Enhanced Rates of C-H Bond Cleavage by a Hydrogen-Bonded Synthetic Heme High-Valent Iron(IV) Oxo Complex.

Secondary coordination sphere interactions are critical in facilitating the formation, stabilization, and enhanced reactivity of high-valent oxidants required for essential biochemical processes. Herein, we compare the C-H bond oxidizing capabilities of spectroscopically characterized synthetic heme iron(IV) oxo complexes, FCmpd-II (F = tetrakis(2,6-difluorophenyl)porphyrinate), and a 2,6-lutidinium triflate (LutH) Lewis acid adduct involving ferryl O-atom hydrogen-bonding, FCmpd-II(LutH). Second-order rate constants utilizing C-H and C-D substrates were obtained by UV-vis spectroscopic monitoring, while products were characterized and quantified by EPR spectroscopy and gas chromatography (GC). With xanthene, FCmpd-II(LutH) reacts 40 times faster ( = 14.2 M s; -90 °C) than does FCmpd-II, giving bixanthene plus xanthone and the heme product [FFeOH]. For substrates with greater C-H bond dissociation energies (BDEs) FCmpd-II(LutH) reacts with the second order rate constants (9,10-dihydroanthracene; DHA) = 0.485 M s and (fluorene) = 0.102 M s (-90 °C); by contrast, FCmpd-II is unreactive toward these substrates. For xanthene vs xanthene-(), large, nonclassical deuterium kinetic isotope effects are roughly estimated for both FCmpd-II and FCmpd-II(LutH). The deuterated H-bonded analog, FCmpd-II(LutD), was also prepared; for the reaction with DHA, an inverse KIE (compared to FCmpd-II(LutH)) was observed. This work originates/inaugurates experimental investigation of the reactivity of authentic H-bonded heme-based Fe═O compounds, critically establishing the importance of oxo H-bonding (or protonation) in heme complexes and enzyme active sites.