Synthetic, structural, and antimicrobial studies of organotin(IV) complexes of semicarbazone, thiosemicarbazone derived from 4-hydroxy-3- methoxybenzaldehyde

Article abstract of DOI:10.1007/s11164-011-0325-8

Reaction of dibutyltin dichloride, dimethyltin dichloride, and tributyltin chloride with ligands derived from thiosemicarbazone and semicarbazone leads to the formation of a new series of organotin(IV) complexes of general formula R₂SnCl_2<

Full text of DOI:10.1007/s11164-011-0325-8

Res Chem Intermed (2012) 38:53–65
DOI 10.1007/s11164-011-0325-8
Synthetic, structural, and antimicrobial studies
of organotin(IV) complexes of semicarbazone,
thiosemicarbazone derived from 4-hydroxy-3-
methoxybenzaldehyde
•
H. L. Singh J. B. Singh K. P. Sharma
•
Received: 1 March 2011 / Accepted: 12 May 2011 / Published online: 26 May 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Reaction of dibutyltin dichloride, dimethyltin dichloride, and tributyltin
chloride with ligands derived from thiosemicarbazone and semicarbazone leads to
the formation of a new series of organotin(IV) complexes of general formula
R2SnCl2ÁL and R3SnClÁL (where L ligands derived from the condensation of thio-
semicarbazide and semicarbazide with 4-hydroxy-3-methoxybenzaldehyde). The
authenticity of these ligands and their metal complexes have been established on the
basis of elemental analysis, conductance measurements, molecular weight deter-
1
minations, infrared, H NMR, 13C NMR, 119Sn NMR, and UV spectral studies.
These studies showed that the ligands coordinate to the metal atom in a bidentate.
An octahedral structure is proposed for the organotin(IV) complexes. The ligands
and its metal complexes are screened for their antimicrobial activities against some
Gram-positive and Gram-negative bacteria, and fungus. The studies demonstrated
that metalation can increase the antimicrobial activity rather than the free ligands.
Keywords Organotin(IV) complexes Á Antimicrobial activity Á
Thiosemicarbazone Á Semicarbazone Á Spectral studies
Introduction
Synthetic vanillin is used in both food and non-food applications, in fragrances, and as
a flavoring agent in pharmaceutical preparation. 4-Hydroxy-3-methoxybenzaldehyde,
H. L. Singh (&) Á J. B. Singh
Department of Chemistry, Faculty of Engineering & Technology, Mody Institute of Technology
and Science, Lakshmangarh, Sikar, India
e-mail: hlsingh9@rediffmail.com; harlalsingh@hotmail.com
K. P. Sharma
Department of Biotechnology, Faculty of Art, Science and Commerce, Mody Institute
of Technology and Science, Lakshmangarh, Sikar, India
123

54
H.L. Singh et al.
which is a major component of natural vanillin, is one of the most widely used and
important flavoring materials worldwide [1, 2]. It is supported by the fact that
approximately 50% of the worldwide production of synthetic vanillin is used as an
intermediate in the chemical and pharmaceutical industries for the production of
herbicides, antifoaming agents, or drugs such as papaverine, L-dopa, L-methyldopa,
and the antimicrobial agents, trimethoprim [3].
Thiosemicarbazones and semicarbazones are of great pharmacological interest
since they show a board spectrum of biological activity including antitumor,
antibacterial, antifungal, and anti-inflammatory activities [4–10]. Considering all of
these, it is of important to synthesize vanillin thiosemicarbazone and semicarbazone
with possible antimicrobial activities. Organotin(IV) complexes are extensively
studied due to the applications in industrial as well as biocidal properties [11–13].
Numerous studies on organotin(IV) complexes have been carried out in order to
study its biological properties against bacterial, fungal, and cancer cell lines [14–
16]. In addition to this, organotin(IV) complexes are also extensively studied due to
its coordination geometries as well as structural diversity [17, 18]. Moreover, the
biological activity of organotin(IV) complexes is greatly influenced by the structure
of the molecule as well as the coordination number of the tin moiety [19, 20].
