Table of Contents
International Journal of Inorganic Chemistry
Volume 2013 (2013), Article ID 135496, 12 pages
http://dx.doi.org/10.1155/2013/135496
Research Article

Synthesis, Spectral, Thermal, Electrochemical, and Biocidal Activity of Tolyl/Benzyl Dithiocarbonates of Zinc(II)

Department of Chemistry, University of Jammu, Baba Saheb Ambedkar Road, Jammu 180006, India

Received 30 August 2013; Accepted 7 October 2013

Academic Editor: Daniel L. Reger

Copyright © 2013 Nidhi Kalgotra 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

Tolyl/benzyl dithiocarbonates of zinc(II) [( CH3C6H4 and C6H5CH2OCS2)2Zn] have been synthesized in 1 : 2 molar ratio by the reaction of zinc chloride, ZnCl2, with sodium salts of tolyl/benzyl dithiocarbonates CS2Na, in aqueous medium. These complexes were further reacted with nitrogen and phosphorous donor ligands to obtain donor stabilized complex of the type [[( CH3C6H4 and C6H5CH2)OCS2]2Zn.nL], (L = Bipy, Phen for and PPh3, Py for ). These complexes were characterised by elemental analysis, mass, IR, and NMR (1H, 13C, and 31P) spectroscopies. The thermal analysis (TGA/DTA), cyclic voltammetry, and SEM have also been done. Distorted tetrahedral and octahedral geometries around the Zn(II) metal are proposed. These complexes have depicted potential antibacterial and antifungal activity.

1. Introduction

Dithiocarbonates are sulfur and oxygen containing ligands which display rich and varied coordination chemistry with a wide range of transition and main group metals [1]. Transition metal dithiolate complexes exhibited versatile and interesting chemistry that have been studied extensively during the last decades [2]. Xanthates can form bidentate, monodentate, or network solids, showing a wide range of coordination behaviour [36]. More recent applications of xanthates and other thiocompounds are in the production of nanoparticles of metal sulphides [7, 8] and NLO properties [9, 10]. Metal xanthates are extensively used as corrosion inhibitors [11] and agricultural reagents [12, 13]. Dithiocarbonates have also found important use in medicine as antitumor agents [14, 15] and for treating Alzheimer’s disease [16]. Sodium and potassium ethylxanthate have antidotal effects in acute mercurial poisoning [17] and recently as coadjuvant in AIDS treatment [18]. The –OCS2 group of xanthates makes them more reactive towards various metals [19].

Zinc is an essential element and plays an important role in biochemical processes [20]. Ability of zinc(II) to coordinate with strategic ligand can lead to a structural and functional model for zinc metalloenzymes [21]. Zinc complexes of 1, 1-dithiolato ligands and their adducts with neutral ligands are known but not all dithiolate ligands have received the same attention [22]. Surprisingly, in spite of years of chemistry of the extensive and long-term use of alkyl xanthates as ligands [2328], structural and spectroscopic characterizations have been rather limited with regard to the aryl xanthates [29]. Fackler et al. [29], however, reported the synthesis of thallium aryl xanthates which in turn were used for the metathetical synthesis of other metal derivatives. A perusal of literature reveals no reports on the zinc(II)tolyl/benzyl dithiocarbonates. Thus, in continuation of our earlier research work on aryl dithiocarbonates, we herein report the synthesis and characterization of tolyl/benzyldithiocarbonates of zinc(II) in different coordination setup.

2. Experimental

2.1. Materials and Methods

Stringent precautions were taken to exclude moisture during the preparation of ligands. Moisture was carefully excluded throughout the experimental manipulations by using standard Schlenk’s techniques. Sodium salts of dithiocarbonates were obtained using literature procedures [30]. Toluene (Thomas Baker, B.P. 110°C) and n-hexane (Thomas Baker, B.P. 68-69°C) were freshly dried over sodium wire. Dichloromethane (Rankem, B.P. 40°C) and methanol (Thomas Baker, B.P. 64°C) were dried over P2O5 and CaCO3, respectively. Cresols (ortho-, meta-, and para-) and benzyl alcohol (Thomas Baker, B.P. 191°C, 203°C, 202°C, and 205°C) were purified by distillation prior to use.

2.2. Physical Measurements

Zinc was estimated gravimetrically as zinc ammonium phosphate [31]. Elemental analyses (C, H, N, and S) were carried out on CHNS-932 Leco Elemental analyzer and ESI mass spectra of the compounds were recorded on ESQUIRE 3000_00037 spectrophotometer from Indian Institute of Integrative Medicine (IIIM), Jammu. The IR spectra were recorded in KBr pallets in the range of 4000–200 cm−1 on a Perkin Elmer spectrum RX1-FT IR spectrophotometer and multinuclear (1H, 13C and 31P) NMR spectra were recorded in CDCl3 on a Brucker Avance II 400 MHz spectrometer using TMS as internal reference for 1H and 13C and 85% H3PO4 as external reference for 31P NMR at Sophisticated Analytical Instrumentation Facility (SAIF), Punjab University, Chandigarh. The thermogram was analyzed by using Perkin Elmer, diamond TG/DTA instrument. The thermogram was recorded in the temperature range from 30°C to 1000°C under nitrogen atmosphere,from National Chemical Lab (NCL), Pune. SEM studies were performed with a Zeiss EVO 50 instrument having magnification range 5x to 1,000,000x and at an accelerating voltage of 0.2 to 30 kV at Indian Institute of Technology (IIT), Delhi. The cyclic voltammograms were recorded on Autolab. Also the antifungal and antibacterial activities were tested under laboratory conditions in the Bioassay Lab, Department of Chemistry, University of Jammu, Jammu, using classical poison food technique and agar well diffusion method.

