by the condensation of
2-amino-4-phenylthiazole or its derivatives with thiophene-2-aldehyde.
RESULTS AND DISCUSSION
Structure of the Ligands
The structure of the ligands was established by UV, IR, and 1H NMR spectral data.
Analytical data and other details of ligands obtained are given in Table I. The electronic
spectra of the ligands exhibited two absorption bands at ∼290 and ∼330 nm, which are
attributed to π–π∗ and n–π∗ transitions of the C N chromophore, respectively.16
The infrared spectral data of the ligands are presented in Table II. The free lig-
ands showed absorption bands in the range ∼1650 cm−1, characteristic of the azomethine
υ(C N) group.17 The band due to the thiazole ring υ(C N) was observed in the range
1600–1610 cm−1, and the vibrational characteristics of the thiazole ring have been found
at ∼2600 cm
occur in the range 701–708 cm
of the substituted thiophene ring occur around 3100, 1590, and 1405 cm−1, and bands due
to υ(C S C) of the thiophene moiety occur in the range 830–850 cm−1 21,22
.
−1 18,19 The C S C group vibrations of the thiazole ring in the free ligands
−1 20 In the spectra of the ligands, vibrational characteristics
.
.
The 1H NMR spectra of the ligands recorded in DMSO-d6 gave added support to the
conclusions drawn on the basis of UV and IR spectral data. The spectral assignments are
detailed in Table III. The singlet observed at 7.25–7.39 ppm in the spectra of the ligands has
been assigned to the azomethine proton.23 The thiazole ring C H resonances are observed
at 7.20–7.23 ppm as a singlet.24 The four hydrogen atoms of the phenyl ring give two
doublets, one at 8.17–8.02 ppm and another at 8.04–7.87 ppm, except for L1. The phenyl

OXOVANADIUM(IV) COMPLEXES
1957
Table II Important IR spectral bands of the ligands and complexes
Thiazole
Thiazole
Thiophene
Compound
υ(C N)
υ(C N)
υ(C
S
C)
υ(C
S
C)
υ(V O)
υ(M N)
L1
1645
1649
1648
1645
1648
1650
1655
1630
1629
1630
1625
1635
1630
1638
1612
1605
1610
1605
1602
1601
1611
1600
1590
1595
1594
1597
1595
1590
708
704
706
704
703
704
701
706
707
704
705
707
708
704
850
838
834
841
831
831
840
852
836
838
844
834
834
842
–
–
–
–
–
–
–
–
–
–
L2
L3
L4
L5
L6
–
–
–
–
L7
[VO(LI)2]SO4
[VO(L2)2]SO4
[VO(L3)2]SO4
[VO(L4)2]SO4
[VO(L5)2]SO4
[VO(L6)2]SO4
[VO(L7)2]SO4
969
972
973
969
973
972
970
405
408
407
411
410
409
412
protons of L1 resonate as a singlet at 8.1 ppm, and the thionyl protons resonate as multiplets
in the range 4.21–5.49 ppm.25
On the basis of the above spectral data, the structure shown in Figure 1 has been
assigned to the free ligands.
Structure of Metal Complexes
The reaction of oxovanadium(IV) sulfate with the Schiff bases (L1–L7) in the molar
ratio 1:2 gave the respective complexes. The formation of the complexes can be represented
as follows.
