Reactions of tin and triorganotin(IV) isopropoxides with thymol derivative: Synthesis, characterization and in vitro antimicrobial screening

Article abstract of DOI:10.2298/JSC120917030M

New series of diisopropoxytin and triorganotin(IV) complexes of H2hbgl (1) of the general formula Sn(OPri)2(hbgl) (2), Sn(OPri)2(Hhbgl)2 (3), Ph3Sn(Hhbgl) (4), Bu3Sn(Hhbgl) (5) and Me3Sn(Hhbgl) (6), where H2hbgl is a ligand of a thymol derivative, namely, N-(2-hydroxy-3-isopropyl-6-methylbenzyl) glycine, were synthesized by reacting tin and triorganotin(IV) chloride with the ligand, with the aid of sodium isopropoxide in appropriate stiochiometric ratios (1:1 and 1:2). These complexes were characterized by elemental analysis, IR and 1H-NMR. The spectral data suggest that the carboxylate group, in complexes 2-5, was bonded in a bidentate manner, while a unidentate bonding was observed in complex 6. All five complexes were tested in vitro for their antibacterial activity against Gram-positive bacteria, namely, Staphylococcus aureus MTCC 96, Bacillus subtilis MTCC 121, and two Gram-negative bacteria, namely, Escherichia coli MTCC 1652 and Pseudomonas aeruginosa MTCC 741. All five complexes were also tested against three pathogenic fungal strains, namely, Aspergillus niger, Aspergillus flavus and Penicillium sp. Copyright 2013 SCS.

Full text of DOI:10.2298/JSC120917030M

J. Serb. Chem. Soc. 78 (9) 1323–1333 (2013)
UDC 546.814+547.563+543.422.25:615.28
JSCS–4500
Original scientific paper
Reactions of tin and triorganotin(IV) isopropoxides with thymol
derivative: synthesis, characterization and in vitro antimicrobial
screening
1
1
2
*
GARIMA MATELA , ROBINA AMAN , CHETAN SHARMA
and SMITA CHAUDHARY3
1Department of Chemistry, Kumaun University, S. S. J. Campus Almora-263 601, India,
2Department of Microbiology, Kurukshetra University, Kurukshetra, Haryana-136 119,
India and 3Institute of Environmental Studies, Kurukshetra University, Kurukshetra,
Haryana-136 119 India
(Received 17 September 2012, revised 6 March 2013)
Abstract: New series of diisopropoxytin and triorganotin(IV) complexes of
H2hbgl (1) of the general formula Sn(OPri)2(hbgl) (2), Sn(OPri)2(Hhbgl)2 (3),
Ph3Sn(Hhbgl) (4), Bu3Sn(Hhbgl) (5) and Me3Sn(Hhbgl) (6), where H2hbgl is a
ligand of a thymol derivative, namely, N-(2-hydroxy-3-isopropyl-6-methyl-
benzyl)glycine, were synthesized by reacting tin and triorganotin(IV) chloride
with the ligand, with the aid of sodium isopropoxide in appropriate stiochio-
metric ratios (1:1 and 1:2). These complexes were characterized by elemental
analysis, IR and 1H-NMR. The spectral data suggest that the carboxylate group,
in complexes 2–5, was bonded in a bidentate manner, while a unidentate bond-
ing was observed in complex 6. All five complexes were tested in vitro for
their antibacterial activity against Gram-positive bacteria, namely, Staphylo-
coccus aureus MTCC 96, Bacillus subtilis MTCC 121, and two Gram-negative
bacteria, namely, Escherichia coli MTCC 1652 and Pseudomonas aeruginosa
MTCC 741. All five complexes were also tested against three pathogenic fun-
gal strains, namely, Aspergillus niger, Aspergillus flavus and Penicillium sp.
Keywords: tri-organotin(IV) complexes; IR, NMR spectral studies; antifungal
activity; antibacterial activity.
INTRODUCTION
Organotin(IV) complexes show a spectrum of biological effects and have
been extensively studied in various biological fields as anticancer, antiviral, anti-
1–12
bacterial and antifungal agents, wood preservatives, pesticides, etc.
