ISRN Inorganic Chemistry

Volume 2013 (2013), Article ID 356802, 8 pages

http://dx.doi.org/10.1155/2013/356802

Research Article

Hypercoordinated Organosilicon(IV) and Organotin(IV) Complexes: Syntheses, Spectral Studies, and Antimicrobial Activity In Vitro

Kiran Singh,¹ Parvesh Puri,¹ Yogender Kumar,¹ and Chetan Sharma²

¹Department of Chemistry, Kurukshetra University, Kurukshetra 136 119, India

²Department of Microbiology, Kurukshetra University, Kurukshetra 136 119, India

Received 27 December 2012; Accepted 14 January 2013

Academic Editors: A. Abu-Surrah, A. Barbieri, A. M. Fonseca, A. Karadag, and S. Turmanova

Copyright © 2013 Kiran Singh et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This paper deals with the syntheses and structural features of some new diorganosilicon(IV) and diorganotin(IV) complexes having general formulae (CH₃)₂MCl(L¹), (CH₃)₂MCl(L²), (CH₃)₂M(L¹)₂, and (CH₃)₂M(L²)₂ with new Schiff bases (M = Si and Sn). The Schiff bases HL¹ and HL² have been derived from the condensation of 3-bromobenzaldehyde with 4-amino-3-ethyl-5-mercapto-1,2,4-triazole and 4-amino-5-mercapto-3-propyl-1,2,4-triazole, respectively. The compounds have been characterized by the elemental analyses, molar conductance, and spectral (UV, IR, ¹H, ¹³C, ²⁹Si, and ¹¹⁹Sn NMR) studies. The resulting complexes have been proposed to have trigonal bipyramidal and octahedral geometries. In vitro antimicrobial activities of the compounds have been carried out.

1. Introduction

The chemistry of complexes with hypercoordinated silicon and tin atoms is interesting from many points of view such as reactivity, biological activity, and structural features as reported in several reviews [1–3]. The Schiff bases bearing additional donor groups represent the most important class of heteropolydentate ligands capable of forming mono-, bi-, and polynuclear complexes with different metal ions. Enhanced reactivities of silicon complexes due to increased coordination number as well as modified electronic properties as a result of modified ligand sphere and coordination geometries have been reported [4, 5]. Singh et al. recently reported the activity of Schiff base complexes with silicon against pathogenic fungi and bacteria [6, 7]. The insecticidal and nematicidal activities were also reported for some hypercoordinated silicon complexes [8]. Organosilicon compounds of N- and S-containing ligands are well known for their anticarcinogenic, tuberculostatic, antimicrobial, and acaricidal activities [9, 10]. Similarly organotin(IV) compounds have been receiving increasing attention in the area of inorganic and metal organic chemistry due to the important industrial [11] (pesticides, antifouling paints, and fire retardants), pharmacological [12, 13] (antifungal, antibacterial, and antitumor drugs), and environmental applications. Organotin(IV) complexes are extensively studied due to their coordination geometries as well as structural diversity (Monomeric, dimeric, hexameric, and oligomeric) [14]. So by keeping in mind the various applications of the organosilicon(IV) and organotin(IV) complexes and further continuation of our work [15, 16], we report here the syntheses, characterization, and biological activity of some new silicon and tin complexes of the Schiff bases derived from the condensation of 3-bromobenzaldehyde with different triazoles.

2. Experimental

2.1. Materials

Adequate care was taken to keep chemicals, glass apparatus, and organosilicon(IV) and organotin(IV) complexes free from moisture. All the chemicals and solvents were used under dry conditions. To attain dry conditions, all the apparatus used during the experimental work were fitted with Quickfit interchangeable standard ground joints. All the reagents, namely, dimethylsilicondichloride (Acros), dimethyltindichloride (TCI-America), and 3-bromobenzaldehyde (Spectrochem) were used as such.

