Identifying the Interactions That Allow Separation of O₂ from N₂ on the Open Iron Sites of Fe₂(dobdc)

The presence of open metal sites along with a high porosity makes the Fe₂(dobdc) metal-organic framework, also called Fe-MOF-74, particularly well suited for separating gaseous mixtures. For instance, since Fe₂(dobdc) adsorbs O₂ more strongly than N₂, it can, in principle, be used to separate O₂ from air [ Bloch et al. J. Am. Chem. Soc. 2011, 133, 14814 ]. In the present work, we investigate the reversible differential adsorption of N₂ and O₂ on Fe₂(dobdc) with Kohn-Sham density functional theory applied to an 88-atom cluster model of the MOF. The cluster is chosen such that it is large enough to allow an accurate description of the most important contributions to the binding enthalpies and small enough to perform high-level quantum mechanical calculations. For the quantum mechanical calculations, we use well-validated exchange-correlation functionals to study the ground-state structures of the Fe-N₂ and Fe-O₂ interacting systems. The calculations agree with experiment in that O₂ binds more strongly than N₂, and they reveal that the ground-state structure of the Fe-O₂ subsystem has the dioxygen unit in a triplet spin state ferromagnetically coupled to the high-spin state (quintet state) of the iron center. Charge Model 5 (CM5) calculations have been performed to determine the partial atomic charges on the adsorbate molecules and the iron atom, and they show that charge transfer from the open iron(II) site is more important in the case of O₂ than in the case of N₂. Furthermore, bond orders, vibrational frequencies, and orbital energies were calculated to rationalize the stronger binding of O₂ compared to N₂ on Fe₂(dobdc).