High-valent oxo compounds of transition metals are often implicated as active species in oxygenation of hydrocarbons through carbon-hydrogen bond activation or oxygen transfer and also in water oxidation. Recently, several examples of cobalt-catalyzed water oxidation have been reported, and cobalt(IV) species have been suggested as active intermediates. A reactive species, formally a dicobalt(IV)-μ-oxo polyoxometalate compound [(α2- P2W17O61Co)2O]14-, [(POMCo)2O], has now been isolated and characterized by the oxidation of a monomeric [α2-P2W17O 61CoII(H2O)]8-, [POMCo IIH2O], with ozone in water. The crystal structure shows a nearly linear Co-O-Co moiety with a Co-O bond length of ∼1.77 Å. In aqueous solution [(POMCo)2O] was identified by 31P NMR, Raman, and UV-vis spectroscopy. Reactivity studies showed that [(POMCo) 2O]2O] is an active compound for the oxidation of H 2O to O2, direct oxygen transfer to water-soluble sulfoxides and phosphines, indirect epoxidation of alkenes via a Mn porphyrin, and the selective oxidation of alcohols by carbon-hydrogen bond activation. The latter appears to occur via a hydrogen atom transfer mechanism. Density functional and CASSCF calculations strongly indicate that the electronic structure of [(POMCo)2O]2O] is best defined as a compound having two cobalt(III) atoms with two oxidized oxygen atoms.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry