Dramatic rate-enhancement of oxygen atom transfer by an iron(iv)-oxo species by equatorial ligand field perturbations

Nonheme iron dioxygenases are efficient enzymes with relevance for human health that regio- and stereospecifically transfer an oxygen atom to substrates. How they perform this task with such selectivity remains unknown, but may have to do with substrate binding, positioning and oxidant approach. To understand substrate approach on a catalytic reaction centre, we investigated the structure and reactivity of a biomimetic oxidant with ligand features that affect the interactions between oxidant and substrate. Thus, we report here the synthesis and characterization of an iron(IV)-oxo complex with pentadentate nonheme ligand, where structurally induced perturbations in the equatorial ligand field affect the spectroscopy and reactivity of the complex. We tested the activity of the complex with respect to oxygen atom transfer to and hydrogen atom abstraction from substrates. This oxidant shows improved reaction rates toward heteroatom oxidation with respect to the nonsubstituted ligand complex by ∼104 fold. The origin of the enhanced reactivity is explained with a series of density functional theory studies that show an enhanced electron affinity of the oxidant through equatorial ligand perturbations.