2.3. Synthetic Procedures
2.3.1. [(o-CH3C6H4OCS2)2Zn] (1)

Zinc(II)chloride (0.33 g, 2.42 mmol) was dissolved in 20 mL distilled water. A solution of sodium salt of o-tolyldithiocarbonate (1.00 g, 4.82 mmol) in 20 mL distilled water was added dropwise with constant stirring at room temperature. A white precipitate was formed immediately and the mixture was stirred for further 45 minutes. The precipitated white complex was filtered off by G-4 sintered funnel. The precipitates were washed first with water followed by petroleum ether three times followed by drying under in vacuo over P2O5, which yielded the complex (1) as white solid. Yield: 75% (0.78 g); M.P.: 203°C (dec.). Analytical data: C16H14O2S4Zn (MW = 431.9) Calcd.: C, 44.49; H, 3.27; S, 29.69; Zn, 15.14; Found: C, 44.16; H, 3.21; S, 29.59; Zn, 15.01. Spectral data: IR(KBr) v = 3359 v(C–H), 1581 v(C–C), 1238 v(C–O–C), 1032 v(S–C–S), 370 v(Zn–S) cm−1, 1H NMR (CDCl3) = 2.21 (s, 6H, CH3), 6.74 (d, 2H, ortho), 7.00 (m, 4H, meta), 6.77 (t, 2H, para) ppm; 13C NMR (CDCl3): = 19.92 (CH3), 114.20 (C-ortho), 120.61 (C-para), 124.80 (C–CH3), 129.47–129.88 (C-meta), 152.88 (C–O), 168.64 (OCS2) ppm. ESI-MS: m/z(%) = [M+] 431.9 (14) [(o-CH3C6H4OCS2)2Zn]; [M+] 183.0 (30) [o-CH3C6H4OCS2]; [M+] 107.0 (18) [o-CH3C6H4O]; [M+] 401.9 (15) [(C6H4OCS2)2Zn].

2.3.2. [(m-CH3C6H4OCS2)2Zn] (2)

Complex (2) was synthesized as white solid according to the protocol as described for complex (1); Yield: 78% (0.81 g); M.P.: 201°C (dec.). Analytical data: C16H14O2S4Zn (MW = 431.9) Calcd.: C, 44.49; H, 3.27; S, 29.69; Zn, 15.14; Found: C, 44.13; H, 3.23; S, 29.51; Zn, 15.11. Spectral data: IR(KBr) v = 3356 v(C–H), 1579 v(C–C), 1236 v(C–O–C), 1034 v(S–C–S), 365 v(Zn–S) cm−1, 1H NMR (CDCl3) = 2.23 (s, 6H, CH3), 6.84 (m, 2H, ortho), 6.94 (d, 2H, para), 7.04 (t, 2H, meta) ppm; 13C NMR (CDCl3): = 20.31 (CH3), 112.01–115.10 (C-ortho), 120.01 (C-para), 131.81 (C-meta), 134.32 (C–CH3), 150.38 (C–O), 166.94 (OCS2) ppm.

2.3.3. [(p-CH3C6H4OCS2)2Zn] (3)

Complex (3) was obtained as white solid following the same procedure as for complex (1). Yield: 74% (0.77 g); M.P.: 204°C (dec.). Analytical data: C16H14O2S4Zn (MW = 431.9) Calcd.: C, 44.49; H, 3.27; S, 29.69; Zn, 15.14; Found: C, 44.36; H, 3.25; S, 29.54; Zn, 15.07. Spectral data: IR(KBr) v = 3353 v(C–H), 1574 v(C–C), 1235 v(C–O–C), 1034 v(S–C–S), 364 v(Zn–S) cm−1, 1H NMR (CDCl3): = 2.10 (s, 6H, CH3), 6.75 (d, 4H, ortho), 7.03 (d, 4H, meta) ppm; 13C NMR (CDCl3): = 20.24 (CH3), 115.20 (C-ortho), 129.47 (C–CH3), 129.88 (C-meta), 150.81 (C–O), 169.02 (OCS2) ppm.

2.3.4. [(C6H5CH2OCS2)2Zn] (4)

Complex (4) was obtained as white solid following the same procedure as for complex (1). Yield: 77% (0.80 g); M.P.: 199°C (dec.). Analytical data: C16H14O2S4Zn (MW = 431.9) Calcd.: C, 44.49; H, 3.27; S, 29.69; Zn, 15.14; Found: C, 44.14; H, 3.22; S, 29.57; Zn, 15.09. Spectral data: IR(KBr) v = 3354 v(C–H), 1574 v(C–C), 1233 v(C–O–C), 1032 v(S–C–S), 360 v(Zn–S) cm−1; 1H NMR (CDCl3): = 4.29 (s, 4H, CH2), 7.01–7.21 (m, 10H, C6H5) ppm; 13C NMR (CDCl3): = 71.40 (CH2), 126.51–126.61 (C-ortho), 127.30–127.40 (C-meta), 128.61 (C-para), 135.96 (C–CH2), 180.96 (OCS2) ppm.

2.3.5. [(o-CH3C6H4OCS2)2Zn 2PPh3] (5)

To a colloidal solution of bis(o-tolyldithiocarbonate)zinc(II) (0.50 g, 1.15 mmol) in dichloromethane (~15 mL) was added a solution of triphenylphosphine (0.59 g, 2.29 mmol) dropwise with constant stirring at room temperature. Immediately the reaction mixture becomes transparent. The desired product [ 2Zn 2PPh3] (5) was obtained as white solid after removal of excess of dichloromethane in vacuo. Yield: 84% (1.84 g); M.P.: 210°C (dec.) Analytical data: C34H29O2S4P2Zn (MW = 956.52) Calcd.: C, 65.29; H, 4.64; S, 13.41; Zn, 6.84; Found: C, 65.16; H, 4.56; S, 13.34; Zn, 6.64. Spectral data: IR(KBr) v = 3356 v(C–H), 1575 v(C–C), 1230 v(C–O–C), 1039 v(S–C–S), 359 v(Zn–S), 535 v(Zn–P) cm−1; 1H NMR (CDCl3) = 2.21 (s, 6H, CH3), 6.71 (d, 2H, ortho), 7.09 (m, 4H, meta), 6.86 (t, 2H, para), 7.24–7.63 (m, 30H, PPh3) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 114.20 (C-ortho), 120.21 (C-para), 124.80 (C–CH3), 129.47–129.88 (C-meta), 152.02 (C–O), 172.49 (OCS2), 125.38, 127.01, 136.64, 141.68 (PPh3) ppm.; 31P NMR (CDCl3): = −5.05 (PPh3) ppm. ESI-MS: m/z(%) = [M+] 946.4 (15) [(o-CH3C6H4OCS2)2Zn 2PPh3]; [M+] 431.9 (6) [(o-CH3C6H4OCS2)2Zn]; [M+] 183.0 (7) [o-CH3C6H4OCS2]; [M] 107.0 (7) [o-CH3C6H4O].

