Experimental and theoretical investigations of the sulfite-based polyoxometalate cluster redox series:: α- and β-[Mo18O54(SO3)2]4-15-16-

The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D-3h)-(C20H44N)(4){alpha-[Mo18O54(SO3)(2)]}(CH3CN)-C-. and beta-(D-3d)(C20H44N)(4){beta-[Mo18O54-(SO3)(2)]}.CH3CN is presented. Voltammetric studies in acetonitrile (0.1 m Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo18O54(SO3)(2)](4--/5-) and [Mo18O54(SO3)(2)](5-/6-) processes produce stable [Mo18O54(SO3)(2)](5-) and [Mo18O54(SO3)(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta ->alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the impaired electron occurs over the alpha-and beta-[Mo18O54(SO3)(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the a cluster than the beta form and also provide insight into the driving force for beta ->alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo18O54(SO4)(2))](4-) sulfate cluster.