Voltammetric and Spectroscopic Studies of α- and β-[PW₁₂O₄₀]^3- Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-Bu₄N)₃][PW₁₂O₄₀] Salts

The structure of the Keggin-type β-[PW₁₂O₄₀]^3- (PW₁₂) polyoxometalate, with n-Bu₄N⁺ 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-PW₁₂ isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH₃CN (0.1 M [n-Bu₄N][PF₆]) reveals that the reversible potential for the β-PW₁₂ isomer always remains ca. 100 mV more positive than that of the α-PW₁₂ isomer on addition of the acid CF₃SO₃H. 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. ¹⁸³W NMR spectra of the oxidized forms of the PW₁₂ isomers are acid dependent and in the case of β-PW₁₂ imply that the bridging oxygens between the W_I and W_II units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PW^VW^VI₁₁O₄₀]^4- indicate that either the W_I or the W_III unit in β-PW₁₂ is reduced in the β-[PW^VI₁₂O₄₀]^3-/β-[PW^VW^VI₁₁O₄₀]^4- process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW₁₂ and [SiW₁₂O₄₀]^4- exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.