Alkanes are a major constituent of natural gas, and their efficient conversion into functionalized chemicals is challenging. High-valent iron–oxo species present in enzymes can activate the strong C–H bonds of alkanes and convert them into useful chemicals, but mimicking this kind of chemistry in synthetic materials is difficult. This work demonstrates the use of novel materials known as metal–organic frameworks (MOFs) to achieve this goal. The magnesium-diluted analogue of the MOF, Fe2(dobdc) (dobdc = 2,5-dioxido-1,4-benzenedicarboxylate), has been shown experimentally to activate the strong C–H bonds of ethane via the formation of transient Fe(IV)–O intermediate and to convert it into ethanol. A detailed mechanism of this catalytic conversion has been explored using Kohn-Sham density functional and multireference wave function theories suggesting that high-spin Fe(IV)–O center is the catalytically active species.[ii] It was found that the activation barrier for the slowest step in the iron(IV)−oxo catalyzed reaction is 82 kJ/mol, while the uncatalyzed reaction has a barrier of 280 kJ/mol. The C–H bond activation energies of methane and ethane were also compared and it was shown that activation of ethane is more facile than methane. The present work aims at tuning the catalytic properties of Fe2(dobdc) by changing the dobdc linker. Analogues of Fe2(dobdc) will be generated by considering biphenyl and the triphenyl derivatives of the linker and adding substituents such as –F, –Cl, and –CH3 on the benzene rings. Changing the oxido group (O–) of the linker to S– is also being considered. Once generated, these analogues will be oxidized to generate the iron(IV)−oxo intermediates, which will be investigated for their C–H bond activation behavior toward various hydrocarbons. Both periodic density functional theory and large cluster models are being employed to study these reactions. Acknowledgments. This work was supported by a Doctoral Dissertation Fellowship (to P. V.) 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. Xiao, et al. Selective Oxidation of Ethane to Ethanol by Nitrous Oxide in a Metal–Organic Framework with Coordinatively–Unsaturated Iron(II) Sites. Nature Chem. 2014, 6, 590–595. [ii] Verma, et al. Mechanism of oxidation of ethane to ethanol at iron(IV)−oxo sites in magnesium-diluted Fe2(dobdc). J. Am. Chem. Soc. 2015, 137, 5770−5781.

C-H bond activation of hydrocarbons at exposed iron sites in metal-organic frameworks designed to improve catalytic efficiency

251st National Meeting (2016)

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