Proton-Coupled Electron Transfer in Ferrocenium–Phenolate Radicals

Abstract

Electron and proton transfer (ET, PT) can be intimately coupled, provided suitable redox and acid/base sites are available. The amide-linked ferrocene–phenol H-1 is deprotonated to the phenolate [1]– by phosphazene bases and oxidized to the ferrocenium ion [H-1]+ by silver hexafluoroantimonate. Concomitant oxidation and deprotonation yields the radical [1]•, featuring a characteristic near-IR absorption band. The ground state of [1]• is best described as the ferrocenium–phenolate zwitterion [1b]• with a dynamic dissymmetric N···H···O hydrogen bond (PT). The ferrocenium–iminolate N···H–O tautomer [1b]•-NHO′ can undergo a thermal structural rearrangement to the high-energy OH···O tautomer [1b]•-OHO, while the amide–phenolate N–H···O tautomer [1b]•-NHO is poised to optical electron transfer to yield the ferrocene–phenoxyl valence isomer [1a]•-NHO (Eop = 1.18–1.19 eV).