Synthesis, spectroscopic, DFT and in vitro biological

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 180 (2017) 127–137

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Synthesis, spectroscopic, DFT and in vitro biological studies of vanadium(III) complexes of aryldithiocarbonates Savit Andotra a, Sandeep Kumar a, Mandeep Kour a, Vikas b, Chayawan b, Vishal Sharma c,d, Sundeep Jaglan c,d, Sushil. K. Pandey a,⁎ a

Department of Chemistry, University of Jammu, Baba Sahib Ambedkar Road, Jammu 180006 (J&K), India Quantum Chemistry Group, Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh 160014, India Quality Control and Quality Assurance Division, CSIR-Indian Institute of Integrative Medicine, Canal Road, Jammu 180001, India d Academy of Scientific and Innovative Research (AcSIR), CSIR-Indian Institute of Integrative Medicine, Canal Road, Jammu 180001, India b c

a r t i c l e

i n f o

Article history: Received 2 September 2016 Received in revised form 24 February 2017 Accepted 1 March 2017 Available online 06 March 2017 Keywords: Dithiocarbonates Antimicrobial TGA Antioxidant DFT method

a b s t r a c t Vanadium(III) tris(dithiocarbonates), [(ROCS2)3V] (R = o-, m-, p-CH3C6H4 and 4-Cl-3-CH3C6H3) and donor stabilized addition complexes [(ROCS2)2V(Cl)·L] [L = NC5H5 or P(C6H5)3] were synthesized and characterized by elemental analyses, IR, mass, TGA/DTA, SEM magnetic susceptibility and heteronuclear NMR (1H, 13C and 31P) spectroscopic studies. The cytotoxicity of the complexes was measured in vitro using the cultivated human cell lines. In addition, the antioxidant activities of the ligands and its vanadium complexes were also investigated through their scavenging effect on DPPH radicals. The antimicrobial activity of ligands and some complexes has been conducted against three bacterial strains and fungus. The density functional theory (DFT) calculations of ligands and vanadium complexes were performed by the DFT/B3LYP/LANL2DZ method to obtain the optimized molecular geometry, vibrational frequencies, the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), thermodynamic properties and various other quantum-mechanical parameters. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The chemistry of metal xanthates has been the subject of study for many years, driven by applications in both metal extraction and analytical chemistry [1–4]. Recent years have seen a renaissance in the area resulting from the use of metal dithiocarbonates as single-source precursors (SSPs) to metal sulfide materials [5]. Xanthates are also known to form a variety of complexes with most of the transition and non-transition metals [6–7]. Transition metal xanthates and their corresponding adducts with variety of Lewis bases such as amine, imine and phosphine are extensively investigated not only for their wide applications in various fields but also due to coordination ability of dithiocarbonate anions, which can act as mono- or bidentate ligands or may also lie outside the coordination sphere of central atom [8–9]. They are widely used as reagents for heavy metal sedimentation in hydrometallurgy and waste water treatment, as collectors in the froth flotation of metal sulfide ores, in cellulose synthesis, as accelerators in vulcanization of rubber and in many other applications [1,6–7,10–14]. One of excellent applications of metal xanthate is the use of cellulose xanthate for the column for the separation of alcohols by chromatographic method [15]. ⁎ Corresponding author. E-mail address: [email protected] (S.K. Pandey).

http://dx.doi.org/10.1016/j.saa.2017.03.003 1386-1425/© 2017 Elsevier B.V. All rights reserved.

Cytotoxic activity of metal xanthates on human cancer cells has also been reported in literature [16–17]. Xanthates have recently been shown to inhibit the replication of both DNA and RNA viruses in vitro [17]. In materials science and nano-technology, xanthates have been proposed as capping agents for the synthesis of metal nano-particles and self-assembled monolayers as alternatives to thiols and as sulfidizing reagents for the preparation of metal sulfide nano-particles [18–22]. Xanthates also find application as catalysts in RAFT polymerization. Newly synthesized S-2(ethyl propionate)-(O-ethyl xanthate) and (S)-2(ethyl isobutyrate)-(O-ethyl xanthate) were used as reversible addition fragmentation chain transfer (RAFT) agents for radical polymerization of N-vinylpyrrolidine (NVP) [23–24]. In view of the interest and importance of xanthate complexes, the work reported herein is focused on synthesis, spectroscopic, DFT and in vitro biological studies of new mono- and di-substituted aryldithiocarbonates of vanadium(III) complexes and their adducts. 2. Materials and Methods Solvents (Toluene, n-hexane and THF) were dried and distilled over sodium before use. Chloroform (Thomas Baker) was dried over P2O5. VCl3 was procured from Sigma Aldrich and it was used as received. Sodium salts of tolyldithiocarbonates (L1 − L3) were synthesized

128

S. Andotra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 180 (2017) 127–137

according to a literature procedure [25]. Moisture was carefully excluded during the experimental manipulations for the synthesis of ligands by using standard Schlenk techniques. Vanadium was estimated as Ag3VO4 [26]. Chlorine was estimated volumetrically by Volhard's method [26]. Elemental analyses (C, H, N, S) were conducted using the Elemental Analyser Vario EL-III. The ESI mass spectra were recorded on ESI-esquires 3000 Bruker Daltonics spectrophotometer. Infrared spectra were recorded from 4000 to 200 cm−1 on a Perkin–Elmer spectrum RX1 FT-IR spectrophotometer. 1H, 13C and 31P (proton-decoupled) NMR spectra were recorded in CDCl3 and DMSO-d6 using TMS as internal reference and H3PO4 (85%) as external reference on a Bruker Avance III 400 MHz. SEM studies were performed with a ZeissEVO50 instrument having magnification range 5 × to 1,000,000 × and at an accelerating voltage of 0.2 to 30 kV at Indian Institute of Technology (IIT), Delhi. 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. 2.1. Synthesis of Ligands and Vanadium Complexes (1−12) (See Supplementary information S1) 2.2. Antifungal Activity The antifungal activities of the complexes were evaluated by the poisoned food technique [27] against pathogenic strain of fungus Fusarium oxysporum. The test solutions were prepared by dissolving the compounds in DMSO. The test solutions were mixed in the PDA and poured in the petriplates in sterilized conditions inside the Laminar flow. After solidification, the plates were inoculated with seven days old culture of pathogen Fusarium oxysporum by placing 2 mm bit in the center of the plates. The inoculated plates were incubated at 27 °C for 4 days. The linear growth of fungus in control and treatment was recorded at different concentrations of the complexes. The percent inhibition was calculated using the formula: %Inhibition ¼ ðC−TÞ=C 100 C = the diameter of the fungus colony in the control plate after 96 h incubation. T = the diameter of the fungus colony in the tested plate after the same incubation period. 2.3. Antibacterial Activity The ligands and few represented complexes were evaluated for their antibacterial activity against three bacterial strains (Gram-positive Staphylococcus aureus ATCC 25923; Gram-negative Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853). The antibacterial assays were performed in accordance with the procedures outlined by Clinical and Standards Institute (CLSI). Mueller-Hinton Broth (MHB) was prepared as per the manufacturer's instructions (HiMedia Laboratories, India). The pre-cultures of the bacterial strains were prepared in MHB by fresh inoculums of the cultures incubated at 37 °C for 18–24 h with 100 rpm shaking to obtain concentrations of approximately 5 to 6 log CFU/mL (evaluated and adjusted photometrically at

