Influence of the Heteroatom Size on the Redox Potentials of Selected Polyoxoanions

The apparent formal potentials for the one-electron redox process of most Keggin-type heteropolytungstates, XW12O40q-, have long been shown to linearly depend on their overall negative charges, in the absence of proton interference in the process. However, for a given overall negative charge, these formal potentials are also shown here to depend on the specific central heteroatom X. In the present work, cyclic voltammetry was used to study a large variety of Keggin-type anions, under conditions where their comparisons are straightforward. In short, apparent potential values get more negative (the clusters are more difficult to reduce) for smaller central heteroatoms within a given family of Keggin-type heteropolyanions carrying the same overall negative charge. Density functional theory calculations were performed on the same family of Keggin compounds and satisfactorily reproduce these trends. They show that internal XO4 units affect differently the tungstate oxide cage. The electrostatic potential created by each internal anionic unit in a fragment-like approach (XO4q-@W12O36) was analyzed, and it is observed that X atoms of the same group show slight differences. Within each group of the periodic table, X atoms with lower atomic numbers are also smaller in size. The net effect of such a tendency is to produce a more negative potential in the surroundings and thus a smaller capacity to accept electrons. The case of [BW12O40](5-) illustrates well this conclusion, with the smallest heteroatom of the Keggin series with group III central elements and a very negative reduction potential with respect to the other elements of the same group. Particularly in this case, the electronic structure of the Keggin anion shows the effects of the small size of boron: the highest occupied molecular orbitals of [BW12O40](5-) appear to be similar to 0.35 eV higher than those in the other clusters of the same charge, explaining that the BO4 unit is more unstable than AlO4 or GaO4 despite carrying the same formal charge.