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 O 2 via H 2 O 2 , while 2 remains unreactive under identical conditions. Electron‐transfer (ET) studies with electron donors such as diacetylferrocene (Ac 2 Fc) 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.