625 nm). The bacterial suspensions were further diluted with MHB to obtain a final inoculum of 5 × 105 CFU/mL. The antibacterial assays were performed in clear 96 well microtiter plates. Initially the test compounds were screened for antibacterial activity at slight higher concentration (128 μg/mL), and those that show inhibition in primary screen were further tested at different concentrations to obtain MIC and MBC values [28]. Each experiment was accompanied by a positive control containing broth, pathogen and a known inhibitory compound (Ciprofloxacin); a negative control containing broth and pathogen. The plates were incubated for 24 h at 37 °C with agitation and then observations were recorded visually. The well containing minimum concentration of the compound in which there is no visual growth is considered as MIC. A loopful inoculum from the wells containing no visual growth was streaked on the Tryptic Soya Agar media and incubated for 24 h at 37 °C to record MBC. 2.4. Cytotoxic Activity The cytotoxicity of few vanadium complexes was measured in vitro using the cultivated human cell lines involving lung adenocarcinoma cell line A-549, leukemia cell line THP-1, lung cervical node cell line NCI-H322 and colorectal cancer cell line HCT-116. The inhibition capacity was assessed using the sulforhodamine B (SRB) protein staining assay by 96-well technique described previously by Skehan et al. [29]. The seeded 96-well plates are incubated for 48 h after addition of test samples. Then the cells were fixed in 30% trichloroacetic acid (TCA) and placed for 1 h at 4 °C followed by washing with distilled water. After air-drying, the fixed cells were stained with 0.4% SRB (prepared in 1% acetic acid), left at room temperature for 30 min washed with 1% acetic acid and dried. Solubilization is carried out with 10 mM tris buffer followed by recording the optical density (OD) with ELISA reader at 540 nm wavelength. 2.5. Antioxidant Activity The antioxidant activity was measured in vitro using Free radicalscavenging of the compounds using DPPH analysis. The DPPH free radical scavenging activity is based on the fact that methanolic solution of DPPH imparts purple color and strong absorption at 517 nm, which get reduced in the presence of an antioxidant compound [30]. Different concentrations of the compounds under evaluation (6.25–100 μg/mL) were added to 4 mL of a DPPH solution (120 μM) in methanol and incubated at 37 °C for 30 min in dark. The absorbance was taken at 517 nm and the percentage free radical scavenging (%) was calculated with the following equation: Scavenging% ¼

Acontrol −Asample =Acontrol 100

Acontrol = is the absorbance of the control reaction (containing all reagents except the test compounds), Asample = is the absorbance of the test compound. Ascorbic acid was used as positive control and tests were conducted in triplicate.

Scheme 1. Synthesis of 4-Cl-3-CH3C6H3OCS2Na (L4).


129

Scheme 2. Synthesis of tris-O,O′-(o-, m-, p-, 4-chloro-3-tolyl)dithiocarbonates of vanadium(III) (1–4).

2.6. Computational Details Density functional theory (DFT) is a computational method that derives properties of the molecule based on a determination of the electron density of the molecule. DFT calculations are less time consuming and include a significant part of the electron correlation leading to good accuracy. In the present work, the DFT calculations for geometry optimization are performed with the help of Gaussian 09 quantum mechanical software [31], employing a widely used hybrid exchange-correlation functional B3LYP, and LANL2DZ basis set. B3LYP exchangecorrelation functional is comprised of Becke's three parameter hybrid exchange functional and Lee-Yang-Parr correlation functional [32,33]. LANL2DZ (Los Alamos National Laboratory 2 Double-Zeta) basis set is an effective core potential (ECP) most often used to model transition metal atoms [34]. The geometry optimization was followed by harmonic vibrational frequency analysis to ensure the absence of imaginary frequencies in the equilibrium structure obtained, and also to obtain the DFT calculated vibrational frequencies, which were further scaled by a recommended scaling factor of 0.961 for B3LYP/LANL2DZ method.

3. Results and Discussion Literature reports describe the isolation of tolyldithiocarbonates. However there are no reports available so far of similar ligand having disubstituted phenyl moiety. Reaction of sodium metal with, 4-chloro-3cresol (4-Cl-3-CH3C6H3OH) in 1:1 stoichiometric ratio in refluxing toluene resulted in the formation of sodium 4–chloro–3–methylphenolate (4-Cl-3-CH3C6H3ONa) as creamish viscous pasty mass. Addition of an equimolar amount of CS2 to this at 0 to 5 °C forms the corresponding sodium salt of 4–Cl–3–methyl dithiocarbonate as a pale yellow solid. These sodium salts appear to be hygroscopic in nature, soluble in methanol, ethanol, dichloromethane and tetrahydrofuran insoluble in most of the hydrocarbon solvents and sparingly soluble in chloroform (Scheme 1). Tris-O,O′-(o-, m-, p- and 4-chloro-3-tolyl)/dithiocarbonates of vanadium(III) were prepared as green solid in fairly good yield by the reaction of vanadium trichloride, VCl3, with sodium salt of O,O′-(o-, m, p- and 4-chloro-3-tolyl)dithiocarbonates, ROCS2Na [R = o-, m-, pCH3C6H4 and 4-Cl-3-CH3C6H3] (1–5), in a 1:3 M ratio in THF under nitrogen atmosphere (Scheme 2). The donor stabilized vanadium dithiocarbonates were prepared by the reaction of vanadium trichloride, (VCl3), donor ligands (pyridine or triphenylphosphine) and sodium O,O′-(o-, m-, p- and 4-chloro-3tolyl)dithiocarbonates, ROCS2Na (R = o-, m-, p-CH3C6H4 and 4-Cl-3CH3C6H3), in 1:1:2 M stoichiometry in THF under under nitrogen

