Biological screening, DNA interaction studies, and catalytic activity of organotin(IV) 2-(4-ethylbenzylidene) butanoic acid derivatives: Synthesis, spectroscopic characterization, and X-ray structure

Article abstract of DOI:10.1080/00958972.2014.884217

A series of organotin(IV) carboxylates, [Me₂SnL₂] (1), [Bu₂SnL₂] (2), [Oct₂SnL₂] (3), [Me₃SnL] (4), and [Ph₃SnL] (5), where L = 2-(4-ethylbenzylidene) butanoic acid, have been synthesized and characterized by elemental analysis, FT-IR, and NMR (¹H, ¹³C, and ¹¹⁹Sn). [Me₃SnL] (4) was analyzed by single crystal X-ray analysis which showed polymeric structure with distorted trigonal bipyramidal geometry. The complexes were screened for biological activities including antibacterial, antifungal, and cytotoxic activities. UV-vis absorption studies of HL, 1 and 4 with SS-DNA revealed groove binding as well as intercalation, which may be due to the presence of planar phenyl groups that facilitate interaction with DNA. The determined intrinsic binding constants, 6.04 × 10³ M^-1 (HL), 9.6 × 10³ M^-1 (1), and 1.7 × 10⁴ M^-1 (4), showed that HL and 1 have less binding strength than 4. The catalytic activities of di- and tri-organotin(IV) complexes were assessed in transesterification of triglycerides (linseed oil) into fatty acid methyl esters (biodiesel). The tri-organotin(IV) complexes have better catalytic activity than their di-analogs. 2014

Full text of DOI:10.1080/00958972.2014.884217

Journal of Coordination Chemistry, 2014
Vol. 67, No. 2, 323–340, http://dx.doi.org/10.1080/00958972.2014.884217
Biological screening, DNA interaction studies, and catalytic
activity of organotin(IV) 2-(4-ethylbenzylidene) butanoic acid
derivatives: synthesis, spectroscopic characterization, and
X-ray structure
MUHAMMAD TARIQ†, SAQIB ALI*†, NIAZ MUHAMMAD‡, NASEER ALI SHAH§,
MUHAMMAD SIRAJUDDIN†, MUHAMMAD NAWAZ TAHIR¶, NASIR KHALID‖ and
MUHAMMAD RASHID KHAN§
†Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
‡Department of Chemistry, Abdul Wali Khan University, Mardan, Pakistan
§Department of Biochemistry, Quaid-i-Azam University, Islamabad, Pakistan
¶Department of Physics, University of Sargodha, Sargodha, Pakistan
‖Chemistry Division, Pakistan Institute of Nuclear Science and Technology, Nilore, Pakistan
(Received 2 March 2013; accepted 10 December 2013)
The synthesized compounds interact with SS-DNA via intercalative binding mode of
interaction in which the compound insert itself into the base pair of DNA resulting in
hypochromic and bathochromic shift.
A series of organotin(IV) carboxylates, [Me2SnL2] (1), [Bu2SnL2] (2), [Oct2SnL2] (3), [Me3SnL]
(4), and [Ph3SnL] (5), where L = 2-(4-ethylbenzylidene) butanoic acid, have been synthesized and
characterized by elemental analysis, FT-IR, and NMR (1H, 13C, and 119Sn). [Me3SnL] (4) was
*Corresponding author. Emails: saqibali@qau.edu.pk, mtnazir@yahoo.com
© 2014 Taylor & Francis

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analyzed by single crystal X-ray analysis which showed polymeric structure with distorted trigonal
bipyramidal geometry. The complexes were screened for biological activities including antibacterial,
antifungal, and cytotoxic activities. UV–vis absorption studies of HL, 1 and 4 with SS-DNA
revealed groove binding as well as intercalation, which may be due to the presence of planar
phenyl groups that facilitate interaction with DNA. The determined intrinsic binding constants,
6.04 × 103 M−1 (HL), 9.6 × 103 M−1 (1), and 1.7 × 104 M−1 (4), showed that HL and 1 have less
binding strength than 4. The catalytic activities of di- and tri-organotin(IV) complexes were assessed
in transesterification of triglycerides (linseed oil) into fatty acid methyl esters (biodiesel). The
tri-organotin(IV) complexes have better catalytic activity than their di-analogs.
Keywords: Organotin(IV) carboxylate; Antibacterial activity; Antifungal activity; Cytotoxicity;
Transesterification
1. Introduction
Organotin(IV) carboxylates have a variety of applications as antifungal, antibacterial, anti-
cancer agents [1, 2], agricultural fungicides, and insecticides [3, 4]. Crystallographic studies
revealed that organotin(IV) carboxylates adopt structures which are dependent on both the
nature of the substituent bonded to tin and on the type of carboxylate [5, 6]. Studies of or-
ganotin compounds containing carboxylates with oxygen donors indicate that new structural
types may lead to different activities [7–9].
Organotin(IV) carboxylates have also been used as catalysts in the synthesis of polyes-
ters, polyurethanes [10], in cross-linking of silicones [11], and in esterification and transe-
sterification [12]. No extensive studies have been reported with the use of organotin(IV)
carboxylates as catalysts in transesterification of triglycerides (vegetable oil) into fatty acid
methyl esters (biodiesel). Biodiesel is the monoalkyl ester of long-chain fatty acids derived
from renewable feed stocks, such as vegetable oil or animal fats, for use in compression
ignition engines [13, 14]. It is a possible substitute of conventional diesel fuel [15]. The use
of biodiesel is environmentally friendly due to reduced emission of carbon monoxide,
hydrocarbons, particulate matter, sulfur dioxide, and volatile organic compounds [16].
The present study deals with the synthesis of di- and tri-organotin(IV) carboxylates of
the type R4−nSnLn (R = Me, n-Bu, Ph), L = 2-(4-ethylbenzylidene) butanoic acid and n = 1
or 2. These compounds were characterized by elemental analysis, FT-IR, NMR (1H, 13C,
and 119Sn), and single crystal X-ray analysis. The synthesized compounds were tested for
their antibacterial, antifungal, and cytotoxic activities. DNA interaction studies of the
synthesized compounds were also performed using UV–vis spectrophotometric technique.
The di- and tri-organotin(IV) carboxylates have been assessed as catalysts in transesterifica-
tion of triglycerides (linseed oil) into fatty acid methyl esters (biodiesel).
2. Experimental
2.1. Materials and methods
All di- and tri-organotin(IV) precursors were purchased from Sigma–Aldrich and used
without purification. Solvents used were of analytical grade and dried according to reported
procedures before use [17]. The melting points were recorded on a Gallenkamp (UK)
electrothermal m.p. apparatus. Elemental analyses were done using a Leo CHNS 932 appa-
ratus. FT-IR spectra were recorded from 4000 to 400 cm−1 using a Thermo Nicolet-6700
1
FT-IR spectrophotometer using KBr pellets. H and 13C NMR spectra were recorded at
room temperature in CDCl3 and DMSO on a Bruker Avance Digital 300 MHz NMR spec-
trometer (Switzerland). 119Sn NMR spectra were recorded on a 400 MHz JEOL ECS instru-
ment at a working frequency of 149.4 MHz and the chemical shift was referenced to Me4Sn