2.2. Analytical Methods and Physical Measurements

Silicon and tin were determined gravimetrically as SiO₂ and SnO₂. Melting points were determined on a capillary melting point apparatus. Molar conductance measurements of 10⁻³ M solution of metal complexes in dry DMF were measured at room temperature (°C) with a conductivity bridge type 305 Systronics model. Carbon, hydrogen, nitrogen, and sulfur were estimated using the elemental analyzer Heraeus Vario EL-III Carlo Erba 1108 at the Central Drug Research Institute, Lucknow, India. The electronic spectra of the ligands and their metal complexes were recorded in dry methanol, on a Systronics, Double Beam spectrophotometer 2203, in the range of 600–200 nm. The IR spectra of the ligands and metal complexes were recorded in Nujol mulls/KBr pellets using BUCK scientific M5000 grating spectrophotometer in the range of 4000–350 cm⁻¹. Nuclear magnetic resonance spectra (¹H, ¹³C) were recorded on Bruker-300ACF, and ²⁹Si and ¹¹⁹Sn were recorded on Bruker-400ACF spectrometer in DMSO-d₆ using Me₄Si as an internal standard for ¹H, ¹³C, ²⁹Si, and Me₄Sn for ¹¹⁹Sn spectra.

2.3. Syntheses of Ligands

4-Amino-3-ethyl-5-mercapto-1,2,4-triazole and 4-amino-5-mercapto-3-propyl-1,2,4-triazole were synthesized by the reported method (Figure 1) [17].

Figure 1: Structure of Schiff bases.

2.3.1. 4-(3-Bromobenzylidene-Amino)-3-Ethyl-5-Mercapto-s-Triazole (HL¹)

A solution of 4-amino-3-ethyl-5-mercapto-1,2,4-triazole (2.0 g, 14 mmol) in ethanol (40 mL) was treated with 3-bromobenzaldehyde (2.57 g, 14 mmol). The reaction mixture was refluxed for 4 h. After the completion of the reaction, the reaction mixture was kept overnight at room temperature, and the product was filtered, washed, and recrystallized from the same solvent:

 m.p. 198°C, greenish white, yield: 86%, (found: C, 42.30; H, 3.02; N, 18.73; S, 10.71%. Calcd. for C₁₁H₁₁BrN₄S: C, 42.45; H, 3.56; N, 18.00; S, 10.30%).

2.3.2. 4-(3-Bromobenzylidene-Amino)-5-Mercapto-3-Propyl-s-Triazole (HL²)

An ethanolic solution of 3-bromobenzaldehyde (2.34 g, 13 mmol) was added with stirring to an ethanolic solution of 4-amino-5-mercapto-3-propyl-1,2,4-triazole (2.0 g, 13 mmol) and refluxed for 4 h. After the completion of the reaction, the reaction mixture was kept overnight at room temperature, and the product was filtered, washed, and recrystallized from the same solvent:

 m.p. 188°C, light green, yield: 83%, (found: C, 44.30; H, 4.19; N, 17.23; S, 9.87%. Calcd. for C₁₂H₁₃BrN₄S: C, 44.32; H, 4.03; N, 17.23; S, 9.86%).

2.4. Synthesis of Organometallic Complexes

2.4.1. Synthesis of 1 : 1 Organosilicon Complexes

To the Me₂SiCl₂ (0.100 g, 0.8 mmol) in ~30 mL of dry methanol, was added the sodium salt of the corresponding ligands in 1 : 1 molar ratio. The sodium salts of the ligands were prepared by dissolving the sodium metal (0.018 g, 0.8 mmol), HL¹ (0.242 g, 0.8 mmol), and HL² (0.252 g, 0.8 mmol) in ~30 mL dry methanol. The reaction mixture was refluxed for about 12 h and then allowed to cool at room temperature. Sodium chloride formed during the reaction was separated by filtration through sintered funnel. The excess of solvent was removed under reduced pressure by vacuum pump, and the resulting solid was repeatedly washed with 5–10 mL dry cyclohexane and again dried under vacuum:

