Characterization of the FeV=O Complex in the Pathway of Water Oxidation

Hypervalent Fe^V=O species are implicated in a multitude of oxidative reactions of organic substrates, as well as in catalytic water oxidation, a reaction crucial for artificial photosynthesis. Spectroscopically characterized Fe^V species are exceedingly rare and, so far, were produced by the oxidation of Fe complexes with peroxy acids or H₂O₂: reactions that entail breaking of the O?O bond to form a Fe^V=O fragment. The key Fe^V=O species proposed to initiate the O?O bond formation in water oxidation reactions remained undetected, presumably due to their high reactivity. Here, we achieved freeze quench trapping of six coordinated Fe^V=O,(OH)(Pytacn)]²⁺ (Pytacn=1‐(2′‐pyridylmethyl)‐4,7‐dimethyl‐1,4,7‐triazacyclononane) ( 2 ) generated during catalytic water oxidation. X‐ray absorption spectroscopy (XAS) confirmed the Fe^V oxidation state and the presence of a Fe^V=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 Fe^V center. 2 is the first spectroscopically characterized high spin oxo‐Fe^V complex and constitutes a paradigmatic example of the Fe^V=O(OH) species proposed to be responsible for catalytic water oxidation reactions.