A GREEN HETEROPOLY BLUE - ISOLATION OF A STABLE, ODD OXIDATION LEVEL IN A DAWSON MOLYBDATE ANION, [S2MO18O62]5-

Four consecutive and reversible one-electron reduction processes are observed as the initial reduction steps in the voltammetry of alpha-[S2Mo18O62]4- in acetonitrile. This contrasts with the initial four reversible two-electron two-proton reduction processes identified previously in acidified acetonitrile. The one- and two-electron reduction products in acetonitrile have been isolated as the green and blue salts, (Bun4N)5[S2Mo18O62] and (Bun4N)6[S2-Mo18O62] respectively. The disproportionation constant for [S2Mo18O62]5- has been estimated (K(dis) = 8.8 X 10(-5)) and indicates less than 1% disproportionation in acetonitrile solution. In the presence of aqueous acid, [S2Mo18O62]5- readily disproportionates to give equal concentrations of [S2Mo18O62]4- and protonated [S2Mo18O62]6-. The microcrystalline EPR spectrum at 253 K (-20-degrees-C) of (Bu(n)4N)5[S2Mo18O62] displays at least 91 lines due to hyperfine interaction with the Mo-95, Mo-97 nuclei (I = 5/2; 25.5 atom %). A number of isotopomers contribute to the observed spectrum. These observations, together with the dependence of the EPR linewidth on temperature (E(a), 0.0045 eV), are consistent with intramolecular thermal delocalization of the odd ''green'' electron over all of the molecular framework in the temperature range 77-253 K. Above 253 K, the temperature dependence increases by a factor of about 20 (E(a), 0.087 eV). This observation is consistent with an intermolecular hopping process at the higher temperatures.