Selective Binding of O_2 over N_2 in a Redox-Active Metal-Organic Framework with Open Iron(ll) Coordination Sites

摘要

The air-free reaction between FeCl_2 and H_4dobdc (dobdc~(4-) = 2,5-dioxido-l,4-benzenedicarboxylate) in a mixture of N,N-dimethylforma-mide (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 Fe_2(dobdc) binds O_2 preferentially over N_2, with an irreversible capacity of 9.3 wt 96, corresponding to the adsorption of one O_2 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 O_2 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 O_2 bound to iron in a symmetric side-on mode with d_(O-O) = 1-25(1) A at low temperature and in a slipped side-on mode with d_(O-O) = 1.6(1) A 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 O_2 from air at temperatures well above those currently employed in industrial settings.