X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity

Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge XAS, enables deeper insight into geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into 3d(z²) orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3d(x²-y²,xy), providing structural insight and allowing the distinction of 6- vs. 5- coordinate active sites as shown through application to the Fe(IV)=O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)=O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3d(xz/yz) orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e. the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.