A new isomeric porous coordination framework showing single-crystal to single-crystal structural transformation and preferential adsorption of 1,3-butadiene from C4 hydrocarbons

Abstract

Reaction of Zn(NO₃)₂ and 4-(3,5-dimethyl-1H-pyrazol-4-yl) benzoic acid (H₂mpba) in a mixed solvent of methanol and toluene gives a new supramolecular isomer for [Zn(Hmpba) ₂]•guest (1•g). Single crystal X-ray diffraction analysis showed that 1•g possesses a similar local coordination structure with the three known [Zn(Hmpba) ₂]•guest isomers with 4-fold interpenetrated diatopologies (dissimilar interpenetration directions, void ratio varies from 0% to 28%), but displays a rare 4-connected afw topology consisting of three- and seven-membered rings and remains a large void ratio of 36% even after 4-fold interpenetration. Interestingly, 1•g can undergo obvious framework contraction of 8.5% in the single-crystal to single-crystal manner upon guest removal to give [Zn(Hmpba) ₂] (1′). Comparison of the single-crystal structures of 1•g and 1′ showed that the structural transformation arises mainly from ligand conformation change. Single-component N₂, CO₂ and C4 hydrocarbon gas adsorption measurements for 1′ show different isotherm steps and gate-opening pressures, in which 1,3-butadiene is the lowest in the five C4 hydrocarbons, likely because of it has the smallest molecular cross-section area. Breakthrough simulations on mixed C4 hydrocarbons showed that 1,3-butadiene is preferentially adsorbed from other C4 hydrocarbons.