Structure and electrochemical properties of the new triply bridged molybdenum(V) complex [NBun4]2[Mo2O2(µ-O)(SCH2CO2)2(µ-SCH2CO2)]

Abstract

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 [NBuⁿ₄]₂[Mo₂O₃(SCH₂CO₂)₃]·2H₂O 2. It crystallizes in the monoclinic space group P2₁/a with a= 20.761 (8), b= 17.441(7), c= 14.587(6)Å, β= 101.08(6)°, and Z= 4. The structure contains two cofacial Mo^VO₂(SCH₂CO₂)₂ 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_1,b 2.489, Mo–O_l,b 2.337, Mo–O_b 1.929 Å, Mo–S_l,b–Mo 64.1, Mo–O_l,b–Mo 68.9, Mo–O_b–Mo 86.5°; 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 Me₂S when the reaction is carried out in the presence of Me₂SO.