ABSTRACT
High-valent Fe(IV)-oxo species derived upon reactions of N2O with Fe(II) centers-embedded in the framework of tri-iron oxo-centered-based metal-organic frameworks (MOFs)- selectively affect the conversion of benzene-to-phenol via electrophilic addition to arene C-H bonds akin to oxygen transfer mechanisms in the P450 enzyme. The Fe(II) species identified by Mössbauer spectroscopy can be titrated in situ by the addition of NO to completely suppress benzene oxidation, verifying the relevance of Fe(II) centers. Observed inverse kinetic isotope effects in benzene hydroxylation preclude the involvement of H atom transfer steps from benzene to the Fe(IV)-oxo species and instead suggest that the electrophilic iron-oxo group adds to an sp2 carbon of benzene, resulting in a change in the hybridization from sp2-to-sp3. These mechanistic postulates are affirmed in Kohn-Sham density functional calculations, which predict lower barriers for additive mechanisms for arene oxidation than H atom abstraction steps. The calculations show that the reaction proceeds on the pentadectet spin surface and that a non-innocent ligand participates in the transfer of the H atom. Following precedent literature which demonstrates that these Fe(IV)-oxo species react with C-H bonds in alkanes via hydrogen atom abstraction to form alcohols, it appears that iron(IV)-oxo species in MOFs exhibit duality in their reactions with inert hydrocarbon substrates akin to enzymes-if the C-H bonds are in saturated aliphatic hydrocarbons, then activation occurs via hydrogen abstraction, while if the C-H bonds are aromatic, then activation occurs by addition rearrangement.
ABSTRACT
Recent work has exploited the ability of metal-organic frameworks (MOFs) to isolate Fe sites that mimic the structures of sites in enzymes that catalyze selective oxidations at low temperatures, opening new pathways for the valorization of underutilized feedstocks such as methane. Questions remain as to whether the radical-rebound mechanism commonly invoked in enzymatic and homogeneous systems also applies in these rigid-framework materials, in which resisting the overoxidation of desired products is a major challenge. We demonstrate that MOFs bearing Fe(II) sites within Fe3-µ3-oxo nodes active for conversion of CH4 + N2O mixtures (368-408 K) require steps beyond the radical-rebound mechanism to protect the desired CH3OH product. Infrared spectra and density functional theory show that CH3OH(g) is stabilized as Fe(III)-OCH3 groups on the MOF via hydrogen atom transfer with Fe(III)-OH groups, eliminating water. Consequently, upon addition of a protonic zeolite in inter- and intrapellet mixtures with the MOF, we observed increases in CH3OH selectivity with increasing ratio and proximity of zeolitic H+ to MOF-based Fe(II) sites, as methanol is protected within the zeolite. We infer from the data that CH3OH(g) is formed via the radical-rebound mechanism on Fe(II) sites but that subsequent transport and dehydration steps are required to protect CH3OH(g) from overoxidation. The results demonstrate that the radical-rebound mechanism commonly invoked in this chemistry is insufficient to explain the reactivity of these systems, that the selectivity-controlling steps involve both chemical and physical rate phenomena, as well as offering a strategy to mitigate overoxidation in these and similar systems.
ABSTRACT
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 overoxidation 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 illustrate 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)âO intermediate. The speciation of Fe(II) and Fe(III) in the nodes and their dynamic interchange was characterized by in situ X-ray absorption spectroscopy and ex situ Mössbauer spectroscopy. The catalytic relevance of Fe(II) sites and the number of such sites were 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 show that Fe-trimer nodes affecting reaction contain antiferromagnetically coupled iron species, and highlight the importance of stabilizing high-spin (S = 2) Fe(II) species for effecting alkane oxidation at low temperatures (<408 K).