 Me₂SiCl(L¹): m.p. 210°C, yellow, yield: 78%, (found: C, 38.81; H, 4.01; N, 13.78; S, 7.71; Si, 5.42%. Calcd. for C₁₃H₁₆BrClN₄SSi: C, 38.67; H, 3.99; N, 13.87; S, 7.94; Si, 6.96%). Me₂SiCl(L²): m.p. 232°C, yellow, yield: 81%, (found: C, 41.24; H, 3.35; N, 13.44; S, 7.70; Si, 5.73%. Calcd. for C₁₄H₁₈BrClN₄SSi: C, 40.24; H, 4.34; N, 13.41; S, 7.67; Si, 6.72%).

2.4.2. Synthesis of 1 : 2 Organosilicon Complexes

To the Me₂SiCl₂ (0.100 g, 0.8 mmol) in ~30 mL of dry methanol, was added the sodium salt of the corresponding ligands in 1 : 2 molar ratio. The sodium salts of the ligands were prepared by dissolving the sodium metal (0.036 g, 1.6 mmol), HL¹ (0.282 g, 1.6 mmol), and HL² (0.504 g, 1.6 mmol), in ~30 mL dry methanol. The reaction mixture was refluxed for about 12 h and then allowed to cool at room temperature. Sodium chloride formed during the reaction was separated by filtration through sintered funnel. The excess of solvent was removed under reduced pressure by vacuum pump, and the resulting solid was repeatedly washed with 5–10 mL dry cyclohexane and again dried under vacuum:

 Me₂Si(L¹)₂: m.p. 242°C, brown, yield: 74%, (found: C, 42.12; H, 3.80; N, 16.48; S, 9.39; Si, 4.91% Calcd. for C₂₄H₂₆Br₂N₈S₂Si: C, 42.48; H, 3.86; N, 16.51; S, 9.45; Si, 4.14%). Me₂Si(L²)₂: m.p. 246°C, light green yield: 82%, (found: C, 43.20; H, 4.11; N, 15.83; S, 9.11; Si, 3.02% Calcd. for C₂₆H₃₀Br₂N₈S₂Si: C, 44.19; H, 4.28; N, 15.86; S, 9.08; Si, 3.97%).

2.4.3. Synthesis of 1 : 1 Organotin Complexes

The sodium salts of the ligands were prepared by dissolving the sodium metal (0.011 g, 0.46 mmol) and HL¹ (0.142 g, 0.46 mmol), HL² (0.148 g, 0.46 mmol) in ~30 mL dry methanol. Then, to the Me₂SiCl₂ (0.100 g, 0.46 mmol) in ~30 mL of dry methanol, was added the sodium salt of the corresponding ligands in 1 : 1 molar ratio. The reaction mixture was refluxed for about 12 h and then allowed to cool at room temperature. Sodium chloride formed during the reaction was separated by filtration through sintered funnel. The excess of solvent was removed under reduced pressure by vacuum pump, and the resulting solid was repeatedly washed with 5–10 mL dry cyclohexane and again dried under vacuum:

 Me₂SnCl(L¹): m.p. 204°C, off-white, yield: 75%, (found: C, 31.50; H, 3.29; N, 11.30; S, 6.51; Sn, 23.22% Calcd. for C₁₃H₁₆BrClN₄SSn: C, 31.58; H, 3.26; N, 11.33; S, 6.49; Sn, 24.01%). Me₂SnCl(L²): m.p. 252°C, off-white, yield: 83%, (found: C, 33.11; H, 3.50; N, 11.12; S, 6.29; Sn, 22.39%. Calcd. for C₁₄H₁₈BrClN₄SSn: C, 33.07; H, 3.57; N, 11.02; S, 6.31; Sn, 23.35%).

