STRUCTURE AND ELECTROCHEMICAL PROPERTIES OF THE NEW TRIPLY BRIDGED MOLYBDENUM(V) COMPLEX [NBU(4)(N)](2)[MO2O2(MU-O)- (SCH2CO2)(2)(MU-SCH2CO2)]

The reaction of molybdate ions with an excess of thioglycolic acid has been studied in aqueous solution at room temperature by UV/VIS and NMR spectroscopy. As previously observed, spectroscopic data suggest initial formation of the complex [MoO2(SCH2CO2)(2)](2-) 1, which is subsequently reduced by the excess of acid. Two molybdenum(V) complexes are formed in equilibrium in this redox reaction. The main product has been isolated as an orange-red solid with the formula [NBu(4)(n)](2)[Mo2O3(SCH2CO2)(3)]. 2H(2)O 2. It crystallizes in the monoclinic space group P2(1)/a with a = 20.761(8), b = 17.441(7), c = 14.587(6) Angstrom, beta = 101.08(6)degrees, and Z = 4. The structure contains two cofacial (MoO)-O-V,(SCH2CO2), distorted octahedra sharing one oxygen atom and one thioglycolate ligand on a pseudo-symmetry plane (O-t-Mo 1.677, Mo-Mo 2.643, Mo-S-l,S-b 2.489, Mo-O-l,O-b 2.337, Mo-O-b 1.929 Angstrom, Mo-S-l,S-b-Mo 64.1, Mo-O-l,O-b-Mo 68.9, Mo-O-b-Mo 86.5 degrees; t = terminal, b = bridging). Cyclic voltammetry shows that complexes 1 and 2 undergo a two-electron irreversible reduction at -1.27 and -1.38 V vs. the saturated calomel electrode, respectively, in methanol. In both reduction processes the monooxomolybdenum(IV)) species [MoO(SCH2CO2)(2)](2-) and [MoO(SCH2CO2)(solv)(2)] are generated. A reaction mechanism for the oxidation of thioglycolic acid by molybdate ions is proposed from the combined analysis of spectrophotometric, NMR, structural. and voltammetric data. Dimeric molybdenum(V) products are generated from reaction of the above monooxomolybdenum(IV) species with the starting dioxomolybdenum(VI) complex 1. A particularly remarkable supporting feature is the formation of Me(2)S when the reaction is carried out in the presence of Me(2)SO.