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 ─O 2 ˙ˉ)/peroxo (Fe III ─O 2 2 ˉ) redox couple in a heme model complex using conventional cyclic voltammetry. Urea groups installed in the second coordination sphere (SCS) of an iron‐porphyrin ( αα UrFe ) form a hydrogen‐bonding network that stabilizes the Fe III ─O 2 ˙ˉ 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 ─O 2 ˙ˉ species generates a low‐spin end‐on Fe III ─O 2 2 ˉ 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 O 2 activation pathways in bioinspired catalysts.