Selective Binding of O2 over N2 in a Redox-Active Metal-OrganicFramework with Open Iron(II) Coordination Sites

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) 34DMF, a metalorganic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a BrunauerEmmettTeller (BET) surface area of 1360 m2/g and features a hexagonal array of onedimensional channels lined with coordinatively unsaturated FeII centers. Gas adsorption isotherms at 298 K 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. M€ossbauer and infrared spectra are consistent with partial charge transfer from iron(II) toO2 at low temperature and complete charge transfer to form iron(III) andO2 2 at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealingO2 bound to iron in a symmetric sideon mode with dOO = 1.25(1) Å at low temperature and in a slipped side-on mode with dOO = 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.