In this study, ligand (L1 and L2) is obtained by the condensation reaction between
thiosemicarbazide, semicarbazide hydrochloride and 4-hydroxy-3-methoxybenzal-
dehyde with this hope that it may provide us valuable theoretical information for
exploring metal-based bacteriostatic and carcinostatic pharmaceuticals with high
efficacy and low toxicity. In this effort, we have also introduced an azomethine
([C=N–) linkage with the concern that it may permit a notable variety in the
remarkable chemistry and behavior of such compounds. The synthesized ligands
(L1 and L2) have been exposed to act as bidentate towards metal atom solely
through the azomethine nitrogen and thioketonic/ketonic sulfur/oxygen forming a
stable five-member ring. Ligands and their corresponding organotin(IV) complexes
have also been screened for their antibacterial and antifungal activities.
Experimental
All chemicals and organotin precursors used were procured from Aldrich and
Merck. All the solvents were dried before use by the literature methods [21] and
moisture was excluded from the glass apparatus using CaCl2 drying tubes. The
melting points were determined in an open glass capillary and were uncorrected.
Analytical methods and spectral measurements
Tin was estimated gravimetrically as SnO2, and chlorine was estimated volu-
metrically using Volhard’s method. Nitrogen and sulfur were estimated by
Kjeldahl’s and Messenger’s methods, respectively. Molar conductance measure-
ments were made in anhydrous dimethylformamide at 45 5 °C using a Systronics
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Synthetic, structural, and antimicrobial studies of organotin(IV)
55
conductivity bride model 305. Molecular weight determinations were carried out by
the Rast camphor method.
The electronic spectra were recorded in DMSO on a Thermo spectrophotometer.
Infrared spectra were recorded on a Perkin-Elmer RX1 FTIR spectrophotometer in
1
the region of 4,000–400 cm-1. H NMR was recorded on a Jeol (model FX 90Q)
using DMSO-d6 as solvent at 89.55 MHz. 13C NMR was recorded on a 90 MHz
Jeol (FX 90Q) NMR spectrometer using dry DMSO as the solvent at 84.25 MHz.
1
TMS was used as internal reference for H NMR and 13C NMR. The 119Sn NMR
spectra with proton noise decoupling were recorded on a 90-MHz Jeol spectrometer
using dry DMSO as the solvent at 22.7 MHz and tetramethyltin (TMT) as an
external standard.
Microwave-assisted synthesis of the ligands
The ligands were prepared by the condensation of 4-hydroxy-3-methoxybenzalde-
hyde (2.435 g, 0.016 mol) with thiosemicarbazide (1.458 g, 0.016 mol) or semi-
carbazide hydrochloride (1.849 g, 0.016 mol), in the presence of sodium acetate.
The reaction mixture was irradiated by the conventional microwave oven by using
3–4 mL of solvent. The reactions were completed in a short period (5–6 min). The
excess solvent was removed on a rotary evaporator. It was dried further and then
purified by recrystallization from the same solvent. These were characterized and
analyzed before use. Elemental analyses (N and S) were conducted using the
methods mentioned above and their results were found to be in good agreement with
the calculated values. The structures of the ligands are shown in Fig. 1.
Compound L1 was prepared by reacting 4-hydroxy-3-methoxybenzaldehyde with
semicarbazide, color, brick red; mp, 190 °C (d) and elemental analysis (%), calcd.
for C9H11N3O3: C, 51.67; H, 5.30; N, 20.09; found, C: 51.72; H, 5.25; N, 19.75;
molecular weight: found, 209.21, calcd. 201.09.
Compound L2 was prepared by reacting 4-hydroxy-3-methoxybenzaldehyde with
thiosemicarbazide, color, pale yellow; mp, 210 °C (d) and elemental analysis (%),
Fig. 1 Structure of the ligands
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56
H.L. Singh et al.
calcd. for C9H11N3O2S: C, 47.99; H, 4.92; N, 18.65; S, 14.23; found, C: 47.65; H,
4.98; N, 18.46; S, 14.03; molecular weight: found, 229.65, calcd. 225.27.
