Crystal Structure of Na9[H3W12O42]·24H2O, a Compound Containing the Protonated Paratungstate B Anion ('Acid Paratungstate'), and Cyclic Voltammetry of Acidified [H2W12O42]10- Solutions

Abstract

Crystallization of a solid at pH 4.0 from an aqueous acidified Rh³⁺–[WO₄]²⁻ solution resulted in the isolation of Na₉[H₃W₁₂O₄₂]·24H ₂ O, which contains the protonated paratungstate B anion and which is likely the species identified previously as ‘acid paratungstate’. The compound is triclinic, space group P^1– , a 10.603(2), b 12.134(3), c 14.042(3) Å, α 114.78(1), β 101.84(1), γ 108.34(1)˚, V 1432.9(5) ų , Z 1, and the structure was solved to an R₁ value of 0.0404 (wR ₂ 0.1108) for 5997 independent observed reflections. The anion exhibits essentially the same isopolytungstate framework as paratungstate B, [H₂W₁₂O₄₂]¹⁰⁻ , consisting of two W₃O₁₃ and two W₃O₁₄ structural subunits linked by shared vertices. Bond valence arguments place two of the hydrogen atoms unequivocally in the internal cavity of the anion, with the remaining hydrogen atom also likely located in this cavity, but disordered over several internal oxygen atoms. The protonation of [H₂W₁₂O₄₂ ]¹⁰⁻ is shown to lead to species that are electrochemically reducible. Extended-HÜckel molecular orbital calculations confirm that protonation of paratungstate B within the internal cavity leads to a change in composition of the LUMO, now based mainly on electrochemically reducible W₃O₁₃ as opposed to (essentially) non-reducible W₃O₁₄ structural subunits. This results in species that are considerably more electrochemically active than the unprotonated anion. The role of [H₃W₁₂O₄₂]⁹⁻ as an intermediate in the polymerization of [WO₄]²⁻ to give the solution form of ψ-metatungstate, [H₇W₁₁O₄₀]⁷⁻ , which crystallizes as [H₄W₁₁O₃₈]⁶⁻ , is also discussed.