Methanol
VOSO4 · xH2O + 2L −−−−−→ [VOL2]SO4 + xH2O
Table III 1H NMR data of the ligands
Compound
1H NMR (DMSO-d6) (ppm)
L1
L2
L3
L4
L5
L6
L7
7.20 (s, 1H, C H thiazole), 7.33 (s, H, HC N), 8.10 (s, 5H, aromatic), 4.26–5.41 (m,
thionyl)
7.25(s, 1H, C H thiazole), 7.28(s, HC N), 8.05 (d, 2H, aromatic),7.91 (d, 2H, aromatic),
4.28–5.46 (m, thionyl), 9.82 (s, H, OH)
7.26 (s, 1H, C H thiazole), 7.25 (s, HC N), 8.02 (d, 2H, aromatic),7.87 (d, 2H,
aromatic), 4.21–5.43 (m, thionyl), 3.45 (s, 3H, OCH3)
7.23 (s, 1H, C H thiazole), 7.39 (s, H, HC N), 8.17 (d, 2H, aromatic), 8.04
(d, 2H,aromatic), 4.26–5.44(thionyl)
7.21 (s, 1H, C H thiazole), 7.31 (s, H, HC N), 8.06 (d, 2H,aromatic),8.00
(d, 2H,aromatic), 4.26–5.45(m, thionyl)
7.22 (s, 1H, C H thiazole), 7.32 (s, H, HC N), 8.00 (d, 2H, aromatic), 8.02
(d, 2H,aromatic), 4.27–5.44 (m, thionyl)
7.25 (s, 1H, C H thiazole), 7.30 (s, H, HC N), 8.07 (d, 2H, aromatic), 7.96 (d, 2H,
aromatic), 4.22–5.48 (m, thionyl), 2.46(s, 3H, CH3)

1958
Y. SINDHU ET AL.
Figure 1 Structure of the ligand (For L1—R H, L2—R OH, L3—R OCH3, L4—R NO2, L5—R Cl,
L6—R Br, L7
CH3)
R
The complexes formed are sparingly soluble in common organic solvents, whereas
they possess good solubility in DMSO. All the complexes decompose above 250◦C. For-
mulations of these complexes have been done on the basis of elemental analysis, molar
conductance, magnetic susceptibility measurements, and various spectral data. Analytical
data and other details of the complexes obtained are listed in Table IV. The analytical
data of the complexes are in accordance with the general formula [VOL2]SO4, where L =
L1–L7. Moreover, the molar conductance values of the complexes measured in DMSO were
obtained in the range 53.2–58.1 ꢀ−1 cm2 mol−1 (Table IV). This adequately confirmed the
electrolytic nature of the Complexes.26
In order to determine the coordination sites involved in complexation, the infrared
spectra of the complexes were compared with those of the free ligands. In all the complexes,
the absorption bands characteristic of the azomethine ν(C N) group have been shifted to
lower frequencies by 15–20 cm−1, indicating coordination of azomethine nitrogen with the
metal ion.27 The band due to ν(C N) of the thiazole ring has been displaced to a lower
frequency, by about 10–15 cm−1 in the complexes, indicating coordination by the thiazole
ring nitrogen.18 In addition, the frequency at ∼2600 cm−1 attributed to the thiazole ring is
shifted to ∼2500 cm−1 in metal complexes, which gives added support to the coordination
of the endocyclic nitrogen.19 In the spectra of the metal complexes. the bands due to
substituted thiophene ring remained almost unaffected, indicating the nonparticipation of
the heterocyclic ring sulfur atom of the thiophene ring during coordination. Moreover, the
bands due to ν(C S C) of the thiophene moiety that persist in the complexes confirmed
the noncoordination of thiophene ring sulfur.22 In addition to these observations, absence of
ν(M S) bands in the far IR spectra of the metal complexes agreed with the non-involvement
of sulfur atoms in chelation.
In all the metal complexes, the band occurring in the region 969–973 cm−1 has been
assigned to the ν(V O) vibration, and this value is in the range observed for monomeric
oxovanadium(IV) complexes.28 The presence of the ionic sulfate group in the complexes
has been confirmed by the appearance of bands at ∼ 1100 cm−1 and ∼ 616 cm−1 in the
spectra of the complexes.21
The nonligand broad bands with medium intensities in the range 405–412 cm−1
observed in the far IR spectra of the complexes could be tentatively assigned for ν(M N)
stretching vibrations.29 Thus it is clear that the ligand is coordinated to the metal ion in a
bidentate fashion through the azomethine nitrogen and thiazole ring nitrogen.
Electronic Spectra and Magnetic Moment
The assignments for the electronic spectrum of VOSO4.XH2O had been made
using extended the Huckel molecular orbital treatment.30 The band in the region

1959

1960
Y. SINDHU ET AL.
Figure 2 Structure of the complexes (R = H, OH, OCH3, NO2, Cl, Br, CH3).