Syntheses of organotin(IV) complexes
A solution of semicarbazone (0.387 g; 1.85 mmol) and thiosemicarbazone
(0.4167 g; 1.85 mmol) in THF (50 mL) was slowly added to a solution of
Me2SnCl2 (0.4064 g; 1.85 mmol), Bu2SnCl2 (0.5621 g; 1.85 mmol), and Bu3SnCl
(0.6022 g; 1.85 mmol) in the same solvent (20 mL) with constant shaking. The
color of the contents immediately changed. The reaction mixture was stirred for 6 h.
Excess solvent was removed under reduced pressure and the solid was dried in
vacuum at 35 5 °C after repeated washing with cyclohexane (Table 1).
Microbiological studies
Antibacterial assay
Synthesized compounds were screened for their antibacterial activity against
Bacillus cereus (MTCC 0430), Nocardia sp. (MTCC 1534), and Enterobacter
aerogenes (MTCC 2824) at various concentrations of 5, 20, and 50 lg by the agar
well-diffusion method 22. An aliquot (5 mL) of nutrient broth was inoculated with
the test organisms and incubated at 37 °C for 24 h. Sterile nutrient agar plates were
also prepared and holes of 5 mm diameter were cut using a sterile cork borer
ensuring proper distribution. The test organisms after 24 h of incubation were
spread onto separate agar plates. The chemical compounds were dissolved in
DMSO were poured into appropriately labeled holes using a pipette in aseptic
conditions. A hole containing DMSO served as a control. Triplicate plate of each
bacterial strain was prepared. The plates were incubated aerobically at 37 °C for
24 h. The antimicrobial activity was determined by measuring the diameter of the
zone (mm) showing complete inhibition with respect to control (DMSO).
Antifungal assay
Besides antibacterial activity, the chemical compounds were also checked for their
potential to inhibit the growth of fungus Aspergillus niger (MTCC 404) at a
concentration of 20 lg. The assay was prepared in triplicate. The fungal cells were
grown in 5 mL of Sabouraud’s broth medium for 3 days at 28 °C. Sterile
Sabouraud’s agar medium plates were prepared and holes of 5-mm diameter were
cut using a sterile cork borer ensuring proper distribution. The test organism after
72 h incubation was inoculated on Sabouraud’s agar medium plate. The inoculated
plate was incubated at 28 °C for 7 days to develop a fungal mat. The synthesized
compounds were dissolved in DMSO and were poured into appropriately labeled
holes in separate plates. These plates were left at room temperature for 2 h to allow
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Synthetic, structural, and antimicrobial studies of organotin(IV)
57
123

58
H.L. Singh et al.
Scheme 1 Representative equation illustrating the formation of Organotin(IV) complexes
the diffusion of the sample followed incubation for 7 days at 28 °C. The inhibition
of growth was determined by the measuring the growth of fungal mat towards the
hole in the plate having compound with respect to DMSO and control plates.
Results and discussion
The reactions of organotin(IV) chloride with thiosemicarbazone and semicarbazone
may be depicted by the following equation (Scheme 1).
The above reactions were found to be quite facile and could be completed in
3–4 h of stirring. All these complexes are intensively colored and solids. They are
soluble in common organic solvents, DMF and DMSO. The compounds were
dissolved in DMF and molar conductance 10-3 M of solution at 25 °C was
measured. The molar conductance valves of the complexes fall in the range of
15–25 ohm-1 cm2 mol-1 indicating that these compounds are non-electrolytes. The
elemental analyses data (Table 1) agree with the proposed formulae for the ligands
and also confirmed the R2SnCl2ÁL (Fig. 2) and R3SnClÁL (Fig. 3) composition of the
organotin(IV) complexes.
Fig. 2 Structure of
diorganotin(IV) complexes
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Synthetic, structural, and antimicrobial studies of organotin(IV)
59
Fig. 3 Structure of
triorganotin(IV) complexes
Electronic spectra
The electronic spectra of the ligands and their complexes were recorded in absolute
ethanol. The various bands observed were assigned to inter-ligands and charge
transfer of n–p* transition according to their energies and intensities. It was found
that the electronic spectra of the ligands exhibit two bands at 342 and 410 nm,
which are assignable to p–p* and n–p* transitions, respectively. A red shift is
observed in the spectra of the complexes, as these bands appear at 348 and 415 nm.
