Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62]4-: a unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media.

The synthesis and characterization of (Bu4N)4[S2W18O62].1.23MeCN.0.27H2O are reported. It crystallizes in the monoclinic space group C2/c with a = 22.389(6) A, b = 22.104(3) A, c = 25.505(5) A, beta = 95.690(15) degrees, V = 12560(5) A3, and Z = 4. The anion exists as the gamma* isomer, the second example of this isomer type to be characterized structurally. The equivalent molybdenum salt occurs as the alpha isomer. gamma*-[S2W18O62]4- in MeCN solution displayed four electrochemically reversible one-electron redox processes at E1/2 values of -0.24, -0.62, -1.18, and -1.57 V versus the Fc+/Fc couple. Upon addition of acid in MeCN/H2O (95/5 v/v), the two most cathodic processes converted to an overall two-electron process at -0.71 V. The total data suggested that this process actually comprises two one-electron transfer processes, occurring at different potentials, with associated proton-transfer reactions. The interpretation is supported by simulation of the effect of acid titration upon the cyclic voltammetry. While multiple pathways for correlated reduction and protonation are present in both the molybdenum and tungsten systems, only a single fast oxidation pathway is available. As the reduced forms of [S2W18O62]4- are much weaker bases than those of [S2Mo18O62]4-, the individual oxidation pathways are not the same. However, their existence determines the highly reversible electrochemical behavior that is characteristic of these anions, and that of polyoxometalate systems in general.