2.2.1 [Ph₂Sn(OPrⁱ)(Hhbgl)] (1)

The solution of diphenyltin(IV) diisopropoxide (1.955 g, 0.005 mol) and H₂hbgl (1.067 g, 0.0045 mol) was refluxed in benzene (30 ml) for 8–10 h at 95–100°. The complex, [Ph₂Sn(OPrⁱ)(Hhbgl)], isolated as a yellow colour solid, was purified by recrystallization from alcohol at room temperature and dried under reduced pressure.

Yield: 1.746 g; colour: yellowish brown; m.p. >300 °C; Molar conductance: 30 μS cm⁻¹; elemental analysis calculated for C₂₈H₃₅NO₄Sn: C 59.12%, H 6.16%, N 2.46%, Sn 20.89%; Found: C 59.16%, H 6.20%, N 2.40%, Sn 20.92%; Isopropanol in azeotrope calculated: 0.299 g; Found: 0.270 g.

IR (ν_max, cm⁻¹; in KBr): 3405 ν(OH), 3055 ν(C–H), 2959, 2872 ν_as(C–H)/ν_s(C–H), 1639 ν_as(COO), 1416 ν_s(COO), Δν: 223, 1077, 1028ν(C–O) Sn, 1220 ν(C–O), 551 ν(Sn–O), 269, 228 ν_as(Sn–C)/ν_s(Sn–C).

¹H NMR (DMSO, ppm): δ 7.29–7.86 (m, 12H, Ar–H and Sn–C₆H₅), δ 8.06 (s, 1H, phenolic –OH), δ 6.9 (br/s, 1H, –NH), δ 2.10 (s, 3H, –CH₃), δ 1.09 (d, 3H, J = 4 Hz, –CH(CH₃)₂), δ 1.04 (d, 3H, J = 4.4 Hz, –CH(CH₃)₂).

2.2.2 [Ph₂Sn(Hhbgl)₂] (2)

Complex 2 was prepared in the similar way as complex 1 using diphenyltin(IV) diisopropoxide (1.955 g, 0.005 mol) and H₂hbgl (2.135 g, 0.009 mol).

Yield: 2.288 g; colour: cream; m.p. >300 °C; Molar conductance: 22 μS cm⁻¹; elemental analysis calculated for C₃₈H₄₆N₂O₆Sn: C 61.17%, H 6.17%, N 3.76%, Sn 15.92%; Found: C 61.20%, H 6.21%, N 3.72%, Sn 15.98%; Isopropanol in azeotrope calculated: 0.598 g; Found: 0.540 g.

IR (ν_max, cm⁻¹; in KBr): 3406 ν(OH), 3056 ν(C–H), 2959, 2863 ν_as(C–H)/ν_s(C–H), 1639 ν_as(COO), 1413 ν_s(COO), Δν: 226, 1081, 1022 ν(C–O) Sn, 1221 ν(C–O), 570 ν(Sn–O), 281, 222 ν_as(Sn–C)/ν_s(Sn–C), 441 ν(Sn ← N).

¹H NMR (DMSO, ppm): δ 6.58–7.86 (m, 14H, Ar–H and Sn–C₆H₅), δ 8.11 (s, 2H, phenolic –OH), δ 6.8 (br/s, 2H, –NH), δ 2.10 (s, 6H, CH₃), δ 1.14 (d, 6H, J = 6.8 Hz, –CH(CH₃)₂), δ 1.05 (d, 6H, J = 7.2 Hz, –CH(CH₃)₂).

2.2.3 [Bu₂Sn(OPrⁱ)(Hhbgl)] (3)

Complex 3 was prepared in the similar way as complex 1 using dibutyltin(IV) diisopropoxide (1.754 g, 0.005 mol) and H₂hbgl (1.067 g, 0.0045 mol).

