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).