title: Dioxygen Activation by a Non-Heme Iron(II) Complex: Theoretical Study toward Understanding Ferric-Superoxo Complexes.
abstract: We present a systematic study using density functional theory (DFT) and coupled cluster (CCSD(T)) computations with an aim of characterizing a non-heme ferric-superoxo complex [(TMC)Fe(O2)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) that was proposed to perform allylic C-H activation of cyclohexene (Lee, Y.-M. et al. J. Am. Chem. Soc.2010, 132, 10668). As such, we investigated a series of iron-O2 species without and with a sixth ligand bound to the iron ion in different O2 coordination modes (end-on and side-on) and different spin states. Most of the iron-O2 complexes were found to be iron(III)-superoxo species, Fe(III)(O2(-)), with high-spin (S = 5/2) or intermediate-spin (S = 3/2) ferric centers coupled ferromagnetically or antiferromagnetically to the superoxide anion radical. One iron(IV)-peroxo state, Fe(IV)(O2(2-)), was also examined. The preference for ferromagnetic or antiferromagnetic coupling modes between the superoxo and ferric radicals was found to depend on the FeOO angle, where a side-on tilt favors ferromagnetic coupling whereas the end-on tilt favors antiferromagnetic states. Experimental findings, e.g., the effects of solvent, spin state, and redox potential of non-heme Fe(II) complexes on O2 activation, were corroborated in this work. Solvent effects were found to disfavor O2 binding, relative to the unbound ferrous ion and O2. The potential H-abstraction reactivity of the iron(III)-superoxo species was considered in light of the recently proposed exchange-enhanced reactivity principle (Shaik, S.; Chen, H.; Janardanan, D. Nat. Chem.2011, 3, 19). It is concluded that localization and/or decoupling of an unpaired electron in the d-block of high-spin Fe(III) center in the S = 2 and 3 ferric-superoxo complexes during H abstractions enhances exchange stabilization and may be the root cause of the observed reactivity of [(TMC)Fe(O2)](2+).
authors.affiliation: Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences , Beijing, 100190, China.|Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , Givat Ram Campus, 91904 Jerusalem, Israel.
authors.forename: Hui
authors.initials: H
authors.lastname: Chen
authors.affiliation: Department of Bioinspired Science, Department of Chemistry and Nano Science, Ewha Womans University , Seoul, 120-750, Korea.
authors.forename: Kyung-Bin
authors.initials: KB
authors.lastname: Cho
authors.affiliation: Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , Givat Ram Campus, 91904 Jerusalem, Israel.|Department of Chemistry, Renmin University of China , Beijing, 100872, China.
authors.forename: Wenzhen
authors.initials: W
authors.lastname: Lai
authors.affiliation: Department of Bioinspired Science, Department of Chemistry and Nano Science, Ewha Womans University , Seoul, 120-750, Korea.
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
authors.affiliation: Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem , Givat Ram Campus, 91904 Jerusalem, Israel.
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik