Fluorinated Squareimines for Molecular Sieving of Aromatic over Aliphatic Compounds

Abstract

The development of more energy-efficient separation technologies is essential. Especially the separation of cyclic aliphatic hydrocarbons from their aromatic counterparts remains a significant challenge due to azeotrope formation and similar physical properties, often requiring energy-intensive processes. Herein, we introduce a novel class of electron-deficient macrocycles with a unique rectangular structure to optimise interactions within the pore, enabling the highly selective molecular sieving of aromatic compounds from mixtures. Utilising dynamic covalent imine chemistry, the squareimine NDI²F4²-based crystalline functional material is directly obtained from the reaction mixture in a single self-assembly step in high yields of 83 %, alongside the larger NDI²F8² congener, which can be obtained in 69 % yield. In vapour sorption and diffusion experiments, NDI²F4² demonstrates rapid adsorption kinetics with selectivities of 97 : 3 for benzene over cyclohexane and 93 : 7 for toluene over methylcyclohexane, while single-crystal and powder X-ray diffraction studies indicate that the selectivity is primarily governed by directed interactions between the electron-deficient panels and aromatic guests.