Quantum chemical characterization of the generation of high-valent oxoruthenium species of Keggin type polyoxometalates: electronic structure and bonding features

Abstract

High-valent transition-metal-substituted Keggin-type polyoxometalates (POMs) are active and robust oxidation catalyst. The important oxidized intermediates of these POM complexes are very difficult to be characterized by using the experimental method, and thus no detail information is available on such species. In the present paper, density functional theory (DFT) calculations have been carried out to characterize the electronic structures of a series of mono-ruthenium-substituted Keggin-type POMs. We find that the aquaruthenium(II/III/IV) species possess d_xy²d_xz²d_yz², d_xy²d_xz²d_yz¹, and d_xy²d_xz¹d_yz¹ electronic configuration, respectively, and hydroxyl/oxoruthenium(IV/V/VI) species possess d_xy²d_xz¹π*_yz¹, d_xy²π*_xz¹π*_yz¹, d_xy¹π*_xz¹π*_yz¹, and d_xy¹π*_xz¹π*_yz⁰ electronic configuration, respectively. Mulliken spin population shows that spin density is localized on the ruthenium center in aquaruthenium(II/III/IV) POM complexes, and the RuO_a unit in hydroxyl/oxoruthenium(IV/V/VI) POM complexes. The O_a atom has substantial radical character in oxoruthenium(IV/V) species, and the radical character of the O_a atom are significantly weakened in the oxoruthenium(VI) species. The relevant energy of the important Ru–O_a π*-antibonding unoccupied orbitals with high RuO_a compositions of oxoruthenium(IV/V/VI) POM complexes decrease in the order: oxoruthenium(IV) > oxoruthenium(V) > oxoruthenium(VI). The pH-independent multiple reduction energies for Ru(III/II), Ru(V/IV), and Ru(VI/V) couples are calculated, which is in agreement with the experimental data.