ESR analysis of the oxidation reactions of phosphorus-containing nitrone-type spin traps with gold(III) ion

Abstract

Phosphorus-containing cyclic nitrones, such as DEPMPO, CYPMPO, and DPPMPO, were oxidized by hydrogen tetrachloroaurate(III) to DEPMPOX, CYPMPOX, and DPPMPOX with the precipitation of Au(0). The reaction was depressed by the addition of chloride or hydroxide ions. The peculiar pH dependency was observed in DEPMPOX, CYPMPOX, and DPPMPOX formation, which should be caused by the diethoxyphosphoryl group in DEPMPO, the 1,3-propoxy cyclophosphoryl group in CYPMPO, and the diphenylphosphinoyl group in DPPMPO. The oxidation of the nitrones proceeded through the ligand exchange of Cl– in AuCl4– with >N+–O– in nitrone and the nucleophilic addition of the water molecule to the C-2 position in the nitrones, the stepwise intra-molecular transfer of three electrons from the nitrones to Au(III), and the release of the resulting Au(0). The phosphoryl group in the nitrones suppressed the first ligand-exchange interaction by its electronegativity, while the group promoted the electron transfer from the nitrones to Au(III) by its inductive effect.