2.3.6. [(o-CH3C6H4OCS2)2Zn Bipy] (6)

A solution of 2,2′-bipyridyl (0.18 g, 1.15 mmol) in dichloromethane (~15 mL) was added dropwise with constant stirring to a colloidal solution of [ 2Zn] (0.50 g, 1.15 mmol) in dichloromethane (~15 mL) at room temperature. The reaction mixture becomes transparent with pale yellow tint. The solvent was evaporated in vacuo and finally dried under reduced pressure which resulted in the formation of the compound (6) as off white solid. Yield:80% (1.08 g); M.P.: 223°C (dec.). Analytical data: C26H22O2S4ZnN2 (MW = 588.93) Calcd.: C, 53.10; H, 3.77; S, 21.81; Zn, 11.12; N, 4.76; Found: C, 53.02; H, 3.72; S, 21.61; Zn, 11.10; N, 4.69. Spectral data: IR(KBr) v = 3354 v(C–H), 1574 v(C–C), 1234 v(C–O–C), 1040 v(S–C–S), 352 v(Zn–S), 750 v(Zn–N) cm−1, 1H NMR (CDCl3) = 2.27(s, 6H, CH3), 6.72 (d, 2H, ortho), 7.01 (m, 4H, meta), 6.83 (t, 2H, para), 7.09–8.53 (m, 10H, Bipy) ppm; 13C NMR (CDCl3): = 19.28 (CH3), 113.02 (C-ortho), 119.10 (C-para), 124.96 (C–CH3), 130.75–130.78 (C-meta), 152.94 (C–O), 172.33 (OCS2), 124.81, 128.95, 136.85, 147.44, 147.56 (Bipy) ppm.

2.3.7. [(o-CH3C6H4OCS2)2Zn 2Py] (7)

For the synthesis of the compound [(o-CH3C6H4OCS2)2Zn 2Py] approximately, 20 mL dichloromethane colloidal solution of bis(o-tolyldithiocarbonate)zinc(II) (0.50 g, 1.15 mmol) was taken in a 100 mL of round bottom flask. To this suspension was added pyridine (0.18 g, 2.29 mmol) dropwise with constant stirring at room temperature. The reaction mixture becomes clear immediately after addition of pyridine. Evaporation of excess of solvent and final drying in vacuo result in the production of an off white solid. Yield: 85% (1.15 g); M.P.: 209°C (dec.) Analytical data: C26H24O2S4ZnN2 (MW = 590.15) Calcd.: C, 52.92; H, 4.10; S, 21.73; Zn, 11.08; N, 4.75; Found: C, 52.80; H, 4.03; S, 21.59; Zn, 11.01; N, 4.69. Spectral data: IR(KBr) v = 3353 v(C–H), 1575 v(C–C), 1230 v(C–O–C), 1038 v(S–C–S), 362 v(Zn–S), 754 v(Zn–N) cm−1; 1H NMR (CDCl3) = 2.27 (s, 6H, CH3), 6.72 (d, 2H, ortho), 7.01 (m, 4H, meta), 6.83 (t, 2H, para), 7.09–8.51 (m, 10H, Py) ppm; 13C NMR (CDCl3): = 19.28 (CH3), 113.02 (C-ortho), 119.10 (C-para), 124.70 (C–CH3), 130.75–130.78 (C-meta), 152.94 (C–O), 172.53 (OCS2), 124,23, 131.88, 149.63 (Py) ppm.

2.3.8. [(o-CH3C6H4OCS2)2Zn Phen] (8)

A solution of 1,10-phenanthroline 0.22 g (1.15 mmol) in dichloromethane (~15 mL) was added dropwise with constant stirring to a colloidal solution of [ 2Zn] 0.50 g (1.15 mmol) in dichloromethane (~15 mL) at room temperature. Immediately a clear solution with yellow tint was obtained. The excess of solvent was evaporated in vacuo which resulted in the formation of the compound (8) as off white solid. Yield: 84% (1.18 g); M.P.: 230°C (dec.). Analytical data: C28H22O2S4ZnN2 (MW = 612.15) Calcd.: C, 54.94; H, 3.62; S, 20.95; Zn, 10.69; N, 4.58; Found: C, 54.90; H, 3.68; S, 20.81; Zn, 10.60; N, 4.49. Spectral data: IR(KBr) v = 3360 v(C–H), 1572 v(C–C), 1227 v(C–O–C), 1036 v(S–C–S), 358 v(Zn–S), 753 v(Zn–N) cm−1; 1H NMR (CDCl3): = 2.10 (s, 6H, CH3), 6.75 (d, 2H, ortho), 7.00 (d, 4H, meta), 6.77 (t, 2H, para), 7.25–8.90 (m, 12H, Phen) ppm; 13C NMR (CDCl3): = 20.04 (CH3), 114.20 (C-ortho), 127.89 (C–CH3), 120.61 (C-para), 129.47–129.88 (C-meta), 150.81 (C–O), 172.63 (OCS2), 120.61, 124.23, 136.64, 138.64, 142.88, 149.63 (Phen) ppm.

2.3.9. [(m-CH3C6H4OCS2)2Zn 2PPh3] (9)

The complex (9) was synthesized as white solid according to the protocol described for complex (5). Yield: 84% (1.85 g); M.P.: 210°C (dec.). Analytical data: C34H29O2S4P2Zn (MW = 956.52) Calcd.: C, 65.29; H, 4.64; S, 13.41; Zn, 6.84; Found: C, 65.16; H, 4.56; S, 13.34; Zn, 6.64. Spectral data: IR(KBr) v = 3356 v(C–H), 1575 v(C–C), 1230 v(C–O–C), 1039 v(S–C–S), 359 v(Zn–S), 535 v(Zn–P) cm−1; 1H NMR (CDCl3) = 2.21 (s, 6H, CH3), 6.81 (m, 2H, ortho), 6.91 (d, 2H, para), 7.09 (t, 2H, meta), 7.24–7.63 (m, 30H, PPh3) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 112.20–115.84 (C-ortho), 120.61 (C-para), 131.81 (C-meta), 133.44 (C–CH3), 150.81 (C–O), 171.42 (OCS2), 125.12, 127.01, 136.64, 141.68 (PPh3) ppm.; 31P NMR (CDCl3): = −5.01 (PPh3) ppm.

