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

## TL;DR

Scientists studied a cobalt complex that helps split water into oxygen, finding that one form is much more efficient than another due to differences in electron transfer.

## Contribution

The paper identifies a reactive intermediate and explains how redox localization controls oxygen formation in water oxidation.

## Key findings

- The ligand-centered Co(III)–OH species is highly reactive and produces O2 via H2O2.
- The metal-centered Co(IV)=O species is unreactive under the same conditions.
- Marcus theory analysis shows a much lower reorganization energy for the reactive species.

## Abstract

A cobalt(III) −TAML complex, [(TAML)CoIII]− (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•⁺)CoIII(OH)]− (1), and a metal‐centered Co(IV)= O species, [(H‐TAML)CoIV(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.

Two redox tautomers of a cobalt–TAML complex show strikingly different reactivity in water oxidation. The ligand‐centered Co(III)–OH species evolves O2 via H2O2, while the metal‐centered Co(IV) = O is inactive. Electron‐transfer kinetics and the Marcus analysis reveal how redox localization influences reactivity and O─O bond formation in high‐valent cobalt–oxo systems.