Density functional theory calculations were carried out to investigate the electronic structures of Keggin-typed [XMo12O40]n− and [XW12O40]n− anions with different heteroatoms (X = Zn2+, B3+, Al3+, Ga3+, Si4+, Ge4+, P5+, As5+, and S6+). The influence of solvent on redox properties of heteropolyanions was discussed. For [XW12O40]n− systems two linear correlation: first, between the experimental redox potential and energies of LUMO orbital; and second, between the experimental redox potential and total energy interaction (calculated between internal tetrahedron (XO4n−), and rest of Kegging anion skeleton, (W12O36)) were designated. Taking into account the similarity of XW12O40n− and XMo12O40n− systems (in geometry and electronic structure), the estimated redox potential of molybdenum heteropolyanions (with X being p block elements) in different solvent were proposed.
The influence of the cobalt cation geometric environment on catalytic activity, namely, oxygen adsorption and its activation, was investigated by exploring two groups of systems. The first group was formed by cobalt cation complexes, in which the Co2+ was surrounded by water-H2O or acetonitrile-CH3CN solvent molecules. This represents heteropolyacids salts (ConH3-nPW(Mo)12O40), where the Co2+ acts as a cation that compensates for the negative charge of the Keggin anion and is typically surrounded by solvent molecules in that system. The second group consisted of tungsten or molybdenum Keggin anions (H5PW11CoO39 and H5PMo11CoO39), having the Co2+ cation incorporated into the anion framework, in the position of one addenda atom. Detailed NOCV (Natural Orbitals for Chemical Valence) analysis showed that, for all studied systems, the σ-donation and σ-backdonation active channels of the electron transfer were responsible for the creation of a single Co-OO bond. Depending on the chemical/geometrical environment of the Co2+ cation, the different quantities of electrons were flown from the Co2+ 3d orbital to the π* antibonding molecular orbitals of the oxygen ligand, as well as in the opposite direction. In molybdenum and tungsten heteropolyacids, modified by Co2+ in the position of the addenda atom, activation of O2 was supported by a π-polarization process. Calculated data show that the oxygen molecule activation changed in the following order: H5PMo11CoO39 = H5PW11CoO39 > Co(CH3CN)52+ > Co(H2O)52+.
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