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

摘要

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, F(8)Cmpd-II (F-8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a 2,6-lutidinium triflate (LutH(+)) Lewis acid adduct involving ferryl O-atom hydrogen-bonding, F(8)Cmpd-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, F(8)Cmpd-II(LutH(+)) reacts 40 times faster (k(2) = 14.2 M-1 s(-1); -90 degrees C) than does F(8)Cmpd-II, giving bixanthene plus xanthone and the heme product [(F8FeOH2)-O-III](+). For substrates with greater C-H bond dissociation energies (BDEs) F(8)Cmpd-II(LutH(+)) reacts with the second order rate constants k(2)(9,10-dihydroan-thracene; DHA) = 0.485 M-1 s(-1) and k(2)(fluorene) = 0.102 M-1 s(-1) (-90 degrees C); by contrast, F(8)Cmpd-II is unreactive toward these substrates. For xanthene vs xanthene-(d(2)), large, nonclassical deuterium kinetic isotope effects are roughly estimated for both F(8)Cmpd-II and F(8)Cmpd-II(LutH(+)). The deuterated H-bonded analog, F(8)Cmpd-II(LutD(+)), was also prepared; for the reaction with DHA, an inverse KIE (compared to F(8)Cmpd-II(LutH(+))) was observed. This work originates/inaugurates experimental investigation of the reactivity of authentic H-bonded heme-based Fe-IV=0 compounds, critically establishing the importance of oxo H-bonding (or protonation) in heme complexes and enzyme active sites.