2.4.4. Synthesis of 1 : 2 Organotin Complexes

To the Me₂SnCl₂ (0.100 g, 0.46 mmol) in ~30 mL of dry methanol, was added the sodium salt of the corresponding ligands in 1 : 2 molar ratio. The sodium salts of the ligands were prepared by dissolving the sodium metal (0.022 g, 0.92 mmol) and HL¹ (0.281 g, 0.92 mmol), HL² (0.296 g, 0.92 mmol) in ~30 mL dry methanol. Sodium chloride formed during the reaction was separated by filtration through sintered funnel. The reaction mixture was refluxed for about 12 h and then allowed to cool at room temperature. The excess of solvent was removed under reduced pressure by vacuum pump, and the resulting solid was repeatedly washed with 5–10 mL dry cyclohexane and again dried under vacuum:

 Me₂Sn(L¹)₂: m.p. 262°C, light yellow, yield: 79%, (found: C, 37.51; H, 3.31; N, 14.53; S, 8.40; Sn, 14.49%. Calcd. for C₂₄H₂₆Br₂N₈S₂Sn: C, 37.48; H, 3.41; N, 14.57; S, 8.34; Sn, 15.43%). Me₂Sn(L²)₂: m.p. 206°C, light yellow, yield: 78%, (found: C, 39.20; H, 3.86; N, 14.12; S, 8.08; Sn, 13.91%. Calcd. for C₂₆H₃₀Br₂N₈S₂Sn: C, 39.17; H, 3.79; N, 14.06; S, 8.04; Sn, 14.89%).

3. Results and Discussion

The resulting complexes have been obtained as colored solids which are soluble in DMSO, DMF, and MeOH but insoluble in other organic solvents. The ligands show a sharp melting point, while the complexes decompose in a range of temperature (204–262°C). The molar conductivity values measured for 10⁻³ M solutions in anhydrous DMF are in the range of 10–17 Ω⁻¹ cm² mol⁻¹, showing that all 1 : 1 and 1 : 2 complexes are nonelectrolytic in nature [3].

3.1. Electronic Spectra

The electronic spectra of ligands HL¹ and HL² exhibit maxima at 365 nm and 364 nm, respectively, which could be assigned to the n-π* transition of the azomethine group. These bands show a blue shift in 1 : 1 and 1 : 2, Si(IV) and Sn(IV) metal complexes and appear at 359 nm, 355 nm, 352 nm, and 355 nm, for Me₂SiCl(L¹), Me₂Si(L¹)₂, Me₂SiCl(L²), and Me₂Si(L²)₂ and at 358 nm, 355 nm, 356 nm, and 358 nm for Me₂SnCl(L¹), Me₂Sn(L¹)₂, Me₂SnCl(L²), and Me₂Sn(L²)₂, respectively. This blue shift is due to the polarization within the >C=N chromophore group caused by the metal-ligand interaction which clearly indicates the coordination of azomethine nitrogen atom to the metal atom [18]. Further, the electronic spectra of both the ligands exhibit medium intensity band at 260 nm due to π-π* transition which remain unchanged in the spectra of metal complexes.

3.2. IR Spectra

The IR spectra of the free ligands were compared with the spectra of organosilicon(IV) and organotin(IV) complexes in order to study the binding mode of the Schiff bases to the metal ions in the new complexes. Several significant changes with respect to the ligands are observed in the corresponding organometallic complexes, which are listed in Table 1. The IR spectra of the ligands HL¹ and HL², show medium intensity bands in the region of 3109 cm⁻¹ and 2754 cm⁻¹ which may be assigned to ν(N–H) and ν(S–H) vibrations, respectively. Other bands in the region of 1165 cm⁻¹ due to ν(C=S) [19] suggest that ligands exist as in the thiol-thione tautomerism (Figure 1). The disappearance of these bands in the corresponding metal complexes and the appearance of a new band ~770 cm⁻¹ due to ν(C–S) indicate the deprotonation of the thiol group of triazole which support the complexation through sulfur atom. The metal sulphur bond formation is further supported by a band at ~446 cm⁻¹ and ~420 cm⁻¹ for ν(Si–S) and ν(Sn–S) in the spectra of organosilicon(IV) and organotin(IV) complexes, respectively [20]. A sharp and strong band in the region of 1582 cm⁻¹ for ν(N=CH) in case of ligands was shifted to a higher wavelength number and appears in the region of 1602–1610 cm⁻¹ in the spectra of metal complexes, indicating the coordination of ligands through azomethine nitrogen to the metal atom. The metal nitrogen bond was further supported by the presence of a band at about ~562 cm⁻¹ for ν(Si–N) and ~538 cm⁻¹ for ν(Sn–N) [21]. A strong band in the region of 412–362 cm⁻¹ was assigned to ν(M–Cl) for 1 : 1 metal complexes.

