<i>In Crystallo</i> O<sub>2</sub> Cleavage\nat a Preorganized Triiron Cluster

In Nature, the four-electron reduction of O<sub>2</sub> is catalyzed\nat preorganized multimetallic active sites. These complex active sites\noften feature low-coordinate, redox-active metal centers precisely\npositioned to facilitate rapid O<sub>2</sub> activation processes\nthat obviate the generation of toxic, partially reduced oxygen species.\nVery few biomimetic constructs simultaneously recapitulate the complexity\nand reactivity of these biological cofactors. Herein, we report solid-state\nO<sub>2</sub> activation at a triiron(II) active site templated by\nphosphinimide ligands. Insight into the structure of the O<sub>2</sub> reduction intermediates was obtained via <i>in crystallo</i> O<sub>2</sub> dosing experiments in conjunction with spectroscopic,\nstructural, magnetic, and computational studies. These data support\nthe in situ formation of an Fe<sub>2</sub><sup>III</sup>Fe<sup>IV</sup>-dioxo intermediate upon exposure to O<sub>2</sub> that participates\nin oxygen atom and hydrogen atom transfer reactivity with exogenous\nsubstrates to furnish a stable Fe<sup>II</sup>Fe<sub>2</sub><sup>III</sup>-oxo species. Combined, these studies provide an extraordinary level\nof detail into the dynamics of bond-forming and -breaking processes\noperative at complex multimetallic active sites.