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 approximate to 1.60 angstrom. 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.