Identifying an Active Intermediate and Monitoring O─O Bond Formation in Water Oxidation by a Cobalt(III)-TAML Complex

A cobalt(III) -TAML complex, [(TAML)Co^III]^- (TAML = tetraamido macrocyclic ligand), catalyzes water oxidation using one-electron oxidants like cerium(IV) ammonium nitrate (CAN) or tris(4-bromophenyl)ammonium hexachloroantimonate radical cation (TBPA^•+). Two redox tautomers were characterized: a ligand-centered Co(III)-OH species, [(TAML^•⁺)Co^III(OH)]^- (1), and a metal-centered Co(IV)= O species, [(H-TAML)Co^IV(O)(HOTf)]^- (2), formed by oxidation with iodosylbenzene (PhIO) in the presence of triflic acid or with one-electron oxidants. 1 readily reacts with water to evolve O₂ via H₂O₂, while 2 remains unreactive under identical conditions. Electron-transfer (ET) studies with electron donors such as diacetylferrocene (Ac₂Fc) revealed that 1 is over 10⁴ times more reactive than 2. The Marcus theory analysis showed a much lower reorganization energy (λ) for 1 (0.74 eV) compared to 2 (2.7 eV), explaining the dramatic rate difference. The detection of 1 during catalysis and kinetic isotope effect studies offers insight into O─O bond formation and underscores how redox-site localization controls water oxidation reactivity.