Adsorbate-Induced Changes in Magnetic Interactions in Fe₂(dobdc) with Adsorbed Hydrocarbon Molecules

Metal-organic frameworks, which are a special case of coordination polymers, form a class of materials with numerous applications due to their high porosities and large internal surface areas and in some cases also due to paramagnetic metal ions. A family of such materials of general formula M₂(dobdc) (where M is a divalent metal ion, and dobdc^4- is 2,5-dioxido-1,4-benzenedicarboxylate) has attracted considerable attention for gas separation, catalysis, and magnetism. In this work, we explore the magnetic properties of a member of this family, Fe₂(dobdc), both in the activated form (bare MOF after solvent removal) and when hydrocarbons are bound to the open coordination sites of the metal. We report quantum mechanical electronic structure calculations using both cluster models and periodic models, and we compare our results to previously reported theoretical studies. We find that hydrocarbon adsorption only mildly affects the isotropic couplings but that the isotropic magnetic couplings obtained with hybrid exchange-correlation functionals for the cluster models were found to be in good agreement with the experimental values, and the ones obtained using local exchange-correlation functionals with empirical Coulomb and exchange integrals for the periodic models were also found to agree well with experiments. Furthermore, local density functionals with empirical Coulomb and exchange integrals for periodic models are found to give good agreement with the experimental result that the adsorption of ethylene changes the magnetic ordering. We also used second-order n-electron valence state perturbation theory and contracted spin-orbit configuration interaction to study the role of adsorption on the crystal-field splitting of the quintet manifold and on the single-ion anisotropy of the iron center.