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

Hypervalent FeV=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 FeV 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 FeV=O fragment. The key FeV=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 [FeV=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 FeV oxidation state and the presence of a FeV=O bond at ≈1.60 Å. Combined EPR and DFT methods indicate that 2 contains a S=3/2 FeV center. 2 is the first spectroscopically characterized high spin oxo‐FeV complex and constitutes a paradigmatic example of the FeV=O(OH) species proposed to be responsible for catalytic water oxidation reactions.