Table 1: IR-spectroscopic data (cm⁻¹) of the ligands and their metal complexes.

Compound
(N–H)
(–C=N)
(C=S)a(C–S)b
(S–H)
(M–S)
(M–N)
(M–Cl)

HL¹
3109
1582
1165
2754
—
—
—

Me₂SiCl(L¹)
—
1605
765
—
454
576
422

Me₂Si(L¹)₂
—
1602
741
—
442
562
—

Me₂SnCl(L¹)
—
1600
725
—
420
536
362

Me₂Sn(L¹)₂
—
1610
771
—
432
542
—

HL²
3109
1582
1165
2770
—
—
—

Me₂SiCl(L²)
—
1607
779
—
449
564
418

Me₂Si(L²)₂
—
1603
779
—
446
558
—

Me₂SnCl(L²)
—
1605
771
—
421
538
378

Me₂Sn(L¹)₂
—
1610
756
—
423
544
—

a: Ligands.b: Complexes.

3.3. ¹HNMR Spectra

The proton magnetic resonance spectra of the ligands and their corresponding organosilicon and organotin complexes were recorded in DMSO-d₆ using TMS as internal standard. The bonding pattern is further supported by ¹HNMR spectral studies of the ligands and their corresponding organometallic complexes. The chemical shift values (δ, ppm) of the different protons are given in Table 2. The ¹HNMR spectra of the ligands exhibit peaks at δ 11.66 ppm (s, 1H) and δ 11.57 ppm (s, 1H) characteristic of the –SH proton of triazole for HL¹ and HL², respectively [16]. The disappearance of the signal due to –SH proton in the spectra of metal complexes indicates the deprotonation of the thiol group and that supports the coordination of ligand through sulphur atom to the central metal atom. A signal at δ 10.56 ppm (s, 1H) and 10.54 (s, 1H) ppm was observed due to azomethine proton in the spectra of free ligands HL¹ and HL², respectively, which moves downfield in the ¹HNMR spectra of metal complexes [22] and indicates the bonding through the azomethine nitrogen atom to the central metal atom. Other peaks that are found around the δ value 7.16–8.15 (4H) are due to aromatic protons. The signal in the region δ 0.6–1.5 ppm is also observed in the spectra of complexes due to CH₃-M group. Some additional signals, due to the aliphatic chain attached to the triazole moiety, also appeared in the ¹HNMR spectra of the ligands and their metal complexes, as reported in Table 2.

Table 2: 1HNMR chemical shifts of the ligands and their metal complexes.

Compound
–CH=N
–SH
Aromatic–H
Triazole–CH ₂–CH ₃, CH ₂–CH ₂–CH ₃

HL¹
10.56
11.66
7.35–8.15(s, d, t)
2.85–2.92(q), 1.35–1.40(t)

Me₂SiCl(L¹)
10.89
—
7.20–8.04(s, d, t)
2.27–2.38(q), 1.12–1.17(t)

Me₂Si(L¹)₂
11.22
—
7.34–8.05(s, d, t)
2.29–2.37(q), 1.15–1.20(t)

