Million-Fold Activation of C–H Bonds by Fluorinated Nonheme Fe^IV═O Complexes via Second Sphere Equatorial Substitution and Catalytic Epoxidation to Boot

FeIV═O units found in the active sites of nonheme iron oxygenases and related synthetic analogs are intriguing intermediates capable of performing challenging oxygenation reactions. The first crystal structure of such a crucial species in a synthetic complex, [FeIV(Oanti)(TMC)(MeCN)]2+ (TMC-anti), reported in 2003, utilizes a 14-TMC (tetramethylcyclam) N4-macrocyclic ligand. With a half-life of 10 h at 25 °C, TMC-anti is quite a sluggish oxidant, but axial ligand replacements enhance TMC-anti reactivity by as much as 50-fold. Herein we switch to an N4-equatorial modification approach by replacing the N-methyl groups in TMC-anti with N–CH2–aryl groups and fluorinated analogs in the secondary coordination sphere to generate even more reactive FeIV(O)L complexes, namely [FeIV(Oanti)(TBF8C)(MeCN)]2+ (2-anti, t1/2 = 6 min at 25 °C), [FeIV(Osyn)(TBF8C)(MeCN)]2+ (2-syn, t1/2 = 2 min at 25 °C) and [FeIV(Osyn)(TBF8C)(Cl)]+ (3-syn, t1/2 = 1.5 min at −20 °C). Surprisingly, despite the increased steric bulk introduced around the FeIV═O moiety, 2-syn and 3-syn exhibit reaction rates as much as a million-fold higher than TMC-anti in C–H bond cleavage as well as oxo-transfer reactions, including unprecedented catalytic epoxidation of olefins by 2-syn. Computations confirm the dramatic reactivity enhancement upon introduction of polyfluorinated arenes into the second coordination sphere of the nonheme FeIV═O complexes, which distort the Me4cyclam that decreases the energy gap between the ground S = 1 and the excited S = 2 spin states.