Reactions of MoOCl4 and MoO2Cl2 with Heterocyclic Thioamides

MoOCl4/MoO2Cl2 were reacted with 2-mercaptopyridine (IUPAC: pyridine-2-thiol)/4phenylimidazole-2-thiol (IUPAC: 4-phenyl-1,3-dihydroimidazole-2-thione)/6-mercaptopurine (IUPAC: 1,7-dihydro-purine-6-thione) in acetonitrile medium using equal/twice molar concentrations at normal temperature. The reactions yielded products: MoOCl3(C5H4NS-SNH4C5).2HCl,[1]; Mo3O3Cl12(C5H4NSSNH4C5)(CH3CN)2,[2]; Mo2OCl6(C9H7N2S)4,[3] and Mo2O2Cl8(C5H4N4S)(CH3CN),[4]. The various techniques used for characterization of compounds are: Fourier transform infrared, protron nuclear magnetic resonance, 13C nuclear magnetic resonance, liquid/gas chromatography, mass proton spectrometry and C, H, N, S, Mo, Cl analysis. The products are prone to oxidation/hydrolysis by air/ moisture, so all procedures were executed in vacuum line using dry nitrogen atmosphere. Elemental analysis and fragments recorded in mass spectrometry are in tune with the formulae proposed.

Purine ring system represents skeleton for nucleic acid bases guanine and adenine. 6-Mercaptopurine 1-2 is used as chemotherapy drugs to treat various types of cancers. This drug is also used to cure autoimmune diseases. 6-Mercaptopurine is marketed as purinethol. Purine is antagonists drug because it stops the growth of cancer cells.
There is thione-thiol tautomerism. In gaseous state, thiol form is favoured, whereas in solution form thione form is favoured 10 . There is also prototropic tautomerism 10 in 6-mercaptopurine between 7 and 9 positions.

4-Phenylimidazole-2-thiol (IUPAC: 4-phenyl-1, 3-dihydroimidazole-2-thione)
Thioamides have higher reactivity 11 than ordinary amides, because (a) thioamides have lesser C=S double bond character than C=O due to larger size of S atom than that of C atom and bond dissociation energy of C=O bond is 50 k.cal./mole higher than that of C=S bond, (b) greater participation of the sulphur to lower occupied molecular orbitals of the thioamide group, (c) there is more electron density on nitrogen atom in thioamides, so protons on the nitrogen atom is are more labile, (d) higher lability of the protons on carbons next to the thioamide group. The sulphur atom of thioamide group accepts nucleophiles and electrophiles.

Aim of investigation
Many ligands have been found to react with MoO 2 Cl 2 and MoOCl 4 .

MATERIALS AND METHODS
Synthesis of MoOCl 4 was carried out from thionyl chloride and molybdenum trioxide at reflux temperature. Unreacted thionyl chloride was evacuated into liquid nitrogen cooled traps. Dark green crystals of MoOCl 4 isolated on evacuation were dissolved in dry dichloromethane. Dark red solution was obtained. It was filtered to eliminate 2-Mercaptopyridine and 1H-pyridine-2-thione are tautomers. We can represent 2-mercaptopyridine as pySH. There are two probable coordination sites 11 pySH and pyS-(pyridine-2thiolate ion), but pySH is more prevalent with any unreacted/undissolved MoO 3 . Red solution was evaporated to get dark green crystals of MoOCl 4 .
The products are prone to oxidation/ hydrolysis by air/moisture, so all procedures were executed in vacuum line using dry nitrogen atmosphere. Ligand solution in dry dichloromethane acetonitrile was added dropwise to MoOCl 4 /MoO 2 Cl 2 with continuous stirring from pressure stabilised dropping funnel. The reactants were agitated for 6-7 hours. The products were filtered through G-4 sintered glass crucible filtration unit and isolated.
Oxinate 43 gravimetric method was used in the lab for molybdenum estimation. Silver chloride 43 gravimetric method was used in the lab for chlorine estimation. Remaining elements were analysed with Thermo Finnigan Elemental Analyser. Infrared spectra were recorded with Perkin-Elmer 400 FTIR Spectrometer. Proton nuclear magnetic resonance, 13 C nuclear magnetic resonance spectra in DMSO-d 6 were taken with Multinuclear Brucker Avance-II 400 NMR spectrometer. Liquid/Gas Chromatography-Mass spectra were recorded in the range 0-1100 m/z. Above instruments were used at P. U. Chandigarh.