atmosphere (Scheme 3). All these complexes and adducts are soluble in common organic solvents and insoluble in solvents like n-hexane and carbon tetrachloride. 3.1. IR Spectra IR spectral data were interpreted on the comparison basis of literature reports [35–41]. In case of complexes and adducts the 1050– 950 cm−1 region has been shown to be reliable for determining whether the ligand is bidentate or unidentate. According to Bonati and Ugo [40], presence of only one strong bond in the 1050–1005 cm−1 region, without any splitting which is associated with v(C S) stretching vibrations, indicates complete symmetric bidentate bonding by dithiocarbonate ligand. The ν(CS2) stretching vibrations which appeared in the parent dithiocarbonate ligand are replaced by a strong intensity band in the region 1045–1015 cm−1which remains unsplited in all cases. This shifting and arising of single sharp band due to ν(CS2) vibrations in the complexes are quite diagnostic to propose bidentate mode of bonding of the dithio moiety with vanadium, as the appearance of only one band without any splitting in the same region is attributed to the bidentate mode of binding of the dithiocarbonate ligand [37– 39]. The medium intensity bands in the range of 391–360 cm−1 are typically assigned to the ν(V\\S) vibrations in all the complexes. The strong intensity bands that were present in the region 1285–1236 cm−1 were assigned to ν(C\\O\\C) stretching vibrations. The IR spectra of the adducts showed all the bands observed in the parent metal(II)bis(dithiocarbonate) moiety (1–4) and bands characteristic of the donor ligands like pyridine (5–8) and triphenylphosphine (11 − 12). The presence of direct bond of donor ligands to metal via the nitrogen atom in complexes and phosphorous atom in complexes can be further supported by an existence of band at 460–440 cm− 1 and 470– 445 cm−1, which may be assigned to ν(V\\N) and ν(V\\P) vibrations respectively. These peaks are shifted to higher wave numbers compared to the complexes without pyridine and triphenylphoshine molecule, suggesting the addition of Lewis base to the vanadium atom. 3.2. 1H NMR The 1H NMR spectral data of all complexes (1–12) shows characteristic proton resonances of the corresponding tolyl protons. The splitting patterns of the peaks in the spectra of all the complexes were found to be consistent with the structure. There were two resonances for the ring protons of para derivative whereas four resonances were observed for ortho (1) and meta. The 4-chloro-3-tolyl derivative shows three signals. Complexes 1–12 did not show any appreciable deviation

Scheme 3. Synthesis of adducts of vanadium(III) tolyldithiocarbonates (5–12).

130


Table 1 Effective magnetic moment values (μeff) of the complexes 1, 4, 7, 8 and 9. S.No.

Molecular weight of complex (g)

Sample weight (g)

Lab temperature (°C)

Effective magnetic moment (μeff)

1. 4. 7. 8. 9.

600.7 704.0 532.0 600.9 715.2

0.02270 0.02300 0.02155 0.01223 0.02230

21.9 21.9 23.0 23.0 23.0

2.80 2.87 2.70 2.76 2.81

compared to the sodium salts of the dithiocarbonate ligands. Hence, there are no significant changes that were observed as a result of being linked to vanadium. The chemical shifts of methyl (CH3) protons of the tolyl rings were observed as singlet in the region 2.0–2.4 ppm. The aromatic protons of the tolyl rings were observed in the region 6.5– 7.3 ppm. The aryl protons of the free heteroaromatic nitrogen and phosphorous donor bases viz. pyridine and triphenylphosphine, exhibited signals with different multiplicities because of the aromatic methine – CH protons. For pyridine complexes, three signals were observed as doublet, triplet and triplet for proton at position in the range 7.3–7.6,

7.7–7.9 and 8.2–8.7 ppm, respectively and for triphenylphosphine complexes signals were observed for protons (2′– 6′) of the aromatic ring in the range 7.2–7.8 ppm. 3.3. 31P NMR The 31P NMR spectra of the complexes (9–12) were recorded in CDCl3. The appearance of a singlet with a downfield shift was observed for each complex, which may be considered as an authentication of the formation of the compounds. A phosphorus signal in the range 40.1 to 44.3 ppm was observed in the complexes for the triphenylphosphine i.e. deshielded with respect to uncoordinated triphenylphosphine moiety. This deshielding confirms the coordination of phosphorus atom with the vanadium and was attributed to the mesomeric drift of electrons from the phosphorus to the vanadium atom [42]. 3.4. 13C NMR The 13C NMR spectra (CDCl3) of complexes 1–12 showed that the chemical shifts due to the carbon atoms of the phenyl rings were retained with a marginal shift in their values compared to the parent

Fig. 2. SEM image of [(p-CH3C6H4OCS2)V(Cl)·NC5H5] (7) complex (a) low magnification; (b) high magnification.

Fig. 3. TGA curve of [(p-CH3C6H4OCS2)3V] (3).

55.5 100 100 100 100 250 250 250 250 250 48.1 98.1 100 100 100

2.4 0 0 0 0

Concentration Colony (ppm) diameter (cm)

2.8 0.1 0 0 0 200 200 200 200 200 40.7 96.2 96.2 100 100 150 150 150 150 150 16.6 75.9 70.3 96.2 100

3.2 0.2 0.2 0 0

Concentration Colony (ppm) diameter (cm) Inhibition over control (%)

4.5 1.3 1.6 0.2 0 100 100 100 100 100 5.3 1.5 2.0 1.5 1.5 50 50 50 50 50 L1. L4. 4. 7. 9.

1.8 72.2 62.9 77.7 72.2


ligands. Evidence for the formation of these complexes is clearly exhibited in the 13C NMR spectra recorded in CDCl3 by occurrence of a sharp peak for CS2 carbon with an upfield shift. But in case of adducts position of\\CS2 carbon moved to a higher field (2–3 ppm) as compared to the parent vanadium-dithiocarbonate (1–4) moiety and was at 170.7– 195.5 ppm, indicating the participation of the donor groups in coordination with the metal. The chemical shifts for the methyl (\\CH3) carbons attached to the phenyl rings were found in the region 16.9–21.3 ppm. The carbon nuclei of the aryl groups displayed their resonances in the region 114.1–137.5 ppm. The chemical shifts for C\\O carbon nuclei were found in the region 151.2–156.5 ppm. The chemical shifts for the C-(CH3) carbon nuclei were observed in the region 115.0–129.9 ppm. The 13C NMR spectra of the complexes (5–12) have also exhibited the chemical shifts of the carbon nuclei of the donor moieties. There were three resonances for the aryl carbon nuclei of the pyridine in the region 123.6–149.1 ppm and four for the triphenylphosphine in the region 128.2–136.9 ppm.

Mass spectrometry is one of the most important methods to determine molecular weight of the complexes and to identify the fragments formed during bombardment. Two important peaks were observed in the mass spectrum i.e. the molecular ion peak and the base peak. The molecular ion peak indicates the molecular mass of the complex, which is very weak in the case of the complexes investigated while the base peak is the strongest peak. The electron impact (EI) mass spectral data shows the presence of molecular ion peak [M+] at 600.7 (m/z) (1), 704.0 (m/z) (4), 532.0 (m/z) (5) and 715.0 (m/z) (11) which suggested the monomeric nature of these complexes. In addition to the molecular ion peak several other peaks were also observed, which are corresponding to the fragmented species after the consecutive removal of different groups. The occurrence of molecular ion peak in the complexes is supporting the monomeric nature of the complexes. Successive fragmentation gives many more important peaks free xanthate ligand (ROCS2), a two-ligand complex VL2, one ligand complex VL, CS2, CH3C6H4O and donor ligands are the expected products of the various pathways. Moreover, in addition complexes 4, 5 and 11 contain chlorine atom that results in appearance of isotopic peaks in the mass spectrum. The masses of the fragmented ions, listed in the table, are calculated using one chlorine atom mass equal to 35 amu, as it is the most abundant isotope of chlorine atom. The mass spectrum of both the complexes has a prominent base peak at 107 (m/z) (100%). 3.6. Magnetic Susceptibility Measurements


Inhibition over control (%)

131

3.5. Mass Spectra

S. No

Table 2 In vitro evaluation of the ligands and their complexes against the fungus Fusarium oxysporum f. sp.