This may be due to the polarization of the [C=N– bond caused by metal ligand
electron transfer interaction. Furthermore, there is a sharp band observed in the
255–270 nm regions in the spectra of the complexes, which is ascribed to the
charge-transfer band, since it is known that metal is capable of forming dp–pp
bands with ligands containing nitrogen or oxygen or sulfur as donor atoms. The tin
atom has vacant 5d orbitals and hence L?M electronic orbital overlap can take
place by the acceptance of a pair of electronic form as oxygen or sulfur donor atoms
of the ligands.
Infrared spectra
The IR spectra of the Schiff base exhibited characteristic bands due to m(NH),
m(NH2), and m(OH) in the region 3,155; 3,385; and 3,450 cm-1, respectively. In
addition, the characteristic high-intensity band in the region 1,620–1,605 cm-1 is
assigned to m(C=N) [23, 24] and a band around 1,060–1,045 cm-1 is assigned to
m(C=S), respectively, in the IR spectra of the Schiff bases. A high-intensity band in
the region 1,260–1,220 cm-1 and a band around 1,730–1,725 cm-1 are assigned to
phenolic m(C–O) and m(C=O), respectively [25, 26]. In comparison to the spectra of
the Schiff bases, all the metal complexes exhibited the band of m(C=N) in the region
1,590–1,580 cm-1 showing the shift to lower wave-numbers, indicating that the
nitrogen of azomethine group is coordinated to metal ions.
The m(C=S) [25, 27] and m(C=O) [25] stretching vibrations have been shifted
towards lower frequency and occurred at 1,035–1,015 cm-1 and 1,710–1,695 cm-1
,
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H.L. Singh et al.
respectively, in the metal complexes. This indicates that the ligands have been
coordinated to the metal ions through the thionic sulphur and ketonic oxygen atoms.
In Me2SnCl2ÁL and Bu2SnCl2ÁL type of complexes, two bands at 615 and 508 cm-1
are also observed, which may be assigned to (Sn–C) asymmetric and symmetric
modes, respectively, thus indicating the presence of a bend (C–Sn–C) moiety [28].
The new bands appearing in the region of 525, 330, and 440 cm-1 in the spectra of
the complexes are assigned to stretching frequencies of m(Sn/O) [28], m(Sn/S)
[29], and m(Sn/N) [29] bond formations, respectively. The unaltered positions of
bands due to m(NH), m(NH2), m(C–O), and m(OH) in all the metal complexes indicate
the non-involvement of these groups in the coordination.
On the basis of the above analysis, we suggest binding the ligands to the metal
atom through the azomethine nitrogen, thiolic sulphur, and ketonic oxygen atoms of
the thiosemicarbazone/semicarbazone ligands.
1H NMR spectra
In the 1H NMR spectra of the Schiff base (L1), the NH proton exhibited a signal at d
8.02 ppm (s, 1H). A characteristic proton signal at d 9.76 ppm (s, 1H), d 13.44 ppm
(s, 1H), and d 2.85 ppm (s, 2H) are assigned to CH=N, OH, and NH2, respectively.
In addition, the signal in the region d 6.95–7.82 ppm (m, 3H) and sharp signal at d
3.47 ppm (s, 3H) are due to aromatic and CH3 protons, respectively. In case of
Schiff base (L2), the signal observed at d 8.22 ppm (s, 1H) is attributed to the NH,
and a signal at d 10.32 ppm (s, 1H) is ascribed to CH=N. The signal observed as a
multiplet in the region d 7.01–7.87 ppm (m, 3H) and signal at d 3.46 ppm are due to
aromatic and CH3 protons, respectively, and a signal at d 10.45 ppm is due to
phenolic –OH proton.
In comparison to the spectra of the Schiff bases and metal complexes,
azomethine proton signals move downfield (*d 0.30 ppm) in the complexes in
comparison to its original position in the ligands due to coordination of azomethine
nitrogen to the metal atom. The complexes show additional signals at d 0.55–1.85
(m) ppm and d 1.28 (s) ppm for the protons of butyl and methyl groups,
1
respectively. The H NMR spectra of the ligands exhibit the –CH3 protons signals
at *d 4.20 ppm, –OH proton signal at *d 13.40 ppm, aromatic proton signal
around d 7.60–6.60 ppm, –NH2 protons signal at *d 2.85 ppm, and its remaining at
the same position in the spectra of the metal complexes indicates the non-
involvement of these groups in the coordination. The 1H NMR spectroscopic data of
the complexes are given in Table 2.