2.3.10. [(m-CH3C6H4OCS2)2Zn Bipy] (10)

Complex (10) was obtained as off white solid following the same procedure as for complex (6). Yield: 78% (1.06 g); M.P.: 219°C (dec.). Analytical data: C26H22O2S4ZnN2 (MW = 588.93) Calcd.: C, 53.10; H, 3.77; S, 21.81; Zn, 11.12; N, 4.76; Found: C, 53.02; H, 3.70; S, 21.61; Zn, 11.10; N, 4.67. Spectral data: IR(KBr) v = 3358 v(C–H), 1579 v(C–C), 1220 v(C–O–C), 1038 v(S–C–S), 360 v(Zn–S), 750 v(Zn–N) cm−1; 1H NMR (CDCl3) = 2.21 (s, 6H, CH3), 6.81 (m, 2H, ortho), 6.91 (d, 2H, para), 7.09 (t, 2H, meta), 7.12–8.59 (m, 10H, Bipy) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 112.20–115.84 (C-ortho), 120.61 (C-para), 131.81 (C-meta), 134.14 (C–CH3), 150.81 (C–O), 171.81 (OCS2), 124.70, 136.82, 142.88, 149.63 (Bipy) ppm. ESI-MS: m/z(%) = [M+] 588.1 (6) [(m-CH3C6H4OCS2)2Zn Bipy]; [M+] 431.9 (8) [(m-CH3C6H4OCS2)2Zn]; [M+] 183.0 (5) [m-CH3C6H4OCS2]; [M+] 107.0 (18) [m-CH3C6H4O].

2.3.11. [(m-CH3C6H4OCS2)2Zn 2Py] (11)

Complex (11) was obtained as off white solid following the same protocol as for complex (7). Yield: 86% (1.16 g); M.P.: 210°C (dec.). Analytical data: C26H24O2S4ZnN2 (MW = 590.15) Calcd.: C, 52.92; H, 4.10; S, 21.73; Zn, 11.08; N, 4.75; Found: C, 52.80; H, 4.03; S, 21.59; Zn, 11.01; N, 4.69. Spectral data: IR(KBr) v = 3353 v(C–H), 1575 v(C–C), 1230 v(C–O–C), 1038 v(S–C–S), 362 v(Zn–S), 754 v(Zn–N) cm−1; 1H NMR (CDCl3) = 2.30 (s, 6H, CH3), 6.80 (m, 2H, ortho), 6.90 (d, 2H, para), 7.01 (t, 2H, meta), 7.54–8.43 (m, 10H, Py) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 112.20–115.84 (C-ortho), 120.61 (C-para), 131.88 (C-meta), 134.61 (C–CH3), 150.81 (C–O), 171.63 (OCS2), 124,23, 131.88, 149.63 (Py) ppm.

2.3.12. [(m-CH3C6H4OCS2)2Zn Phen] (12)

Complex (12) was also obtained as off white solid, following the same procedure as for complex (8). Yield: 82% (1.15 g); M.P.: 229°C (dec.). Analytical data: C28H22O2S4ZnN2 (MW = 612.15) Calcd.: C, 54.94; H, 3.62; S, 20.95; Zn, 10.69; N, 4.58; Found: C, 54.90; H, 3.68; S, 20.81; Zn, 10.60; N, 4.49. Spectral data: IR(KBr) v = 3360 v(C–H), 1572 v(C–C), 1227 v(C–O–C), 1036 v(S–C–S), 358 v(Zn–S), 753 v(Zn–N) cm−1; 1H NMR (CDCl3): = 2.30 (s, 6H, CH3), 6.80 (m, 2H, ortho), 6.90 (d, 2H, para), 7.01 (t, 2H, meta), 7.25–8.90 (m, 12H, Phen) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 112.20–115.84 (C-ortho), 120.61 (C-para), 131.88 (C-meta), 134.64 (C–CH3), 150.81 (C–O), 171.45 (OCS2), 120.61, 124.23, 136.64, 138.64, 142.88, 149.63 (Phen) ppm.

2.3.13. [(p-CH3C6H4OCS2)2Zn 2PPh3] (13)

Complex (13) was obtained as white solid, following the same procedure as for complex (5). Yield: 82% (1.81 g); M.P.: 211°C (dec.). Analytical data: C34H29O2S4P2Zn (MW = 956.52) Calcd.: C, 65.29; H, 4.64; S, 13.41; Zn, 6.84; Found: C, 65.16; H, 4.56; S, 13.34; Zn, 6.64. Spectral data: IR(KBr) v = 3356 v(C–H), 1575 v(C–C), 1230 v(C–O–C), 1039 v(S–C–S), 359 v(Zn–S), 535 v(Zn–P) cm−1; 1H NMR (CDCl3) = 2.12 (s, 6H, CH3), 6.78 (d, 4H, ortho), 7.04 (d, 4H, meta), 7.24–7.64 (m, 30H, PPh3) ppm; 13C NMR (CDCl3): =20.05 (CH3), 115.84 (C-ortho), 130.81 (C-meta), 129.64 (C–CH3), 150.81 (C–O), 175.02 (OCS2), 126.21, 127.01, 136.64, 141.68 (PPh3) ppm.; 31P NMR (CDCl3): = −4.99 (PPh3) ppm.

2.3.14. [(p-CH3C6H4OCS2)2Zn Bipy] (14)

Complex (14) was obtained as off white solid, following the same procedure as for complex (6). Yield: 79% (1.07 g); M.P.: 219°C (dec.). Analytical data: C26H22O2S4ZnN2 (MW = 588.93) Calcd.: C, 53.10; H, 3.77; S, 21.81; Zn, 11.12; N, 4.76; Found: C, 53.02; H, 3.70; S, 21.61; Zn, 11.10; N, 4.67. Spectral data: IR(KBr) v = 3358 v(C–H), 1579 v(C–C), 1220 v(C–O–C), 1038 v(S–C–S), 360 v(Zn–S), 750 v(Zn–N) cm−1; 1H NMR (CDCl3) = 2.14 (s, 6H, CH3), 6.74 (d, 4H, ortho), 7.03 (d, 4H, meta), 7.12–8.59 (m, 10H, Bipy) ppm; 13C NMR (CDCl3): = 20.25 (CH3), 115.24 (C-ortho), 127.02 (C–CH3), 129.88 (C-meta), 150.81 (C–O), 175.84 (OCS2), 124.70, 136.82, 142.88, 149.63 (Bipy) ppm.

