Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state

Abstract

Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C–H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C–H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe–oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C–H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO) 2+ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the “σ-pathway” that requires an essentially linear attack; by contrast a “π-channel” is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.