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 and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz 2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx 2 -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 p-pathway involving the 3dxz/ yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the a/ss splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory..

Automated summary

Fe K-edge XAS is widely used for studying high-valent iron intermediates in catalysts. The 4p-mixing into the 3d orbitals complicates the analysis, but understanding it correctly enables deeper insights into the structure and reactivity. This study reveals that the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy orbitals, providing structural insights for distinguishing 6- vs 5-coordinate active sites. Furthermore, the study investigates the electronic structure of Fe(IV)=O active sites and their reactivity selectivity through O K-edge XAS.