Organotin(IV)
325
as an external standard. The X-ray diffraction data were collected on a Bruker SMART
APEX CCD diffractometer equipped with a 4 K CCD detector set 60.0 mm from the crystal.
The crystals were cooled to 100 1 K using the Bruker KRYOFLEX low-temperature
device and intensity measurements were performed using graphite-monochromated Mo-Kα
radiation from a sealed ceramic diffraction tube (SIEMENS). Generator settings were
50 kV/40 mA. The structure was solved by Patterson method and extension of the model
was accomplished by direct method using DIRDIF or SIR2004. Final refinement on F2 was
made by full-matrix least-squares techniques using SHELXL-97, a modified version of
PLUTO (preparation of illustrations) and PLATON. Absorption spectra were recorded on a
Shimadzu 1800 UV–vis spectrophotometer. The linseeds were purchased from a local mar-
ket. The seeds were washed with distilled water to remove dirt and were oven-dried at
60 °C till constant weight. The oil was extracted by using an electric oil expeller (KEK
P0015–10127 Germany, KEK, EGON KELLER GMBH & Co. KG).
2.2. Synthesis
2.2.1. Synthesis of 2-(4-ethylbenzylidene) butanoic acid. Mixture of 4-ethylbenzalde-
hyde (3.75 g, 28 mM), ethylmalonic acid (7.39 g, 56 mM), and piperidine (4.76 g, 56 mM)
in molar ratios of 1 : 2 : 2 (scheme 1) was refluxed in a two-necked round-bottom flask in
pyridine as solvent for 24 h on a steam bath. After cooling, the reaction mixture was poured
into ice water and acidified with conc. HCl until pH 3. The precipitates were filtered,
washed with water, recrystallized in ethanol, and dried. Yield: 3.92 g, 69.6%. m.p.
109–110 °C. Anal. Calcd for C13H16O2 (%): C, 76.44; H, 7.90. Found (%): C, 76.28; H,
7.92. FT-IR (KBr, cm−1): 3329 ν(OH), 1672 ν(OCO)asym, 1421 ν(OCO)sym, (Δν = 251
1
cm−1). H NMR (CDCl3, ppm): 11.90 (s, H1, 1H), 7.81 (s, H3, 1H), 7.40 (d, H5,5ʹ, 2H),
7.26 (d, H6,6ʹ, 2H), 2.78 (q, H8, 2H), 1.28 (t, H9, 3H), 2.58 (q, H10, 2H), 1.23 (t, H11, 3H).
13C NMR (CDCl3, ppm): 173.7 (C-1), 131.4 (C-2), 142.2 (C-3), 132.2 (C-4), 128.6 (C-5),
129.7 (C-6), 138.5 (C-7), 29.3 (C-8), 14.1 (C-9), 21.4 (C-10), 15.2 (C-11).
2.2.2. Synthesis of Na-salt of 2-(4-ethylbenzylidene) butanoic acid. The sodium salt of
ligand, R/COONa, was prepared by dropwise addition of an equimolar amount of sodium
hydrogen carbonate solution to a methanolic solution of R/COOH. The solution was stirred
for 2 h at room temperature, evaporated under reduced pressure to give a white solid, and
vacuum dried.
Scheme 2 represents numbering in ligand and R groups attached to Sn for 1H and 13C NMR
interpretation.
2.2.3. Dimethyltin(IV) [bis(2-(4-ethylbenzylidene) butanoate)] (1). R/COONa (0.4 g,
1.76 mM) was refluxed for 10 h with dimethyltin(IV) dichloride (0.194 g, 0.88 mM) in 2 : 1
M ratio in dry toluene (100 mL) contained in a 250 mL two-necked round-bottom flask. A
turbid solution obtained was left overnight at room temperature. The sodium chloride
formed was filtered off and the filtrate was rotary evaporated. The resultant solid mass was
recrystallized from chloroform and n-hexane (4 : 1) mixture. Yield: 0.44 g, 89.7%. m.p.
123–125 °C. Anal. Calcd for C28H36O4Sn (%): C, 60.56; H, 6.53. Found (%): C, 60.02; H,
6.48. FT-IR (KBr, cm−1): 1537 ν(OCO)asym, 1400 ν(OCO)sym, (Δν = 137 cm−1), 574.7
1
ν(Sn–C), 494 ν(Sn–O). H NMR (CDCl3 and 〈DMSO〉 ppm): 7.81 〈7.85〉 (s, H3, 2H), 7.36

