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

Abstract

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 H₂ O₂ : 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.

Keywords: Fe-based catalysts; X-ray spectroscopy; artificial photosynthesis; highly oxidized species; water oxidation.