13C NMR spectra
13C NMR spectra of 4-hydroxy-3-methoxybenzaldehyde semicarbazone (L1) and
4-hydroxy-3-methoxybenzaldehyde thiosemicarbazone (L2) and their corresponding
organotin(IV) complexes have also been recorded in dry DMSO. A considerable
shift (Table 3) in the position of carbons attached to the different participating
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Synthetic, structural, and antimicrobial studies of organotin(IV)
61
Table 2 1H NMR spectral data of the ligands and their corresponding organotin(IV) complexes
Compounds
Chemical shift in (d ppm)
–CH=N –NH– –OCH3 –OH
–NH2
Aromatic protons Sn–R
L1
9.76 (s) 8.02 (s) 3.82 (s) 13.44 (s) 2.85 (s) 6.62–7.26 (m)
9.98 (s) 8.22 (s) 3.79 (s) 13.48 (s) 2.87 (s) 7.90–6.75 (m)
9.90 (s) 8.18 (s) 3.80 (s) 13.42 (s) 2.88 (s) 7.88–6.76 (m)
9.94 (s) 8.20 (s) 3.81 (s) 13.41 (s) 2.90 (s) 7.89–6.70 (m)
10.32 (s) 8.22 (s) 3.84 (s) 13.32 (s) 2.84 (s) 6.52–7.32 (m)
–
Me2SnCl2ÁL1
Bu2SnCl2ÁL1
Bu3SnClÁL1
L2
1.20 (s)
0.65–1.90 (m)
0.60–1.95 (m)
–
Me2SnCl2ÁL2 10.68 (s) 8.45 (s) 3.85 (s) 13.30 (s) 2.86 (s) 8.06–6.68 (m)
1.22 (s)
Bu2SnCl2ÁL2 10.65 (s) 8.50 (s) 3.84 (s) 13.34 (s) 2.85 (s) 8.01–6.70 (m)
10.70 (s) 8.51 (s) 3.85 (s) 13.33 (s) 2.87 (s) 8.10–6.75 (m)
0.62–1.95 (m)
0.65–1.92 (m)
Bu3SnClÁL2
s singlet, m multiplet
Table 3 13C NMR spectral data of the ligands and their corresponding organotin(IV) complexes
Compounds
Chemical shift in (d ppm)
C=N C=S/ OCH3 Aromatic carbon
C=O
Sn–Me/Sn–Bu
L1
162.3 179.6 56.7
149.1, 134.3, 118.7, 148.1, 120.3, 128.7
–
Me2SnCl2ÁL1 155.9 169.5 56.8
148.5, 134.7, 119.1, 147.6, 121.3, 128.2 9.2
Bu2SnCl2ÁL1 156.5 170.8 56.4
148.2, 133.8, 118.3, 146.7, 120.1, 127.5 26.9, 28.3, 26.2, 13.7
148.1, 135.2, 119.1, 147.5, 121.1, 126.5 27.9, 27.3, 26.5, 13.4
Bu3SnClÁL1
156.1 170.1 56.5
160.4 178.6 56.1
L2
148.2, 133.4, 118.6, 146.6, 121.4, 130.1
–
Me2SnCl2ÁL2 154.7 170.7 56.3
147.8, 133.7, 119.6, 145.4, 121.8, 129.3 8.6
Bu2SnCl2ÁL2 151.6 172.9 56.7
152.3 172.7 56.5
147.9, 132.9, 120.1, 145.7, 122.1, 129.5 26.7, 27.5, 26.1, 13.8
148.5, 133.1, 119.2, 146.1, 121.9, 129.8 28.9, 27.5, 27.1, 14.2
Bu3SnClÁL2
groups suggests that the bonding through the azomethine nitrogen and thiketonic
sulfur/oxygen atoms to the tin atom. The carbon of the methyl and butyl groups are
observed at d 15.5 ppm and d 26.8–14.3 ppm, respectively, a position comparable
to other similar compounds. The alkyl group attached to tin displays resonance for
chemical equivalent carbon, however, the butyl compounds displays three
resonances. The tin carbon coupling nJ(119Sn–13C) values of n = 1, 925 Hz;
n = 2, 41.5 Hz, and n = 3 125.6 Hz indicate the six coordinate around tin in such
organotin(IV) complexes.
