Ligand Torsion Engineering in Three Flexible Metal–Organic-Frameworks Exhibiting Distinct Breathing Behavior for Enhancing C2H2 Capture

Acetylene is an important industrial gas for the production of vinyl chloride and 1,4-butynediol, but its storage remains a major challenge because it is highly explosive. Flexible metal-organic frameworks (FMOFs) are always at the forefront of porous materials due to the transformation of the structure under the external stimuli. In this work, divalent metal ions and multifunctional aromatic N,O-donor ligands were chosen, and three FMOFs [M(DTTA)₂]·guest [M = Mn (1), Cd (2), and Cu (3)] (H₂DTTA = 2,5-bis(1H-1,2,4-trazol-1-yl) terephthalic acid) have been successfully constructed. Single-crystal X-ray diffractions show that these compounds are isostructural and feature a three-dimensional framework. Topological analysis shows a (4, 6)-connected network with a Schläfli symbol of {4⁴.6¹⁰.8}{4⁴.6²}. All three compounds exhibit breathing behavior on N₂ adsorption at 77 K, and due to the difference of ligand torsion angles, compounds 2 and 3 exhibit extraordinary adsorptions for C₂H₂ of 101 and 122 cm³ g^-1 at 273 K under 1 bar, respectively. Compared with previous work, successfully obtaining compound 3 with an innovative structure can be attributed to the solvent-induced effect in the process of crystal synthesis, leading to the structure transformation promoting the significantly increased C₂H₂ adsorption performance. This study provides a platform for improvement of synthetic structures, which can effectively boost gas adsorption performance.