A mononuclear nonheme iron complex with higher affinity for O₂ than CO via hydrogen bonding

Dioxygen activation at iron centers is central to many biological and synthetic oxidation processes. In proteins, the reactivity and stability of iron-dioxygen intermediates are often controlled by secondary-sphere interactions such as hydrogen bonding. For example, hemoglobin stabilizes a Fe-O₂ adduct through distal hydrogen bonding, while hemerythrin employs hydrogen bonding to stabilize reduced oxygen species within a diiron active site, enabling reversible O₂ binding. Here we show that a mononuclear nonheme iron complex, [Fe^II(DIG₃tren)]²⁺ (DIG₃tren = tris(N',N"-diisopropylguanidinyl-2-ethyl)amine), reversibly reduces O₂ by two electrons to generate an iron(IV)-peroxido species. Strong hydrogen bonds from N - H groups of the ligand stabilize the O₂^2- ligand, while the electron-rich guanidine donors promote the unusual Fe^II-mediated two-electron reduction of O₂. As a result, the complex exhibits higher affinity for O₂ than for CO due to preferential hydrogen-bond stabilization of the peroxido intermediate. These results demonstrate how secondary-sphere design can control both O₂ activation and ligand selectivity at iron centers.