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 ECEC 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.