No abstract

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.

The heteropoly anion clusters [SzMoigOea]4-and [S2Moi8062]5may be photochemically activated to generate extremely powerful oxidants. Quantitative photoelectrochemical measurements using laser excitation at 325 nm of acetonitrile solutions of [S2M018O62]4containing the electron donors (ED), toluene and tetrahydrofuran, with hydrodynamic voltammetric monitoring at a platinum channel electrode are consistent with the following CECE mechanism: S2Mo,8062]4" + 2e" (E); ED+ (fast) --products. Photochemical oxidation of other electron donors such as methanol, [BPI14]-, ferrocene, and Cr(CO)s (arene) also occurs to generate [S2Moi8062]5-and [S2Moi8062]6-but not the more highly reduced [S2M0180e2]7-and [S2Moi8C>62]8-anions. It is concluded on the basis of these studies that [S2Moi8062]4-is a potentially valuable photooxidant in the synthetic sense, since it may be readily synthesised in large quantities from inexpensive materials.