Metal–Ligand Cooperative Control of Reactivity in a Porphyrin Manganese(IV)–Oxo–Phenoxyl Radical Complex for Water Activation

Abstract

The Oxygen Evolving Complex (OEC) is a Mn4Ca cluster at the heart of Photosystem II that catalyzes the water oxidation reaction. Recent studies suggest the formation of a Mn(IV)‐oxyl species in the OEC, accompanied by a nonbonded tyrosinyl radical in the S3′ state, prior to O─O bond formation and O2 release. In this study, we designed and synthesized a manganese‐based porphyrin laterally appended with an imidazole–phenol pair. We found that the [MnIIIPImiPhOH]+ can promote electrocatalytic water oxidation in basic medium, resulting in O2 evolution. UV–Vis, EPR, and resonance Raman spectroscopic studies support the formation of a doubly oxidized intermediate, best described as a MnIV═O species weakly coupled to a neighboring phenoxyl radical, formulated as [MnIV(O)PImiPhO•]+. DFT calculations provided further insight into the electronic structure of this reactive intermediate and revealed that its reaction with OH− should lead to the formation of a MnIII–OOH species. For comparison, the parent MnIIITPP does not exhibit water oxidation activity under identical experimental conditions. This study therefore provides new perspectives for the rational design of first‐row transition‐metal complexes catalysts for the water oxidation reaction, highlighting the important role of secondary sphere functionalities in promoting O─O bond formation and catalytic activity.