Although it is also possible that the shifting of azomethine carbon is due to the
change in hybridization of nitrogen attached to the ketonic group, in the light of IR,
UV, and 1H NMR spectral studies, it seems more plausible that the shifting in these
carbons is due to the involvement of thiolic sulphur/ketonic oxygen and azomethine
nitrogen in bonding.
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H.L. Singh et al.
119Sn NMR spectra
The value of d 119Sn NMR reflects the coordination number of the nucleus in the
corresponding metal complexes. In general, 119Sn chemical shifts move to lower
frequency with increasing coordination number of the nuclei. In order to confirm the
geometry of the complexes, 119Sn NMR spectra were recorded. In the 119Sn NMR
spectra of the organotin(IV) complexes, (Bu3SnClÁL, Bu2SnCl2ÁL, and Me2SnCl2ÁL)
give a sharp signal around d –360 to –380 ppm, which strongly supports the six
coordination around tin in a distorted octahedral geometry. Values [30–32] for
similar six-coordinated organotin(IV) complexes have been reported in the range of
d –355 to –503 ppm. These results are in agreement with the hypothesis of an
increase in the coordination number of the tin atom in the complexes, and hence, of
tin nuclear shielding.
On the basis of the above-mentioned different spectral studies, it is suggested that
the bonding through the azomethine nitrogen and thioketonic sulfur/oxygen atoms
to the tin atom. Finally, octahedral geometries around the tin atom have been
proposed, as shown in Figs. 2 and 3.
Antimicrobial activities
In vitro biocidal screening tests of synthesized ligands and their organotin
complexes were carried out as antibacterial and antifungal activity. The antibacterial
activity was tested against three bacterial strains; two Gram-positive (MTCC 0430,
Bacillus cereus; MTCC 1534, Nocardia sp.) and one Gram-negative (MTCC 2824,
Enterobacter aerogenes) strains. The agar well-diffusion method was used in these
assays and each experimental was performed in triplicate. The zone of inhibition
value represents the mean value of three readings, which are shown in Table 4. The
antibacterial studies demonstrated that all complexes have activity toward tested
bacteria whereas their ligands show no activity (L1 and L2). Compound Bu3SnClÁL2
shows the potential to inhibit the growth of all bacterial strains. The compounds
Bu2SnCl2ÁL2 and Bu2SnCl2ÁL1 inhibit the growth of Gram-positive bacteria MTCC
0430 (Bacillus cereus) and MTCC 1534 (Nocardia sp.) while Enterobacter
aerogenes growth is not suppressed by the presence of these compounds in nutrient
agar medium. Compound Bu2SnCl2ÁL1 inhibits the growth of Nocardia sp. only.
Besides these, the inhibition of bacterial growth increases upon enhancing the
concentration of these compounds.
Few representative compounds were also subjected to antifungal activity against
Aspergillus niger (MTCC 404) by using the agar well-diffusion method. The results
are shown in Table 5. The investigation shows that all the synthesized organotin
compounds in general have more activity than the parent ligands. Analysis of
growth pattern of Aspergillus niger in the presence of these synthesized compounds
reveals that only compounds Bu2SnCl2ÁL2 and Bu2SnCl2ÁL1 have potential to inhibit
the growth of fungus tested.
Further, the organotin complexes are more active than the free ligand, which
indicates that metalation increases antimicrobial activity. The above studies indicate
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Synthetic, structural, and antimicrobial studies of organotin(IV)
63
Table 4 Antibacterial activity of ligands and their organotin(IV) complexes
Compounds
Zone of inhibition (mm)a
MTCC 0430
MTCC 2824
MTCC 1534
5 (lg) 20 (lg) 50 (lg) 5 (lg) 20 (lg) 50 (lg) 5 (lg) 20 (lg) 50 (lg)
L2
Inact.
4.45
Inact.
