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.

Polyoxometalates (POMs) have been proposed as electromaterials for lithium-based batteries because they provide access to multiple electron transfer reactions coupled to fast lithium ion transport processes and high stability over many redox cycles. Consequently, knowledge of reversible potentials and Li+ cation–POM anion interactions provides a strategic basis for their further development. In this study, detailed cyclic voltammetric studies of a series of [XVVM11O40] n− (XVM11 n–) POMs (where X (heteroatom) = P (n = 4), As (n = 4), and S (n = 3) and M (addenda atom) = Mo, W) have been undertaken in CH3CN in the presence of LiClO4, with n-Bu4NPF6 also present when required to keep the ionic strength close to constant value of 0.1 M. An analysis of the data has allowed the impact of the POM charge, and addenda and hetero atoms on the reversible potentials and the interaction between Li+ and the oxidized XVVM11 n– and reduced XVIVM11 (n+1)– forms of the VV/IV redox couple to be determined. The SVV/IVM11 3–/4– process is independent of the Li+ concentration, implying the absence of the association of this cation with either SVVM11 3– or SVIVM11 4– redox levels. However, lithium-ion association constants for both VV and VIV redox levels were obtained from a comparison of simulated and experimental cyclic voltammograms for the reduction of the more negatively charged XVVM11 4– (X = P, As; M = Mo, W), since the Li+ interaction with these more negatively charged POMs is much stronger. The interaction between Li+ and the oxidized, XVVM11 n–, and reduced, XVIVM11 (n+1)–, forms was also investigated by 51V NMR and EPR spectroscopy, respectively, and it was confirmed that, due to their lower charge density, SVVM11 3– and SVIVM11 4– interact significantly less strongly with the lithium ion than XVVM11 4– and XVIVM11 5– (X = P, As). The lithium–POM association constants are substantially smaller than the corresponding proton association constants reported previously, which is attributed to a smaller surface charge density. The much stronger impact of Li+ on the WVI/V- and MoVI/V-based reductions that occur at more negative potentials than the VV/IV process also has been qualitatively evaluated.