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 H2 O2 : 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.