Selective Binding of O₂ Over N₂ in a Redox-active Metal-organic Framework with Open Iron(II) Coordination Sites


The air-free reaction between FeCl 2 and H 4dobdc (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 Fe 2(dobdc) binds O 2 preferentially over N 2, with an irreversible capacity of 9.3 wt %, 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. Mössbauer 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) 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 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. © 2011 American Chemical Society.



Document Type

Article - Journal

Document Version


File Type





© 2011 American Chemical Society (ACS), All rights reserved.