Yield: 1.593 g; colour: light yellow; m.p. >300 °C; Molar conductance: 25 μS cm⁻¹, elemental analysis calculated for C₂₄H₄₃NO₄Sn: C 54.51%, H 8.14%, N 2.65%, Sn 22.47%; Found: C 54.56%, H 8.18%, N 2.60%, Sn 22.50%; Isopropanol in azeotrope calculated: 0.299 g; Found: 0.269 g.

IR (ν_max, cm⁻¹; in KBr): 3417 ν(OH), 2956 cm⁻¹, 2862 ν_as(C–H)/ν_s(C–H), 1602 ν_as(COO), 1412 ν_s(COO), Δν: 190, 1071 ν(C–O) Sn, 1212 ν(C–O), 566 ν(Sn–O)/ν(Sn–C).

¹H NMR (MeOD, ppm): δ 6.35–6.95 (m, 2H, Ar-H), δ 8.21 (s, 1H, phenolic –OH), δ 0.88–1.59 (m, 18 H, Sn–C₄H₉).

2.2.4 [Bu₂Sn(Hhbgl)₂] (4)

Complex 4 was prepared in the similar way as complex 1 using dibutyltin(IV) diisopropoxide (1.754 g, 0.005 mol) and H₂hbgl (2.135 g, 0.009 mol).

Yield: 2.195 g; colour: dark brown; m.p. >300 °C; Molar conductance: 18 μS cm⁻¹; elemental analysis calculated for C₃₄H₅₄N₂O₆Sn: C 57.83%, H 7.65%, N 3.97%, Sn 16.82%; Found: C 57.86%, H 7.68%, N 3.94%, Sn 16.86%; Isopropanol in azeotrope calculated: 0.598 g; Found: 0.538 g.

IR (ν_max, cm⁻¹; in KBr): 3416 ν(OH); 2958, 2865 ν_as(C–H)/ν_s(C–H); 1605 ν_as(COO); 1413 ν_s(COO); Δν: 192; 1088, 1034 ν(C–O) Sn; 566 ν(Sn–O)/ν(Sn–C);

¹H NMR (MeOD, ppm): δ 6.99 (d, 2H, J = 8.4 Hz, Ar–H), δ 6.92 (d, 2H, J = 8.2 Hz, Ar–H), δ 8.10 (s, 2H, phenolic –OH), δ 5.42 (br, 2H, –NH), δ 0.88, δ 1.03–1.59 (t, m, 18 H, Sn–C₄H₉).

2.2.5 [Me₂Sn(OPrⁱ)(Hhbgl)] (5)

The solution of dimethyltin(IV) diisopropoxide (1.334 g, 0.005 mol) and H₂hbgl (1.067 g, 0.0045 mol) was refluxed in toluene (30 ml) for 15–16 h at 95–100°. The complex, [Me₂Sn(OPrⁱ)(Hhbgl)], isolated as a yellow brown solid, was purified by recrystallization from alcohol at room temperature and dried under reduced pressure.

Yield: 1.329 g; colour: light yellow; m.p. >300 °C; Molar conductance: 55 μS cm⁻¹; elemental analysis calculated for (C₁₈H₃₁NO₄Sn): C 48.68%, H 6.99%, N 3.16%, Sn 26.75%; Found: C 48.72%, H 7.05%, N 3.10%, Sn 26.80%; Isopropanol in azeotrope calculated: 0.299 g; Found: 0.271 g.

IR (ν_max, cm⁻¹; in KBr): 3432 ν(OH), 2961, 2876 ν_as(C–H)/ν_s(C–H), 1638 ν_as(COO), 1408 ν_s(COO), Δν: 230, 1076 ν(C–O) Sn; 1212 ν(C–O), 571, 549 ν_as(Sn–C)/ν_s(Sn–C), 558 ν(Sn–O), 443 ν(N → Sn).