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M. Tariq et al.
〈7.32〉 (d, H5,5ʹ, 4H), 7.24 〈7.28〉 (d, H6,6ʹ, 4H), 2.66 〈2.60〉 (q, H8, 4H), 1.25 〈1.27〉 (t, H9,
6H), 2.57 〈2.53〉 (q, H10, 4H), 1.20 〈1.16〉 (t, H11, 6H), 1.09 〈1.01〉 (s, Hα, 6H,
2J(119/117Sn–1H) = 82/79, C–Sn–C = 134.06°). 13C NMR (CDCl3 and 〈DMSO〉 ppm): 178.2
〈177.3〉 (C-1), 133.0 〈128.5〉 (C-2), 144.9 〈140.7〉 (C-3), 133.3 〈130.1〉 (C-4), 129.5
〈127.9〉 (C-5), 128.0 〈127.5〉 (C-6), 140.3 〈139.2〉 (C-7), 28.7 〈28.1〉 (C-8), 13.8 〈14.3〉
1
(C-9), 21.1 〈22.6〉 (C-10), 15.3 〈14.8〉 (C-11), 1.0 〈1.05〉 (C-α, J(119Sn–13C) = 704/670 Hz,
C–Sn–C = 138.5°). 119Sn NMR (CDCl3, ppm): –136.9.
2.2.4. Dibutyltin(IV) [bis (2-(4-ethylbenzylidene) butanoate)] (2). Compound 2 was
prepared in the same way as 1 using R/COONa (0.4 g, 1.76 mM) and dibutyltin(IV) dichlo-
ride (0.27 g, 0.88 mM) in 2 : 1 M ratio. The product was recrystallized from chloroform and
n-hexane (4 : 1) mixture. Yield: 0.46 g, 82%. m.p. 93–95 °C. Anal. Calcd for C34H48O4Sn
(%): C, 63.86; H, 7.57. Found (%): C, 64.04; H, 7.61. FT-IR (KBr, cm−1): 1508 ν(OCO)asym
,
1367 ν(OCO)sym, (Δν = 141 cm−1), 503 ν(Sn–C), 459 ν(Sn–O). 1H NMR (CDCl3 and
〈DMSO〉 ppm): 7.81 〈7.84〉 (s, H3, 2H), 7.37 〈7.41〉 (d, H5,5ʹ, 4H), 7.27 〈7.25〉 (d, H6,6ʹ, 4H),
2.67 〈2.61〉 (q, H8, 4H), 1.26 〈1.29〉 (t, H9, 6H), 2.60 〈2.55〉 (q, H10, 4H), 1.21 〈1.25〉
(t, H11, 6H), 1.75 〈1.69〉 (t, Hα, 4H), 1.39–1.45 〈1.30–1.37〉 (m, Hβ, 4H), 1.45–1.51 〈1.39–
1.48〉 (m, Hγ, 4H), 0.92 〈0.90〉 (t, Hδ, 6H). 13C NMR (CDCl3 and 〈DMSO〉 ppm): 178.2
〈180.1〉 (C-1), 133.2 〈134.2〉 (C-2), 144.8 〈140.4〉 (C-3), 133.6 〈132.8〉 (C-4), 128.0 〈127.6〉
(C-5), 129.5 〈128.2〉 (C-6), 139.9 〈140.1〉 (C-7), 28.7 〈28.1〉 (C-8), 13.8 〈14.2〉 (C-9), 21.1
1
〈21.5〉 (C-10), 15.3 〈14.9〉 (C-11), 25.2 〈25.6〉 (C-α, J(119Sn–13C) = 785 Hz, C–Sn–C =
2
3
152.1°), 26.3 〈25.9〉 (C-β, J(119Sn–13C) = 42 Hz), 26.8 〈26.1〉 (C-γ, J(119Sn–13C) = 131
Hz), 13.6 〈14.0〉 (C-δ). 119Sn NMR (CDCl3, ppm): –147.9.
2.2.5. Dioctyltin(IV) [bis (2-(4-ethylbenzylidene) butanoate)] (3). Compound 3 was
prepared from R/COOH (0.32 g, 1.56 mM) and dioctyltin(IV) oxide (0.28 g, 0.78 mM). The
reactant mixture was suspended in 100 mL dry toluene in a single-necked round-bottom flask
(250 mL), equipped with a Dean-Stark apparatus. The mixture was refluxed for 10 h and
water formed during the condensation was removed at regular intervals. A clear solution thus
obtained was cooled to room temperature and solvent was removed under reduced pressure.
The solid obtained was recrystallized from chloroform and n-hexane (4 : 1) mixture. Yield:
0.44 g, 76%. m.p. semi-solid. Anal. Calcd for C42H64O4Sn (%): C, 67.11; H, 8.58. Found
(%): C, 66.34; H, 8.13. FT-IR (KBr, cm−1): 1509 ν(OCO)asym, 1365 ν(OCO)sym, (Δν = 144
cm−1), 560 ν(Sn–C), 420 ν(Sn–O). 1H NMR (CDCl3 and 〈DMSO〉 ppm): 7.80 〈7.86〉 (s, H3,
2H), 7.37 〈7.40〉 (d, H5,5ʹ, 4H), 7.26 〈7.31〉 (d, H6,6ʹ, 4H), 2.66 〈2.59〉 (q, H8, 4H), 1.75
〈1.68〉 (t, H9, 6H), 2.59 〈2.49〉 (q, H10, 4H), 1.41 〈1.43〉 (t, H11, 6H), 1.30–1.21 〈1.25–1.19〉
(bs, Hα,β,γ,δ,αʹ,β′,γ′, 28H), 0.84 〈0.89〉 (t, Hδ′, 6H). 13C NMR (CDCl3 and 〈DMSO〉 ppm):
178.1 〈180.2〉 (C-1), 133.2 〈131.5〉 (C-2), 144.8 〈141.1〉 (C-3), 133.6 〈132.4〉 (C-4), 128.0
〈127.6〉 (C-5), 129.5 〈128.7〉 (C-6), 130.8 〈130.1〉 (C-7), 33.4 〈32.1〉 (C-8), 14.0 〈14.2〉
(C-9), 21.1 〈22.4〉 (C-10), 15.3 〈14.8〉 (C-11), 22.6 〈21.7〉 (C-α, 1J(119Sn–13C) = 575 Hz, C–
Sn–C = 127.1°), 31.8 〈30.9〉 (C-β), 29.2 〈29.5〉 (C-γ), 29.1 〈29.3〉 (C-δ), 28.7 〈28.4〉 (C-α′),
25.5 〈25.0〉 (C-β′), 24.6 〈24.9〉 (C-γ′), 13.8 〈14.1〉 (C-δ′). 119Sn NMR (CDCl3, ppm): –162.7.
2.2.6. Trimethyltin(IV) 2-(4-ethylbenzylidene) butanoate (4). Compound 4 was prepared
in the same way as 1 using R/COONa (0.4 g, 1.76 mM) and trimethyltin(IV) chloride (0.35 g,
1.76 mM) in 1 : 1 M ratio. The product was recrystallized from chloroform and n-hexane

