Million-Fold Activation of C–H Bonds by Fluorinated Nonheme FeIV═O Complexes via Second Sphere Equatorial Substitution and Catalytic Epoxidation to Boot
Bittu Chandra, Chupeng Li, Mursaleem Ansari, Jin Xiong, Alex Lovstedt, Yisong Guo, Marcel Swart, Lawrence Que, Jr
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引用次数: 0
Abstract
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.
期刊介绍:
ACS Catalysis is an esteemed journal that publishes original research in the fields of heterogeneous catalysis, molecular catalysis, and biocatalysis. It offers broad coverage across diverse areas such as life sciences, organometallics and synthesis, photochemistry and electrochemistry, drug discovery and synthesis, materials science, environmental protection, polymer discovery and synthesis, and energy and fuels.
The scope of the journal is to showcase innovative work in various aspects of catalysis. This includes new reactions and novel synthetic approaches utilizing known catalysts, the discovery or modification of new catalysts, elucidation of catalytic mechanisms through cutting-edge investigations, practical enhancements of existing processes, as well as conceptual advances in the field. Contributions to ACS Catalysis can encompass both experimental and theoretical research focused on catalytic molecules, macromolecules, and materials that exhibit catalytic turnover.