Synthesis and oxidation catalysis of a Ti-substituted phosphotungstate, and identification of the active oxygen species

Abstract

In this paper, we report the synthesis of a Ti-substituted phosphotungstate, TBA₆[(γ-PW₁₀O₃₆)₂Ti₄(μ-O)₂(μ-OH)₄] (I, TBA = tetra-n-butylammonium), and its application to H₂O₂-based oxidation. Firstly, an organic solvent-soluble dilacunary phosphotungstate precursor, TBA₃[γ-PW₁₀O₃₄(H₂O)₂] (PW10), has been synthesized. By the reaction of PW10 and TiO(acac)₂ (acac = acetylacetonate) in an organic medium (acetonitrile), I can be obtained. Compound I possesses a tetranuclear Ti core which can effectively activate H₂O₂ and shows high catalytic performance for several oxidation reactions, such as epoxidation of alkenes, oxygenation of sulfides, oxidative bromination of unsaturated compounds, and hydroxylation of anisole, giving the corresponding oxidation products with high efficiencies and selectivities. The catalytic performance of I is much superior to those of previously reported Ti-substituted polyoxometalates. In addition, I is highly durable during catalysis and can be reused several times while keeping its high catalytic performance. Furthermore, we have successfully isolated the truly catalytically active species for the present I-catalyzed oxidation, TBA₆[(γ-PW₁₀O₃₆)₂Ti₄(μ-η²:η²-O₂)₄] (II), and its anion structure has been determined by X-ray crystallographic analysis. All of the four Ti₂-μ-η²:η²-peroxo species in II are active for stoichiometric oxidation (without H₂O₂), and II is included in the catalytic cycle for I-catalyzed oxidation.