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−O 2 adduct through distal hydrogen bonding, while hemerythrin employs hydrogen bonding to stabilize reduced oxygen species within a diiron active site, enabling reversible O 2 binding. Here we show that a mononuclear nonheme iron complex, [Fe II (DIG 3 tren)] 2+ (DIG 3 tren = tris(N’,N”-diisopropylguanidinyl-2-ethyl)amine), reversibly reduces O 2 by two electrons to generate an iron(IV)-peroxido species. Strong hydrogen bonds from N − H groups of the ligand stabilize the O 2 2− ligand, while the electron-rich guanidine donors promote the unusual Fe II -mediated two-electron reduction of O 2 . As a result, the complex exhibits higher affinity for O 2 than for CO due to preferential hydrogen-bond stabilization of the peroxido intermediate. These results demonstrate how secondary-sphere design can control both O 2 activation and ligand selectivity at iron centers.