Organotin(IV)
327
(4 : 1) mixture. Yield: 0.44 g, 89%. m.p. 80–81 °C. Anal. Calcd for C16H24O2Sn (%): C,
52.35; H, 6.59. Found (%): C, 52.16; H, 6.41. FT-IR (KBr, cm−1): 1539 ν(OCO)asym, 1376
1
ν(OCO)sym, (Δν = 163 cm−1), 547 ν(Sn–C), 435 ν(Sn–O). H NMR (CDCl3 and 〈DMSO〉
ppm): 7.65 〈7.77〉 (s, H3, 1H), 7.32 〈7.38〉 (d, H5,5′, 2H), 7.21 〈7.25〉 (d, H6,6′, 2H), 2.64
〈2.55〉 (q, H8, 2H), 1.24 〈1.25〉 (t, H9, 3H), 2.53 〈2.45〉 (q, H10, 2H), 1.16 〈1.08〉 (t, H11, 3H),
2
0.52 〈0.53〉 (s, Hα, 9H) J(119/117Sn–1H) = 58/56 Hz, C–Sn–C = 110.66°). 13C NMR (CDCl3
and 〈DMSO〉 ppm): 173.8 〈175.2〉 (C-1), 133.7 〈134.5〉 (C-2), 144.3 〈141.5〉 (C-3), 135.4
〈136.2〉 (C-4), 127.9 〈126.8〉 (C-5), 129.3 〈128.6〉 (C-6), 137.9 〈137.1〉 (C-7), 28.6 〈29.3〉
(C-8), 13.9 〈14.3〉 (C-9), 21.4 〈22.3〉 (C-10), 15.3 〈16.1〉 (C-11) –2.2 〈2.4〉 (C-α),
1J (119Sn–C) = 388 Hz, C–Sn–C = 110.78°). 119Sn NMR (CDCl3, ppm): +127.8.
2.2.7. Triphenyltin(IV) 2-(4-ethylbenzylidene) butanoate (5). Compound 5 was prepared
in the same way as 1 using R/COONa (0.4 g, 1.76 mM) triphenyltin(IV) chloride (0.68 g,
1.76 mM) in 1 : 1 M ratio. The product was recrystallized from chloroform and n-hexane
(4 : 1) mixture. Yield: 0.73 g, 74%. m.p. 145–146 °C. Anal. Calcd for C31H30O2Sn (%): C,
67.30; H, 5.47. Found (%): C, 66.23; H, 5.06. FT-IR (KBr, cm−1): 1537 ν(OCO)asym, 1393
1
ν(OCO)sym, (Δν = 144 cm−1), 537 ν(Sn–C), 442 ν(Sn–O). H NMR (CDCl3 and 〈DMSO〉
ppm): 7.34 〈7.46〉 (s, H3, 1H), 7.32 〈7.38〉 (d, H5,5′, 2H), 7.10 〈7.0〉 (d, H6,6′, 2H), 2.62
〈 2.69〉 (q, H8, 2H), 1.21 〈1.29〉 (t, H9, 3H), 2.54 〈2.47〉 (q, H10, 2H), 1.17 〈1.11〉 (t, H11,
3H), 7.35–7.30 〈7.29–7.21〉 (Hβ, Hγ, Hδ, 15H). 13C NMR (CDCl3 and 〈DMSO〉 ppm):
173.6 〈176.2〉 (C-1), 133.2 〈135.1〉 (C-2), 145.3 〈142.3〉 (C-3), 135.8 〈134.2〉 (C-4), 129.1
〈130.6〉 (C-5), 130.3 〈130.2〉 (C-6), 137.2 〈138.7〉 (C-7), 28.2 〈29.5〉 (C-8), 14.3 〈14.6〉 (C-
9), 22.4 〈21.7〉 (C-10), 15.7 〈16.1〉 (C-11), 131.0 〈132.2〉 (C-α, J(119Sn–13C) = 648 Hz, C–
Sn–C = 116.4°), 139.5 〈140.4〉 (C-β, 2J(119Sn–13C) = 49 Hz), 130.5 〈129.6〉 (C-γ,
3J(119Sn–13C) = 65 Hz), 130.7 〈129.8〉 (C-δ). 119Sn NMR (CDCl3, ppm): +101.8.
2.3. Antibacterial studies
HL and its organotin(IV) complexes were tested against five bacterial strains; two
Gram-positive (Micrococcus luteus and Staphylococcus aureus) and three Gram-negative
(Escherichia coli, Bordetella bronchiseptica, and Pseudomonas aeruginosa). The agar-well
diffusion method was used for the determination of antibacterial activities [18]. Broth
culture (0.75 mL) containing ca. 106 colony-forming units per mL of the test strain was
added to 75 mL of nutrient agar medium at 45 °C, mixed well, and then poured into a 14
cm sterile Petri plate. The media were allowed to solidify and 8 mm wells were dug with a
sterile metallic borer. Then a DMSO solution of test sample (100 μL) at 1 mg/mL was
added to the respective wells. DMSO served as negative control, and the standard antibacte-
rial drugs Roxyithromycin (1 mg/mL) and Cefixime (1 mg/mL) were used as positive control.
Triplicate plates of each bacterial strain were prepared and incubated aerobically at 37 °C
for 24 h. The activity was determined by measuring the diameter of zone showing complete
inhibition (mm).
2.4. Antifungal studies
Antifungal activities against five fungal strains (Aspergillus flavus, Aspergillus niger,
Fusarium solani, Mucor spp., and Aspergillus fumigatus) were determined by using the

