A cubic coordination framework constructed from benzobistriazolate ligands and zinc ions having selective gas sorption properties

Abstract

Two novel metal coordination polymers, [Zn₅Cl₄(BBTA)₃]·3 DMF (1), and [ZnCl(BBTA)_0.5(DMA)] (2) {H₂-BBTA = 1H,5H-benzo(1,2-d:4,5-d′)bistriazole}, have been synthesized under solvothermal conditions using ZnCl₂ and H₂-BBTA in DMF (DMF = N,N′-dimethylformamide) or DMA (DMA = N,N′-dimethylacetamide). Moreover, a highly efficient microwave synthetic route has been developed for 1. The structures of both compounds have been determined by single crystal X-ray diffraction. Compound 1 represents the first example of a novel family of cubic microporous metal–organic frameworks (MFU-4; Metal-Organic Framework Ulm University-4), consisting of dianionic BBTA^2– linkers and pentanuclear {Zn₅Cl₄}⁶⁺ secondary building units, whereas compound 2 forms a dense 2D layered framework. Phase purity of both compounds was confirmed by X-ray powder diffraction (XRPD), IR spectroscopy, and elemental analysis. TGA and variable temperature XRPD (VTXRPD) experiments carried out on 1 indicate that solvent molecules occluded in the large cavities of 1 can be removed at a temperature >250 °C in high vacuum without significant loss of crystallinity, giving rise to a metal–organic framework with void cavities. Due to the small diameter of the aperture joining the two types of cavities present in 1, the diffusion of guest molecules across the crystal lattice is largely restricted at ambient conditions. Compound 1 therefore exhibits a highly selective adsorption for hydrogenvs. nitrogen at −196 °C. The framework is stable against moisture and has a specific pore volume of 0.42 cm³ g⁻¹ estimated from the water adsorption isotherm.