Reactivity and Kinetic Studies of (NH4)2(MoS4) in Acidic Aqueous Solution: Possible Relevance to the Angiostatic Function of the MoS4(2-) Ligand

Although addition of mineral acids to WS(4)(2-) in water is known to lead to aggregation and formation of various polynuclear thiotungstate anions, acid hydrolysis of the MoS(4)(2-) anion is reported to give mainly MoS(3) or MoS(2) as hydrolysis products. Knowledge of the resulting product(s) from such reactions has implications on the use of tetrathiomolybdate (MoS(4)(2-)) as both a potential anti-tumor drug and for the treatment of Wilson's disease. In this investigation, reaction of HCl with MoS(4)(2-) in water was monitored as a function of time. Reaction mixtures of both 1:1 and 2:1 mole ratios of the acid to MoS(4)(2-) were examined, as well as MoS(4)(2-) reactions in simulated human stomach fluids at pH of approximately 2 and 3. Monitoring by electrospray mass spectrometry (ESMS), Fourier transform infrared (FTIR), and UV-visible spectroscopy clearly has revealed the formation of complex mixtures of polynuclear thiomolybdates (Mo(2)-Mo(18)). Generally, a two-stage consecutive reaction sequence occurs. A faster stage (k=7.0-7.9 x 10(-2)min(-1)), which seems to extend to trinuclear thiomolybdate species, followed by a slower second stage (k=5.4-15.2 x 10(-4)min(-1)) to higher polynuclear thiomolybdates. Under acidic conditions (e.g. pH approximately 3) that could also mimic some human stomach fluids, and under anaerobic atmosphere where the generated hydrogen sulfide is prevented from escaping from the reaction vessel, Mo(3)S(9)(2-) predominates over an extended reaction period. In similar reactions under aerobic conditions and where the hydrogen sulfide is irretrievably lost from the reaction mixture the binuclear (Mo(2)O(a)S(10-a)(2-);a=0-3) and trinuclear (Mo(3)O(b)S(9-b)(2-);b=1-3) anions predominate.