Oxidation of ethane to ethanol by N2O in a metal–organic framework with coordinatively unsaturated iron(II) sites

Abstract

Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although nature's non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.

Understanding Small‐Molecule Interactions in Metal–Organic Frameworks: Coupling Experiment with Theory

“…A recent study by Xiao et al [ 94 ] , further supporting the observation of mixed coordination. When heating the N 2 O-dosed framework to 348 K, there is an irreversible transformation to Fe 2 (OH) 2 (dobdc) with an Fe-OH distance of 1.91(1) Å, a value that was further corroborated both by extended X-ray absorption fi ne structure (EXAFS) measurements and periodic DFT calculations.…”

“…2015, 27, 5785-5796 www.advmat.de www.MaterialsViews.com Figure 6. Ball and stick model of Fe 0.1 Mg 1.9 (dobdc) [ 94 ] showing ethane conversion to ethanol after framework oxidation with N 2 O, which likely forms a transient Fe(IV)-oxo. The green, orange, grey, red, dark blue, and light blue spheres are Mg, Fe, C, O, N, and D, respectively.…”

“…Both methods predict the existence of a quintet ground state with a short Fe-O bond distance of 1.64 Å, a value consistent with previously characterized iron(IV)-oxo species. [ 94 ] It should be additionally noted that the authors synthesized a material that was diluted with Fe 2+ , Fe 0.1 Mg 1.9 (dobdc), in the hope of separating the reactive sites and inhibiting other side reactions. Indeed, this yielded the exclusive formation of ethanol and acetaldehyde (in a 10:1 ratio).…”

An Overview of Recent Advances of the Catalytic Selective Oxidation of Ethane to Oxygenates

“…A biomimetic system for alkane oxidation in the gas phase was reported by Xiao et al, who employed N 2 O as the oxidant for ethane conversion, catalysed by the metal organic frameworks Fe 2 (dobdc) and Fe 0.1 Mg 1.9 (dobdc) where dobdc 4´= 2,5-dioxido-1,4-benzenedicarboxylate [90].…”

“…A biomimetic system for alkane oxidation in the gas phase was reported by Xiao et al, who employed N2O as the oxidant for ethane conversion, catalysed by the metal organic frameworks Fe2(dobdc) and Fe0.1Mg1.9(dobdc) where dobdc 4− = 2,5-dioxido-1,4-benzenedicarboxylate [90]. Reaction of the high spin iron (II) centres within Fe2(dobdc) with N2O was proposed to form a transient iron Hydrogen was shown to reduce the catalyst surface to generate Fe (II) and H + which is absorbed by a neighbouring phosphate group.…”