Structure, dynamics, and reactivity for alkane oxidation of Fe(II) sites situated in the nodes of a Metal-Organic Framework

Simons, M. C.; Vitillo, J. G.; Babucci, M.; Hoffman, A. S.; Boubnov, A.; Beauvais, M. L.; Chen, Z.; Cramer, C. J.; Chapman, K. W.; Bare, S. R.; Gates, B. C.; Lu, C. C.; Gagliardi, L.; Bhan, A.
J. Am. Chem. Soc. 2019, 141, 18142 (doi:10.1021/jacs.9b08686).

Metal organic frameworks (MOFs), with their crystalline, porous structures, can be synthesized to incorporate a wide range of catalytically active metals in tailored surroundings. These materials have potential as catalysts for conversion of light alkanes, feedstocks available in large quantities from shale gas that are changing the economics of manufacturing commodity chemicals. Mononuclear high-spin (S = 2) Fe(II) sites situated in the nodes of the MOF MIL-100(Fe) convert propane via dehydrogenation, hydroxylation, and over-oxidation pathways in reactions with the atomic oxidant N₂O. Pair distribution function analysis, N₂ adsorption isotherms, X-ray diffraction patterns, and infrared and Raman spectra confirm the single-phase crystallinity and stability of MIL-100(Fe) under reaction conditions (523 K in vacuo, 378-408 K C₃H₈ + N₂O). Density functional theory (DFT) calculations indicate a reaction mechanism for the formation of 2-propanol, propylene, and 1-propanol involving the oxidation of Fe(II) to Fe(III) via a high-spin Fe(IV)=O intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange were characterized by in-situ X-ray absorption spectroscopy. Ex-situ Möuat;ssbauer spectroscopy confirms the identity of Fe(II) sites, and the catalytic relevance and number of these sites was determined using in-situ chemical titrations with NO. N₂ and C₃H₆ production rates were found to be first-order in N₂O partial pressure and zero-order in C₃H₈ partial pressure, consistent with DFT calculations that predict the reaction of Fe(II) with N₂O to be rate determining. DFT calculations using a broken symmetry method indicate that Fe-trimer nodes affecting oxidation contain anti-ferromagnetically coupled Fe species that reduce the activation enthalpy, and calculations that compute a higher barrier on the low-spin (S = 1) Fe(II) site highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (< 408 K).