Two metal–organic frameworks based on a flexible benzimidazole carboxylic acid ligand: selective gas sorption and luminescence

Abstract

Two new three-dimensional (3D) porous metal–organic frameworks (MOFs), {[Cd₂(L)(NMP)(H₂O)₂]·2H₂O}_n (1) and {[Zn(L)_0.5(H₂O)]·NMP·H₂O}_n (2), have been solvothermally synthesized by using Zn(II) and Cd(II) ions with H₄L (H₄L = 1,4-di(5,6-bicarboxylbenzimidazol-lylmethyl)benzene) in different ratios of H₂O and NMP (NMP = 1-methyl-2-pyrrolidinone) solvents, all of which have been characterized by elemental analysis, FT-IR spectroscopy, powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). As revealed by structural analysis, the H₄L ligand took different coordination fashions in 1 and 2, and as a result, two MOFs with a novel (3,4,7)-connected trinodal net and (3,6)-connected binodal net were obtained, respectively. The structure comparison reveals that the coordination modes of ligands and the types of metal ions play decisive roles in regulating the final networks of MOFs. The desolvated frameworks 1 and 2 possess polar channels decorated with uncoordinated carboxylate groups, leading to an excellent ability to attract CO₂ as well as significant adsorption selectivity for CO₂ over CH₄ at 273 and 298 K. Solid state 1 and 2 show blue photoluminescence.