It is note-
worthy that, for a long time, organotin(IV) complexes have been widely used in a
13–15
variety of industrial and agricultural applications.
A unique fluorescent pro-
*Corresponding author. E-mail: matelagarima28@yahoo.in
doi: 10.2298/JSC120917030M
1323
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MATELA et al.
perty of organotin carboxylates has also been investigated in the last few
16,17
years.
Thymol, a p-cymene-derived compound, is widely used in medicine
18–23
for its antimicrobial, antiseptic, disinfectant and wound-healing properties.
As derivatives of p-cymene have leishmanicidal activity and are considered
important basic structures for the development of novel antiparasitic drugs, in the
present study, a new ligand of a thymol derivative derived from glycine was
synthesized and investigated for its ligating properties. The interaction of tin
metal with the organic groups via O–Sn and N–Sn bonds has attracted consi-
24
derable interest in several research fields. Regarding this, some tin compounds
with a thymol derivative were synthesized as potential antibacterial and antifun-
gal agents.
EXPERIMENTAL
Materials and methods
All the reagents, viz., tin (Merck), triphenyltin(IV) chloride (Merck), tributyltin(IV)
chloride (Merck), trimethyltin(IV) chloride (Merck) and thymol (Sigma–Aldrich) were used
as received. All the chemicals and solvents used were dried and purified by standard methods,
and moisture was excluded from the glass apparatus using CaCl2 drying tubes. The melting
points were determined in open capillaries with an electronic melting point apparatus. C, H
and N analyses of the ligand and complexes were performed on a Vario-EL, CHNS elemental
analyzer. The tin contents in the synthesized complexes were determined gravimetrically as
SnO2. Infrared spectra of the solid compounds were recorded on a Perkin-Elmer 1600 series
FT-IR spectrophotometer over the range 4000–500 cm-1 in KBr discs and over 500–200 cm-1
in CsI discs. The 1H-NMR spectra were recorded on a Bruker DRX 300 (300 MHz FT-NMR)
spectrometer at the Central Drug Research Institute, Lucknow, India, using DMSO or MeOD
as solvents with TMS as the internal standard. The conductivity measurements were per-
formed using an Ecotestr EC Low conductometer in 10-3 M DMSO solutions at room
temperature. The antibacterial and antifungal activities of the synthesized complexes were
evaluated by the agar well diffusion method and the poison food technique, respectively.
Synthesis of H2hbgl (1)
An equimolar mixture of thymol (11.86 g, 0.05 mol), glycine (3.75 g, 0.05 mol), sodium
acetate crystals (6.8 g, 0.05 mol) was dissolved in glacial acetic acid (25 mL). Formalin solu-
tion (37 % (w/v), 4.05 mL) was added to it dropwise under stirring and the contents were
heated at 60–80 °C, until a viscous mass was obtained. The viscous mass was then added
dropwise with brisk stirring into an excess of water. The thus obtained crude product was
purified by dissolving it in a requisite quantity of ca. 7 M sodium hydroxide solution followed
by its reprecipitation by 6 M hydrochloric acid. It was further purified by recrystallisation
from ethanol.
Synthesis of the complexes
[Sn(OPri)2(hbgl)] (2). A solution of tin(IV) tetrachloride (1.300 g, 0.005 mol) in benzene
(10 mL) was treated with sodium isopropoxide (1.64 g, 0.020 mol) to produce tin(IV) tetra-
isopropoxide and sodium chloride. The sodium chloride precipitate was removed by filtration
and the solvent by distillation. The solution of tin(IV) tetraisopropoxide (1.770 g, 0.005 mol)
and H2hbgl (1.067 g, 0.0045 mol) was refluxed in benzene (20 mL) for 8–10 h at 95–100 °C.
The liberated 2-propanol was fractionated out azeotropically with benzene. The complex,
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Sn(IV) METAL COMPLEXES
1325
[Sn(OPri)2(hbgl)], isolated as a yellow brown solid, was purified by recrystallisation from
ethanol at room temperature and dried under reduced pressure.
