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

Area of Science:

• Catalysis
• Inorganic Chemistry
• Bioinorganic Chemistry

Background:

• Cobalt-tetraamido macrocyclic ligand (TAML) complexes are investigated for water oxidation catalysis.
• Understanding redox-site localization is crucial for designing efficient catalysts.

Purpose of the Study:

• To characterize redox tautomers of a cobalt-TAML complex and assess their water oxidation activity.
• To elucidate the mechanism of water oxidation and the factors controlling catalytic efficiency.

Main Methods:

• Spectroscopic characterization of cobalt-TAML redox tautomers.
• Water oxidation experiments using one-electron oxidants and iodosylbenzene.
• Electron-transfer studies and Marcus theory analysis.
• Kinetic isotope effect studies.

Main Results:

• Two redox tautomers were identified: a ligand-centered Co(III)-OH species (1) and a metal-centered Co(IV)=O species (2).
• Tautomer 1 readily oxidizes water to O2, while tautomer 2 is unreactive.
• Tautomer 1 is over 10^4 times more reactive than tautomer 2, attributed to a lower reorganization energy.

Conclusions:

• Redox-site localization in cobalt-TAML complexes critically influences water oxidation reactivity.
• The ligand-centered Co(III)-OH tautomer is the active species for O2 evolution.
• This study provides insights into O-O bond formation and catalyst design for water oxidation.