Proton-coupled electron transfer at a [Co-OHx]z unit in aqueous media: evidence for a concerted mechanism

Abstract

The proton-coupled electron transfer (PCET) chemistry associated with the [Co-OH]²⁺/[Co-OH₂]²⁺ redox couple for [Co(PY5)(OH₂)]²⁺ (1; PY5 = 2,6-(bis(bis-2-pyridyl)-methoxymethane)-pyridine) and [Co^II(pz₄depy)(OH₂)]²⁺ (2; pz₄depy = 2,6-bis(1,1-di(1H-pyrazol-1-yl)ethyl)pyridine) is reported. It is found that the couple is acutely sensitive to the geometry of the axially ligated group in addition to the electronic-donating/-withdrawing character of the ligand. Interrogation of the electron-transfer kinetics by electrochemical methods also shows for the first time that the interconversion of [Co^III-OH]²⁺ and [Co^II-OH₂]²⁺ proceeds through a concerted pathway in favour of energetically unfavourable stepwise electron-transfer or proton-transfer reaction steps.