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

ABSTRACT 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.