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

Abstract

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.

Towards a computational treatment of polyoxometalates in solution using QM methods and explicit solvent molecules

“…The next question to answer was how many water molecules the first solvation shell has to contain. To answer this question, a set of calculations was carried out increasing the size of the first solvation shell (24,49,74,99), and the interactions between water molecules and the POM. and water molecules with themselves were analyzed.…”

“…For instance, Weinstock et al 21 reported that the redox potentials of Keggin anions strongly depend on the counter-ion and on the ionic force of the solution. Our group has been pioneering the theoretical study of the electronic structure of POMs during the last two decades [22][23][24] (by using mainly ADF), and realized that computational assessment of the redox properties of these species has to somehow include solvent effects. However, continuum solvent models are not able to introduce in their formulations the explicit nature of the counter-ion (such as Li + , Na + , or K + ), nor the ionic force of the solution.…”

A theoretical study on the efficient reversible redox-based switching of the second-order polarizabilities of two-dimensional nonlinear optical-active donor–acceptor phenanthroline-hexamolybdate

“…Since the calculation of the vibrational frequencies is still unfeasible for large molecules like POMs, the frequency calculations were ignored as in previous reports [27][28][29], and the G R in the equation E • = − G R /nF was approximated as the reduction energy of the POMs in solution (n = 1 for the one-electron reduction). The E • values were measured and compared to a reference electrode, NHE (normal hydrogen electrode), and the free energy change for the half-reaction 1/2H 2 → H + + e − was estimated to be −4.28 eV [30].…”

Redox and acidic properties of chalcogenido-substituted mixed-metal polyoxoanions: a DFT study of α-[PW11O39ME]4− (M = Nb, Ta; E = O, S, Se)

“…13,14 The first step in many catalytic processes involves the adsorption or release of protons. 21 Alternately, the properties of POMs can be modified through the substitution of oxo ligands by other ligands, such as sulfur and selenium. The protonation and acid strength of phosphotungstic acid, which are potential indicators of solid acid activity, are still widely debated in the literature.…”