Laser flash photolysis study of the mechanism of photooxidation of alkanes catalysed by decatungstate anion

Abstract

Generated during light excitation, the O(2p) → W(5d) ligand-to-metal charge transfer (LMCT) excited state of decatungstate anion (W₁₀O₃₂^4-) relaxes for t < 30 ps to a transient X which persists for 5 ns. In the nanosecond time domain, it reacts with all components of the photocatalytic system (substrate, organic counter-ion and solvent) to give a one-electron reduced decatungstate species HW₁₀O₃₂^4- and an organic radical R˙. The latter is quenched by dioxygen at a near diffusion-controlled rate, forming a peroxyl radical ROO˙. In the solvent cage, the peroxyl radical ROO˙ reoxidizes HW₁₀O₃₂^4- to give the hydroperoxide ROOH and W₁₀O₃₂^4-, thus closing the catalytic cycle. The catalyst reoxidation is the rate-limiting step of the alkane oxidation.