Oxidation of tertiary phosphines by molecular oxygen catalysed by Ru^III-EDTA complex. Electronic effect of phosphine substituent on the oxygen atom transfer reaction; X-ray crystal structure of the complex [Ru^III(EDTA-H)PPh₃]

Abstract

The catalytic oxidation of tertiary phosphines, PR₃ (R = p-fluorophenyl, phenyl and cyclohexyl), by molecular oxygen to the corresponding phosphine oxide, (PR₃O), catalysed by Ru^III(EDTA-H)(H₂O) is reported as a function of catalyst, substrate (PR₃) and molecular oxygen concentration at a constant pH 3.0 in water-dioxan (50% v/v) medium. The reactivity of PR₃ towards catalytic oxidation by molecular oxygen decreases in the order tris(p-fluorophenyl)phosphine triphenylphosphine tris-(cyclohexyl)-phosphine. A reverse reactivity order was observed in the case of stoichiometric oxidation of PR₃ by [O=Ru^V(EDTA)]⁻. The proposed mixed-ligand complex Ru^III-EDTA-PR₃ intermediate in the catalytic oxidation of PR₃ with molecular oxygen has been isolated and its structure solved by single-crystal X-ray diffraction. The experimental results are discussed in terms of the σ-basic and π-acidic character of the phosphine substrates in the homolytic bond cleavage of O---O bonds of the μ-peroxo intermediate and oxygen atom transfer to the substrate. The bond dissociation energy for O---O bond cleavage is computed by the kinetic data obtained for oxygen atom transfer from the oxo complex [O=Ru^V(EDTA)]⁻.