Crystal Structure of Na_9[H_3W_(12)O_(42)] centre dot 24H_2O, a Compound Containing the Protonated Paratungstate B Anion ('Acid Paratungstate'), and Cyclic Voltammetry of Acidified [H_2W_(12)O_(42)]~(10-) Solutions

摘要

Crystallization of a solid at pH 4.0 from an aqueous acidified Rh~(3+)-[WO_4]~(2-) solution resulted in the isolation of Na_9[H_3W_(12)O_(42)] centre dot 24H_2O, 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-bar, a 10.603(2), b 12.134(3), c 14.042(3) A,#alpha#114.78(1), #beta# 101.84(1), r 108.34(1) deg, V 1432.9(5) A~3, Z1, and the structure was solved to an R_1 value of 0.0404 (wR_2 0.1108) for 5997 independent observed reflections. The anion exhibits essentially the same isopoly-tungstate framework as paratungstate B,[H_2W_(12)O_(42)]~(10-), consisting of two W_3O_(13) and two W_O_(14) 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 [H_2W_(12)O_(42)]~(10-) is shown to lead to species that are electrochemically reducible. Extended-Huckel 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 W_3O_(13) as opposed to (essentially) non-reducible W_(3)O_(14) structural subunits. This results in species that are considerably more electrochemically active than the unprotonated anion. The role of [H_3W_(12)O_(42)]~(9-) as an intermediate in the polymerization of [WO_4]~(2-) to give the solution form of #psi#-metatungstate, [H_7W_(11)O_(40)]~(7-), which crystallizes as [H_4W_(11)O_(38)]~(6-), is also discussed.