Proton-Coupled Electron Transfer and Rebound Studies of (bTAML)Iron(IV)–Hydroxo Complex

Abstract High-valent iron–oxo intermediates are well established as key oxidants in biological and synthetic oxidation chemistry, whereas their protonated counterparts, iron(IV)–hydroxo (FeIVOH) species, remain comparatively underexplored despite their central role in Cytochrome P450 catalysis. Herein, we report the synthesis, spectroscopic characterization, and reactivity of a room-temperature-stable FeIVOH complex supported by a biuret-modified tetraamido macrocyclic ligand (bTAML). One-electron oxidation of the crystallographically characterized precursor [Et4N]2[(Ph,Me-bTAML)FeIIIOH] using tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue) affords the one-electron-oxidized species [(Ph,Me-bTAML)Fe(OH)]− under ambient conditions. Comprehensive characterization by UV–vis, HRMS, EPR, Mössbauer spectroscopy, and X-ray absorption spectroscopy (XANES/EXAFS) conclusively identifies this species as a mononuclear FeIVOH complex. This species represents the first TAML-supported Compound II analogue and a rare non-heme model of the biologically relevant FeIVOH intermediate. Reactivity studies reveal efficient hydroxyl radical rebound chemistry, achieving hydroxylation of the trityl radical derived from Gomberg’s dimer in 60% yield. Thermodynamic analysis revealed an O–H bond dissociation free energy of 76.3 kcal mol–1, which is reflected in its selective reactivity toward O–H bond oxidation of substituted 2,6-di-tert-butylphenols. Kinetic and mechanistic studies support a concerted proton–electron transfer (CPET) pathway, providing new insights into the structure, thermodynamics, and reactivity of FeIVOH intermediates.