The magnetic moment of a representative complex (1, 4, 7, 8, 9) was carried out on vibrating sample magnetometer (VSM) at room temperature. The effective magnetic moment value (μeff) of the complex (1, 4, 7, 8, 9) is t22g and e0g configuration to vanadium(III). The effective magnetic moment of complex (1, 4, 7, 8, 9) lies in the range is 2.70–2.87 B.M which is close to the spin-only magnetic moment (2.83 B.M) [43] expected for two unpaired electrons of vanadium(III) center and its weak paramagnetic nature. This magnetic moment value of the complex depicts an octahedral geometry around the vanadium(III) atom, due to anti-ferromagnetic interaction The relevant magnetic moment values of these complexes in the VSM range of 0.1–1 are tabulated in Table 1. 3.7. Scanning Electron Microscopy (SEM) Scanning electron microscopy is used to evaluate morphology and particle size of the [(p-CH3C6H4OCS2)V(Cl)·NC5H5] (7) and has been carried out at a low and high magnification Fig. 2(a-b). The information revealed from the signals included external morphology, topography, structure and orientation of materials making up the sample. The

132


Fig. 4. Graph showing comparative result of antifungal activity.

images show the different sized particles with rough texture with grooves and ridges on the surface. The particles are present in the form of clusters. In general, the SEM photograph shows single phase formation with well-defined shape. 3.8. Thermogravimetric Analysis The thermal behavior of the complex 4 was studied under inert atmosphere in the range of 30–1000 °C and displays weight losses in steps with different time intervals and at different temperatures. These losses indicate decomposition and evaporation of the volatile part of the sample. The curved portion indicates weight loss during process of heating. The thermogram from thermal studies performed on the complex [(p-CH3C6H4OCS2)3V] (3) is shown in Fig. 3. The results show a loss of weight 30.5% (obs.) 30.5% (calc.) due to the removal of (CH3C6H4OCS2) group at approximately 43.9 °C. Further heating up to 367.0 °C shows a gradual weight loss of 60.8% (theoretical weight loss 60.8%) attributable to the formation of [(OCS2)2V]. The weight loss continues beyond this temperature and finally attains a constant mass corresponding to VS2 (observed 80.8%, calcd. 80.8%) at 793.5 °C. 3.9. Antifungal Activity The in vitro biological screening effects of the investigated ligands and their vanadium derivatives were tested against the pathogen “Fusarium oxysporum” f. sp. Capsici causing vascular wilt of chilli (Capsicum annuum L. is unequivocally an important condiment throughout the world and is an important source of vitamin C) by the poisoned food method using Potato Dextrose Agar (PDA) nutrient as the medium. The linear growth of fungus in control and treatment was recorded at different concentrations.

Antifungal activities of sodium salt of ligand and their derivatives have been measured and summarized in Table 2. Our results show that parent compounds and the corresponding adducts of the parent compounds both exhibit potent antifungal activities against Fusarium. The impact of the metal was found in the antimicrobial activity against the tested fungal species. The results obtained by the poison food method indicated that the coordination compounds have enhanced activity compared with the ligands. This shows that metal derivatives are more fungitoxic than the chelating agent (ligand) itself. The dithiocarbonates, represented by the general structure\\O\\C(_S)\\S\\Ar, have no hydrophilic group. On the contrary, these compounds are considered to be lipophilic. The enhanced activity of the metal derivatives may be ascribed to the increased lipophilic nature of these derivatives arising due to the chelation and toxicity of the metal chelates increases with increasing concentration of the complexes, the inhibitory effect on the mycelial growth of the fungus also increases, which can be explained on the basis of Overtone's concept and Tweedy's Chelation theory [44]. Metal ions are adsorbed on the cell walls of the microorganisms, disturbing the respiration processes of the cells and thus blocking the protein synthesis that is required for further growth of the organisms. Hence, metal ions are essential for the growth-inhibitory effects. According to Overtone's concept of cell permeability, the lipid membrane that surrounds the cell favors the passage of only lipid-soluble materials, so lipophilicity is an important factor controlling the antifungal activity. Upon chelation, the polarity of the metal ion will be reduced due to the overlap of the ligand orbitals and partial sharing of the positive charge of the metal ion with donor groups. In addition, chelation allows for the delocalization of πelectrons over the entire chelate ring and enhances the lipophilicity of the complexes. This increased lipophilicity enhances the penetration of the complexes into lipid membranes and blocking of the metal binding sites in the enzymes of microorganisms. These complexes also disturb

Table 3 Antimicrobial activity of ligands and complexes expressed in MIC 128 μg/mL. S. No.

Test organisms Staphylococcus aureus (ATCC 25923)

L1. L4. 4. 5. 10.

Escherichia coli (ATCC 25922)

Pseudomonas aeruginosa (ATCC 27853)

Primary screening at 128 μg/mL

MIC

MBC


MIC

MBC


MIC

MBC

− + + + +

− 128 128 64 64

− − 128 64 64

− − + + +

− − 64 64 64

− − 64 64 64

− + + + +

− 128 128 32 32

− − 128 32 32

S. Andotra et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 180 (2017) 127–137 Table 4 In vitro evaluation of cytotoxicity activity of vanadium dithiocarbonates. Cell line type

THP-1

HCT-116

S.No.

Conc (μM)

% Growth inhibition

L2. L4. 4. 5. 10. Cisplatin

100 100 100 100 100 100

0 11 15 25 22 81

65 86 50 60 51 85

NCI-H322

A549

24 29 40 69 41 89

34 69 70 84 78 96

the respiration process of the cell and thus block the synthesis of the proteins that restricts further growth of the organism. All the complexes showed promising result in inhibiting the mycelial growth of the fungus at a concentration of 250 ppm. The different inhibitory effect of the complexes can be correlated by their different structures. The comparison of antifungal activity of the ligand and some of the complexes is described diagrammatically in Fig. 4 and their comparison with the biocidal activities of free ligands and newly formed complexes is summarized as follows: 1. All the aryl dithiocarbonate ligands possess a pronounced antifungal effect against the fungus at higher concentration and less activity at low concentration. Ligand (L4) shows more activity than corresponding sodium salt of dithiocarbonates. 2. All the metal complexes have higher or equal activity against all micro-organisms as compared with the free ligands. 3. By comparison of antifungal behavior of these synthesized complexes with their corresponding free ligands, we found that these complexes exhibited greater antifungal effects over free ligands used due to increase in lipophilic character of the metal complexes. 4. However, adducts show pronounced antifungal activity as compared to the parent dithiocarbonate complexes. As it is evident from the antifungal screening data, adducts of nitrogen and phosphorous donor ligands are more potent than the parent complex which are in turn more potent than the dithiocarbonate ligands. Due to the presence of anions (strong donors) like pyridine and triphenylphosphine. 5. Maximum inhibition is seen at higher concentration i.e. at 250 ppm. 6. Adducts by means of nitrogen donor bases are prove to be more fungicidal as compared to phosphorous donor bases. 3.10. Antibacterial Activity During the present course of investigations the free ligands, their metal derivatives and their corresponding adducts were tested in vitro

