Seeing the key intermediates in bioinspired nonheme iron complex-catalyzed water oxidation

Summary

Inspired by the oxygen-evolving complex in photosystem II, the nature of intermediates and the oxygen–oxygen bond formation mechanism in photocatalytic water oxidation have been the topic of current research. Herein, we report that the key intermediates and the O–O bond formation step in the catalytic water oxidation by [Ru^III(bpy)₃]³⁺ or the excited state of 2,3-dichloro-5,6-dicyano-p-benzoquinone (³DDQ∗) with a nonheme iron complex, [Fe^II(dpaq)]⁺, were captured and identified. Especially, two keyintermediates, such as an iron(V)-oxo ([Fe^V(dpaq)(O)]²⁺ or [Fe^IV(dpaq cation radical)(O)]²⁺) and an iron(III)-hydroperoxo species ([Fe^III(dpaq)(OOH)]⁺), were captured in the water oxidation reactions; [Fe^V(dpaq)(O)]²⁺ (or [Fe^IV(dpaq cation radical)(O)]²⁺) was detected using transient absorption spectroscopy in the oxidation of [Fe^IV(dpaq)(O)]⁺ by ³DDQ∗, whereas the formation of [Fe^III(dpaq)(OOH)]⁺ was directly observed by reacting [Fe^IV(dpaq)(O)]⁺ with [Ru^III(bpy)₃]³⁺ or ³DDQ∗ in the presence of water. The evolution of O₂ upon the oxidation of [Fe^III(dpaq)(OOH)]⁺ by DDQ was also demonstrated in this study.