Me₂SnCl(L¹)
11.04
—
7.31–8.05(s, d, t)
2.28–2.39(q), 1.12–1.20(t)

Me₂Sn(L¹)₂
10.87
—
7.16–8.05(s, d, t)
2.27–2.35(q), 1.13–1.22(t)

HL²
10.54
11.57
7.35–8.04(s, d, t)
2.81–2.85(t), 1.79–1.86(m), 1.03–1.08(t)

Me₂SiCl(L²)
10.98
—
7.34–8.13(s, d, t)
2.63–2.68(t), 1.37–1.66(m), 0.90–0.92(t)

Me₂Si(L²)₂
10.81
—
7.33–7.98(s, d, t)
2.76–3.06(t), 1.25–1.56(m), 0.98–1.07(t)

Me₂SnCl(L²)
11.05
—
7.16–8.05(s, d, t)
2.72–2.91(t), 1.59–1.74(m), 0.86–0.91(t)

Me₂Sn(L¹)₂
10.94
—
7.27–8.04(s, d, t)
2.59–2.64(t), 1.53–1.67(m), 0.89–0.94(t)

3.4. ¹³CNMR Spectra

The ¹³CNMR spectral data of ligands HL¹ and HL² and their corresponding 1 : 1 and 1 : 2 metal complexes were also recorded in DMSO-d₆ and reported in Table 3. The proposed coordination in these complexes has been supported by the shifting in chemical shift values of the carbon atoms attached to the azomethine nitrogen atom and thiolic sulfur atom. The other carbon atoms remain almost undisturbed [15]. The new signal due to the methyl groups attached to the metal atom in the spectra of complexes has also been reported in Table 3. All these data also support the coordination of the ligands through nitrogen and sulfur atoms to the central metal atom.

Table 3: 13C NMR chemical shifts of the ligands and their metal complexes.

Compound
C₁/C₂
C₃/C₄
C₅/C₆
C₇/C₈
C₉/C₁₀
C₁₁/C₁₂
M–CH₃

HL¹
123.18/127.64
130.46/130.87
135.12/134.75
162.36/153.96
158.47/10.29
18.90
—

Me₂SiCl(L¹)
122.06/128.04
130.68/130.96
136.93/135.42
166.84/147.99
154.43/20.19
12.78
31.43

Me₂Si(L¹)₂
121.93/127.05
130.72/130.81
136.26/133.92
167.25/147.09
153.23/21.81
12.05
29.34

Me₂SnCl(L¹)
122.27/127.32
129.92/131.03
136.43/133.98
165.23/149.06
153.92/19.53
12.89
33.89

Me₂Sn(L¹)₂
122.67/127.08
130.77/130.97
136.39/134.41
164.89/149.48
153.54/18.97
14.44
31.54

HL²
123.18/127.58
130.47/130.93
135.13/134.75
162.38/152.88
158.50/13.62
19.47/26.97
—

Me₂SiCl(L²)
122.94/127.43
129.78/131.25
136.78/134.17
163.79/151.56
154.26/22.12
15.65/22.14
30.45

Me₂Si(L²)₂
122.71/127.49
131.75/131.13
136.91/135.54
164.21/150.97
155.03/19.96
24.97/18.58
33.86

Me₂SnCl(L²)
123.69/127.39
130.98/131.06
136.79/134.23
164.02/151.39
154.89/23.53
19.06/19.36
32.42

Me₂Sn(L¹)₂
121.91/127.26
130.74/130.74
137.13/134.72
163.89/152.03
154.38/21.26
23.02/14.77
34.53