133

against three bacterial strains (Gram-positive Staphylococcus aureus ATCC 25923; Gram-negative Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853) to assess their antimicrobial properties. Antibacterial activities of sodium salt dithiocarbonate ligand and derivatives of vanadium (L1, L4, 4, 5, 10) have been measured and summarized in Table 3. As expected, our newly synthesized complexes showed an interesting antibacterial activity. As is clear from the above table the free dithiocarbonate ligands have almost zero activity towards the three tested bacteria than their metal derivatives. Only the ligand (L4) showed antibacterial activity. All of the metal complexes possess higher antibacterial activity than the ligand. The sodium salts of ortho dithiocarbonate showed no activity on primary screening at 128 μg/mL therefore they have not shown any antibacterial activity. Ligand 4-Cl3-CH3C6H3OCS2Na showed antibacterial activity (MIC = 128) against the Staphylococcus aureus and Pseudomonas aeruginosa but found inactive against the bacteria Escherichia coli. Vanadium(III) dithiocarbonate complexes with nitrogen donor ligands (5) were found to exhibit greater inhibitory action against all bacteria. But these complexes were less active as compared to Ciprofloxacin (positive control). 3.11. Cytotoxicity Cytotoxicity of the ligands (L2 and L4) and few complexes (4, 5 and 10) was determined in vitro against cell lines (Table 4) lung adenocarcinoma cell line A-549, leukemia cell line THP-1, lung cervical node cell line NCI-H322 and colorectal cancer cell line HCT-116. The vanadium complex is rather more effective than the pure ligand, and it is likely that the vanadium metal serves as a protective carrier for the ligand, ensuring to intact at the active site. Complex 5 and 10 showed significant cytotoxic effect against cell lines HCT-116, NCI-H322 and A549. However all these complexes show poor anticancer activity against cell line THP-1. The comparative cytotoxicity data is well illustrated in the form of bar graphs in Fig. 5. 3.12. Antioxidant Activity During the present course of investigations DPPH scavenging ability of the free ligands, their metal derivatives and corresponding adducts were screened (Table 5). Comparison of the antioxidant activities of the free ligands and synthesized complexes with some previously investigated antibiotics shows the following results. The antioxidant assay was carried out using different concentrations of the test samples while ascorbic acid (vitamin C) was used as standard. Antioxidant properties investigated in this study are expressed as EC50 values for comparison. Higher EC50 values indicate lower effectiveness in antioxidant properties. 1. Dithiocarbonate ligand (L1) possesses moderate radical scavenging activity (% RSA) with 44.86 ± 0.94 scavenging at concentration of 100 μg/mL. Ligand (L4) exhibited slightly higher radical scavenging activity with 52.42 ± 0.37 scavenging at concentration of 100 μg/mL than corresponding sodium salt of dithiocarbonates. 2. It was observed that all vanadium complexes had higher % radical scavenging activity than that of the free ligand upon complexation. The increased antioxidant activity of these complexes (Table 5) can be attributed to the electron withdrawing effect of the metal ion which facilitates the release of hydrogen to reduce the DPPH radical. 3. Vanadium(III) dithiocarbonate complexes with pydine as donor ligand (5) were found to exhibit higher DPPH activity with a EC50 b 6.25 which is more than ascorbic acid. 4. Vanadium complex 10 was also found to exhibit higher DPPH activity with a EC50 = 6.25 which is more than ascorbic acid.

Fig. 5. Comparative cytotoxic activity of dithiocarbonate ligands and its vanadium (V) complexes.

From the above results, it can be concluded that the scavenging effects of the free ligand are less when compared to that of their corresponding complexes which is due to the chelation of the organic

134


Table 5 In vitro free radical scavenging activity of complexes, measured in DPPH assay. **S. No.

Concentrations (μg/mL) 100

L1. L4. 4. 5. 10. Ascorbic acid

44.86 52.42 80.11 91.16 70.66 87.83

50 ± ± ± ± ± ±

0.94 0.37 0.2 0.27 0.2 0.03

35.02 42.24 61.22 90.69 67.98 85.83

25 ± ± ± ± ± ±

0.79 0.92 0.30 0.14 0.21 0.03

NA 15.31 59.10 89.27 63.09 76.48

± ± ± ± ±

0.79 0.15 0.14 0.31 1.54

12.5

6.25

NA 8.69 ± 0.92 50.11 ± 0.2 66.24 ± 0.01 57.88 ± 0.16 40.94 ± 0.67

NA NA 38.11 60.34 50.01 20.66

EC50 (μg/mL)

± ± ± ±

0.23 0.2 0.2 0.89

N100 50–100 12.5 b6.25 6.25 13.27 ± 1.10

Values expressed as percentage DPPH radical scavenging activity, as means of three replicates ± SD.

molecules with the metal ions. The enhanced inhibition displayed on the DPPH radical by the test samples shows that the compounds are capable of donating electrons to neutralize free radicals and thus, could be promising therapeutic agents for the treatment of pathological diseases and conditions caused as a result of excessive radicals or stress. 3.13. Computational Discussion In present day density functional theory (DFT) has become an effective tool for determining structure, bond angles, bond lengths, mulliken (partial) atomic charges and vibrational frequencies. Since we could not develop the crystal for X-ray analysis, we analyzed the ligands, pCH3C6H4OCS2Na (L3) and 4-Cl-3-CH3C6H4OCS2Na (L4), and their vanadium complexes (4, 7, 11) with the help of DFT computations. The optimized molecular structures (with minimum energies) obtained from the quantum chemical calculations are shown in Figs. 6 and 7. 3.13.1. Computational Studies of Ligands L3 and L4 The computed bond lengths and bond angles help us to analyze the geometrical features of the molecular structure of the ligands (see Supplementary information Table S1). A comparison of the DFT computed geometrical parameters with the already reported experimental results indicates that optimized bond length and bond angle values are slightly different from these experimental ones [45,46]. The literature reported experimentally determined structure of ligand is built up of two inequivalent xanthate ions that are attracted to four different potassium ions through ionic K\\S and K\\O bonds. Eight S and O atoms surround each potassium ion. Selected bond lengths and bond angles in the calculated structure are presented (see Supplementary information Table S1) together with the previously reported experimentally determined data [45,46]. In our present DFT calculations we have only considered a molecular fragment without taking into account the ionic attractions to neighboring sodium ions. This disregard should give the largest discrepancies on the Na, S and O atoms. However, despite this neglect, the bond lengths and bond angles are in very good agreement with the reported experimental data, except for the ones involving K. It is known that DFT calculations on ionic bonds generally give shorter bonds than expected. The only discrepancies are in the ionic region due to the effect of the absence of the surrounding lattice. The calculated S1\\Na1 bonds [2.752 Å

