Dioxygen activation by a non-heme iron(II) complex: Theoretical study toward understanding ferric-superoxo complexes

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(O ₂)] 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-O ₂ species without and with a sixth ligand bound to the iron ion in different O ₂ coordination modes (end-on and side-on) and different spin states. Most of the iron-O ₂ complexes were found to be iron(III)-superoxo species, Fe(III)(O ₂ ^-), 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)(O ₂ ^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 O ₂ activation, were corroborated in this work. Solvent effects were found to disfavor O ₂ binding, relative to the unbound ferrous ion and O ₂. 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(O ₂)] ²⁺.