3.5. ²⁹Si and ¹¹⁹Sn NMR Spectra

The ²⁹Si and ¹¹⁹Sn NMR chemical shifts are very sensitive to the coordination number of the silicon and tin. So in order to propose the geometry of the complexes, ²⁹Si and ¹¹⁹Sn NMR spectra of the organosilicon and organotin complexes of ligand HL¹ were recorded (Figure 2). These ²⁹Si and ¹¹⁹Sn NMR chemical shift values greatly shifted upfield on bonding to the Lewis base. The ²⁹Si NMR spectra of {Me₂SiCl(L¹)} and {Me₂Si(L¹)₂} give sharp signals at −103.35 ppm, −110.40 ppm, respectively, which clearly indicates the penta- and hexa-coordinated environment around the silicon atom. Similarly, the observed ¹¹⁹Sn NMR chemical shifts of the studied complexes, {Me₂SnCl(L¹)} and {Me₂Sn(L¹)₂}, are in the range of penta- and hexa-coordinated environment, which appears at −172.14 ppm, −240.30 ppm, respectively [23].

Figure 2: ²⁹Si NMR spectra of Si (1 : 2) metal complex of ligand (HL¹).

4. Biological Assay

4.1. Test Microorganisms

Four bacteria, Staphylococcus aureus (MTCC 96), Bacillus subtilis (MTCC 121) (Gram positive), Escherichia coli (MTCC 1652), and Pseudomonas aeruginosa (MTCC 741) (Gram negative), were procured from MTCC, Chandigarh; and two fungi, A. niger and A. flavus, the ear pathogens isolated from the patients of Kurukshetra, were used in the present study.

4.2. In Vitro Antibacterial Activity

All the newly synthesized Schiff bases and their organometallic complexes were screened for their antibacterial activities against test bacteria, namely, S. aureus, B. subtilis (Gram positive), E. coli, and P. aeruginosa (Gram negative). The activity is determined by reported agar well-diffusion method [24]. All the 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 Mueller-Hinton agar medium was poured into each Petri plate, and the agar plates were swabbed with 100 μL inocula of each test bacterium 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 hr. Antibacterial activity of each compound was evaluated by measuring the zone of growth inhibition against the test organisms with zone reader (Hi antibiotic 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.

4.3. Determination of Minimum Inhibitory Concentration (MIC)

MIC is the lowest concentration of an antimicrobial compound that will inhibit the visible growth of a microorganism after overnight incubation. MIC of the various compounds against bacterial strains was tested through a modified agar well-diffusion method [25]. In this method, a twofold serial dilution of each chemically synthesized compound was prepared by first reconstituting the compound in DMSO followed by dilution in sterile distilled water to achieve a decreasing concentration range of 256 to 0.5 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 inoculums (10⁶ cfu/mL) of the test microbial strain. All test plates were incubated aerobically at 37°C for 24 hr 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.

4.4. In Vitro Antifungal Activity

The ligands and their metal complexes were also screened for their antifungal activity against two fungi, namely, A. niger and A. flavus, the ear pathogens isolated from the patients of Kurukshetra [26], by poison-food technique [27]. The moulds were grown on Sabouraud dextrose agar (SDA) at 25°C for 7 days and used as inocula. The 15 mL of molten SDA (45°C) was poisoned by the addition of 100 μL volume of each compound having concentration of 4.0 mg/mL, reconstituted in the DMSO, poured into a sterile Petri plate, and allowed it to solidify at room temperature. The solidified poisoned agar plates were inoculated at the center 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:

  = average diameter of fungal colony in negative control sets,  = average diameter fungal colony in experimental sets.

4.5. Observations

The antibacterial data reveals that the free ligands and their metal complexes are active against gram-positive bacteria (S. aureus and B. subtilis) and inactive against gram-negative bacteria (E. coli and P. aeruginosa). It has also been observed that the organometallic complexes are more affective as compared to the free ligands. Efficacy of all the compounds was found to be more potent inhibitors of bacterial growth as compared to the fungal culture. Among the synthesized compounds tested, 1 : 1 and 1 : 2 complexes of silicon and tin, that is, Me₂SiCl(L¹), Me₂Si(L¹)₂, Me₂Si(L²)₂, Me₂SnCl(L²), and Me₂Sn(L²)₂ show more antibacterial activity that is near to standard drug (ciprofloxacin) (Table 4). In the series, the MIC of the compounds ranged between 64 and 128 μg/mL against gram-positive bacteria. Compound Me₂Sn(L¹)₂ and Me₂SnCl(L²) show highest MIC of 64 μg/mL against S. Aureus and B. Subtilis (Table 5). The antifungal activity of compounds (Figure 3) shows more than 50% inhibition of mycelia growth against A. Niger and A. flavus (Table 6). Thus, it can be postulated that further studies of these complexes in this direction could lead to more interesting results.

