The electronic structure of reduced phosphovanadomolybdates and the implications on their use in catalytic oxidation initiated by electron transfer

The geometries and electronic structures of the five isomers of [PV ₂Mo₁₀O₄₀]^5- and its mono- and direduced species were theoretically investigated by means of B3LYP calculations, with an attempt to understand their role as oxidative catalysts in electron-transfer-initiated processes. The calculations reveal the following features: (a) Either in the gas phase or in a solvent, the reduction of the [PV₂Mo₁₀O₄₀]^5- produces [PV ₂Mo₁₀O₄₀]^6,7- ions in which the excess electrons are localized on the vanadium atoms in the corresponding δ orbitals. By contrast, the direduced [PV₂Mo ₁₀O₄₀]^7- isomers where the one electron is localized on molybdenum are high in energy. Consequently, whereas the five isomers of [PV₂Mo₁₀O₄₀]^5- have roughly a statistical distribution, in the direduced species, the 1,4-[PV ₂Mo₁₀O₄₀]^7- isomer becomes more stable than others. (b) The gas-phase reduction of the parent [PMo ¹²O₄₀]^3- anion is exceedingly more favorable (by ca. 250 kcal/mol) than the reduction of [PV₂Mo₁₀O ₄₀]^5-. By contrast, contribution of the solvent exerts a strong leveling effect and makes the reduction potentials of the two species very similar, (c) Since the reduction of [PV₂Mo₁₀O ₄₀O₄₀]^5- concentrates high negative charge near the O=VO₄ moiety, this negative charge accumulation will act as an attractor that binds the organic radical cation (cation), which causes a subsequent proton transfer to an oxo ligand of the vanadium. As such, in terms of an electron-transfer catalytic effect, the presence of vanadium in [PV ₂Mo₁₀O₄₀]^5- creates function for the catalyst, (d) The computations reveal that [PV₂Mo₁₀O ₄₀]^7- is a diradicaloid that possesses two virtually degenerate states, with singlet and triplet spins. Thus, we may expect two-state reactivity (TSR) in the catalytic reactions of [PV₂Mo ₁₀O₄₀]^5-. Some predictions are made based on these features.