Properties and reactivities of nonheme iron(iv)-oxo versus iron(v)-oxo: Long-range electron transfer versus hydrogen atom abstraction

Recent work of Nam and co-workers [J. Yoon, S. A. Wilson, Y. K. Jang, M. S. Seo, K. Nehru, B. Hedman, K. O. Hodgson, E. Bill, E. I. Solomon and W. Nam, Angew. Chem., Int. Ed., 2009, 48, 1257] on a biomimetic iron complex implicated a mixture of iron(iv)-oxo and iron(v)-oxo intermediates but the latter could not be spectroscopically characterized, hence its involvement was postulated. To gain insight into the relative activity of these iron(iv)-oxo versus iron(v)-oxo intermediates, we have performed an extensive density functional theory (DFT) study on the chemical properties of the chemical system of Nam et al., namely [Fe(O)(BQEN)(NCCH₃)]^2+/3+ with BQEN = N, N′-dimethyl-N, N′-bis(8-quinolyl)ethane-1,2-diamine and their reactivity in hydrogen atom abstraction from ethylbenzene. We show that the perceived iron(v)-oxo species actually is an iron(iv)-oxo ligand cation radical, similar to cytochrome P450 compound I. Moreover, this intermediate has an extremely large electron affinity and therefore can abstract electrons from substrates readily. In our particular system, this means that prior to the hydrogen atom abstraction, an electron is abstracted to form an iron(iv)-oxo species, which subsequently abstracts a hydrogen atom from the substrate. Thus, our calculations show for the first time how some nonheme iron complexes react by long-range electron transfer and others directly via hydrogen atom abstraction. We have rationalized our results with detailed thermochemical cycles that explain the observed reactivity patterns.