Oxophosphonium NHC adducts as precursors to phosphinoyl adducts: a combined electrochemical and computational study.

Phosphinoyl radicals are key intermediates in hydrophosphorylation and related transformations, yet persistent unsupported examples remain unknown. Inspired by the recent isolation of CAAC-stabilised phosphinoyl radicals, a series of NHC-supported oxophosphonium salts was investigated as potential precursors to phosphinoyl carbene adducts. Cyclic voltammetry, supported by DFT calculations, reveals that both the carbene framework and the phosphorus substitution pattern strongly influence the accessibility and persistence of the corresponding reduced species. While all investigated adducts undergo reduction, NHC-derived systems remain significantly less persistent than their CAAC counterparts. Correlation of experimental reduction potentials with computed LUMO energies highlights the importance of carbene π-acceptor properties, whereas variation of the phosphorus substituents reveals additional electronic effects that govern radical formation. These results provide new insight into the factors controlling the stability of phosphinoyl carbene adducts and establish guidelines for the development of persistent phosphinoyl radicals.