2
16800–17600 cm−1 is attributed to 2B2 → B1, and the band at about 21200–21640 cm−1
2
2
can be assigned to the B2→ A1 transition. These observations give adequate support to
a square pyramidal geometry for the complexes. In the visible region, the oxovanadium
complexes showed another absorption band in the range 17650–17680 cm−1, presumably
due to ligand–metal charge transfer.31
The room temperature magnetic moment values of the complexes fall in the range
1.73–1.76 B.M. These values correspond to one unpaired spin per vanadium atom, demon-
strating the presence of vanadium(1V) in these complexes, and the values are characteristic
of monomeric oxovanadium complexes.32
Thus on the basis of elemental analyses, conductance and magnetic moment values,
and IR and UV-visible spectral data, a square pyramidal geometry has been proposed for
all the complexes (Figure 2). The difficulty of obtaining crystalline complexes in the proper
symmetric forms rendered the single crystal X-ray crystallographic study unsuitable for the
complexes.
Thermal Decomposition Studies
The complexes [VO(L1)2]SO4 and [VO(L2)2]SO4 were subjected to thermal decom-
position studies in dynamic air. In both cases, a single stage decomposition pattern was
observed in the temperature range 310–510◦C with DTG peaks at 420◦C and 435◦C, respec-
tively. The final mass loss of 87.36% (86.99% theoretical) for [VO(L1)2]SO4 corresponds to
the formation of V2O5. In the case of [VO(L2)2]SO4, the mass loss of 88.93% (87.68% the-
oretical) also indicates the formation of the residue left as V2O5. The above mass loss also
agreed well with the mass loss obtained by independent pyrolysis experiment. Examination
of the mass loss data revealed that [VO(L1)2]SO4 decomposed at a faster rate compared
to [VO(L2)2]SO4. Apart from providing valuable insight into the thermal stability of the
vanadium complexes, this study also supported the formulation of the complexes.

OXOVANADIUM(IV) COMPLEXES
1961
Antibacterial Activity
The ligand and the metal complexes have been screened for their antibacterial activity
and the results obtained are available in Table S1 (see the Supplemental Materials, available
online). The results indicated that all the metal complexes are more potent bactericides than
the ligands and less potent than the standard streptomycin.
EXPERIMENTAL
All the chemicals used were of analytical grade; purchased from Aldrich, Merck,
Sisco (India), etc.; and were used without further purification. The elemental analysis data
were obtained by using a Heraeus Carlo Erba 1180-CHN analyzer. Infrared spectra were
recorded on a Shimadzu FT IR 8000 spectrophotometer, and proton NMR spectra were
recorded in DMSO-d6 on a JEOL GSX 400NB 400 MHz FT-NMR spectrometer. Vanadium
was determined by the standard method.35 Far IR spectra were recorded on a Polytec FIR 30
Fourier spectrometer using CsI discs. The electronic spectra of the complexes were recorded
on a Hitachi 320 UV-visible spectrophotometer using DMSO as solvent. Molar conductance
measurements in DMSO were conducted using 10−3 M solutions of the complexes at room
temperature with a Systronic Conductivity Meter Type-304. Magnetic moment values of the
complexes were measured at room temperature using a Magway MSB Mk1 susceptibility
balance, and X-ray diffraction data were recorded on a Siemens D 5005 model spectrometer.
Thermal decomposition studies were carried out using a thermobalance of the type Mettler
Toledo thermogravimetric system.
Synthesis of the Ligands
2-Amino-4-phenylthiazole and its derivatives were prepared according to a reported
method.36 The ligand N-(2-thiophenylmethylene)-2-amino-4-phenyl thiazole (L1) was pre-
pared by refluxing an equimolar mixture of 2-amino-4-phenyl thiazole (1.76 g, 0.01 mol)
and thiophene-2-aldehyde (1 mL, 0.01 mol) in ethanol (30 mL) for 5 h. The volume of the
reaction mixture was reduced to one-third of its initial volume and kept aside for 24 h. To
the mixture, an excess of diethyl ether was added to precipitate the Schiff base (L1). The
yellow precipitate that formed was filtered, washed with ether, recrystallized from ethanol,
and dried in vacuum over P4O10 (yield 76%).
