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

Tolyl/benzyl dithiocarbonates of zinc(II) [(o-, m-, p-CH 3 C 6 H 4 and C 6 H 5 CH 2 OCS 2 ) 2 Zn] have been synthesized in 1 : 2 molar ratio by the reaction of zinc chloride, ZnCl 2 , with sodium salts of tolyl/benzyl dithiocarbonates (o-, m-, p-CH3C6H4O/ C6H5CH2O)CS2Na, in aqueous medium. These complexes were further reacted with nitrogen and phosphorous donor ligands to obtain donor stabilized complex of the type [[(o-, m-, p-CH 3 C 6 H 4 and C 6 H 5 CH 2 )OCS 2 ] 2 Zn.nL], (L = Bipy, Phen for n = 1 and PPh 3 , Py for n = 2). These complexes were characterised by elemental analysis, mass, IR, and NMR (H, C, and P) 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.


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 [3][4][5][6]. 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 -OCS 2 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 [23][24][25][26][27][28], 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). Complex (2) was synthesized as white solid according to the protocol as described for complex (1) (3) was obtained as white solid following the same procedure as for complex (1). Yield: 74% (0.77 g); M.P.: 204 ∘ C (dec.

[(p-CH
. The complex (9) was synthesized as white solid according to the protocol described for complex (5) (10). Complex (10) was obtained as off white solid following the same procedure as for complex (6).  (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.   (12). Complex (12) was also obtained as off white solid, following the same procedure as for complex (8) (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.   (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.   (16). Complex (16) was obtained following the same procedure as for complex (8) (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. was prepared in a flask and sterilized. Now 100 L 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 = (( − )/ ) × 100, 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.

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 L 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.
3.4. 31 P NMR. 31 P 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 31 P 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.

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-CH 3 C 6 H 4 OCS 2 ) 2 Zn] (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 [(CH 2 OCS 2 ) 2 Zn]. The weight loss continues beyond this temperature and finally attains a constant mass corresponding to ZnS (observed 77.3%, calcd. 77.8%).
Similarly, the complex, [(C 6 H 5 CH 2 OCS 2 ) 2 Zn⋅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 [(C 6 H 4 CH 2 OCS 2 ) 2 Zn], 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 [(OCS 2 ) 2 Zn] 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.

Cyclic Voltammetry.
The redox behaviour of a complex [(p-CH 3 C 6 H 4 OCS 2 ) 2 Zn] (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, pc = 0.60455 V, corresponds to the Zn +2 / Zn +1 redox couple and anodic peak, pc = −0.52338 V, corresponds to Zn +1 /Zn +2 redox couple. The cathodic peak current is = −1.31 × 10 −6 A and anodic peak current is = 1.66 × 10 −6 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.
International Journal of Inorganic Chemistry

Scanning Electron Microscopy (SEM).
Scanning electron micrography is used to evaluate morphology and particle size of the[(o-CH 3 C 6 H 4 OCS 2 ) 2 Zn] (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.
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.

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).

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 ( 1 H, 13 C and 31 P). In conjunction with the literature reports [10,22,[24][25][26][35][36][37][38], a probable geometry may be assigned to these compounds. The formation of the complexes (5,9,13,17) is also supported by the 31 P NMR spectra which exhibited the signal due to triphenylphosphine moiety with a slight downfield shift. It is evident from the 13 C NMR spectra of all compounds that the peak in the region 166.94-184.72 ppm which is characteristic of the CS 2 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 (1-4) and donor stabilized complexes (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), respectively (Figures 6, 7(a), and 7(b)).

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.