Two-State Reactivity in Alkane Hydroxylation by Non-Heme Iron−Oxo Complexes

Abstract

Density functional theory is used to explore the mechanisms of alkane hydroxylation for four synthetic non-heme iron(IV)-oxo complexes with three target substrates (Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde; J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126, 472-473; Rohde, J.-U.; Que, L., Jr. Angew. Chem. Int. Ed. 2005, 44, 2255-2258.). The iron-oxo reagents possess triplet ground states and low-lying quintet excited states. The set of experimental and theoretical reactivity trends can be understood if the reactions proceed on the two spin states, namely two-state reactivity (TSR); an appropriate new model is presented. The TSR model makes testable predictions: (a) If crossing to the quintet state occurs, the hydroxylation will be effectively concerted; however, if the process transpires only on the triplet surface, stepwise hydroxylation will occur, and side products derived from radical intermediates would be observed (e.g., loss of stereochemistry). (b) In cases of crossing en route to the quintet transition state, one expects kinetic isotope effects (KIEs) typical of tunneling. (c) In situations where the two surfaces contribute to the rate, one expects intermediate KIEs and radical scrambling patterns that reflect the two processes. (d) Solvent effects on these reactions are expected to be very large.

Axial ligand tuning of a nonheme iron(IV)–oxo unit for hydrogen atom abstraction

“…In sharp contrast, the COH and OOH activation rate constants increase as ⌬E Q-T decreases, indicating that the greater accessibility of the quintet state enhances the COH and OOH activation rates. Indeed, as shown and rationalized in a previous theoretical study (44), the quintet transition state lies below the triplet one and has much smaller barriers for COH bond activation (SI Figs. 14 and 15).…”

“…5) and the quintet state becomes more accessible along the reaction trajectory. As explained previously (44), this trend is rooted in the greater destabilizing effect on the localized ␦ orbital compared with the delocalized * xy orbital as the donor ability of X increases from the neutral to the anionic ligands. Indeed, this tuning of reactivity by the ⌬E Q-T parameter is apparent from Fig.…”

“…It is postulated that the observed reactivity reflects the availability of two closely lying spin states for all of the 1-X reagents, the so-called TSR hypothesis fully detailed in ref. 44, wherein the excited quintet state has a much lower reaction barrier than the ground triplet state. Thus, increasing the electron-donating character of the axial ligand results in a decrease in the triplet-quintet gap and an increased participation of the quintet state in determining the rate of reaction.…”

“…5) possess two adjacently lying states: a ground state with a triplet spin quantum number (T) and a low-lying state with a quintet (Q) spin state (44). The two states are very close in energy for all of the complexes, lying within Ͻ7 kcal/mol; as X becomes a better electron donor, the ⌬E Q-T gap decreases (Fig.…”

A synthetic precedent for the [Fe IV 2 (μ-O) 2 ] diamond core proposed for methane monooxygenase intermediate Q

“…MMO-Q oxidizes methane and is thus a superior oxidant than any of these synthetic low-spin iron(IV) complexes. Its greater reactivity may arise from the fact that it has high-spin iron(IV) centers, which DFT calculations find to be more reactive than their low-spin counterparts in hydrogen-atom abstraction (40)(41)(42). However, our observations suggest in addition that its [Fe IV 2 ( -O) 2 ] core may be further activated in the course of the reaction with methane by isomerization to a more reactive ring-opened form with a terminal Fe V AO unit, such as Fe III OOOFe V AO, as originally proposed in 1997 by Siegbahn and Crabtree (7).…”