Accurate Ab Initio Description of Adsorption on Coordinatively Unsaturated Cu2+ and Fe3+ Sites in MOFs
Abstract
The performance of different exchange-correlation functionals was evaluated for the description of the interaction of small molecules with (i) cluster models containing Cu2+ and Fe3+ coordinatively unsaturated metal sites and (ii) HKUST-1 metal organic framework (MOF). Adsorbates forming dispersion-bound complexes (CH4), complexes with important dispersion and electrostatic contributions (H-2, N-2, CO2), and complexes stabilized also by a partial dative bond (CO, H2O, and NH3) were considered.
The interaction with coordinatively unsaturated sites was evaluated with respect to the coupled-cluster calculations for Cu2+ and Fe3+ centers represented by cluster models. The adsorption on dispersion-stabilized sites was examined for the cage-window and the cage-center sites in HKUST-1 with respect to the experimental and DFT/CC results.
None of the functionals considered can accurately describe the interaction of all seven adsorbates with Cu2+ and Fe3+ sites and with dispersion-dominated adsorption sites. The interaction with coordinatively unsaturated sites was frequently underestimated, for adsorbates forming a partial dative bond in particular, while the adsorption at dispersion-stabilized sites was overestimated.
Consequently, interaction energies calculated for different adsorption sites were often in qualitatively incorrect order. The optimal exchange-correlation functional for a particular adsorbate/MOF can thus be found by comparing the performance of various functionals with respect to highly accurate calculations on smaller cluster models as a good representative of MOF structural building blocks.