¹H NMR (MeOD, ppm): δ 6.59 (d, 1H, J = 8.2 Hz, Ar–H), δ 7.01 (d, 1H, J = 7.9 Hz, Ar–H), δ 7.80 (s, 1H, phenolic –OH), δ 6.5 (br, 1H, –NH), δ 0.52–0.84 (s, 6H, Sn–CH₃, ²J ^117/119Sn–H = 96 Hz)

2.2.6 [Me₂Sn(Hhbgl)₂] (6)

Complex 6 was prepared in the similar way as complex 5 using dimethyltin(IV) diisopropoxide (1.334 g, 0.005 mol) and H₂hbgl (2.135 g, 0.009 mol).

Yield: 1.898 g; colour: dark brown; m.p. >300 °C; Molar conductance: 45 μS cm⁻¹; elemental analysis calculated for C₂₈H₄₂N₂O₆Sn: C 54.13%, H 6.77%, N 4.51%, Sn 19.12%; Found: C 54.18%, H 6.82%, N 4.46%, Sn 19.17%; Isopropanol in azeotrope calculated: 0.598 g; Found: 0.540 g.

IR (ν_max, cm⁻¹; in KBr): 3419 ν(OH), 2960, 2864 ν_as(C–H)/ν_s(C–H), 1632 ν_as(COO), 1409 ν_s(COO), Δν: 223, 1052 ν(C–O) Sn; 1221 ν(C–O), 547, 520 ν_as(Sn–C)/ν_s(Sn–C), 557 ν(Sn–O), 445 ν(N → Sn).

¹H NMR (MeOD, ppm): δ 6.96 (d, 2H, J = 7.5 Hz, Ar–H), δ 6.57 (d, 2H, J = 8.4 Hz, Ar–H), δ 8.06 (s, 1H, phenolic –OH), δ 8.50 (s, 1H, phenolic –OH), δ 1.17 (d, 6H, J = 6.6 Hz, –CH(CH₃)₂), δ 1.07 (d, 6H, J = 6.9 Hz, –CH(CH₃)₂), δ 0.46 (s, 6H, Sn–CH₃, ²J ^117/119Sn–H = 105 Hz).

2.3.1 Antibacterial activity

The diorganotin(IV) complexes were screened for their antibacterial activity against four bacterial strains Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa, using the agar well diffusion method. All the microbial cultures were adjusted to 0.5 McFarland standards, which are visually comparable to a microbial suspension of approximately 1.5 × 10⁸ cfu/ml. 20 ml of the Mueller Hinton agar medium was poured into each Petri plate and plates were swabbed with 100 μl inocula of the test microorganisms and kept for 15 min for adsorption. Using sterile cork borer of 8 mm diameter, wells were bored into the seeded agar plates and these were loaded with a 100 μl volume with concentration of 2.0 mg/ml of each compound reconstituted in the dimethylsulphoxide (DMSO). All the plates were incubated at 37 °C for 24 h. Antibacterial activity of each synthetic compound 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 a positive control. This procedure was performed in three replicate plates for each organism (Aneja et al., 2011).

2.3.2 Determination of minimum inhibitory concentration (MIC) of chemical compounds

MIC of various complexes against bacterial strains was determined by a modified agar well diffusion method (Aneja et al., 2011). In this method, a twofold serial dilution of each chemically synthesized complex was prepared by first reconstituting the complex in DMSO followed by dilution in sterile distilled water to achieve a decreasing concentration range of 512–1 μg/ml. A 100 μl volume of each dilution was introduced into wells (in triplicate) in the agar plates already seeded with 100 μl of standardized inoculum (10⁶ cfu/ml) of the test microbial strain. All test plates were incubated aerobically at 37 °C for 24 h and observed for the inhibition zones. MIC, taken as the lowest concentration of the chemical compound that completely inhibited the growth of the microbe, showed by a clear zone of inhibition, was recorded for each test organism. Ciprofloxacin was used as the positive control while DMSO as the negative control.

2.3.3 Antifungal activity

The antifungal activity of chemical compounds was evaluated by the poisoned food technique.

