Secondary-Sphere Hydrogen Bonds Regulating Spin-Redox Interplay in Hemes.

Hydrogen bonding (H-bonding) plays a pivotal role in regulating the chemical and electrochemical properties of metalloproteins by influencing substrate recognition, binding orientation, and active-site geometry. In heme enzymes, conserved H-bonding networks are directly linked to catalytic efficiency by modulating redox potentials and spin states of the iron center. Despite extensive studies on biological systems, the molecular origin of H-bonding effects on the electronic structure and redox properties of heme groups remains underexplored. We report here iron(III) porphyrin-phenoxide complexes where secondary-sphere H-bonding interactions exert a large influence on geometry, spin state, and redox properties. The H-bonding interactions elongate the axial Fe─O bond, contract the porphyrin core, and stabilize the intermediate-spin (S = 3/2) state of iron, while the absence of H-bonding favors the high-spin (S = 5/2) state. Similar effects are also observed in the iron(III)-chloro complex in which the axial ligand is engaged in secondary-sphere H-bonding interactions. Electrochemical studies reveal positive shifts in the Fe(III)/Fe(II) couple and 1e⁻ oxidation, highlighting H-bonding as a regulator of redox noninnocence. Supported by computational studies, our findings provide fundamental insights into the interplay between H-bonding, spin state, and redox chemistry, thereby offering insight into enzymatic regulation for its biological functions.