3D Zinc–Organic Frameworks Based on Mixed Thiophene Dicarboxylate and 4-Amino-3,5-bis(4-pyridyl)-1,2,4-triazole Linkers : Syntheses, Structural Diversity, and Single-Crystal-to-Single-Crystal Transformations

Abstract

In this study, for the first time two new interpenetrated 3D pillared-layer metal–organic frameworks (MOFs), namely, {[Zn(bpt)(tdc)]·dmf}n (MOF-1-Zn) and {[Zn2(bpt)(tdc)2]·2(dmf)}n (MOF-2-Zn), as well as a homochiral 3D framework {[Zn(bpt)(tdc)(H2O)]·dmf}n (MOF-3-Zn), were prepared under different synthesis conditions (bpt = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole, H2tdc = 2,5-thiophenedicarboxylic acid, and dmf = N,N′-dimethylformamide). Synthesis products were thoroughly characterized by single-crystal and powder X-ray diffraction and thermoanalytical methods. Remarkably, in the case of MOF-2-Zn, dmf molecules in the voids can be exchanged for different solvents, e.g., chloroform, methanol, ethanol, isopropyl alcohol, acetonitrile, acetone, tetrahydrofuran, 1,4-dioxane, and cyclohexane in a single-crystal-to-single-crystal (SCSC) manner. With different solvent types, the bpt ligands adapted slightly different conformations by adjusting the orientation of the NH2-triazole core unit and the tilting of the pyridyl groups. In comparison, most changes for planar tdc ligands were observed on the dihedral angles of the adjacent thiophene rings (e.g., 69.5 vs 78.5° for MOF-2-Zn and 2-CHCl3, respectively). Desolvation of the MOF illustrated systematic structural adaptability of {Zn2}-“paddlewheel” in SCSC fashion, and the MOF’s crystallinity and 3D networked structure were preserved even after vacuum and heat-assisted desolvation. Finally, preliminary CO2 adsorption tests for MOFs were conducted utilizing a custom-built fixed-bed adsorption/desorption device, resulting in about 0.1 mmolCO2/gsorbent at 400–4000 ppmv CO2 and about 1 mmolCO2/gsorbent at 15 vol % CO2.