[Sn(OPri)2(Hhbgl)2] (3). Complex 3 was prepared in the similar way as complex 2.
Tin(IV) tetraisopropoxide (1.770 g, 0.005 mol) and H2hbgl (2.135 g, 0.009 mol).
[Ph3Sn(Hhbgl)] (4). A solution of triphenyltin(IV) isopropoxide (2.045 g, 0.005 mol)
and H2hbgl (1.067 g, 0.0045 mol) was refluxed in benzene (20 mL) for 8–10 h at 95–100 °C.
The complex, [Ph3Sn(Hhbgl)], isolated as a brown solid, was purified by recrystallisation
from ethanol at room temperature and dried under reduced pressure.
[Bu3Sn(Hhbgl)] (5). Complex 5 was prepared in the similar way as complex 4. Tribu-
tyltin(IV) isopropoxide (1.745 g, 0.005 mol) and H2hbgl (1.067 g, 0.0045 mol).
[Me3Sn(Hhbgl)] (6). A solution of trimethyltin(IV) isopropoxide (1.115 g, 0.005 mol)
and H2hbgl (1.067 g, 0.0045 mol) was refluxed in toluene (20 mL) for 16–18 h at 95–100 °C.
The complex, [Me3Sn(Hhbgl)], isolated as a yellow solid, was purified by recrystallisation
from ethanol at room temperature and dried under reduced pressure.
Antimicrobial assays
Antibacterial activity. The antibacterial activities of the synthesized complexes were
evaluated by the agar well diffusion method. All the bacterial cultures were adjusted to 0.5
McFarland standards, which is visually comparable to a bacterial suspension of approximately
1.5×108 cfu mL-1. 20 mL of Mueller Hinton agar medium was poured into each Petri plate and
plates were swabbed with 100 µL inocula of the test micro-organisms and kept for 15 min for
adsorption. Using a sterile cork borer of 8 mm diameter, wells were bored into the seeded agar
plates and these were loaded with a 100 µL of each complex reconstituted in dimethyl sulph-
oxide (DMSO) at a concentration of 2.0 mg mL-1. All the plates were incubated at 37 °C for
24 h. The antibacterial activity of each organotin complex was evaluated by measuring the
zone of growth inhibition against the test organisms with a zone reader (HiAntibiotic zone
scale). DMSO was used as a negative control whereas ciprofloxacin was used as positive con-
trol. This procedure was performed in three replicate plates for each organism.24-27
Determination of the minimum inhibitory concentration (MIC) of the synthesized com-
plexes. The MIC value is the lowest concentration of an antimicrobial complex that will in-
hibit the visible growth of a micro-organism after overnight incubation. The MIC of the
various complexes against the bacterial strains was evaluated through a modified agar well
diffusion method.28 In this method, a twofold serial dilution of each complex was prepared by
first reconstituting the complex in DMSO followed by dilution in sterile distilled water to
achieve a decreasing concentration range of 256 to 0.5 µg mL-1. A 100 µL volume of each
dilution was introduced into the wells (in triplicate) in the agar plates already seeded with 100
µL of standardized inoculum (106 cfu mL-1) of the test bacterial strains. All the test plates
were incubated aerobically at 37 °C for 24 h and observed for an inhibition zone. The MIC of
an organotin(IV) complex, taken as the lowest concentration that completely inhibited the
growth of the bacteria, showed by a clear zone of inhibition, was recorded for each test
organism. Ciprofloxacin was used as the positive control while DMSO was the negative
control.
Antifungal activity. The antifungal activities of the five organotin complexes were eva-
luated by the poisoned food technique.27,29 The moulds were grown on Sabouraud dextrose
agar (SDA) at 25 °C for 7 days and used as inoculates. The 15 mL of molten SDA (45 °C)
was poisoned by the addition of 100 µL of each organotin complex reconstituted in the
DMSO at a concentration of 2.0 mg mL-1, poured into a sterile Petri plate and allowed it to
solidify at room temperature. The solidified poisoned agar plates were inoculated at the centre
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MATELA et al.
with fungal plugs (8 mm diameter) obtained from the colony margins and incubated at 25 °C
for 7 days. DMSO was used as the negative control whereas fluconazole was used as the
positive control. The experiments were performed in triplicates. The diameter of fungal
colonies was measured and expressed as percent mycelial inhibition by applying the formula:
(dc − dt )
Inhibition =
×100
dc
where dc is the average diameter of fungal colony in the negative control sets and dt is the
average diameter of the fungal colony in the experimental sets.