2.3.15. [(p-CH3C6H4OCS2)2Zn 2Py] (15)

Complex (15) was obtained following the same procedure as for complex (7) as off white solid. Yield: 78% (1.06 g); M.P.: 209°C (dec.). Analytical data: C26H24O2S4ZnN2 (MW = 590.15) Calcd.: C, 52.92; H, 4.10; S, 21.73; Zn, 11.08; N, 4.75; Found: C, 52.83; H, 4.05; S, 21.69; Zn, 11.04; N, 4.65. Spectral data: IR(KBr) v = 3363 v(C–H), 1573 v(C–C), 1228 v(C–O–C), 1035 v(S–C–S), 356 v(Zn–S), 752 v(Zn–N) cm−1; 1H NMR (CDCl3): = 2.22 (s, 6H, CH3), 6.73 (d, 4H, ortho), 7.04 (d, 4H, meta), 7.54–8.43 (m, 10H, Py) ppm; 13C NMR (CDCl3): = 20.05 (CH3), 114.20 (C-ortho), 124.70 (C–CH3), 129.47 (C-meta), 150.81 (C–O), 174.63 (OCS2), 129.88, 136.82, 149.63 (Py) ppm. ESI-MS: m/z(%) = [M+] 590.1 (10) [(p-CH3C6H4OCS2)2Zn 2Py]; [M+] 431.9 (12) [(p-CH3C6H4OCS2)2Zn]; [M+] 183.0 (8) [p-CH3C6H4OCS2]; [M+] 107.0 (16) [p-CH3C6H4O].

2.3.16. [(p-CH3C6H4OCS2)2Zn Phen] (16)

Complex (16) was obtained following the same procedure as for complex (8) as off white solid. Yield: 82% (1.15 g); M.P.: 229°C (dec.). Analytical data: C28H22O2S4ZnN2 (MW = 612.15) Calcd.: C, 54.94; H, 3.62; S, 20.95; Zn, 10.69; N, 4.58; Found: C, 54.90; H, 3.68; S, 20.81; Zn, 10.60; N, 4.49. Spectral data: IR(KBr) v = 3360 v(C–H), 1572 v(C–C), 1227 v(C–O–C), 1036 v(S–C–S), 358 v(Zn–S), 753 v(Zn–N) cm−1; 1H NMR (CDCl3): = 2.22 (s, 6H, CH3), 6.75 (d, 4H, ortho), 7.03 (d, 4H, meta), 7.25–8.90 (m, 12H, Phen) ppm; 13C NMR (CDCl3): = 20.04 (CH3), 114.20 (C-ortho), 127.01 (C–CH3), 129.88 (C-meta), 150.81 (C–O), 174.23 (OCS2), 120.61, 124.23, 136.64, 138.64, 142.88, 149.63 (Phen) ppm.

2.3.17. [(C6H5CH2OCS2)2Zn 2PPh3] (17)

Complex (17) was obtained as white solid, following the same procedure as for complex (5). Yield: 80% (1.76 g); M.P.: 208°C (dec.). Analytical data: C34H29O2S4P2Zn (MW = 956.52) Calcd.: C, 65.29; H, 4.64; S, 13.41; Zn, 6.84; Found: C, 65.14; H, 4.53; S, 13.30; Zn, 6.78. Spectral data: IR(KBr) v = 3360 v(C–H), 1570 v(C–C), 1233 v(C–O–C), 1036 v(S–C–S), 360 v(Zn–S), 539 v(Zn–N) cm−1; 1H NMR (CDCl3): = 4.28 (s, 4H, CH2), 7.27–7.34 (m, 10H, C6H5), 7.34–7.64 (m, 30H, PPh3) ppm; 13C NMR (CDCl3): = 71.41 (CH2), 126.14–126.36 (C-ortho), 127.11–127.14 (C-meta), 128.69 (C-para), 136.45 (C–CH2), 184.61 (OCS2), 126.38, 129.92, 134.36, 139.14 (PPh3) ppm.; 31P NMR (CDCl3): = −5.11 (PPh3) ppm.

2.3.18. [(C6H5CH2OCS2)2Zn Bipy] (18)

Complex (18) was obtained following the same procedure as for complex (6) as off white solid. Yield: 81% (1.10 g); M.P.: 220°C (dec.). Analytical data: C26H22O2S4ZnN2 (MW = 588.93) Calcd.: C, 53.10; H, 3.77; S, 21.81; Zn, 11.12; N, 4.76; Found: C, 53.01; H, 3.58; S, 21.79; Zn, 10.99; N, 4.46. Spectral data: IR(KBr) v = 3365 v(C–H), 1572 v(C–C), 1230 v(C–O–C), 1034 v(S–C–S), 359 v(Zn–S), 749 v(Zn–N) cm−1; 1H NMR (CDCl3): = 4.29 (s, 4H, CH2), 7.27–7.32 (m, 10H, C6H5), 7.12–8.49 (m, 10H, Bipy) ppm; 13C NMR (CDCl3): = 71.41 (CH2), 126.61-126.71 (C-ortho), 127.01–127.11 (C-meta), 128.69 (C-para), 136.71 (C–CH2), 184.02 (OCS2), 124.61, 127.41, 128.95, 136.85, 147.98 (Bipy) ppm.