Table 4: In vitro antibacterial activity of the ligands and their metal complexes.

Compounds
Zone of inhibition (mm)a

S. Aureus
B. Subtilis
E. Coli
P. Aeruginosa

HL¹
16.3
16.6
—
—

Me₂SiCl(L¹)
17.6
18.6
—
—

Me₂Si(L¹)₂
16.6
19.6
—
—

Me₂SnCl(L¹)
17.3
16.8
—
—

Me₂Sn(L¹)₂
16.9
15.3
—
—

HL²
16.3
15.3
—
—

Me₂SiCl(L²)
17.6
17.3
—
—

Me₂Si(L²)₂
18.6
15.6
—
—

Me₂SnCl(L²)
16.9
19.6
—
—

Me₂Sn(L²)₂
16.6
18.0
—
—

Ciprofloxacin
26.6
24
25.0
22.0

—: No activity. a Values, including diameter of the well (8 mm), are means of three replicates.

Table 5: Minimum inhibitory concentration (MIC) (g/mL) of the ligands and their metal complexes.

Compounds
S. Aureus
B. Subtilis

HL¹
128
128

Me₂SiCl(L¹)
128
64

Me₂Si(L¹)₂
64
128

Me₂SnCl(L¹)
64
128

Me₂Sn(L¹)₂
64
64

HL²
128
128

Me₂SiCl(L²)
64
64

Me₂Si(L²)₂
128
128

Me₂SnCl(L²)
64
64

Me₂Sn(L²)₂
64
128

Ciprofloxacin
5
5

Table 6: In vitro antifungal activity of the ligands and their metal complexes.

Compounds
Mycelial growth inhibition (%)

A. niger
A. flavus

HL¹
46.6
44.4

Me₂SiCl(L¹)
51.3
55.6

Me₂Si(L¹)₂
49.5
50.5

Me₂SnCl(L¹)
54.7
49.4

Me₂Sn(L¹)₂
59.1
50.9

HL²
41.1
45.5

Me₂SiCl(L²)
48.8
62.5

Me₂Si(L²)₂
56.6
48.4

Me₂SnCl(L²)
54.8
63.3

Me₂Sn(L²)₂
49.5
55.7

Fluconazole
81.1
77.7

Figure 3: Comparison of antifungal activity of compounds with commercial antibiotic.

5. Conclusions

Trigonal bipyramidal and octahedral geometries have been proposed for 1 : 1 and 1 : 2 organosilicon(IV) and organotin(IV) complexes (Figure 4) with the help of various spectral studies like UV, IR, ¹H, ¹³C, ²⁹Si, and ¹¹⁹Sn NMR. The antimicrobial studies suggested that all the Schiff bases were found to be biologically active and their metal complexes showed significantly enhanced antibacterial and antifungal activity against microbial strains in comparison to the free ligands, thus, exhibiting their broad spectrum nature and can be further used in pharmaceutical industry.

Figure 4: Proposed structures of the 1 : 1 and 1 : 2 complexes.

Acknowledgments

The financial assistance from UGC, New Delhi, India, vide Major Research Project F. no. 34-317/2008(SR), which provided Project fellowship to one of the authors (P. Puri), is gratefully acknowledged. The authors are also thankful to the Head of SAIF, Panjab University, Chandigarh, India and the Head of SAIF, IIT, Bombay, India, for providing elemental analyses and metal NMR.

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