RESULTS AND DISCUSSION
The ligand of the thymol derivative was prepared adopting a procedure
30
almost identical to that described by Mehrotra et al. The synthetic route used to
synthesise the ligand and its complexes is outlined in Schemes 1 and 2, respec-
tively. A new methodology was used to synthesise the tin and triorganotin(IV)
complexes. Organotin complexes are usually prepared by reacting organotin hyd-
roxide or organotin oxide with the corresponding ligand and by reacting organo-
tin halide with the sodium or potassium salt of the ligand. In present study, the
halogen of tin or organotin was replaced with the isopropoxide group by reacting
them with sodium isopropoxide (Scheme 2). The tin and triorganotin isopro-
poxides were isolated and reacted with the ligand. These reactions proceeded
with the liberation of 2-propanol, which was fractionated out azeotropically and
estimated to monitor the completion of reaction. The obtained values are given in
Table S-I of the Supplementary material to this paper. Owing to highly hydro-
scopic nature of tin and tri-organotin(IV) alkoxides, all the reactions were per-
formed under strictly anhydrous conditions.
Scheme 1. Synthesis of the ligand 1.
Characterisation of the ligand and its complexes
The physical, analytical and spectral data for the ligand and the synthesised
complexes are given in the Supplementary material to this paper.
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Sn(IV) METAL COMPLEXES
1327
Scheme 2. Synthesis of the complexes.
Elemental analysis. The analytical data were in good agreement with the
proposed stoichiometry of the complexes.
Molar conductance. The molar conductance values of the synthesized com-
31
plexes showed very low values indicating their non-electrolytic nature.
Infrared spectra. The IR spectra of the complexes displayed a broad vibra-
–1
tional band at 3000–3500 cm , which is assignable to the unbonded –OH
32,33
stretching of the phenolic group.
These complexes gave a strong asymmetric stretching frequency ν (COO)
as
–1
near 1588–1626 cm and a medium symmetrical stretching frequency ν (COO)
s
–1
near 1406–1417 cm . The magnitude of ν –ν (Δν) was used to explain the
as
s
34,35
type of boding of the carboxylate group to tin metal.
For complex 2, Fig. 1,
–1
36
the Δν value of 201 cm was attributed to a bidentate carboxyl group. The
magnitude of Δν for 3–5 (Fig. 1) were smaller than 200 cm , indicating bridged
tin and triorganotin(IV) carboxylates. While, for the complex 6 (Fig. 1), the
–1
–1
values of Δν exceeded 210 cm , which clearly demonstrated that these com-
plexes adopt a monodentate carboxylate structure.
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MATELA et al.
(2)
(3)
(4)
(5)
(6)
Fig. 1. Proposed structures of the metal complexes of N-(2-hydroxy-3-isopropyl-6-methyl-
benzyl)glycine.
The bonding of the carboxylate group to the tin metal was further confirmed
–1
by the appearance of a band at 500–558 cm , assignable to the Sn–O stretching
37
frequency. The far IR spectrum of triphenyltin(IV) complex showed bands at
–1
279 and 207 cm , which may be assigned to the ν (Sn–C) and ν (Sn–C), res-
as
s
–1
pectively, whereas the corresponding peaks at 528 and at 469 cm were found in
the spectrum of tributyltin(IV) complex. In the case of trimethyltin(IV) com-
38
plex the appearance of ν(Sn–C) bands were not certain due to overlapping with
the Sn–O stretching vibration.