(L3) and 2.759 Å (L4)] are shorter than expected (reported value 3.18 Å) [45]. The bond length C7\\S1 lies in the range 1.750 Å (L3) and 1.754 Å (L4) Å and there is double bond between C7 and S2 which lies in the range 1.764 Å (L3) and 1.763 Å (L4) Å for both the ligands which are slightly larger than reported results (1.67 and 1.69 Å) [47,48]. The calculated bond lengths for C7\\O1 come out to be 1.381 (L3) and 1.386 (L4) Å (compared to reported value 1.37 Å). The S\\C\\S bond angles for the ligands L3 and L4 are of 126.77 and 127.00°. The bond angles for O1\\C7\\S1 and O1\\C7\\S2 are 113.16 and 120.06° for ligand L3. The bond angles for O1\\C7\\S1 and O1\\C7\\S2 are 113.12 and 119.86° for ligand L4 [45,46]. Mulliken analysis is the most common population analysis method [47]. The partial atomic charges obtained from such analysis have a significant role since these may affect some properties of molecular systems including dipole moment, and molecular polarizability. It also has been used to describe the electrostatic potential surfaces [48–49]. It was shown from the calculations that C7 atom carries highest negative charge and Na atom carries highest positive charge in case of both the ligands. Also S and O atoms in both the ligands carry negative charge. The C2, C3, C5, C6 atoms of the tolyl ring carry negative charge while a C1 and C4 atoms carry positive charge (see Supplementary information Table S2). Vibrational spectroscopy has been widely used as a standard tool for structural characterization of molecular systems by DFT calculations. It is found that the DFT calculated frequencies are closer to the experimental values. Experimental and calculated vibrational frequencies along with corresponding vibrational assignments are given in Table 6. Note that frequencies have been scaled by a scaling factor of 0.961 at the level of DFT/B3LYP/LANL2DZ method. 3.13.2. Computational studies of complex 4 It was shown from the calculations that vanadium atom enjoys octahedral geometry formed by six S- atoms from the three chelating phenyldithiocarbonate ligand. The computed bond lengths and bond angles help us to understand the molecular structure of the complex. The calculated bond lengths for V\\S1 and V\\S2 fall in the range 2.453 and 2.469 Å which are in good agreement with the experimentally reported values 2.43 and 2.48 Å [50]. The computed bond lengths for C7\\S1 and C7\\S2 fall in the range (1.730 and 1.773 Å) and the optimized bond length for O1\\C7 is 1.345 Å. The bond angle S1\\C7\\S2,

Fig. 6. Optimized structure of ligands L3 and L4 at the level of DFT/B3LYP/LANL2DZ theory.


135

Fig. 7. Optimized structure of complex 4, 7 and 11 at the level of DFT/B3LYP/LANL2DZ theory.

S1\\V1\\S2, V1\\S1\\C7, V1\\S2\\C7 and C7\\O1\\C1 is measured as 116.634, 76.026, 82.600, 83.0278 and 123.187° as it is calculated by using DFT (see Supplementary information Table S3). So, comparison of the data reveals that there is close agreement between bond distances and bond angles as compared with the literature reports. The marginal difference in value of bond distances and angles may be attributed to the presence of phenyl ring and substitution on it. The atomic charges on the various atoms are obtained by using Mulliken population analysis [47]. It is worthy to mention that C1, C5 atoms of the tittle complex 4 exihibit positive charge while all the other carbon atoms (C2, C3 and C4, C6) of the tolyl ring exhibit negative charges. The charges on the sulfur atoms of the ligand are strongly affected upon complexation and it acquires a positive charge. The central metal atom acquires slight negative charge (see Supplementary information Table S4). The experimental and calculated vibrational stretching frequencies show negligible difference. However, the theoretical and observed agreements were found to be satisfactory. Experimental IR spectra show the characteristic sharp band for ν(C S) stretching frequencies in the region 1015 cm− 1 and the theoretical data shows band in the range 1010.48 cm−1. Bands for v(V\\S) were appeared in the range 370 cm− 1 which are in agreement with the results computed as 365.44 cm−1 (for DFT). Experimental and optimized IR stretching frequencies were presented in Table 7.

3.13.3. Computational studies complex of 7 and 11 The adducts of vanadium(III) are computed to have octahedral geometry. The vanadium atom has octahedral coordination geometry formed by tertiary chlorine and nitrogen of one donor moiety (pyridine/triphenylphosphine) and four sulfur atoms of two coordinated dithiocarbonate ligands which are situated in the equatorial plane. The V\\S bond lengths for complex 7 and 11 are (2.501 and 2.525 Å) and (2.450 and 2.539 Å) which are also in good agreement with the experimentally reported bond length (2.43 and 2.48 Å) [50]. The computed bond lengths C7\\S1/S2 are 1.730 and 1.840 Å (7) and 1.773 and 1.847 Å (11). The computed bond lengths V\\N and V\\P (2.200 and 2.516 Å) were found to be in close agreement with those compared with literature values (2.146 and 2.543) Å [51–52]. The bond angle S1\\C7\\S2 and S1\\V1\\S2 is measured as 118.338, 117.539, 73.866 and 74.126° for both the complexes as it is calculated by using DFT. The V\\S bond lengths in complexes 7 and 11 are marginally longer than those in complex 4. This increase in V\\S bond length in complexes 7 and 11 may be attributed to the formation of an adduct. Coordination of the aromatic donor ligands to the V atom results in widening of the S1\\C7\\S2 bond angle, lengthening of V\\S distance and decrease in the S\\V\\S angle due to the steric and electronic effects of the Lewis base when compared to the V(III) octahedral complexes (see Supplementary information Table S3). The C3 atom has greater negative charge than other C atoms in an aromatic benzene ring. In complex 7 vanadium carries positive charge

Table 6 Selected experimental and calculated vibrational frequencies (cm−1) for ligands L3 and L4. Assignment

L3 Experimental

v(C_S) v(C\ \S) v(C\ \O\ \C) ⁎ Scale factor = 0.961.

1145, s 1007, s 1225, vs

L4 Theoretical

Experimental

Unscaled

Scaled⁎

1191.10 1037.62 1245.60

1144.64 997.15 1197.02

1166, s 999, s 1246, vs

Theoretical Unscaled

Scaled⁎

1178.75 1026.14 1304.51

1132.77 986.12 1253.63

136


Table 7 Selected experimental and calculated vibrational frequencies (cm−1) of complex 4, 7 and 11. Assignment

Complex 4

Complex 7

Experimental

v(C\ \O\ \C) v(V\ \S) v(V\ \N/P)

Theoretical

1015, s 1236, s 370, w –

Experimental

Scaled

Unscaled

1051.49 1280.12 380.28 –

1010.48 1230.19 365.44

1015, s 1265, s 391, w 441, w

while in complex 11 it carries negative charge. Nitrogen of pyridine moiety carries negative charge while phosphorous of triphenylphosphine carries positive charge. The charges on the sulfur atoms of the complexes are strongly affected upon complexation and it acquires a positive charge (see Supplementary information Table S4). The vibrational frequencies are calculated with the optimized geometry, the computed and experimental data are in close agreement. Thus, in complexes 7 and 11, the sharp band for v(C S) as observed in the region 1015 cm−1 and the theoretical data for v(C S) vibration are in the region 1011.32 and 1013.85 cm− 1. The observed bands in the range 379–391 cm−1 for v(V\\S) match with the corresponding theoretical data 384.96 and 369.10 cm− 1. The observed bands in the region 441 cm−1 and 465 cm−1 for v(V\\N) and v(V\\P) are close to theoretical data and are found as 447.88 and 466.20 cm−1. Experimental and optimized IR stretching frequencies were presented in Table 7.

