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-[PW 12 O 40 ] 3– (PW 12 ) polyoxometalate, with n -Bu 4 N + 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 12 isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH 3 CN (0.1 M [ n -Bu 4 N][PF 6 ]) reveals that the reversible potential for the β-PW 12 isomer always remains ca. 100 mV more positive than that of the α-PW 12 isomer on addition of the acid CF 3 SO 3 H. 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. 183 W NMR spectra of the oxidized forms of the PW 12 isomers are acid dependent and in the case of β-PW 12 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 V W VI 11 O 40 ] 4– indicate that either the W I or the W III unit in β-PW 12 is reduced in the β-[PW VI 12 O 40 ] 3– /β-[PW V W VI 11 O 40 ] 4– process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW 12 and [SiW 12 O 40 ] 4– exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.