Voltammetric and Spectroscopic Studies of α- and β-[PWO] Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-BuN)][PWO] Salts.

The structure of the Keggin-type β-[PWO] (PW) polyoxometalate, with n-BuN as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CHCN (0.1 M [n-BuN][PF]) reveals that the reversible potential for the β-PW isomer always remains ca. 100 mV more positive than that of the α-PW isomer on addition of the acid CFSOH. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0-5 mM mimic experimental data exceptionally well. These simulation-experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W(VI)/W(V) processes converge into a single two-electron process if sufficient acid is present. W NMR spectra of the oxidized forms of the PW isomers are acid dependent and in the case of β-PW imply that the bridging oxygens between the W and W units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PWWO] indicate that either the W or the W unit in β-PW is reduced in the β-[PWO]/β-[PWWO] process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW and [SiWO] exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.