Theoretical study of cyclohexane hydroxylation by three possible isomers of [FeIV(O)(R-TPEN)] 2+: does the pentadentate ligand wrapping around the metal center differently lead to the different stability and reactivity?

Density functional theory calculations have been carried out to elucidate the mechanism of cyclohexane hydroxylation by three possible isomers of [Fe(IV)(O)(N-R-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine)](2+) (R is methyl or benzyl) (Klinker et al. in Angew Chem Int Ed 44:3690-3694, 2005). The calculations offer a mechanistic view and reveal the following features: (a) all the three isomers possess triplet ground states and low-lying quintet excited states, (b) the relative stability follows the order isomer A > isomer B > isomer C, in agreement with the conclusions of Klinker et al., (c) the theoretical investigations provide a rationale to explain the interconversion of the three isomers, (d) the reaction pathways of the C-H hydroxylation are initiated by a hydrogen-abstraction step, and (e) the three isomers react with cyclohexane via two-state-reactivity patterns on competing triplet and quintet spin-state surfaces. As such, in the gas phase, the relative reactivity exhibits the trend isomer B > isomer A, while at the highest level, B2//B1 with zero point energy and solvation corrections, the relative reactivity follows the order isomer B > isomer A > isomer C. Thus, the calculated reaction pathway shows that pyridine rings perpendicular to the Fe-O axis result in more reactive species, and a pyridine ring coordinated trans to the oxygen atom leads to the least reactive isomer.