1
H-NMR spectra. The absence of a signal due to the –OH proton at δ 12.00–
–13.00 ppm suggested deprotonation of the carboxylic oxygen atom of the ligand
39
1
on complexation. The H-NMR spectra of the complexes displayed signals in
the region δ 6.45–7.58 ppm due to the aromatic protons of the ligand. A signal at
40
δ 7.80–8.54 ppm may be attributed to the unbonded phenolic group proton. In
the complexes, the –NH resonances were observed either as a broad weak signal
2
41,42
at δ 4.90–6.88 ppm or in conjugation with the phenyl protons.
The values of
2
117/119
1
coupling constants J(
the coordination number.
Sn–C– H) provides important information regarding
38,43
2 117/119
The two-band coupling J(
Sn–C–H) of tri-
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Sn(IV) METAL COMPLEXES
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44
methyltin(IV) complex in DMSO was 68.0 Hz. Using the Lockhart equation,
the estimated value of C–Sn–C bond angle for complex 6 was 116.62°, implying
2
117/119
1
a four-coordinated tetrahedral geometry. The J(
Sn–C– H) value for 5 was
not visible since the protons of butyl group were multiplets in the range of 0.90–
45,46
–1.21 ppm, however a triplet was observed at δ 0.78 ppm.
In the spectrum of
complex 4, the signals for the phenyl group attached to tin were observed in the δ
range 7.22–7.58 ppm, in conjugation with phenyl protons of the ligand.
Antimicrobial evaluation
Antibacterial activity. All the newly synthesized complexes were evaluated
for their antibacterial activity against two Gram-positive bacteria (Staphylococ-
cus aureus and Bacillus subtilis) and two Gram-negative bacteria (Escherichia
coli and Pseudomonas aeruginosa). The results revealed that all the tested orga-
notin(IV) complexes possessed variable antibacterial activity against both Gram-
positive and Gram-negative bacteria.
Based on the maximum inhibitory activity shown against Gram-positive bac-
teria, the complexes 4 and 6 were found to be most effective against S. aureus,
with inhibition zones of 27 and 25.6 mm, respectively, whereas complex 4 was
found to be most effective against B. subtilis, with an inhibition zone of 25 mm.
However, in case of Gram-negative bacteria, two complexes, 4 and 3, were found
to be most effective against E. coli with the zone of inhibition ranging between
21 and 24 mm and one complex, 4, against P. aeruginosa with zone of inhibition
of 22 mm (Table I). The synthesized complexes showed comparatively better
activity against Gram-positive bacteria.
TABLE I. Antibacterial activity of the metal complexes of N-(2-hydroxy-3-isopropyl-6-
-methylbenzyl)glycine; values of the zone of inhibition include the diameter of the well (8 mm)
Diameter of the growth of inhibition zone, mm
Complex
S. aureus
B. subtilis
E. coli
P. aeruginosa
2
3
4
5
22
24
27
22
26
28
22
22
25
22
22
26
19
21
24
19
20
25
16
17
22
15
17
25
6
Ciprofloxacin
In the whole series, the MIC values of organotin complexes ranged between
–1
8 and 32 µg mL against the Gram-positive bacteria. Complex 4 was found to
–1
be best as it exhibited the lowest MIC of 8 µg mL against S. aureus. In case of
the Gram-negative bacteria, the MIC values of complexes ranged from 32 to 128
–1
–1
µg mL . Complex 4 showed lowest MIC of 32 µg mL against E. coli and P.
aeruginosa (Table II). Ciprofloxacin was used as the standard antibiotic. The
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MATELA et al.
positive controls produced significantly sized inhibition zones against the tested
bacteria; however, the negative control produced no observable inhibitory effect
against any of the tested bacteria.
TABLE II. Minimum inhibitory concentration (MIC / μg mL-1) of the metal complexes of
N-(2-hydroxy-3-isopropyl-6-methyl benzyl)glycine
Bacterium
B. subtilis
Complex
S. aureus
E. coli
P. aeruginosa
2
3
4
5
32
32
8
32
16
5
32
32
16
32
16
5
64
64
32
64
64
5
128
128
32
128
128
5
6
Ciprofloxacin
Antifungal activity. All the newly synthesized complexes were evaluated for
their antifungal activity against three fungal pathogens, Aspergillus niger, A. fla-
47
vus and Penicillium sp., isolated from ear patients of Kurukshetra. The com-
plexes 3 and 4 showed a more than 60 % inhibition level of mycelial growth
against A. niger and Penicillium sp., whereas complex 4 showed highest inhibi-
tion level of 69 %, of fungal mycelium against A. flavus. Fluconazole was used
as the standard antibiotic (Table III).
