Direct Electrochemical P(V) to P(III) Reduction of Phosphine Oxide Facilitated by Triaryl Borates

Triaryl borate Lewis acids facilitate the direct two-electron reduction of the P(V) center of triphenylphosphine oxide (TPPO) to the P(III) center of triphenylphosphine at faradaic efficiencies of 37%. Insight from direct P(V) to P(III) reduction is provided from cyclic voltammetry. The electrochemical reduction of TPPO proceeds through an unusual EC r EC i mechanism in which the breaking of the phosphoryl bond in a two-electron-reduced association complex with the triaryl borate is rate-determining. The rate and faradaic efficiency for TPPO reduction are tuned by judicious choice of substituents on triaryl borate, with tris(4-methoxyphenyl) borate demonstrating the highest for both. These results suggest that an attractive route toward the room-temperature reduction of phosphate for phosphorus reclamation is greatly facilitated by the stabilization of reduced phosphate intermediates through their association with Lewis acids.