Multielectron Reduction of alpha-[S(2)Mo(18)O(62)](4-) in Aprotic and Protic Media: Voltammetric Studies.
Detailed voltammetric studies are reported of the reduction of alpha-[S(2)Mo(18)O(62)](4)(-) at glassy carbon and platinum macro- and microdisk electrodes (stationary and rotated) in acetonitrile and in 95/5 acetonitrile/water mixtures containing 0.02 M perchloric acid. Experiments at -30 degrees C and at short time scales in acetonitrile detect eight one-electron processes in the potential range 0.1 to -2.7 V versus Fc(+)/Fc (Fc = ferrocene). The eight processes fall into four pairs. The differences in potential within each pair are remarkably constant (0.26 +/- 0.02 V), as are the separations between adjacent pairs (0.60 +/- 0.06 V). This periodicity is consistent with spin-paired [Mo(V)O](2)(&mgr;-O) fragments providing a significant contribution to the overall superexchange stabilization. The influence of the following chemical reactions (primarily protonation by trace water) increases with the number of electrons added, although the five-electron reduced species [S(2)Mo(18)O(62)](9)(-) has a significant lifetime on the voltammetric time scale. In the presence of aqueous acid and at short time scales, eight chemically reversible processes are observed in the range 0 to -1.5 V in which two, two, two, two, four, four, two, and eight electrons, respectively, are transferred. The four pairs of one-electron processes found in acetonitrile occur as the first four overall two-electron reduction steps, separated by about 0.1, 0.2, and 0.3 V, respectively. Addition of a further 10 electrons occurs in a range of 0.3 V only and results in an 18-electron reduced species, stable on the voltammetric time scale. This corresponds to formal reduction of all Mo(VI) centers in alpha-[S(2)Mo(18)O(62)](4)(-) to Mo(V). The species is able to accept a further eight electrons in a series of unresolved processes.
Way
David M.
DM
Department of Chemistry, La Trobe University, Bundoora, Victoria 3083, Australia, Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia, and School of Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia.
Bond
Alan M.
AM
Wedd
Anthony G.
AG