15.68
10.77
5.88
Inact.
20.81
17.83
8.76
Inact. Inact.
3.55 10.36
Inact.
15.92
Inact.
Inact.
Inact.
Inact.
Inact.
2.87
3.39
1.64
Inact.
2.33
Inact.
5.29
Inact.
12.16
12.62
12.75
Inact.
19.48
Bu3SnClÁL2
Bu2SnCl2ÁL2 2.83
Inact. Inact.
Inact. Inact.
Inact. Inact.
Inact. Inact.
5.55
Me2SnCl2ÁL2 Nil
5.82
L1
Inact.
Inact.
16.56
Inact.
20.28
Inact.
12.65
Bu2SnCl2ÁL1 6.87
MTCC 0430 Bacillus cereus, MTCC 2824 Enterobacter aerogenes, MTCC 1534 Nocardia sp.
a
The zone of inhibition was measured with respect to control. DMSO (dimethyl sulfo-oxide was taken as
control and as well as solvent)
Table 5 Antifungal activity of ligands and their organotin(IV) complexes against Aspergillus niger
(MTCC 404) on Sabouraud’s agar medium
Compounds
Compound dissolved
in DMSO (20 lg)
DMSO alone
(40 ll)
Control (without DMSO
and compound)
L2
28.79
26.56
18.98
20.36
22.54
19.48
25.25
25.38
21.57
17.54
20.65
23.86
32.64
30.64
24.52
23.38
25.85
28.92
Bu3SnClÁL2
Bu2SnCl2ÁL2
Me2SnCl2ÁL2
L1
Bu2SnCl2ÁL1
that the organotin complexes synthesized in the present studies are highly active
against all these microorganisms. The organotin complexes of thiosemicarbazones
are more active for all organisms than corresponding semicarbazones complexes
and this also indicates that sulphur is a more effective antimicrobial agent than
oxygen, as was suggested by Tandon [33]. The increase in the activity of
organotin(lV) complexes as compared to the parent ligand may be due to the
complex formation in which the ligand is coordinated to the central tin atom through
the thioketonic sulphur and azomethine nitrogen leading to an increased biocidal
action. Almost all the compounds were found to be more active against all the
microorganisms used than the ligands themselves. The mode of action of the
compounds may involve the formation of a coordinate bond through (-N–C=S)
groups with the active centers of the cell constituents resulting in an interference
with the cell process. The screening data of a particular ligand and its metal
complexes show that the former has greater activity than the latter from the
biochemical point of view. On comparing the results in general, it may be concluded
that the organotin(IV) complexes have greater inhibiting power than the free ligands
against all the microbes.
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H.L. Singh et al.
Although it is difficult to make out an exact structure–activity relationship
between the antimicrobial activity and the structure of these complexes, it can
possibly be concluded that the chelation as well as the addition of a substrate
enhance the activity of the complexes. The variation in the toxicity of different
antibacterial agents against various organisms depends on either the impermeability
of the cell or differences in site of action or ability to cause mutations in the
microorganism [34]. Though the results suggest that the ligands have a remarkable
toxic property, their complexes of tin inhibit the growth of microorganisms to a
greater extent. This is in accordance with earlier reports [35]. Further, the greater
activity of the complexes can also be explained on the basis of their higher solubility
of the particles.
Conclusions
The newly synthesized Schiff bases act as bidentate ligands coordinating to metal
ions through azomethine nitrogen, thionic sulphur, and ketonic oxygen atoms.
Octahedral geometries have been proposed for organotin(IV) complexes with the
1
help of different spectral studies like IR, UV, H, 13C, and 119Sn NMR. In addition
to this, the process of microwave-assisted ligand synthesis was found economical.
The Schiff bases and their metal complexes were found to be highly active against
some of the antibacterial and antifungal species. The activity is significantly
increased on coordination. All these observations put together lead us to propose the
structures shown in Figs. 2 and 3.
Acknowledgments The author is thankful to the dean of the Faculty of Engineering and Technology,
Mody Institute of Technology and Science, Deemed University, Lakshmangarh, Sikar for providing the
necessary facilities. The authors are also grateful to Dr. Ankit Gandhi for linguistic corrections.
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