title: How does tunneling contribute to counterintuitive H-abstraction reactivity of nonheme Fe(IV)O oxidants with alkanes?
abstract: This article addresses the intriguing hydrogen-abstraction (H-abstraction) and oxygen-transfer (O-transfer) reactivity of a series of nonheme [Fe(IV)(O)(TMC)(Lax)](z+) complexes, with a tetramethyl cyclam ligand and a variable axial ligand (Lax), toward three substrates: 1,4-cyclohexadiene, 9,10-dihydroanthracene, and triphenyl phosphine. Experimentally, O-transfer-reactivity follows the relative electrophilicity of the complexes, whereas the corresponding H-abstraction-reactivity generally increases as the axial ligand becomes a better electron donor, hence exhibiting an antielectrophilic trend. Our theoretical results show that the antielectrophilic trend in H-abstraction is affected by tunneling contributions. Room-temperature tunneling increases with increase of the electron donation power of the axial-ligand, and this reverses the natural electrophilic trend, as revealed through calculations without tunneling, and leads to the observed antielectrophilic trend. By contrast, O-transfer-reactivity, not being subject to tunneling, retains an electrophilic-dependent reactivity trend, as revealed experimentally and computationally. Tunneling-corrected kinetic-isotope effect (KIE) calculations matched the experimental KIE values only if all of the H-abstraction reactions proceeded on the quintet state (S = 2) surface. As such, the present results corroborate the initially predicted two-state reactivity (TSR) scenario for these reactions. The increase of tunneling with the electron-releasing power of the axial ligand, and the reversal of the "natural" reactivity pattern, support the "tunneling control" hypothesis (Schreiner et al., ref 19). Should these predictions be corroborated, the entire field of C-H bond activation in bioinorganic chemistry would lay open to reinvestigation.
authors.affiliation: Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , 91904 Jerusalem, Israel.
authors.forename: Debasish
authors.initials: D
authors.lastname: Mandal
authors.forename: Rajeev
authors.initials: R
authors.lastname: Ramanan
authors.forename: Dandamudi
authors.initials: D
authors.lastname: Usharani
authors.forename: Deepa
authors.initials: D
authors.lastname: Janardanan
authors.forename: Binju
authors.initials: B
authors.lastname: Wang
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D016877:Oxidants; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D000473:Alkanes; D016877:Oxidants; D006859:Hydrogen; D007501:Iron; D010100:Oxygen