Reactions
I t i s p r o b a bl e t h a t m e c h a n i s m o f these reactions involve oxidation/reduction, disproportionation/rearrangement, isomerization steps. The product source residue (R) or filtrate (F) is shown below the products. Table 1 shows the percentage of the elements. Theoretical values are enclosed in brackets.  Table 2 shows that 2-mercaptopyridine 35,44,48 has N-H stretching (3179 cm -1 ) and S-H stretching (2709 cm -1 ). 3208 cm -1 peak in [1] implies the presence of N-H functional group. S-H peak at ~ 2708 cm -1 disappears in [1] amounting to thiol group elimination. This further indicates that the ligand has reacted in [1] as thione. C=N bond in [1] has absorbed at the same position as in the free ligand. There is positive shift of C-S frequency to 763 cm -1 due to increase in C-S bond order.

FTIR Spectra
Absorption of terminal Mo=O group [45][46][47] (983 cm -1 ) is observed. 3209 cm -1 peak in [2] implies the presence of N-H functional group. S-H peak at ~ 2709 cm -1 disappears in [2] amounting to thiol group elimination. This further indicates that the ligand has reacted in [2] as thione. C=N bond in [2] has absorbed at 1599 cm -1 showing positive shift in frequency. There is positive shift of C-S frequency to 760 cm -1 due to increase in C-S bond order. Absorption of terminal Mo=O group [45][46][47] (974 cm -1 ) is observed.  [49][50][51] υ s = sym. stretching, υ = out-of-plane bending (wagging), υ = in-plane bending (rocking) Table 3 shows N-H stretching peak (3139 cm -1 ) in [3] of 4-phenylimidazole-2-thiol 39,52-54 . No S-H stretching in the range 2551-2603 cm -1 in [3] has been observed referring that S-H group has got dissociated. Stretching of C=S (1255 cm -1 and 1225 cm -1 ) in [3] have been detected. Carbonyl group has greater stretching wave number than that of thio group due to the fact that C=O group is stronger and more polar than C=S group. Intensity of C=O group absorptions is more than that of C=S group. C-S stretching (761 cm -1 ) has been manifested by [3] owing to origin of Mo-S bond. The latter has stretching at 529 cm-1. Mo=O terminal group stretching (992 cm -1 -1010 cm -1 ) has been reported in literature. This peak was observed [49][50][51] at 968 wave number in [3]. Imidazole-2-thiones 39,54 undergoes thiol-thione tautomerism. There is decrease in Mo=O stretching owing to S→Mo coordination 39,55 . Mo=O and ligand are trans to each other. It suggests that ligand reacts as thiol during the reaction. Higher wave number of C=N stretching is associated with it. 968.5 s 6-Mercaptopurine 9,56,57 pyrimidine ring stretching (3398 cm -1 ) in [4] is indicative of N-H group. 6-Mercaptopurine 9,56,57 imidazole ring N-H stretching is absent, because imidazole ring does not have N-H bond in [4] (Table 4). This supports the presence of Mo-N bond. There is no S-H stretching in [4]. It is concluded that pyrimidine of 6-mercaptopurine reacted as thiol. This fact is also reflected by positive shift of C=N stretching frequency and negative shift of C-S frequency.

