Redox properties of polyoxometalates: new insights on the anion charge effect

In this paper we study the electronic structure of Lindqvist, Keggin, Dawson and Preyssler polyoxometalates (POMs) at the DFT level, particularly their LUMOs and reduction energies. Our aim was to revisit the previously reported evidence that a linear relationship exists between reduction potentials and molecular charges in Keggin anions. In this line of thought, we calculated one simple structural parameter-volume of the clusters-so that the corresponding volume charge density, rho(v), could be estimated. Contrary to what we expected, the connection between rho(v) and the experimental reduction potentials is not evident since q/V itself does not justify the scale of oxidizing powers. Complementary calculations were performed using the clathrate model, anion@W(m)O(3m), analyzing separately the effects of the size of the neutral cages and the molecular charge, q, upon the redox properties. The parameter m emulates the size (volume) of the clusters, only approximately, but with the benefit that it is easily accounted for. A linear relationship was found between the difference in LUMO energies for the neutral and charged clusters and the q/m ratio.