2.3.19. [(C6H5CH2OCS2)2Zn 2Py] (19)

Complex (19) was obtained following the same procedure as for complex (7) as off white solid. Yield: 80% (1.08 g); M.P.: 220°C (dec.). Analytical data: C26H24O2S4ZnN2 (MW = 590.15) Calcd.: C, 52.92; H, 4.10; S, 21.73; Zn, 11.08; N, 4.75; Found: C, 52.80; H, 4.03; S, 21.59; Zn, 11.01; N, 4.66. Spectral data: IR(KBr) v = 3365 v(C–H), 1572 v(C–C), 1230 v(C–O–C), 1032 v(S–C–S), 350 v(Zn–S), 740 v(Zn–N) cm−1; 1H NMR (CDCl3): = 4.29 (s, 4H, CH2), 7.27–7.32 (m, 10 H, C6H5), 7.54–8.43 (m, 10 H, Py) ppm; 13C NMR (CDCl3): = 71.41 (CH2), 126.61–126.71 (C-ortho), 127.01–127.11 (C-meta), 128.69 (C-para), 135.99 (C–CH2), 184.62 (OCS2), 129.88, 136.82, 149.64 (Py) ppm.

2.3.20. [(C6H5CH2OCS2)2Zn Phen] (20)

Complex (20) was obtained following the same protocol as for complex (8) as off white solid. Yield: 80% (1.12 g); M.P.: 220°C (dec.). Analytical data: C28H22O2S4ZnN2 (MW = 612.15) Calcd.: C, 54.94; H, 3.62; S, 20.95; Zn, 10.69; N, 4.58; Found: C, 54.81; H, 3.58; S, 20.79; Zn, 10.59; N, 4.46. Spectral data: IR(KBr) v = 3365 v(C–H), 1572 v(C–C), 1230 v(C–O–C), 1034 v(S–C–S), 359 v(Zn–S), 749 v(Zn–N) cm−1; 1H NMR (CDCl3): = 4.29 (s, 4H, CH2), 7.27–7.32 (m, 10H, C6H5), 7.34–8.99 (m, 12 H, Phen) ppm; 13C NMR (CDCl3): = 71.41 (CH2), 126.61–126.71 (C-ortho), 127.01–127.11 (C-meta), 128.69 (C-para), 136.45 (C–CH2), 184.72 (OCS2), 120.11, 127.41, 127.76 129.92, 134.71, 149.14 (Phen) ppm. ESI-MS: m/z(%) = [M+] 612.1 (17) [(C6H5CH2OCS2)2Zn Phen]; [M+] 431.9 (8) [(C6H5CH2OCS2)2Zn]; [M+] 183.0 (9) [C6H5CH2OCS2]; [M+] 107.0 (16) [C6H5CH2O].

2.4. Antimicrobial Activity
2.4.1. Antifungal Activity

Potato dextrose medium (PDA) was prepared in a flask and sterilized. Now 100 μL of each sample was added to the PDA medium and poured into each sterilized petri plate. Mycelial discs taken from the standard culture (Fusarium oxysporum) of fungi were grown on PDA medium for 7 days. These cultures were used for aseptic inoculation in the sterilized petri dish. Standard cultures, inoculated at 28 ± 1°C, were used as the control. The efficiency of each sample was determined by measuring the radial fungal growth. The radial growth of the colony was measured in two directions at right angles to each other and the average of two replicates was recorded in each case. Data were expressed as percent inhibition over the control from the size of the colonies. The percent inhibition was calculated using the formula % Inhibition = ( , where is the diameter of the fungus colony in the control plate after 96 hrs incubation and is the diameter of the fungus colony in the tested plate after the same incubation period.

2.4.2. Antibacterial Activity

Test samples were prepared in different concentrations (250, 500, and 1000 ppm) in DMSO. Agar medium (20 mL) was poured into each petri plate. The plates were swabbed with broth cultures of the respective microorganisms Klebsiella pneumonia and Bacillus cereus and kept for 15 minutes for adsorption to take place. About 6 mm diameter wells were bored in the seeded agar plates using a punch and 100 μL of the DMSO solution of each test compound was poured into the wells. DMSO was used as the control for all the test compounds. After holding the plates at room temperature for 2 hrs to allow diffusion of the compounds into the agar, the plates were incubated at 37°C for 24 hrs. The antibacterial activity was determined by measuring the diameter of the inhibition zone. The entire tests were made in triplicates and the mean of the diameter of inhibition was calculated.

3. Results and Discussion

The complex bis(o-, m-, p-tolyl)/benzyl dithiocarbonates of zinc(II) (14) was prepared as white solid in fairly good yield by reaction of zinc chloride with sodium salts of (o-, m-, p-tolyl)/benzyl dithiocarbonates in 1 : 2 molar ratio in aqueous medium (Scheme 1).

135496.sch.001
Scheme 1

The compounds (14) were reacted with the nitrogen and phosphorous donor ligands like triphenylphosphine, pyridine, 2,2′-bipyridine, and 1,10-phenanthroline in dichloromethane in appropriate stoichiometries. These reactions were quite facile and yielded the compounds [(o-, m-, p-CH3C6H4OCS2)2Zn nL] and [(C6H5CH2OCS2)2Zn nL] (L = Bipy, Phen for and PPh3, Py for ) as white to off white solids (Scheme 2).

135496.sch.002
Scheme 2

These complexes are soluble in common organic solvents, but insoluble in solvents like n-hexane and carbon tetrachloride. The elemental analyses (C, H, N, S, and Zn) were found to be consistent with the molecular formulae of the complexes.

3.1. IR Spectra

IR spectral assignment of the complexes (120) is done on the basis of relevant literature reports [10, 22, 30, 32, 33]. The comparison of IR spectra of these complexes with starting materials has also shown seminal information. The IR spectra show the characteristic sharp band for v(C–O–C) and broad band for v(C=C) (tolyl and benzyl ring stretching) in the ranges 1043–1015 and 1607–1581 cm−1, respectively. The presence of one strong band for v(C–S) in the region 1040–1035 cm−1 without a shoulder favors the bidentate linkage of the dithiocarbonate ligands with zinc atom. The band for (C–S) vibrations has depicted a shift of 20–30 cm−1 toward the lower frequency region in comparison to the parent dithiocarbonate ligands, which may also be attributed to bidentate mode of bonding by dithiocarbonate ligands. The presence of a new band ascribed to v(Zn–S) was present in the region 380–370 cm−1. The IR spectra of the adducts (520) have showed all the bands observed in the parent zinc dithiocarbonates and bands characteristic of donor ligands (PPh3, Py, Bipy and Phen) in addition to the bands in the regions 751–731 cm−1 and 535-534 cm−1, which may be assigned to v(Zn–N) and v(Zn–P) bonding modes, respectively.

