# Hypercoordinated Organosilicon(IV) and Organotin(IV) Complexes: Syntheses, Spectral Studies, and Antimicrobial ActivityIn Vitro

Authors: Kiran Singh; Parvesh Puri; Yogender Kumar; Chetan Sharma
Journal: ISRN Inorganic Chemistry
 (2013)

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## 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.


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## Body

## 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 (25±1°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 (Figure1) [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%).


## 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 (25±1°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 (Figure1) [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%).


## 2.3.1. 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%).


## 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 Table1. 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
v(N–H)
v(–C=N)
v;(C=S)a/v(C–S)b
v(S–H)
v(M–S)
v(M–N)
v(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–CH2–CH3, CH2–CH2–CH3
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

The13CNMR 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

The29Si 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 δ−103.35 ppm, δ−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 δ−172.14 ppm, δ−240.30 ppm, respectively [23].Figure 2
29Si NMR spectra of Si (1 : 2) metal complex of ligand (HL1).


## 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 Table1. 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
v(N–H)
v(–C=N)
v;(C=S)a/v(C–S)b
v(S–H)
v(M–S)
v(M–N)
v(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–CH2–CH3, CH2–CH2–CH3
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

The13CNMR 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

The29Si 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 δ−103.35 ppm, δ−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 δ−172.14 ppm, δ−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:
(1)percentinhibitionofmyelialgrowth=(dc-dt)dc×100,dc = average diameter of fungal colony in negative control sets,dt = 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.
aValues, including diameter of the well (8 mm), are means of three replicates.Table 5
Minimum inhibitory concentration (MIC) (μg/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
5Table 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
77.7Figure 3
Comparison of antifungal activity of compounds with commercial antibiotic.


## 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:
(1)percentinhibitionofmyelialgrowth=(dc-dt)dc×100,dc = average diameter of fungal colony in negative control sets,dt = 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.
aValues, including diameter of the well (8 mm), are means of three replicates.Table 5
Minimum inhibitory concentration (MIC) (μg/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
5Table 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
77.7Figure 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 (Figure4) 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.



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*Source: 356802-2013-02-20.xml*