Abstract

S = 2 FeIV═O species are key intermediates in the catalysis of most nonheme iron enzymes. This article presents detailed spectroscopic and high-level computational studies on a structurally-defined S = 2 FeIV═O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both π- and σ-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These π- and σ-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with FeIV═O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an FeIV═O species in the same enzyme.