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agar tube dilution method [19]. Screw-capped test tubes containing Sabouraud dextrose agar
(SDA) medium (4 mL) were autoclaved at 121 °C for 15 min. Tubes were allowed to cool
at 50 °C and non-solidified SDA was loaded with 66.6 μL of compound from the stock
solution (12 mg/mL in DMSO) to make 200 μg/mL final concentration. Tubes were then
allowed to solidify in slanting position at room temperature. Each tube was inoculated with
4 mm diameter piece of inoculum from seven-day-old fungal culture. The media supple-
mented with DMSO and Terbinafine (200 μg/mL) were used as negative and positive con-
trol, respectively. The tubes were incubated at 28 °C for seven days and growth was
determined by measuring linear growth (mm). The growth inhibition was calculated with
reference to growth in vehicle control,
ꢀ
ꢁ
Linear growth in test sample ðmmÞ
% Growth inhibition ¼ 100 ꢀ
ꢁ 100
Linear growth in control (mm)
2.5. Cytotoxic studies
Cytotoxicity was studied by the brine-shrimp lethality assay method [18]. Brine-shrimp
(Artemia salina) eggs were hatched in artificial sea water (3.8 g sea salt/L) at 23 1 °C.
After two days, these shrimps were transferred to vials containing 5 mL of artificial sea
water (10 shrimps per vial) with 10, 100, and 1000 μg/mL final concentrations of each com-
pound taken from their stock solutions of 12 mg/mL in DMSO. After 24 h, the number of
surviving shrimps was counted. Data were analyzed with a Biostat 2009 computer program
(Probit analysis) to determine LD50 values.
2.6. DNA interaction studies by UV–vis spectroscopy
0.2 g of SS-DNA (salmon sperm) was dissolved in 100 mL of double-deionized water and
kept at 4 °C. Solutions of DNA in 20 mM Tris-HCl (pH 7.4) gave the ratio of UV absor-
bance at 260 and 280 nm, A260/A280, of 1.86 indicating that the DNA is sufficiently free
from protein [20]. The DNA concentration was determined via absorption spectroscopy
using the molar absorption coefficient of 6600 M−1 cm−1 at 260 nm [21], and was found as
7.45 × 10−5 M. From this stock solution, 3, 6, 9, 12, 15, 18, 21, 24, and 27 μM of working
solutions were prepared by dilution. The complexes were dissolved in 10% DMSO at a
concentration of 1 × 10−3 M of HL and 4 while 5 × 10−5 M of 1. The UV absorption titra-
tions were performed by keeping the complex concentration fixed while varying the con-
centration of DNA. Equivalent solutions of DNA were added to the complex and reference
solutions to eliminate the absorbance of DNA itself. Compound–DNA solutions were kept
for 30 min at ambient temperature before absorption measurements were recorded using
cuvettes of 1 cm path length.
2.7. Catalytic experiment
Transesterification of linseed oil was carried out using organotin(IV) carboxylates as cata-
lysts in molar ratio of 400 : 100 : 1 (methanol, oil, and catalyst). Linseed oil (0.01 M) was
transesterified using 0.04 M methanol and 0.1 mM catalyst in a 100 mL three-necked round-
bottom flask equipped with a reflux condenser, magnetic stirrer, thermometer, and sampling

Organotin(IV)
329
outlet. Before the reaction, the organotin catalyst was solubilized in 0.5 mL chloroform.
The reaction mixture was refluxed and stirred with a constant speed of 600 rpm. The sample
1
was taken after 1, 8, 12, and 24 h and analyzed by H NMR to check the % conversion of
triglycerides in linseed oil into fatty acid methyl esters (biodiesel).
3. Results and discussion
3.1. Syntheses of 1–5
Di- and tri-organotin chlorides react with NaL in 1 : 2 and 1 : 1, respectively, while R2SnO
with HL in 1 : 2 M ratios gave complexes according to the following equations:
R2SnCl2 þ 2NaL ! R2SnL2 þ 2NaCl ð1Þ R ¼ CH3ð1Þ; n ꢀ C4H9ð2Þ;
R2SnO þ 2HL ! R2SnL2 þ H2O
R3SnCl þ NaL ! R3SnL þ NaCl
ð2Þ R ¼ n ꢀ C8H17ð3Þ
ð3Þ R ¼ CH3ð4Þ; C6H5ð5Þ
3.2. FT-IR analysis
FT-IR spectra of complexes were recorded from 4000 to 400 cm−1 to determine the mode
of carboxylate coordination to tin. The difference between the asymmetric and symmetric
carboxylate stretch [Δν = νasym(COO−) − νsym(COO−)] gives the mode of binding of COO−
to tin [22]. Difference (Δν) between asymmetric (COO−) and symmetric (COO−) absorption
frequencies below 200 cm−1 represents bidentate carboxylate, and greater than 200 cm−1 repre-
sents unidentate carboxylate [23]. The magnitudes of Δν [(νasym(COO−) − νsym(COO−)] for 1–
5 are in the range 141–163 cm−1, indicating bidentate carboxylates in these complexes [24];
Δν value between 150 and 250 cm−1 indicates bridging while a value less than 150 cm−1
exhibits a chelate structure [1]. The carboxylates are chelated bidentate in 1–3 and 5 in solid
state. The Δν for 4 is compatible with bridging bidentate giving five-coordinate Sn(IV) in the
solid state and is matched with X-ray structure of 4. The structures proposed by IR data are
consistent with reported data [1, 9, 25, 26]. Absorption bands at 420–494 cm−1 for 1–5 are
assigned to Sn–O stretch which indicates the formation of complexes [27].
3.3. NMR studies
All the protons in 1–5 were identified by the position and number with the protons
calculated from the incremental method [28]. The methyl protons of dimethyltin(IV) (1)
and trimethyltin(IV) (4) are sharp singlets with well-defined satellites at 1.09–1.28 and
0.52–0.71 ppm having coupling constants of 82/79 and 58/56 Hz [2J (119/117Sn,1H)], respec-
2
tively. The determined J values indicate five or six coordination for 1 and four coordina-
tion for 4 in solution, consistent with our earlier reports [25, 26]. The protons of n-butyltin
(IV) derivative showed a complex pattern and were assigned according to the literature [29,
1
30]. Despite the complex pattern of H NMR spectra of di-n-butyltin(IV) (2) derivative, a
clear triplet due to terminal methyl appears at 0.92–0.97 ppm. The methylene protons (CH2)
of n-octyltin(IV) 3 exhibited different behavior compared with the n-butyl groups of the
respective complexes and gave broad/multiplets at 0.84–1.30 ppm, which are consistent
with the values calculated by the incremental method [28].