3.2. 1H NMR

The 1H NMR spectra of these complexes in CDCl3 show the characteristic proton resonances of the corresponding tolyl and benzyl protons. In the 1H NMR spectra, the signals for the –CH3 (tolyl ring) and –CH2 (benzyl ring) protons were observed at 2.10–2.30 and 4.28-4.29 ppm as singlet. The protons of the C6H4 (tolyl ring) and C6H5 (benzyl ring) in the zinc(II) complexes gave signals in the range 6.71–7.09 and 7.02–7.33 ppm with their usual splitting pattern. There were two resonances for the ring protons of para complexes whereas four resonances were observed for ortho and meta derivatives. The splitting pattern and intensities of peaks in the spectra of all these complexes are found to be consistent with their structures. The addition complexes also exhibited additional peaks for the aromatic protons of the donorligands in the regions 7.24–7.64 (triphenylphosphine), 7.25–8.99 (1,10-phenanthroline), 7.29–8.51 (pyridine), and 7.09–8.59 (2,2′-bipyridine).

3.3. 13C NMR

The 13C NMR spectra of these complexes showed chemical shifts for all carbon nuclei in their characteristic regions. The signals for methyl (–CH3) and methylene (–CH2) carbon occurred in the ranges 19.28–20.25 and 71.40-71.41 ppm, respectively. The carbon nuclei of phenyl groups (–C6H5 and –C6H4) displayed their resonance in the region 112.01–131.88 ppm. The carbon attached to the methyl and methylene group appeared at 124.70–134.64 and 135.96–136.71 ppm, respectively. The signal in the region 150.30–152.94 ppm was due to the carbon attached to the oxygen in the tolyl derivatives. The chemical shift for the dithiocarbonate carbon (–OCS2) appeared at 166.94–184.72 ppm with a upfield shift (30–36 ppm) compared to the parent ligands [30]. Presumably, this reflects the fact that environment around the CS2 carbon is the one most affected by the formation of the Zn–S bond. The 13C NMR spectra of the complexes (520) have exhibited the chemical shifts of the carbon nuclei of the donor moieties. There were four resonances for the aryl carbon nuclei of the triphenylphosphine, three for pyridine, four for 2,2′-bipyridine, and six for 1,10-phenanthroline moieties in the regions 125.12–139.14, 124.23–149.63, 124.70–149.73, and 120.11–149.65 ppm, respectively.

3.4. 31P NMR

31P NMR spectra of the complexes (5, 9, 13, and 17) exhibited the chemical shift for the phosphorus atom of the triphenylphosphine moiety as a singlet at −4.99 to −5.11 ppm [22]. The 31P NMR resonances of the bound triphenylphosphine ligand exhibited a slight downfield shift compared to those of the free triphenylphosphine (−5.51 ppm), which employs the coordination of triphenylphosphine moiety with the central metal atom.

3.5. Mass Spectra

The mass spectra of few representative zinc(II) complexes (1, 5, 10, 15, and 20) depicted molecular ion peaks [M+] at m/z = 431.9 (1), 946.4 (5), 588.1 (10), 590.1 (15), and 612.1 (20). In addition to the molecular ion peak, several other peaks of different fragments were also observed, which were formed after consecutive dismissal of different groups. The occurrence of molecular ion peak in the complexes is supporting the monomeric nature of the complexes.

3.6. Thermogravimetric Analysis

The results are in good agreement with the composition of the complexes. The calculated mass change agrees favorably with experimental values. The thermogram from thermal studies performed on the complex [(p-CH3C6H4OCS2)2Zn] (3) is shown in Figure 1(a). The results show a loss of weight 7.3% (obs.) 6.9% (calc.) due to the removal of methyl group at approximately 43.9°C and also an endothermal peak at 90°C. Further, heating up to 294.3°C shows a gradual weight loss of 42.3% (theoretical weight loss 42.0%) attributable to the formation of [(CH2OCS2)2Zn]. The weight loss continues beyond this temperature and finally attains a constant mass corresponding to ZnS (observed 77.3%, calcd. 77.8%).

fig1
Figure 1: (a) TGA curve for zinc complex [(p-CH3C6H4OCS2)2Zn] (3), (b) TGA curve for zinc complex [(p-CH3C6H4OCS2)2Zn 2Py] (15), and (c) TGA curve for zinc complex [(C6H5CH2OCS2)2Zn Bipy] (18).

In the thermogravimetric analysis of [(p-CH3C6H4OCS2)2Zn 2Py] (15), an initial weight loss was 26.8% calc. (26.4% obs.) and occurred at 206.5°C [(p-CH3C6H4OCS2)2Zn], indicating the loss of pyridine. Another fragments that were formed due to the loss of various different groups were [(C6H4OCS2)2Zn], weight loss 31.9% calc. (32.0% obs.) at 276.5°C and [(OCS2)2Zn], weight loss 57.7% calc. (58.1% obs.) at 486.5°C. The TGA curve shows the maximum weight loss 83.5% calc. (83.6% obs.) at 571.6°C leading to the formation of stable metal sulphide (Figure 1(b)).

Similarly, the complex, [(C6H5CH2OCS2)2Zn Bipy] (18) displayed a thermolysis step that covers a temperature range from 150 to 900°C. The thermogram (Figure 1(c)) exhibited the decline curve characteristic for dithiocarbonate complexes. The diagnostic weight loss of initial weight occurs in the steeply descending segment of the TGA curve. The weight loss, that is, 26.4% (obs.) at 206.5°C, is due to the formation of the dithiocarbonate corresponding to [(C6H4CH2OCS2)2Zn], weight loss 26.6% (calc.) as an intermediate product, which agrees with thermogravimetric data for dithiocarbonates. Another important weight loss 56.8% (obs.) occurs at 481.5°C temperature corresponding to the formation of [(OCS2)2Zn] 57.3% (calc.). The decomposition continue to about 580°C at which most of the organic parts of the compound have been lost. This sharp decomposition period brings about 80–83% weight loss in the zinc complex and led to the complete formation of ZnS. Thus, in all cases the final products are the metal sulfides.

