No 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.

Six metal-organic frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) structure type are demonstrated to bind carbon monoxide reversibly and at high capacity. Infrared spectra indicate that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C-O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions. Structure determinations reveal M-CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M-C-O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms indicate reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm(3)/cm(3). The six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving-Williams stability order. The reversible CO binding suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H2 and CO/N2. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas compositions at 1 bar and 298 K indicate that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases.