<i>In Crystallo</i> O ₂ Cleavage at a Preorganized Triiron Cluster

In Nature, the four-electron reduction of O₂ is catalyzed at preorganized multimetallic active sites. These complex active sites often feature low-coordinate, redox-active metal centers precisely positioned to facilitate rapid O₂ activation processes that obviate the generation of toxic, partially reduced oxygen species. Very few biomimetic constructs simultaneously recapitulate the complexity and reactivity of these biological cofactors. Herein, we report solid-state O₂ activation at a triiron(II) active site templated by phosphinimide ligands. Insight into the structure of the O₂ reduction intermediates was obtained via <i>in crystallo</i> O₂ dosing experiments in conjunction with spectroscopic, structural, magnetic, and computational studies. These data support the in situ formation of an Fe₂^IIIFe^IV-dioxo intermediate upon exposure to O₂ that participates in oxygen atom and hydrogen atom transfer reactivity with exogenous substrates to furnish a stable Fe^IIFe₂^III-oxo species. Combined, these studies provide an extraordinary level of detail into the dynamics of bond-forming and -breaking processes operative at complex multimetallic active sites.