3.7. Cyclic Voltammetry

The redox behaviour of a complex [(p-CH3C6H4OCS2)2Zn] (3) with respect to metal centre has been studied. The potential is applied between the reference electrode (Ag/AgCl) and the working electrode (Gold electrode) and the current is measured between the working electrode and the counter electrode (platinum wire). 0.1 M phosphate buffer solution (pH = 7.0) was used. The cyclic voltagramm (Figure 2) of the complex was recorded in the potential range of +1.0 to −1.0 V, which exhibited that the cathodic peak,  V, corresponds to the Zn+2/Zn+1 redox couple and anodic peak,  V, corresponds to Zn+1/Zn+2 redox couple. The cathodic peak current is  A and anodic peak current is  A and they are discussed in Table 1. The value of ratio is close to unity which corresponds to simple one electron process and the couple is found to be quasireversible. All the metal complexes and the adducts have almost same redox behaviour because of Zn(II) metal centre.

tab1
Table 1: Cyclic voltammetric data of [(p-CH3C6H4OCS2)2Zn] (3).
135496.fig.002
Figure 2: Cyclic voltammetric curve of [(p-CH3C6H4OCS2)2Zn] (3).
3.8. Scanning Electron Microscopy (SEM)

Scanning electron micrography is used to evaluate morphology and particle size of the[(o-CH3C6H4OCS2)2Zn] (1) and has been carried out at a low and high magnification, Figures 3(a) and 3(b). The information revealed from the signals included external morphology, topography, structure, and orientation of materials making up the sample. The images show the round shape of particles with rough texture. The particles are present in the form of clusters. In general, the SEM photograph shows single phase formation with well-defined shape.

fig3
Figure 3: (a) SEM image of [(o-CH3C6H4OCS2)2Zn] (1) complex at low magnification, (b) SEM image of [(o-CH3C6H4OCS2)2Zn] (1) complex at high magnification.
3.9. Antimicrobial Activity
3.9.1. Antifungal

The antifungal screening data are given in Table 2, which points toward two significant conclusions. Firstly, on increasing concentration of the complex the colony diameter of the fungus decreases (percent inhibition increases); that is, all the complexes show potent antifungal activity. Secondly, adducts with nitrogen and phosphorus donor ligands exhibit greater antifungal activity than the complex without donor ligands. The increase in antimicrobial activity is due to faster diffusion of metal complexes as a whole through the cell membrane or due to the combined activity of the metal and ligand. Further, the greater potency of adducts against the fungus Fusarium oxysporum can be explained on the basis of Overtone’s concept and Tweedy’s chelation theory [33]. These complexes are also supposed to disturb the respiration process of the cell and thus block the synthesis of the proteins that restricts further growth of the organism. The comparison of antifungal activity of all the ligands, and some of the complexes is described diagrammatically in Figure 4.

tab2
Table 2: In vitro evaluation of Zn(II) complexes against the fungus Fusarium oxysporum f. Sp. Capsici.
135496.fig.004
Figure 4: Graph showing comparative result of antifungal activity.
3.9.2. Antibacterial

The free ligands and a few complexes were screened for their in vitro antibacterial study by well diffusion method [34]. Antibacterial screening data are given in Table 3. These studies revealed that free ligands are inactive against the bacterial strains but metal complexes show higher activity than free ligands but lower activity than reference drug, that is, penicillin. However, complex (15) shows pronounced activity against Klebsiella pneumonia and Bacillus cereus even more than reference drug. The comparison of antibacterial activity of ligands and some of the complexes is described diagrammatically in Figures 5(a) and 5(b).

tab3
Table 3: In vitro evaluation of Zn(II) complexes for antibacterial activity.
fig5
Figure 5: (a) Graph showing comparative result of antibacterial activity for bacteria Klebsiella pneumonia (−), (b) graph showing comparative result of antibacterial activity for bacteria Bacillus cereus (+).
3.10. Structural Features

The outcome of the above results confirms the formation of the zinc(II)dithiocarbonate complexes as indicated from elemental analyses, TGA, cyclic voltammetry, SEM, and spectral analysis including IR, mass, and multinuclear NMR (1H, 13C and 31P). In conjunction with the literature reports [10, 22, 2426, 3538], a probable geometry may be assigned to these compounds. The formation of the complexes (5, 9, 13, 17) is also supported by the 31P NMR spectra which exhibited the signal due to triphenylphosphine moiety with a slight downfield shift. It is evident from the 13C NMR spectra of all compounds that the peak in the region 166.94–184.72 ppm which is characteristic of the CS2 group, shows an up field shift as compared with the free ligand [30]. Moreover, the C–S band is shifted to higher frequencies in the IR spectra of the complexes. This shows the complexation of zinc with the free ligand, which is authenticated by presence of band for vZn–S. Therefore, bidentate mode of bonding by dithiocarbonate ligands would lead to distorted tetrahedral and octahedral geometry around zinc in bis-dithiocarbonates (14) and donor stabilized complexes (620), respectively (Figures 6, 7(a), and 7(b)).

135496.fig.006
Figure 6: Proposed distorted tetrahedral geometry for [ 2Zn] (13).
fig7
Figure 7: (a) Proposed distorted octahedral geometry for [(o-, m- and p-CH3C6H4OCS2)2Zn 2L]; [L = PPh3 (5, 9, 13) or = Py (7, 11, 15)]; (b) [(o-, m- and p-CH3C6H4OCS2)2Zn L]; [ = Bipy (6, 10, 14) or = Phen (8, 12, 16)].

4. Conclusion

We have synthesized and characterized a series of twenty new (o-, m-, and p-tolyl/benzyl)dithiocarbonate derivatives of zinc. Fourfold and sixfold coordinations are proposed for zinc(II)dithiocarbonate complexes wherein bidentate chelation by the ligand to the zinc(II) ion may be postulated. The thermal decomposition behaviour of the complexes proceeds in one major decomposition step to give the respective zinc sulphide. The molecular weight determination and mass spectra of the complexes suggested the monomeric nature of the complexes. In addition, appearance of Zn–S band in the IR spectra also indicates the complexation between the zinc and sulphur atom of the ligand. SEM shows the morphology of the zinc complexes. The cyclic voltammetric analysis predicted the redox behaviour of the zinc. The complexes are found to have higher biological activities (antifungal and antibacterial) as compared to the respective ligand and the parent drug.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

Sushil Kumar Pandey gratefully acknowledges the financial assistance from the University Grants Commission, New Delhi.

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