O−O Bond Formation and Liberation of Dioxygen Mediated by N5-Coordinate Non-Heme Iron(IV) Complexes

Abstract

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non-heme, N5-coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O−O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.