The 15 ml of molten SDA (45 °C) was poisoned by the addition of 100 μl volume of each diorganotin(IV) complex having concentration of 2.00 mg/ml, reconstituted in the DMSO, poured into a sterile Petri plate and allowed it to solidify in the laminar flow at room temperature. A fungal plug (8 mm diameter), from the stock of different fungal strains, was inoculated at the centre of the solidified poisoned agar plates in the laminar air flow with a sterile inoculating needle. The inoculated agar slants were then incubated further at 25 °C for 7 consecutive days and were daily checked. DMSO was used as the negative control whereas; Fluconazole was used as the positive control. The experiments were performed in triplicates. Diameter of fungal colonies was measured and expressed as percent mycelial inhibition by applying the formula (Al-Burtamani et al., 2005) $$\text{Percent inhibition of myelial growth}=(\text{dc}-\text{dt})/\text{dc}\times 100$$

dc = average diameter of fungal colony in negative control sets,

dt = average diameter fungal colony in experimental sets.

3.1.1 Elemental analysis

Analytical data were in good agreement with the proposed stoichiometry of the diorganotin(IV) complexes.

3.1.2 Molar conductance

Molar conductance of the synthesized complexes showed very low values, indicating their non-electrolytic nature (Jamil et al., 2010).

3.1.3 Infrared spectra

IR spectra of the diorganotin complexes displayed a broad band at 3405 (1), 3406 (2), 3417 (3), 3416 (4), 3432 (5), and 3419 (6) cm⁻¹ (Singh, 1985; Abdellah et al., 2009), which were assignable to the unbonded –OH stretching of the phenolic group. The bridging carboxylates in organotin complexes were simply characterized by infrared spectroscopy by the shift of ν(COO) bands, as compared with the parent ligand. The magnitude of Δν [ν_as − ν_s] was used to determine the coordination status of the carboxylate group (Sandhu and Verma, 1987; Deacon and Phillips, 1980). In infrared spectra of [Ph₂Sn(OPrⁱ)(Hhbgl)] (1), [Ph₂Sn(Hhbgl)₂] (2), [Bu₂Sn(OPrⁱ)(Hhbgl)] (3), [Bu₂Sn(Hhbgl)₂] (4), [Me₂Sn(OPrⁱ)(Hhbgl)] (5) and [Me₂Sn(Hhbgl)₂] (6), the asymmetric stretching frequencies ν_as(COO) and symmetric stretching frequencies ν_s(COO) appeared at 1639, 1416; 1639, 1413; 1602, 1412; 1605, 1413; 1638, 1408; 1632, and 1409 cm⁻¹ respectively. The magnitude of ν_as − ν_s (Δν) for 1, 2, 5 and 6 were above 200 cm⁻¹, indicating the unidentate bonding of the carboxylate group to the tin atom (Fig. 1). While, for the complexes 3 and 4, the values of Δν were below 200 cm⁻¹, this clearly demonstrated that these complexes adopted the bidentate carboxylate structure (Fig. 1). A medium peak at 550–570 cm⁻¹ was assignable to the Sn–O stretching frequency, which further confirmed the bonding of the carboxylate group to the tin atom (Nath and Yadav, 1998, 1997). The proposed coordination of the diphenyl- and dimethyltin(IV) complexes was further supported by the appearance of a band in the region of 441–456 cm⁻¹ due to the ν(N → Sn) (Nath et al. 2003). The far IR spectra of diphenyltin(IV) complexes showed Sn–C stretching vibrations, ν_as(Sn–C) and ν_s(Sn–C) bands at 269–281 cm⁻¹ and at 225 ± 3 cm⁻¹, respectively, as reported in the literature (Nath et al., 2001; Nath and Saini, 2011), whereas the corresponding peaks at 559 ± 12 and at 535 ± 15 cm⁻¹, (Nath et al., 2001) have also been assigned in the spectra of dimethyltin(IV) complexes. While in dibutyltin(IV) complexes the appearance of Sn–C stretching bands was not certain due to overlapping with the Sn–O stretching vibrations.

