The metal–organic frameworks (MOFs) Mg2(dobdc) and Fe2(dobdc) have open metal sites [Mg(II) or Fe(II)] at the nodes, connected by 2,5‒dioxido‒1,4‒benzenedicarboxylate (dobdc4–) linkers. The mixed-metal MOF, Fe0.1Mg1.9(dobdc), catalyzes hydroxylation of ethane, and this was interpreted to occur by oxidation involving a fleeting iron(IV)–oxo species [Xiao, Bloch, Mason, Queen, Hudson, Planas, Borycz, Dzubak, Verma, Lee, Bonino, Crocellà, Yano, Bordiga, Truhlar, Gagliardi, Brown, Long. Nature Chem. 6 (2014) 590], similar to the chemistry found in several Fe-containing heme and non-heme enzymes and synthetic complexes for activating strong C–H bonds. Here, we present a theoretical study of this reaction by Kohn-Sham density functional theory and wave function-based multireference calculations to characterize the electronic structures of key intermediates and transition strcutures and to unravel the detailed mechanism. The strong C–H bonds of ethane have been demonstrated to be activated by the highly reactive iron(IV)–oxo (S = 2) intermediate generated after the oxidation of Fe(II) sites by nitrous oxide. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol, has also been investigated; the activation barrier for the uncatalyzed reaction is 280 kJ/mol while that for the catalyzed one is 82 kJ/mol, which clearly indicates that the formation of the iron(IV)–oxo intermediate is crucial for the reaction to occur. Also investigated are the competing pathways that follow after the formation of ethyl radical and that may lead to desaturation or dissociation products besides the hydroxylated product observed experimentally. Research supported by a Phillips 66 Fellowship (PV) and by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences under Award DE-FG02-12ER16362.

Mechanism of C–H bond activation of C2H6 by iron(IV)–oxo sites in magnesium-diluted Fe2(dobdc)

249th National Meeting (2015)

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