3.13.4. Electronic Properties The frontier orbitals, namely, the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) are very important parameters from quantum chemistry calculations. The HOMO-LUMO gap is an important parameter to determine the electrical transport properties in the molecules [53]. The eigenvalues of HOMO and LUMO and their energy gap can reflect the biological activity of the molecule. A molecule having a small frontier orbital gap is more polarizable and is generally associated with a high chemical reactivity and low kinetic stability [54–55]. The popular Koopmans' theorem describes ionisation potential (I) and electron affinity (A) as the negative of energy eigenvalues of HOMO and LUMO respectively. In both the ligands (L3 and L4), it has been found that HOMO is largely distributed over the oxygen, carbon and sulfur atoms of anionic dithiocarbonates while LUMO is distributed over the sodium atoms. LUMO and HOMO (see Supplementary information Figs. S1–S5) are distributed over the V, O, C and S atoms in complex 4. In the pictorial plot of the HOMO-LUMO, the red color indicates the positive phase and blue color denotes negative

Table 8 Quantum-mechanical molecular properties (in a.u., 1 a.u. = 27.211 eV, except dipole moment which is in the units of Debye) calculated for L3, L4, 4, 7 and 11 at the level of DFT/ B3LYP/LANL2DZ theory. Parameters

L3

L4

Complex 4 Complex 7 Complex 11

EHOMO ELUMO EHOMO − 1 ELUMO+1 EHOMO-LUMO gap EHOMO − 1-LUMO + 1 gap Ionisation potential (I) Electron affinity (A) Chemical hardness (η) Chemical softness (S) Chemical potential (μ) Electronegativity (χ) Electrophilicity (ω) Dipole moment (D)

−5.519 1.343 −5.653 −1.069 4.176 4.584 5.519 1.343 2.088 0.478 −3.431 3.431 2.818 6.632

−5.704 −1.443 −5.830 −1.251 4.261 4.579 5.704 1.443 2.130 0.469 −3.573 3.573 2.996 8.201

−5.799 −3.503 −6.971 −3.415 2.296 3.556 5.799 3.503 1.148 0.574 −4.651 4.651 9.421 2.373

−5.656 −3.096 −6.353 −2.692 2.560 3.661 5.656 3.096 1.280 0.781 −4.376 4.376 7.480 9.623

−5.519 −2.939 −6.402 −2.587 2.580 3.815 5.519 2.939 1.290 0.775 −4.229 4.229 6.931 6.954

Complex 11 Theoretical

Experimental

Scaled

Unscaled

1052.37 1261.13 400.59 466.06

1011.32 1211.94 384.96 447.88

1015, s 1240, s 379, w 465, w

Theoretical Scaled

Unscaled

1055.00 1282.06 384.08 485.13

1013.85 1232.05 369.10 466.20

phase of the orbitals. For complexes 7 and 11, HOMO is located mainly on the vanadium and sulfur atoms and LUMO is mainly distributed over sulfur, chlorine and pyridine moiety. And in the case of triphenylphosphine adduct, LUMO is mainly localised over sulfur, chlorine and oxygen atoms while HOMO is localised over sulfur, oxygen and central metal atom. Other electronic parameters viz. EHOMO − 1, EHOMO, ELUMO and ELUMO + 1, energy gap (EGAP), ionisation potential (I), electron affinity (A), hardness (η), softness (s), electronegativity (χ), electrophilicity (ω) and dipole moment (D) are also listed in Table 8. 3.13.5. Thermodynamic Parameters To get consistent relations between energetic, structural and reactivity characteristics of the dithiocarbonate complexes, the thermodynamic parameters have also been calculated quantum-mechanically. The values of thermodynamic parameters such as zero-point vibrational energy, entropy, enthalpy, internal energy, specific heat capacity and rotational constants of both the complexes by the DFT/B3LYP/LANL2DZ method at 298.15 K and 1 atm pressure were calculated and listed in Table 9. 4. Conclusion We have reported the synthesis and characterization of some vanadium dithiocarbonates and their corresponding donor stabilized complexes for the first time using mono and di-substituted aryldithiocarbontes having methyl and chlorine substituents. Various physico-chemical studies viz. elemental analyses, IR, mass, magnetic susceptibility, TGA, SEM and heteronuclear NMR (1H, 13C and 31P) and in conjunction with literature reports octa-coordinate geometry may be proposed for vanadium(III) trisdithiocarbonates and their adducts, in which dithiocarbonate ligands behaved in bidentate manner (see Supplementary information Figs. S6–S8). The formation of complexes (9, 10, 11, 12) is also supported by 31P NMR spectra which exhibited the signal due to triphenylphosphine moiety with downfield shift. Antimicrobial screening data show that donor stabilized vanadium complexes are more potential compared to parent vanadium dithiocarbonates. Antioxidant activity reveals that the complexes 5 and 10 exhibit higher DPPH activity compared to that of standard ascorbic acid. The cytotoxic properties of the complex (5) exhibited maximum antitumor potency against cell line A549. The molecular structures were examined theoretically by DFT calculations. The geometrical parameters of the optimized geometries are in good agreement with the experimental and literature values. The present quantum chemical study may lead to understanding of properties and activities of complexes. Acknowledgments We are grateful to the Sophisticated Analytical Instrumentation Facility, Panjab University, Chandigarh and NMR Lab (PURSE Programme), Department of Chemistry, University of Jammu, Jammu, for providing spectral facilities. We are also thankful to the Division of Pharmacology, IIIM, Jammu, for providing cytotoxicity analysis.