The Schiff bases of other thiazole derivatives (L2–L7) were prepared by adopting the
same method.
For the synthesis of ligand L2, thiophene-2-aldehyde (1 mL, 0.01 mol) was dissolved
in methanol (10 mL) and was added in small portions to a solution of 2-amino-4-(p-
hydroxy)phenylthiazole (1.92 g, 0.01 mol) in hot methanol (50 mL). The solution was then
refluxed on a water bath for 4 h. The brown colored product that was precipitated by the
addition of diethyl ether was filtered, washed with methanol, and dried in vacuum. The
ligand was further purified by recrystallization from ethanol (yield 78%).
The ligand L3 was prepared by refluxing an equimolar mixture of 2-amino-4-(p-
methoxy)phenylthiazole (2.06 g, 0.01 mol) and thiophene-2-aldehyde (1 mL, 0.01 mol) in
ethanol (30 mL) for 4 h. The yellowish brown colored product obtained upon addition of
diethyl ether was filtered, washed several times with ether, recrystallized from ethanol, and
dried in vacuum over P4O10 (yield 76%).

1962
Y. SINDHU ET AL.
For the preparation of ligand L4, 2-amino-4-(p-nitro)phenylthiazole (2.21 g, 0.01
mol) was condensed with thiophene-2-aldehyde (1 mL, 0.01 mol) in 1:1 molar ratio in
ethanol medium. The resulting mixture was refluxed on a water bath for 5 h. The solution
changed to yellowish brown, and it was then concentrated to ca. 20 mL and allowed to
cool. Addition of diethyl ether results in the precipitation of the ligand, which was filtered
and dried. It was further purified by recrystallization from ethanol (yield 75%).
The ligand L5 was prepared by refluxing equimolar mixtures of 2-amino-4-(p-
chloro)phenylthiazole (2.1 g, 0.01 mol) and thiophene-2-aldehyde (1 mL, 0.01 mol) in
ethanol (30 mL) for 4 h. The yellow colored product precipitated by adding diethyl ether
was filtered, washed several times with diethyl ether, recrystallized from ethanol, and dried
in vacuum over P4O10 (yield 78%).
The ligand L6 has been obtained by refluxing together equimolar solutions of 2-
amino-4-(p-bromo)phenylthiazole (2.1 g, 0.01 mol) and thiophene-2-aldehyde (1 mL, 0.01
mol) in ethanol (30 mL) for 4 h. The volume of the reaction mixture was reduced to one-
third of its initial volume, and excess of diethyl ether was added to precipitate the product.
The product that was obtained was filtered, washed with ether, purified by recrystallization
from ethanol, and dried in vacuum (yield 74%).
Condensation of thiophene-2-aldehyde (1 mL, 0.01 mmol) with an 2-amino-4-(p-
methyl)phenylthiazole (2.1 g, 0.01 mol) in 1:1 molar ratio for about 5 h yielded ligand L7.
The yellow colored product that was obtained upon addition of ether was filtered, washed
with ether, purified by recrystallization from methanol, and dried in vacuum over P4O10
(yield 74%).
Synthesis of Complexes
The metal complexes were synthesized according to the following general procedure.
(To avoid repetition, only a general method is given in the case of the complexes.) Vanadyl
sulfate (0.005 mol) dissolved in ethanol (20 mL) was added to the solution of the appropriate
ligand (0.01 mol) dissolved in ethanol (25 mL). After adjusting the pH of the solution to
6.5–7.0, the mixture was refluxed for about 4–5 h on a water bath. The mixture was then
allowed to cool, and the complex that separated was filtered, washed with ethanol, and
dried in vacuum over P4O10.
Antibacterial Experiments
The ligands and their metal complexes were screened for their antibacterial activity
against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Salmonella typhi,
and Bacillus substilis, at a concentration of 300 µg/mL in DMSO by the agar diffusion
method described in the Supplemental Materials.33,37
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