Water-Soluble α-Amino Acid Complexes of Molybdenum as Potential Antidotes for Cyanide Poisoning: Synthesis and Catalytic Studies of Threonine, Methionine, Serine, and Leucine Complexes

Water-soluble complexes are desirable for the aqueous detoxification of cyanide. Molybdenum complexes with α-amino acid and disulfide ligands with the formula K[(L)Mo2O2(μ-S)2(S2)] (L = leu (1), met (2), thr (3), and ser (4)) were synthesized in a reaction of [(DMF)3MoO(μ-S)2(S2)] with deprotonated α-amino acids; leu, met, thr, and ser are the carboxylate anions of l-leucine, l-methionine, l-threonine, and l-serine, respectively. Potassium salts of α-amino acids (leu (1a), met (2a), thr (3a), and ser (4a)) were prepared as precursors for complexes 1–4, respectively, by employing a nonaqueous synthesis route. The ligand exchange reaction of [Mo2O2(μ-S)2(DMF)6](I)2 with deprotonated α-amino acids afforded bis-α-amino acid complexes, [(L)2Mo2O2(μ-S)2] (6–8). A tris-α-amino acid complex, [(leu)2Mo2O2(μ-S)2(μ-leu + H)] (5; leu + H is the carboxylate anion of l-leucine with the amine protonated), formed in the reaction with leucine. 5 crystallized from methanol with a third weakly bonded leucine as a bridging bidentate carboxylate. An adduct of 8 with SCN— coordinated, 9, crystallized and was structurally characterized. Complexes 1–4 are air stable and highly water-soluble chiral molecules. Cytotoxicity studies in the A549 cell line gave IC50 values that range from 80 to 400 μM. Cyclic voltammetry traces of 1–8 show solvent-dependent irreversible electrochemical behavior. Complexes 1–4 demonstrated the ability to catalyze the reaction of thiosulfate and cyanide in vitro to exhaustively transform cyanide to thiocyanate in less than 1 h.