137

Table 9 Calculated thermodynamical parameters of L3, L4, 4, 7 and 11 (1 a.u. = 627.51 kcal/mol). Parameters

L3

L4

Complex 4

Complex 7

Complex 11

Zero-point vibrational energies (a.u) Gibbs free energy (a.u) Enthalpy (a.u) Internal energy (a.u) Entropy (calmol−1 K−1) Specific heat, Cv (calmol−1 K−1) Rotational constants (GHz)

−404.551 −404.607 −404.550 −404.551 119.695 43.537 1.7767 0.29359 0.27723

−418.912 −418.956 −418.897 −418.898 124.300 47.457 1.32477 0.20201 0.20039

−1327.339 −1327.424 −1327.298 −1327.299 266.604 141.641 0.05340 0.03369 0.02247

−1143.157 −1143.227 −1143.124 −1143.125 216.511 112.971 0.19650 0.04999 0.04845

−1596.028 −1596.114 −1596.984 −1596.985 274.592 159.116 0.09738 0.03938 0.03525

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.saa.2017.03.003. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

[28] [29]

R.S. Rao (Ed.), Xanthates and Related Compounds, Dekker, New York, 1971. B.S. Rao, K. Ramakrishna, J. Ind. Council Chem. 20 (2003) 13–16. R.J. Magee, Rev. Anal. Chem. 1 (1973) 335–377. E.M. Donaldson, Talanta 23 (1976) 417–426. M.S. Hill, G. Kociok-Kohn, K.C. Molloy, D.C. Stanton, Main Group Met. Chem. 38 (2015) 61–67. G. Winter, Rev. Inorg. Chem. 2 (1980) 253–342. E.R.T. Tiekink, G. Winter, Rev. Inorg. Chem. 12 (1992) 183–302. Z. Travnicek, M. Malon, Z. Sinetar, Transit. Met. Chem. 24 (1999) 38–41. L. Ballester, A. Gutierrez, M.F. Perpinan, R.J. Motekaitis, I. Murase, A.E. Martell, Polyhedron 7 (1996) 1103–1107. G.H. Harrison, Xanthates, in: M. Howe-Grant (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, New York 1998, pp. 713–734. S.M. Bulatovic, Handbook of Flotation Reagents-Chemistry, Theory and Practices,– Flotation of Sulfides Ores, vol. 1, Elsevier, Amsterdam, 2007. A.P. Chandra, L. Puskar, D.J. Simpson, A.R. Gerson, Int. J. Miner. Process. 16 (2012) 114–117. Y.-K. Chang, J.-E. Chang, T.-T. Lin, Y.-M. Hsu, J. Hazard. Mater. B94 (2002) 89–99. S. Chakraborty, V. Tare, Bioresour. Technol. 97 (2006) 2407–2413. W. Friebolin, G. Schilling, M. Zoller, E. Amtmann, J. Med. Chem. 47 (2004) 2256–2263. W. Friebolin, G. Schilling, M. Zoller, E. Amtmann, J. Med. Chem. 48 (2005) 7925–7931. S. Efrima, N. Pradhan, C. R. Chim. 6 (2003) 1035–1045. H.J. Moore, R. Colorado Jr., H.J. Lee, A.C. Jamison, T.R. Lee, Langmuir 29 (2013) 10674–10683. N. Pradhan, B. Katz, S. Efrima, J. Phys, Chem.B 107 (2003) 13843–13854. H.C. Leventis, S.P. King, A. Sudlow, M.S. Hill, K.C. Molloy, S.A. Haque, Nano Lett. 10 (2010) 1253–1258. Y. Mikhlin, S. Vorobyev, S. Saikova, Y. Tomashevich, O. Fetisova, S. Kozlova, S. Zharko, New J. Chem. 40 (2016) 3059–3065. J.W. Spanyer, J.P. Phillips, Anal. Chem. 28 (1956) 253. J. Schmitt, N. Blanchard, J. Polym. Chem. 2 (2011) 2231–2238. S. Perrier, P. Takolpuckdee, J. Poly, Sci. Part A. 43 (2005) 5347–5393. B. Gupta, N. Kalgotra, S. Andotra, S.K. Pandey, Monatsh. Chem. 143 (2012) 1087–1095. A.I. Vogel, Quantitative Inorganic Analysis, third ed. Longmans, London, 1961. P.A. Wayne, Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard, nineth ed. Clinical and Laboratory Standards Institute (CLSI), 2012 M07–MA9. D.P. Singh, V. Malik, R. Kumar, K. Kumar, J. Serb. Chem. Soc. 75 (2010) 763–772. P. Skehan, R. Storeng, D. Scudiero, A. Monks, J. McMahon, D. Vistica, J.T. Warren, H. Bokesch, S. Kenney, M.R. Boyd, J. Natl. Cancer Inst. 82 (1990) 1107–1112.

[30] V. Bondet, W. Brand-Williams, C. Berset, Lebensm. Wiss. Technol. 30 (1997) 609–615. [31] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Revision A. 02, Gaussian, Inc., Wallingford, CT, 2009. [32] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652. [33] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B37 (1988) 785–789. [34] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270–283. [35] A.A.S. El-Khaldy, R.M. Barakra, R.A. Abu El-Gomboz, Synth. React. Inorg. Met.-Org. Nano -Met. Chem. 33 (2003) 195–204. [36] R. Chander, B.L. Kalsotra, S.K. Pandey, Transit. Met. Chem. 28 (2003) 405. [37] N. Kalgotra, B. Gupta, K. Kumar, S.K. Pandey, Phosphorus Sulfur Silicon Relat. Elem. 187 (2012) 364–375. [38] S. Andotra, N. Kalgotra, S.K. Pandey, Bioinorg. Chem. Appl. 2014 (2014) 1–12. [39] N. Kalgotra, B. Gupta, S. Andotra, S. Kumar, S.K. Pandey, Int. J. Inorg Chem. 2013 (2013) 1–12. [40] F. Bonati, R. Ugo, J. Organomet. Chem. 10 (1967) 257–268. [41] K. Nakamoto, Infrared and Raman Spectra of Inorganic Compounds, fourth ed. Wiley-Interscience, New York, USA, 1986. [42] G. Socrates, Infrared Characteristics Group Frequencies, John Wiley and Sons Ltd., Great Britain, 1980. [43] L.F. Larkworthy, M.W. O' Donoghue, Inorg. Chim. Acta 74 (1983) 155–158. [44] B.G. Tweedy, C. Loeppky, Phytopathology 58 (1968) 1522. [45] C. Larsson, S. Oberg, J. Phys, Chem. A 115 (2011) 1396–1407. [46] N.A. Frolova, V.Kh. Kravtsov, M.D. Mazus, A.A. Kashaev, S.B. Leonov, Kristallografiya 31 (1986) 806–808. [47] R.S. Mulliken, J. Chem. Phys. 23 (1995) 1833–1840. [48] V. Balachandran, K. Parimala, Spectrochim. Acta A 96 (2012) 340–351. [49] A. Lakshmi, V. Balachandran, J. Mol. Struct. 1033 (2013) 40–50. [50] A.V. Rogachev, A.V. Virovets, M.N. Sokolov, Russ. J. Coord. Chem. 40 (2014) 722–725. [51] M.F. Davis, M. Jura, A. Leung, W. Levason, B. Littlefield, G. Reid, M. Webster, Dalton Trans. (2008) 6265–6273. [52] S. Tofke, U. Behrens, Acta Cryst 42 (1986) 161–163. [53] M. Govindarajan, S. Periandy, K. Carthigayen, Spectrochim. Acta A 97 (2012) 231–245. [54] L.A. Curtiss, P.C. Redfern, K. Raghavachari, J.A. Pople, J. Chem. Phys. 42 (1998) 117–122. [55] I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley and Sons, New York, 1976.