TABLE III. Antifungal activity of the metal complexes of N-(2-hydroxy-3-isopropyl-6-methyl-
benzyl)glycine; all the values are the means of three replicates
Mycelial growth inhibition, %
Complex
A. niger
A. flavus
Penicillium sp.
2
3
4
5
57
63
61
51
56
81
56
58
69
44
53
78
59
61
66
50
59
83
6
Fluconazole
CONCLUSIONS
1
Based on various studies, such as elemental analysis, IR and H-NMR spec-
troscopy, five- and six-coordinate geometry are proposed for complexes 2–5,
while four-coordinate geometry is proposed for complex 6. The synthesized com-
plexes were screened against various fungi and bacteria to assess their potential
as antimicrobial agents. The antimicrobial data revealed that the triphenyltin(IV)
complex (4) was superior to the other complexes. The synthesized complexes
showed remarkable antimicrobial activity.
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Sn(IV) METAL COMPLEXES
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SUPPLEMENTARY MATERIAL
Table S-I, as well as physical, analytical and spectral data of the ligand and complexes
are available electronically from http://www.shd.org.rs/JSCS/, or from the corresponding
author on request.
Acknowledgements. One of the authors (R. A.) thanks the U-COST, Dehradun, India, for
financial support in the form of a Research Project and another of the authors (G. M.) for the
award of Project Fellowship under the same project.
И З В О Д
РЕАКЦИЈЕ КАЛАЈ(IV)- И ТРИОРГАНОКАЛАЈ(IV)-ИЗОПРОПОКСИДА СА ДЕРИВАТОМ
ТИМОЛА: СИНТЕЗА, КАРАКТЕРИЗАЦИЈА И IN VITRO АНТИМИКРОБНО
ИСПИТИВАЊЕ
GARIMA MATELA1, ROBINA AMAN1, CHETAN SHARMA2 и SMITA CHAUDHARY3
1
2
Department of Chemistry, Kumaun University, S. S. J. Campus Almora-263 601, India, Department
3
of Microbiology, Kurukshetra University, Kurukshetra, Haryana-136 119 India и Institute of
Environmental Studies, Kurukshetra University, Kurukshetra, Haryana-136 119 India
У реакцијама калај(IV)- и триорганокалај(IV)-хлорида са H2hbgl лигандом (1;
H2hbgl је дериват тимола, N-(2-хидрокси-3-изопропил-6-метилбензил)глицин), уз дода-
так натријум-изопропоксида у одговарајућим стехиометријским односима (1:1 и 1:2),
синтетизовани су нови комплекси опште формуле Sn(OPri)2(hbgl) (2), Sn(OPri)2(Hhbgl)2
(3), Ph3Sn(Hhbgl) (4), Bu3Sn(Hhbgl) (5) и Me3Sn(Hhbgl) (6). Добијени комплекси су
окарактерисани применом елементалне анализе, као и IR и 1H-NMR спектроскопије. На
основу спектроскопских података, претпостављено је да је у комплексима 2–5 карбок-
силна група бидентатно координована, док је у комплексу 6 монодентатно коорди-
нована за јон метала. Испитивана је in vitro антибактеријска активност на две грам-
-позитивне бактерије, Staphylococcus aureus MTCC 96 и Bacillus subtilis MTCC 121, и на
две грам-негативне бактерије, Escherichia coli MTCC 1652 и Pseudomonas aeruginosa
MTCC 741. Поред тога, испитивана је антифунгална активност на три врсте гљива,
Aspergillus niger, Aspergillus flavus и Penicillium sp.
(Примљено 17. септембра 2012, ревидирано 6. марта 2013)
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