Terminal Oxygen as a Proton Acceptor: S–H Activation via Proton-Coupled Electron Transfer in Decavanadate-Catalyzed P(O)–S Coupling

Abstract

Polyoxometalates-catalyzed P–S oxidative coupling under mild conditions has emerged as a promising sustainable route to thiophosphates for pharmaceuticals and agrochemicals. This work reveals that the decavanadate cluster-catalyzed oxidative coupling between phosphine oxide and thiophenol involves four elementary steps, in which the rate-determining step is S–H bond activation via proton-coupled electron transfer (PCET). The polyoxovanadate cluster acts as a proton relay station, stabilizing the key proton in the transition state and facilitating its transfer. Interestingly, the proton preferentially transfers to a terminal oxygen rather than a bridging oxygen, with simultaneous electron transfer to the adjacent vanadium center. The terminal oxygen atom is more reactive than the bridging one because the [H3V10VO28]3– skeleton incurs a smaller electronic reorganization penalty for electron transfer in the terminal oxygen via a PCET pathway. On the other hand, the preferential activation of the S–H bond over the P–H bond in the [H3V10VO28]3–-catalyzed reaction arises from both the smaller S–H bond dissociation energy and the better orbital energy match with the thiol substrate. These findings provide a theoretical foundation for understanding PCET in polyoxometalate catalysis and expanding the application of POMs in organic synthesis.