Synthesis and Characterization of Novel Wells–Dawson-Type Mono Vanadium(V)-Substituted Tungsto-polyoxometalate Isomers: 1- and 4‑[S₂VW₁₇O₆₂]^5–

Two vanadium­(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S2VW17O62]5–, were prepared as their tetra-alkyl ammonium salts from a WVI–H2SO4–VV reaction mixture in aqueous CH3CN solution. X-ray crystallographic structural analysis revealed that both isomers have a Wells–Dawson-type structure with a higher occupancy of vanadium at polar sites and belt sites for 1- and 4-[S2VW17O62]5–, respectively. The isomers were also characterized by elemental analysis, infrared, Raman, UV–vis, and 51V NMR spectroscopies as well as voltammetry, and the data obtained were compared with that derived from [S2W18O62]4–. Significantly, the reversible potentials for the vanadium­(V/IV) couple for both 1- and 4-[S2VW17O62]5– in CH3CN (0.1 M n-Bu4NPF6) are considerably more positive than the tungstate reduction process exhibited by the [S2W18O62]4– framework, implying that the presence of vanadium should be useful in catalytic reactions. The one-electron-reduced [S2VIVW17O62]6– forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy.