MOLYBDENUM COMPLEXES WITH STERICALLY-HINDERED THIO-CARBOXYLATE LIGANDS - ELECTROCHEMICAL PROPERTIES OF THE ANIONIC COMPLEX BIS(2,2-DIPHENYL-2-MERCAPTOETHANOATE)DIOXOMOLYBDATE(VI) IN PROTIC SOLVENTS

The electrochemistry of the cis-dioxo-Mo(VI) complex [MoVIO2(O2CC(S)Ph2)2]2- (1), containing two coordinated thiobenzilate ligands, has been investigated in methanol and water by cyclic voltammetry and differential pulse polarography. It undergoes an irreversible proton-assisted reduction at -0.88 V versus SCE which yields two monooxo Mo(IV) species, [MOIVO(O2CC(S)Ph2)2]2- and [MoIVO(O2CC(S)Ph2)(Solv)2], which are involved in a ligand dissociation equilibrium. Even in aqueous solution, steric factors on the ligand can stabilize the reduced Mo(IV) monomers thus eliminating their very common reaction with the starting Mo(VI) complex to give monooxobridged Mo(V) dimers. The reduced monomeric Mo(IV) species are, in turn, reversibly oxidized at more positive potentials ([MoIVO(O2CC(S)Ph2)2]2-, -0.48 V; [MoIVO(O2CC(S)Ph2)(Solv)2], -0.28 V), to yield the corresponding monooxo Mo(V) species. In order to examine chemical and electrochemical differences between Mo-S and Mo-O ligation, we have also studied the electrochemical behaviour of the fully oxygenated benzilate complex, [MoVIO2(O2CC(O)Ph2)2]2. The bielectronic reduction of the complex occurs at the more negative potential of -1.35 V, indicating a low pi-donor interaction of the oxygen atoms on the ligand with the molybdenum d orbital. For both thiobenzilic and benzilic systems, the electrochemical generation of aqueous stable Mo(IV) species, which are reversibly oxidized to stable monomeric Mo(V) complexes, mimics the redox behaviour assumed for molybdenum hydroxylase enzymes.