330
M. Tariq et al.
1
The carbons attached to Sn atom have satellites due to J [119Sn, 13C] coupling. The
1
values of the coupling constant J [119Sn, 13C] are helpful to find coordination behavior of
1
tin(IV) in solution. Complexes 1–3 exhibit J [119Sn, 13C], 704 Hz, 705 Hz, and 575 Hz,
respectively, which correspond to five-coordinate tin(IV) in solution [31]. Complexes 4 and
1
5 exhibit J [119Sn, 13C], 388 Hz, and 648 Hz which correspond to four-coordinate tin(IV)
in solution [32]. The C–Sn–C angles, calculated by Lockhart’s equation [33], also support
five-coordinate di-organotin(IV) complexes 1–3 and four coordinate for tri-organotin(IV)
complexes 4 and 5 in solution.
119Sn NMR spectroscopy is valuable for the study of coordination number of tin(IV) with
wide range of tin chemical shifts and strong dependence of tin chemical shifts upon the sub-
stituents. 119Sn NMR spectra show only a sharp singlet, indicating the formation of a single
species. A representative 119Sn NMR spectrum is shown in figure 1. 119Sn NMR values for
1–3 are –136.9, –147.9, and –162.7 ppm, respectively, falling in the range of five coordina-
tion and for 4 and 5 are +127.8 and +101.8 ppm, respectively, suggesting four-coordinate
tin(IV) in non-coordinating solvent [33].
3.4. Crystal structure of 4
The molecular and polymeric bridging structure of 4 is shown in figures 2 and 3, respec-
tively. The crystal data, selected bond lengths, and bond angles of 4 are listed in tables 1
and 2, respectively. Sn1 is bonded with three methyl carbons and two oxygens from the
Figure 1. 119Sn NMR of 1.

Organotin(IV)
331
Figure 2. Molecular structure of Me3SnL (4).
Figure 3. Polymeric structure of 4.

332
M. Tariq et al.
CH3
CH2
C
CH3
CH2
a) pyridine
b) piperidine
CO2
H2O
H
COOH
CH=C
COOH
+
+
+
CHO
c) 24 h stirring
d) 100 oC
COOH
Scheme 1.
Table 1. Crystal data and structure refinement parameters for Me3SnL (4).
Chemical formula
Formula mass (g M−1
Crystal system
Space group
Unit cell dimensions
a (Å)
C16H24O2Sn
367.06
Monoclinic
P21/n
)
6.9755(2)
b (Å)
9.9214(2)
c (Å)
25.2694(6)
α (°)
90.00
β (°)
97.806(1)
γ (°)
90.00
1732.61(7)
4
Volume (Å3)
Z
Temperature (K)
296(2)
1.407
1.472
744
Density (Calcd) (g cm−3
)
Absorption coefficient (mm−1
F (0 0 0)
)
Radiation (A0) (Mo-Kα)
Index ranges
0.71073
h: −8→8; k: −10→12; l: −31→31
2.62–26.0
3411
θ (°) Min Max
Total reflections
Restraints/parameters
Rall, Rgt
WRref, WRgt
Goodness of fit
06/206
0.0316, 0.0241
0.0647, 0.0594
1.034
Table 2. Selected bond lengths (Å) and angles (°) of Me3SnL (4).
Bond lengths (Å)
Sn1–C14
Sn1–C13
Sn1–C12
2.112(3)
2.112(3)
2.118(3)
O1–C1
O2–C1
Sn1–O1
Sn1–O2
1.241(3)
1.282(3)
2.457(2)
2.1687(19)
Bond angles (°)
C14–Sn1–C13
C14–Sn1–C12
C13–Sn1–C12
C14–Sn1–O1
C13–Sn1–O1
C12–Sn1–O1
120.78(15)
121.87(15)
116.23(16)
85.41(10)
83.48(11)
90.63(11)
O1–Sn–O2
172.15(7)
98.04(11)
88.71(11)
93.49(11)
143.91(19)
C14–Sn1–O2
C13–Sn1–O2
C12–Sn1–O2
C1–O1–Sn1
two carboxylates. The Sn1–O1 and Sn1–O2 bond distances are 2.457(2) and 2.1687(19) Å,
respectively, showing the former is coordinate covalent (shorter than the sum of the van der
Waals radii of connected atoms, 3.68 Å) and the latter is a covalent bond (covalent bond
length, 2.13 Å) [34]. The two oxygens bonded with Sn1 have axial positions O1–Sn1–O2

Organotin(IV)
333
11
9
10
5
6
3
HC
8
2
7
4
C
1
HOOC
5'
6'
Scheme 2.
angle of 172.15(7)° and the three methyl groups are equatorial having angles C14–Sn1–
C13 = 120.78(15) Å, C14–Sn1–C12 = 121.87(15) Å, and C13–Sn1–C12 = 116.23(16) Å.
Therefore, Sn1 is best assigned as distorted trigonal bipyramidal, consistent with FT-IR and
literature [9, 34, 35]. The geometry around Sn can also be deduced by the value of
τ = (β–α)/60, where β is the largest and α is the second largest basal angles around Sn [36].
The angle values α = β = 180° correspond to a square-pyramidal geometry and the value of
α = 120° corresponds to a perfect trigonal-bipyramidal geometry. Thus, τ = 0 for a perfect
Table 3. Antibacterial data of HL and its organotin(IV) complexes.
Average zone of inhibition (mm) mean SD
Staphylococcus
aureus
Escherichia
coli
Bordetella
bronchiseptica
Pseudomonas
aeruginosa
Micrococcus
luteus
Compound
HL
1
2
3
4
5
0
0
14
13
17
20
17
15
13.3 0.6
15
17
19
25.7 0.6
26
2
1
0
0
20
19
17
0
1
19
17
17
20
19
21
24
1
1
2
1
1
1
1
1
1
1
1
1
1
11
13
25
14
26
30
1
1
1
1
1
1
1
1
1
17.7 0.6
25
26
24
25
1
1
2
1
Cefixime
Roxythromycin
22.3 0.6
27
1
1
Note: Concentration: 1 mg/mL of DMSO. Reference drugs, Roxythromycin and Cefixime 1 mg/mL.

