Direct Observation of a Hydrogen-Bonding Assisted Quasi-Reversible Superoxo/Peroxo Interconversion in a Heme Model

The activation and reduction of molecular oxygen at transition metal centers are central to both enzymatic catalysis and energy conversion. Here, we report the first direct observation of a hydrogen-bonding-assisted, quasi-reversible superoxo (Fe^III─O₂˙-)/peroxo (Fe^III─O₂ ²-) redox couple in a heme model complex using conventional cyclic voltammetry. Urea groups installed in the second coordination sphere (SCS) of an iron-porphyrin (ααUrFe) form a hydrogen-bonding network that stabilizes the Fe^III─O₂˙- intermediate through interaction with a trapped water molecule. Spectroscopic studies and DFT calculations reveal that one-electron reduction of the low-spin end-on Fe^III─O₂˙- species generates a low-spin end-on Fe^III─O₂ ²- adduct with minimal structural reorganization. These findings provide the first synthetic demonstration of a biologically relevant end-on peroxo species stabilized by hydrogen-bonding "pull effect" in the SCS and highlight the critical role of second-sphere interactions in modulating O₂ activation pathways in bioinspired catalysts.