Selective binding of O₂ over N₂ in a redox-active metal-organic framework with open iron(II) coordination sites

Abstract

The air-free reaction between FeCl₂ and H₄dobdc (dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe₂(dobdc)·4DMF, a metal-organic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a Brunauer-Emmett-Teller (BET) surface area of 1360 m²/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated FeII centers. Gas adsorption isotherms at 298 κ indicate that Fe₂(dobdc) binds O₂ preferentially over N₂, with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O₂ molecule per two iron centers. Remarkably, at 211 K, O₂ uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O₂ molecule per iron center. Mossbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O₂ at low temperature and complete charge transfer to form iron(III) and O₂^2- at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O₂ bound to iron in a symmetric side-on mode with d_O-O = 1.25(1) Å at low temperature and in a slipped side-on mode with d_O-O = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe₂(dobdc) to be a promising material for the separation of O₂ from air at temperatures well above those currently employed in industrial settings.