Million-Fold Activation of C¬H Bonds by Fluorinated Non-heme FeIV=O Complexes via Second Sphere Equatorial Sub-stitution and Catalytic Epoxidation to Boot

FeIV=O units found in the active sites of nonheme iron oxygenases and related synthetic analogs are intriguing inter-mediates 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 (tetramethyl-cyclam) 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 sec-ondary 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 introduc-tion 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.