Demonstration of the Radical Coupling Pathway in a Fast Fe-Based Water Oxidation Catalyst

Optimization of water oxidation reaction (WOR) catalysts is critical for the development of clean energy technology based on the concept of artificial photosynthesis. Deep mechanistic insights at the molecular and electronic levels are required. Theoretically, the radical coupling (RC) mechanism should allow for a virtually barrier-less process of O-O bond formation in the WOR. This mechanism has been proposed for a number of multinuclear Fe-based water oxidation catalysts (WOCs), but its firm experimental confirmation for Fe systems is lacking. Here, we describe a RC mechanism in [(MeOH)-Fe-(Hbbpya)-μ-O-(Hbbpya)-Fe-(MeOH)]-(OTf)₄ (1) (Hbbpya = N,N-bis-(2,2'-bipyrid-6-yl)-amine) and its methylated analog [(MeOH)-Fe-(CH₃bbpya)-μ-O-(CH₃bbpya)-Fe-(MeOH)]-(OTf)₄ (1-CH ₃ ). In situ XAS revealed that 1 (dimer species) breaks into monomers under catalytic conditions. The kinetic analysis has shown a second-order reaction in the catalyst, and the kinetic isotope effect has shown a minimal k _H/k _D ≈ 1 effect consistent with the RC pathway; EPR and XAS detected an Fe^VO intermediate. DFT confirmed the preference for the RC pathway. This Fe-based WOC shows a high rate of O₂ evolution in chemical and photochemical WOR, comparable with some well-known Ru-based systems. These results highlight the direction for designing Fe-based WOCs with high activity and the future engineering of WOCs with the RC mechanism for functional and scalable applications for artificial photosynthesis.