Selective Binding of O(2) over N(2) in a Redox-Active Metal-Organic Framework with Open Iron(II) Coordination Sites

The air-free reaction between FeCl2 and H4dobdc (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixt. of N,N-dimethylformamide (DMF) and methanol affords Fe2(dobdc)·4DMF, a metal-org. 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 unsatd. FeII centers. Gas adsorption isotherms at 298 K indicate that Fe2(dobdc) binds O2 preferentially over N2, with an irreversible capacity of 9.3%, corresponding to the adsorption of one O2 mol. per two iron centers. Remarkably, at 211 K, O2 uptake is fully reversible and the capacity increases to 18.2%, corresponding to the adsorption of one O2 mol. per iron center. Mossbauer and IR spectra are consistent with partial charge transfer from iron(II) to O2 at low temp. and complete charge transfer to form iron(III) and O22- at room temp. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O2 bound to iron in a sym. side-on mode with dO-O = 1.25(1) Å at low temp. and in a slipped side-on mode with dO-O = 1.6(1) Å when oxidized at room temp. Application of ideal adsorbed soln. theory in simulating breakthrough curves shows Fe2(dobdc) to be a promising material for the sepn. of O2 from air at temps. well above those currently employed in industrial settings.