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

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(pz(4)depy)(OH2)](2+) (2; pz(4)depy 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 {[}CoII-OH2](2+) proceeds through a concerted pathway in favour of energetically unfavourable stepwise electron-transfer or proton-transfer reaction steps.