H NMR Spectra
When 2-mercaptopyridine 35,46 spectrum is compared with that of [1], it is observed that there is deshielding of protons due to N → Mo coordination (Table 5). N-H resonance occurs at 14.93 δ. C-N bond π electron density increases on coordination.
When 2-mercaptopyridine 35,46 spectrum is compared with that of [2], it is observed that there is deshielding of protons due to N → Mo coordination (Table 5). There is no N-H peak. C-N bond π electron density increases on coordination. Notations ↓ & ↑ stand for shielding and deshielding.  N-H chemical shift of 4-phenylimidazole-2thiol 39,59-60 appears at 12.9 δ. There is no particular chemical shift of alcoholic, amino, phenolic and thiolic protons in a solvent, because these protons are labile. There is no existence of N-H and S-H peaks in [3] due to above reasons. There is upfield chemical shift of ring protons and H-5 due to N → Mo coordination.    (Table 7) is not observed may be due to its participation in bonding. There is no S-H peak may be due to reaction of the ligand as thione. Acetonitrile peak is observed at 2.05 δ.

C NMR Spectra
In 13 CNMR, the C=S absor ption of 2-mercaptopyridine 46,61-63 in [2] is shifted upfield by 20 ppm (Table 8). This high chemical shift is characteristic of S-bonded thiones. This is due to Mo→S back donation of molybdenum d-orbitals to the sulfur empty π*-orbitals. It increases the C=S π-electron density and shielding of the thiocarbon. C-2 has moved upfield while other carbons have moved downfield. Notations ↓ & ↑ stand for shielding and deshielding.

CONCLUSION
N-H group in [1] absorbs at 3208 cm -1 . There is no peak in the region 2709 cm -1 , because S-H group has disappeared. There is positive shift of C-S frequency to 763 cm -1 due to increase in C-S bond order confirming the Mo-S bond formation. Ligand has reacted in [1] as thione. Mo=O stretching at 983 cm -1 in [1] confirms that Mo=O is situated at terminal position. Deshielding of protons shows that there is N → Mo coordination. Ions detected in GCmass spectrum convince the suggested formula. [2] shows N-H peak (3209 cm -1 ). There is no peak in the region 2709 cm -1 , because S-H group has disappeared. There is positive shift of C-S frequency to 760 cm -1 due to increase in C-S bond order confirming the Mo-S bond formation. Ligand has reacted in [2] as thione. Mo=O stretching at 974 cm -1 in [2] confirms that Mo=O is situated at terminal position. Ions detected in LC-mass spectrum convince suggested formula. Deshielding of protons shows that there is N → Mo coordination. Presence of CH 3 CN is confirmed by the presence of 1 H NMR peak at 1.86 δ. 13 C NMR also justifies the presence of 2-mercaptopyridine.
Thiol group is absent in [3] due to lack of absorptions in the frequency span 2549-2600.
Thiocarbonyl group (1255, 1225, 1102 cm -1 ) exists. C-S stretching (761 cm -1 ) manifests the origin of Mo-S bond. Mo=O terminal group (968 wave number) is noticed. S-H chemical shift is missing due to absence of thiol group. Ions detected in LC-mass spectrum convince the suggested formula.
S-H is lacking in [4] because of presence of υ(C=S) at 1027 cm -1 and absence of absorption ~ 2672 cm -1 . There is no S-H stretching in [4]. Pyrimidine of 6-mercaptopurine reacted as thiol. This fact is also reflected by positive shift of C=N stretching frequency and negative shift of C-S frequency. Here ligand may be acting as multidentate ligand as thione. N→Mo coordination of pyrimidine decreases N-H stretching. There is S→Mo coordination. Mo=O terminal group (977 cm -1 ) is observed. There is no chemical shift of S-H in 1 H NMR of [4]. Presence of CH 3 CN is confirmed by the presence of peak at 2.05 δ in 1 H NMR. Derived formula is supported by LC-mass spectrum.

ACKNOWLEDGEMENT
We, the authors express our gratitude to P. U. Chandigarh, India for providing characterizing facility.

Conflict of interest
There is no conflict of interest among the authors.