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

The air-free reaction between FeCl2 and H(4)dobdc (dobdc(4) = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe-2(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 m(2)/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe-II centers. Gas adsorption isotherms at 298 K indicate that Fe2(dobdc) binds 02 preferentially over N-2, with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one 02 molecule per two iron centers. Remarkably, at 211 K, O-2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one 02 molecule per iron center. Mossbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O-2 at low temperature and complete charge transfer to form iron (III) and O-2(2-) at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing 02 bound to iron in a symmetric side-on mode with d(O-O) = 1.25(1) angstrom at low temperature and in a slipped side-on mode with d(O-O), = 1.6(1) angstrom when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe-2(dobdc) to be a promising material for the separation of 02 from air at temperatures well above those currently employed in industrial settings.