Redox chemistry of the acetato-bridged clusters [M3(µ3-O)n(µ-O2CCH3)6(H2O)3]2+ (M = Mo, W, n = 1, 2): reversible redox between mono-µ3-oxo d8 MIII2MIV and d9 MIII3 forms

Abstract

A cyclic voltammogram of aqueous 0.1 mol dm⁻³ triflic acid solutions of the d⁶ bioxo-capped M–M bonded cluster [Mo₃(µ₃-O)₂(O₂CCH₃)₆(H₂O)₃]²⁺ at a glassy carbon electrode at 25 °C gives rise to an irreversible 3e⁻ cathodic wave to a d⁹ Mo^III₃ species at −0.8 V vs. SCE which on the return scan gives rise to two anodic waves at +0.05 V vs. SCE (E_1/2, 1e⁻ reversible to d⁸ Mo^III₂Mo^IV) and +0.48 V vs. SCE (2e⁻ irreversible back to d⁶ Mo^IV₃). The number of electrons passed at each redox wave has been confirmed by redox titration and controlled potential electrolysis which resulted in 90% recovery of [Mo₃(µ₃-O)₂(O₂CCH₃)₆(H₂O)₃]²⁺ following electrochemical re-oxidation at +0.8 V. A corresponding CV study of the d⁸ monoxo-capped W^III₂W^IV cluster [W₃(µ₃-O)(O₂CCH₃)₆(H₂O)₃]²⁺ gives rise to a reversible 1e⁻ cathodic process at −0.92 V vs. SCE to give the d⁹ W^III₃ species [W₃(µ₃-O)(O₂CCH₃)₆(H₂O)₃]⁺; the first authentic example of a W^III complex with coordinated water ligands. However the cluster is too unstable (O₂/water sensitive) to allow isolation. Comparisons with the cv study on [Mo₃(µ₃-O)₂(O₂CCH₃)₆(H₂O)₃]²⁺ suggest irreversible reduction of this complex to monoxo-capped [Mo^III₃(µ₃-O)(O₂CCH₃)₆(H₂O)₃]⁺ followed by reversible oxidation to its d⁸ counterpart [Mo₃(µ₃-O)(O₂CCH₃)₆(H₂O)₃]²⁺ (Mo^III₂Mo^IV) and finally irreversible oxidation back to the starting bioxo-capped cluster. Exposing the d⁹ Mo^III₃ cluster to air (O₂) however gives a different final product with evidence of break up of the acetate bridged framework. Corresponding redox processes on d⁶ [W₃(µ₃-O)₂(O₂CCH₃)₆(H₂O)₃]²⁺ are too cathodic to allow similar generation of the monoxo-capped W^III₃ and W^III₂W^IV clusters at the electrode surface.