Inspired by the commercially available azoimidazolium dyes (e.g., Basic Red 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes, we developed the synthesis of a unique set of arylazophosphonium salts. A range of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids. With cyclic voltammetry, a general procedure was designed to establish whether the reaction between a Lewis acid and a Lewis base occurs by single-electron transfer or electron-pair transfer.

Inspired by the commercially available azoimidazolium dyes (e.g., Basic Red 51) that can be obtained from aryldiazonium salts and N-heterocyclic carbenes,w ed eveloped the synthesis of aunique set of arylazophosphonium salts. Arange of colours were obtained by applying readily tuneable phosphine donor ligands and para-substituted aryldiazonium salts as nitrogen-based Lewis acids.W ith cyclic voltammetry, ag eneral procedure was designed to establish whether the reaction between aL ewis acid and aL ewis base occurs by single-electron transfer or electron-pair transfer.Scheme 3. Reaction of tBu 3 Pwith [NO][BF 4 ]t ogether with the experimental (black) and simulated (red) EPR spectra of tBu 3 P-NOC. Simulated g value and hyperfine coupling constants(A): g iso = 2.0071, A N iso =+29.55 MHz, A P iso = À34.10 MHz.