Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)–Oxo Sites in Magnesium-Diluted Fe₂(dobdc)

Abstract

The catalytic properties of the metal-organic framework Fe2(dobdc), containing open Fe(II) sites, include hydroxylation of phenol by pure Fe2(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron(IV)-oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe0.1Mg1.9(dobdc), activates the strong C-H bonds of ethane. Kohn-Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C-H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron-oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N2O to the Fe(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron(IV)-oxo catalytic mechanism. The energetics of the C-H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway.

Homogenous Meets Heterogenous and Electro‐Catalysis: Iron‐Nitrogen Molecular Complexes within Carbon Materials for Catalytic Applications

“…Though strictly speaking not a materials characterisation method, increasingly sophisticated systems increasingly rely on supporting understanding of characterisation techniques by computational modelling. This is none the least relevant for catalytic reactions, including single‐site catalysts, an area that has undergone a tremendous amount of progress thanks to method development, increased computational power . Computationally assessing catalytic activity by relating it to descriptors instead of calculating the activation energies of elementary reaction steps in heterogeneous catalytic reactions has been significantly relying on screening linear free energy relationships (LFERs), or linear scaling relations .…”

Metal–Organic Frameworks as Catalysts for Oxidation Reactions

“…The catalytic non‐heme iron‐catalyzed hydroxylation of the strong C−H bond of ethane was shown to proceed by a quintet single‐state σ‐attack pathway after the formation of highly reactive iron–oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N 2 O to the Fe II center . The uncatalyzed reaction of N 2 O with ethane to form ethanol was found to have an activation barrier of 280 kJ mol −1 , which is in contrast to 82 kJ mol −1 for the slowest step in the iron(IV)–oxo mediated process …”

“…Later, the same group also reported detailed mechanistic studies justifying how the catalytic material Fe 0.1 Mg 1.9 (dobdc) activates the strong C−H bonds of ethane . Kohn–Sham density functional and multireference wavefunction calculations were performed to characterize the electronic structure of the key active site.…”

“…[28] Later,t he same group also reported detailed mechanistic studies justifying how the catalytic material Fe 0.1 Mg 1.9 (dobdc) activates the strong CÀHb onds of ethane. [29] Kohn-Shamd ensity functional and multireference wavefunction calculations were performed to characterize the electronic structure of the key active site. The catalytic non-heme iron-catalyzed hydroxylation of the strongC ÀHb ond of ethane wass hown to proceed by aq uintet single-state s-attack pathway after the formation of highly reactive iron-oxo intermediate.…”

Direct Conversion of Benzothiadiazole to Benzimidazole: New Benzimidazole‐Derived Metal–Organic Frameworks with Adjustable Honeycomb‐Like Cavities

“…Although elucidating reaction mechanisms is very important for delicately modulating reactions and target products, in fact a large number of reaction mechanisms currently are still unclear because of the difficulty of monitoring reaction intermediates. Single‐crystal X‐ray diffraction (SCXRD) is a powerful tool for the structural analysis of reaction intermediates to reveal the nature of a reaction . Moreover, because of the particular features of solvothermal reactions, SCXRD analysis can also be used to observe the presence of new reactions.…”