Detection of host–guest interactions in clathrates of heterocyclic molecules adsorbed in a porous MOF with Cu2 cluster nodes via vibration spectra and magnetic properties

Abstract

A three-dimensional NbO-type MOF with the formula Cu₂(EBTC)(H₂O)₂·[G] (1) (EBTC⁴⁻ = 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylate; G = guest molecules and represent DMF, DMSO and H₂O) was synthesized using a solvothermal method. The sufficient cavity space in 1 can be used to encapsulate the heterocyclic compounds benzopyrrole (C₈H₇N), benzofuran (C₈H₆O) and benzothiophene (C₈H₆S). The guest-free framework (2) was obtained by heating methanol-exchanged 1 at 130 °C under vacuum. The clathrates [Cu₂(EBTC)(H₂O)₂·1.6C₈H₇N]_∞ (3), [Cu₂(EBTC)(H₂O)₂·2.5C₈H₆O]_∞ (4) and [Cu₂(EBTC)(H₂O)₂·2C₈H₆S]_∞ (5) were prepared by soaking 2 in C₈H₇N, C₈H₆O and C₈H₆S at 55 °C for 48 h, respectively. 1–5 showed quite similar PXRD patterns, indicating that they have the same framework structure. The IR and Raman spectra and the variable-temperature magnetic susceptibilities were comparatively studied for 1–5; the results disclosed the existence of weakly intermolecular interactions between the –CC– in the organic linkers and guest molecules, whereas an almost complete absence of intermolecular interactions between the paddle-wheel-type dinuclear Cu₂ units and large guest molecules owing to steric hindrance. This study suggests the possibility of these rather useful MOF materials in the removal of organosulfur or organonitrogen compounds that are widely known contaminants in petroleum and fuels via their rationally designed organic linkers.