Dioxygen Binding to Complexes with FeII2(μ-OH)2 Cores: Steric Control of Activation Barriers and O2-Adduct Formation

A series of complexes with [FeII2(μ-OH)2] cores has been synthesized with N3 and N4 ligands and structurally characterized to serve as models for nonheme diiron(II) sites in enzymes that bind and activate O2. These complexes react with O2 in solution via bimolecular rate-limiting steps that differ in rate by 103-fold, depending on ligand denticity and steric hindrance near the diiron center. Low-temperature trapping of a (μ-oxo)(μ-1,2-peroxo)diiron(III) intermediate after O2 binding requires sufficient steric hindrance around the diiron center and the loss of a proton (presumably that of a hydroxo bridge or a yet unobserved hydroperoxo intermediate). The relative stability of these and other (μ-1,2-peroxo)diiron(III) intermediates suggests that these species may not be on the direct pathway for dioxygen activation.