Dioxygen Activation by a Non-Heme Iron(II) Complex: Theoretical Study toward Understanding Ferric鈥揝uperoxo Complexes
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- 作者：Hui Chen ; Kyung-Bin Cho ; Wenzhen Lai ; Wonwoo Nam ; Sason Shaik
- 刊名：Journal of Chemical Theory and Computation
- 出版年：2012
- 出版时间：March 13, 2012
- 年：2012
- 卷：8
- 期：3
- 页码：915-926
- 全文大小：642K
- 年卷期：v.8,no.3(March 13, 2012)
- ISSN：1549-9626
文摘
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鈥搒uperoxo complex [(TMC)Fe(O2)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) that was proposed to perform allylic C鈥揌 activation of cyclohexene (Lee, Y.-M. et al. J. Am. Chem. Soc.2010, 132, 10668). As such, we investigated a series of iron鈥揙2 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鈥揙2 complexes were found to be iron(III)鈥搒uperoxo species, Fe(III)(O2鈥?/sup>), 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)鈥損eroxo state, Fe(IV)(O22鈥?/sup>), 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)鈥搒uperoxo 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鈥搒uperoxo complexes during H abstractions enhances exchange stabilization and may be the root cause of the observed reactivity of [(TMC)Fe(O2)]2+.