Origin of the Strong Interaction between Polar Molecules and Copper(II) Paddle-Wheels in Metal Organic Frameworks

The copper paddle-wheel is the building unit of many metal organic frameworks. Because of the ability of the copper cations to attract polar molecules, copper paddle-wheels are promising for carbon dioxide adsorption and separation. They have therefore been studied extensively, both experimentally and computationally. In this work we investigate the copper-CO₂ interaction in HKUST-1 and in two different cluster models of HKUST-1: monocopper Cu(formate)₂ and dicopper Cu₂(formate)₄. We show that density functional theory methods severely underestimate the interaction energy between copper paddle-wheels and CO₂, even including corrections for the dispersion forces. In contrast, a multireference wave function followed by perturbation theory to second order using the CASPT2 method correctly describes this interaction. The restricted open-shell Møller-Plesset 2 method (ROS-MP2, equivalent to (2,2) CASPT2) was also found to be adequate in describing the system and used to develop a novel force field. Our parametrization is able to predict the experimental CO₂ adsorption isotherms in HKUST-1, and it is shown to be transferable to other copper paddle-wheel systems.