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

Abstract

Two vanadium­(V)-substituted tungsto-polyoxometalate isomers, 1- and 4-[S₂VW₁₇O₆₂]^5–, were prepared as their tetra-alkyl ammonium salts from a W^VI–H₂SO₄–V^V 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 <i>n</i>-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 [S₂V^IVW₁₇O₆₂]^6– forms of both isomers were prepared in solution by controlled potential bulk electrolysis and characterized by voltammetry and EPR spectroscopy