334
M. Tariq et al.
Table 4. Antifungal activity data of HL and its organotin(IV) complexes.
Mean value of percent growth inhibition (%) mean SD
Aspergillus niger Aspergillus fumigatus Fusarium solani
13
Compound
Aspergillus flavus
Mucor spp.
HL
11
13
15
15
19
22
3
2
2
2
2
3
1
0
0
38
60
24
22
74
86
2
3
2
1
3
3
1
14.3 0.6
13
2
1
3
2
2
12
2
1
1
2
3
2
3
4
5
17
14
25
64
1
3
2
3
14
15
17
13
16
16
52
50
Terbinafine
100
100
100
100
100
Note: In vitro agar tube dilution method, concentration: 200 μg/mL of DMSO; % inhibition of fungal growth = 100 – gt/gc × 100.
gt = linear growth in test (mm) and gc = linear growth in vehicle control (mm).
Table 5. Cytotoxicity data of HL and its organotin(IV) complexes.
No. of shrimps killed out of 20 per dilution
Compound
1000 μg/mL
100 μg/mL
10 μg/mL
LD50 (μg/mL)
HL
1
2
3
4
5
10
20
15
12
20
20
0
06
12
8
0
06
05
06
08
15
0
729
108
137
620
19
8
15
20
0
6
Vehicle control
square-pyramidal and unity for perfect trigonal-bipyramidal. In 4, β = 172.15° and
α = 121.87°, so the calculated τ value is 0.84 which indicates distorted trigonal-bipyramidal
arrangement around Sn.
3.5. Biological studies
3.5.1. Antibacterial studies. In vitro antibacterial activities of HL and 1–5 were carried
out by agar-well diffusion. Roxyithromycin and Cefixime were used as positive controls.
The results are shown in table 3. Criteria for activity are based on zone of inhibition (mm)
with inhibition zone more than 20 mm showing significant activity, 18–20 mm activity is
good, 15–17 mm is low, and below 14 mm is non-significant [20]. The antibacterial study
showed that all the complexes were active compared to HL but lower than standard
reference drugs except 4 and 5 whose antibacterial activities against P. aeruginosa is com-
parable with the standard reference drugs. In general, 4 and 5 (tri-organotin(IV) derivatives)
exhibited greater activity than 1–3 (di-organotin(IV) derivatives), consistent with the
literature [9].
3.5.2. Antifungal studies. HL and 1–5 were also screened for antifungal activities against
five fungal strains (A. flavus, A. niger, A. fumigatus, F. solani, and Mucor spp.) by using
agar tube dilution method. The results are shown in table 4. Terbinafine was used as stan-
dard drug in this assay. Criteria for activity are based on percent growth inhibition with

Organotin(IV)
335
Figure 4. Absorption spectra of 1 mM HL in the absence (a) and the presence of 3–27 μM (a–j) DNA. The arrow
direction indicates increasing concentrations of DNA. Inside graph is the plot of A0/(A – A0) vs. 1/[DNA] for the
determination of binding constant and Gibb’s free energy of the HL – DNA adduct.
Figure 5. Absorption spectra of 50 μM of compound in the absence (a) and the presence of 3–27 μM (a–j)
DNA. The arrow direction indicates increasing concentrations of DNA. Inside graph is the plot of A0/(A – A0)
vs. 1/[DNA] for the determination of binding constant and Gibb’s free energy of the 1 – DNA adduct.

336
M. Tariq et al.
Figure 6. Absorption spectra of 1 mM of compound in the absence (a) and the presence of 3–27 μM (a–j)
DNA. The arrow direction indicates increasing concentrations of DNA. Inside graph is the plot of A0/(A – A0) vs.
1/[DNA] for the determination of binding constant and Gibb’s free energy of the 4 – DNA adduct.
more than 70% growth inhibition considered significant, 60–70% good, 50–60% moderate
while below 50% non-significant. The data showed that the antifungal activity of 1–5 was
higher than HL but lower than the reference drug.
3.5.3. Cytotoxic studies. Cytotoxicities of HL and 1–5 were studied in vitro using the
brine-shrimp lethality method by using reference drug MS-222 (Tricaine methanesulfonate);
the results are shown in table 5. The data are based on mean value of two replicates each of
10, 100, and 1000 μg mL−1. The LD50 data exhibited that 4 and 5 are toxic while ligand
HL and 1–3 are less toxic.
3.6. DNA binding studies
The binding interactions of HL, 1 and 4 with SS-DNA were investigated by comparing
their absorption spectra with and without SS-DNA. Both HL, 1 and 4 showed minor
bathochromic shift of the spectral band with significant hypochromicity, suggesting groove
binding as well as intercalation of the compounds with the DNA helix (figures 4–6). This
may be attributed to the presence of phenyl group that facilitates interaction with double-
stranded DNA [37]. After 24 h, the spectrum was taken again and the same results were
obtained which confirm the stability of the drug-DNA complex.
The intrinsic binding constant K of HL, 1 and 4 were calculated to compare binding
strengths of ligand-DNA and complex-DNA by using the Benesi–Hildebrand equation [38]:
A0
eG
eG
1
¼
þ
ꢁ
A ꢀ A0 eHꢀG ꢀ e0 eHꢀG ꢀ eG K½DNAꢂ