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Figure 1

Proposed structure of diorganotin(IV) complexes.

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3.1.4 ¹HNMR spectroscopy

¹HNMR data for the synthesized complexes and ligand in DMSO and MeOD solutions are given in the experimental section. ¹H NMR spectra of the complexes, displayed signals in the region of δ 7.80–8.50 ppm which may be attributed to the unbonded phenolic group proton (Williams and Fleming 2004). In the complex 1 and 2, the signals for the phenyl group attached to tin were observed in the range of δ 6.58–7.86 ppm, in conjugation with phenyl protons of the ligand. ¹H NMR spectra of the dibutyltin(IV) complexes showed the triplet at δ 0.88 ppm and multiplets in the range of δ 0.88–1.59 ppm (Matela and Aman, 2012; Nath et al., 1997; Shahzadi et al., 2008) due to butyl protons attached to the tin atom. The ²J (^117/119Sn–C–¹H) value was only observed in the spectrum of the dimethyltin(IV) complexes. The estimated value of C–Sn–C bond angle, in complex 6, was found to be 154° by substituting ²J (^117/119Sn–C–H) value in the Lockhart equation (Lockhart and Manders, 1986), indicating an octahedral geometry (Torres et al., 2011). While the value of coupling constant ²J (^117/119Sn–C–H) for the complex 5, in MeOD, was found to be 96 Hz, indicating five-coordination (Casas et al., 2003).

3.2.1 Antibacterial activity

The antibacterial activity result is presented in Tables 1 and 2, respectively. These results show that diorganotin(IV) complexes possessed a variable antibacterial activity against both Gram-positive and Gram negative bacteria. Complexes namely, 1 and 2 showed the highest zone of inhibition of 28.3 mm and 29.6 mm against S. aureus which is more than the standard antibiotic used in this study, while complex 5 showed the zone of inhibition of 26.6 mm against B. subtilis which was equal to the standard antibiotic used in this study. However in case of Gram negative bacteria, two complexes, 2 and 5 were found to be most effective against E. coli with the zone of inhibition 21.3 and 21.6 mm, respectively and complex 5 against P. aeruginosa with the zone of inhibition 20.3 mm (Table 1). Thus it has been shown that the diorganotin(IV) complexes showed a higher activity against Gram positive bacteria as compared to Gram negative bacteria.

In the whole series, the MIC of diorganotin(IV) complexes ranged between 4 and 32 μg/ml against Gram positive bacteria and 128 and 32 μg/ml against Gram negative bacteria, respectively. Complexes 1 and 2 were found to be the best as they exhibit the lowest MIC of 4 μg/ml against S. aureus and complex 5 against B. subtilis with the lowest MIC of 8 μg/ml. In case of Gram negative bacteria, complexes 2 and 5 showed the lowest MIC of 32 μg/ml against E. coli, whereas diorganotin(IV) complexes were found to be exhibit very low activity against P. aeruginosa (Table 2). Comparison of MIC of organotin complexes with standard antibiotic, ciprofloxacin is represented in Fig. 2.

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Figure 2

Comparison of MIC (μg/ml) of synthesized complexes with standard antibiotic. C, ciprofloxacin.

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3.2.2 Antifungal activity

In case of fungal strains four complexes 1, 2, 4 and 5 showed more than 60% inhibition of mycelial growth against A. niger and Penicillium sp. whereas only one complex 5 showed more than 60% inhibition of mycelial growth against Aspergillus flavus (Table 3). Comparison of antifungal activity of newly synthesized complexes with a standard drug fluconazole against the tested fungal strains in terms of % mycelial growth inhibition is shown in Fig. 3.

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Figure 3

Comparison of (%) mycelial growth inhibition of complexes with standard antifungal drug. F, fluconazole.

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