Visualizing the distinctly different crystal-to-crystal structural dynamism and sorption behavior of interpenetration-direction isomeric coordination networks

Abstract

We show here that the interpenetration direction can be more important than the interpenetration number for the porosity, stability, framework flexibility and sorption behaviors of porous coordination frameworks. Solvothermal reactions of Zn(NO₃)₂ and a shape-asymmetric ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H₂mpba) in different solvents/templates gave three isomeric frameworks [Zn(Hmpba)₂] (1, 2, 3) possessing the same polar dia networks and 4-fold interpenetration. However, their polar nets adopt parallel, orthogonal, and anti-parallel arrangements, respectively, giving very similar voids but totally different pore shapes for 1 and 2, and a nonporous structure for 3. Thermogravimetry, powder X-ray diffraction, and sorption analyses revealed remarkably different framework flexibilities and multi-step gas sorption behavior, in which 1 selectively adsorbs CO₂ over N₂, 2 adsorbs both CO₂ and N₂, while 3 adsorbs neither CO₂ nor N₂. Although these compounds cannot retain their single-crystallinity after structural transformations and their complicated structures hinder conventional structural characterization techniques, we successfully combined first-principle calculations and computational simulations to solve the crystal structures of their activated phases, and developed a dynamic computational simulation method combining sorption simulated annealing, molecular mechanics and grand canonical Monte Carlo modelling to perfectly simulate the CO₂ adsorption/desorption isotherms and visualize the accompanying continuous/abrupt structural transformations, demonstrating the important role of interpenetration-direction isomerism in functional porous materials.