Organotin(IV)
337
where K = binding constant, A0 = absorbance of the drug, A = absorbance of the drug and its
complex with DNA, εG = absorption coefficient of the drug and εH–G = absorption coeffi-
cient of the drug-DNA adduct. The binding constants were obtained from the intercept to
slope ratios of A0/(A−A0) versus 1/[DNA] plots. The binding constants were 6.04 × 103 M−1
(HL), 9.6 × 103 M−1 (1), and 1.7 × 104 M−1 (4).
The Gibb’s free energies (ΔG) of HL, 1 and 4 were determined at 298 K to be –21.57
(HL), –9.9 (1), and –24.14 kJ M−1 (4), respectively.
3.7. Catalytic activities of organotin(IV) complexes
The catalytic activities of the complexes were investigated in a transesterification reaction
of triglycerides in linseed oil with methanol to produce biodiesel. The general transesterifi-
cation can be represented as:
The tin complexes were selected due to the Lewis acid character of tin. Tin can expand its
coordination, an important factor in the catalytic activity. The tin may activate carbonyl
groups in triglycerides and cause interaction by increasing the electrophilicity of carbonyl
carbon.
The transesterification of triglycerides in linseed oil was carried out with molar ratio of
400 : 100 : 1 (methanol : oil : catalyst) to assess the catalytic activity of organotin(IV)
carboxylates. The conditions for experiments were not optimized and no attempt was made
to remove the added catalyst from the reaction mixture. However, the effect of reaction time
on % conversion of triglycerides into fatty acid methyl esters (biodiesel) was studied.
1
Samples were taken from the reaction mixture at 1, 8, 16, and 24 h and analyzed by H
NMR to calculate % conversion of triglycerides into fatty acid methyl esters. The equation
used [39, 40] to quantify the extent of transesterification was
2AMe
C ¼ 100 ꢁ
3ACH
2
where C = percentage conversion of triglycerides to corresponding methyl esters, AMe = inte-
gration value of the methoxy protons of the methyl esters, and ACH2 = integration value of
1
α-methylene protons. The results are shown in figure 7. A representative H NMR spectrum
showing % conversion (30.82%) of oil (triglycerides) into biodiesel (fatty acid methyl esters)

338
M. Tariq et al.
is shown in Supplementary material (see online supplemental material at http://dx.doi.org/
10.1080/00958972.2014.884217). The characteristic peak of methoxy protons was observed
as a strong singlet at 3.65 ppm and a triplet of α-CH2 protons at 2.33 ppm. These two peaks
are distinct for the confirmation of conversion of oil (triglycerides) into biodiesel [13]. The
peaks at 4.11–4.32 ppm indicate glyceridic protons and incomplete conversion of oil (triglyc-
erides) into biodiesel (fatty acid methyl esters). These glyceridic protons peaks are absent in
complete conversion of oil (triglycerides) into biodiesel [13]. The tested complexes showed
less to good catalytic activity in the order n-Bu2SnL2 < Me2SnL2 < Ph3SnL < Me3SnL. The
proposed mechanisms for the catalytic activity of organotin(IV) carboxylates involve Lewis
acid character of tin as in polyesterification [41–43]. All the tested complexes showed cata-
lytic activity but Me3SnL and Ph3SnL showed better activity which may be due to the pres-
ence of methyl groups which cause less hindrance during attack on triglyceride molecules
and greater Lewis acidity in Ph3SnL. The Lewis acidities of Sn increase, Me < Bu < Ph,
because methyl and butyl groups are electron-donating while phenyl is electron-withdrawing.
The stronger Lewis acidity should have greater catalytic efficiency but in our studies catalytic
performance is in the reverse order, due to size of groups (Me, Bu, Ph) attached to Sn. How-
ever, catalytic activity (not Lewis acidity) may be related to geometry because in R3SnL com-
pared to R2SnL2, there would be more space for attack of nucleophilic group.
3.8. Structure–activity relationship of organotin(IV) complexes
The structure–activity relationship is required to understand the relationship between the
structure and biological activity of compounds. This relationship is also helpful to predict
the activity of compounds even before synthesizing. The behavior of tin is exceptional,
i.e. non-toxic in its inorganic forms and highly toxic in organotin form probably due to its
structures. Organotin(IV) complexes display a broad spectrum biological effect and have
been extensively studied as bactericides, fungicides, wood preservatives, antitumor, and
anticancer agents [44–46]. The biological activities may depend on substituted aromatic
rings in the ligand and organic groups attached with tin [47–50]. To correlate structure and
activity of biocidal complexes, we have studied antibacterial results of 47 and antifungal
results of 38 organotin(IV) complexes [1, 9, 25, 26, 51–53]. Oxygen and sulfur donor
ligands having substituted aromatic rings have increased biological activities. Muhammad
et al. [1, 9] showed that attachment of halo group, e.g. bromo derivatives [9], gave better
Figure 7. Percentage conversion of linseed oil triglycerides into biodiesel.

Organotin(IV)
339
biocidal results than methoxy derivatives [1]. However, fluoro- and cyano-substituted aro-
matic derivatives [25, 26] show interesting and better results than bromo and methoxy
derivatives [1, 9]. Rehman et al. [51] used sulfur donor ligands with more aromatic and
cyclic parts with better results than Jabbar et al. [52]. Electron-withdrawing groups in the
aromatic moiety provided better biocidal results [9, 25, 26] than electron-donating groups
[1] due to electron-withdrawing character that make the benzene ring electron deficient with
delocalized positive charge promoting interaction with negatively charged outer layer of
Gram-negative bacteria. For Gram-positive bacteria, complexes having substituted aromatic
rings exhibit more activity [1, 25, 26]; complexes with no substituent on aromatic ring
showed less activity [53].
4. Conclusion
Five organotin(IV) complexes of 2-(4-ethylbenzylidene) butanoic acid were synthesized and
characterized by elemental analysis, FT-IR, and NMR (1H, 13C, and 119Sn). 4 was also ana-
lyzed by single-crystal X-ray analysis; 4 has tetrahedral geometry in solution and distorted
trigonal bipyramidal with polymeric bridging behavior in the solid state. The antimicrobial
studies revealed that 4 and 5 have greater activity than 1–3. Cytotoxic studies revealed that
5 shows more toxicity than 1–4. DNA binding studies showed that HL, 1 and 4 bind with
DNA via groove binding as well as intercalation, due to the presence of phenyl that facili-
tates interaction with double-stranded DNA. Me3SnL exhibited better catalytic activity in
transesterification for the synthesis of biodiesel due to small methyl groups which cause less
hindrance during attack on triglyceride molecules.
Supplementary material
Crystallographic data for the structure reported in this article has been deposited with the
Cambridge Crystallographic Data Center, CCDC 891311 of complex 4. Copies of this
information may be obtained free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ (Fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
Acknowledgement
M. Tariq (Pin no. 074-0616-PS4-099) is thankful to higher education commission of Pakistan for
financial support during the present study. He is also thankful to Muhammad Sirajuddin (PhD
HEC scholar on IRSIP program) in the Department of Chemistry, Loughborough University, UK
for 119Sn NMR studies of the complexes.
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