Use of the ditopic ligand 1,4-benzenedi(1H-1,2,3-triazole) (H2BDTri) enabled isolation of two new three-dimensional metal–organic frameworks of formulae Cu(BDTri)L in which L=DMF (1) and diethylformamide (DEF; 2). These compounds have the same primary structure, featuring one-dimensional channels with the bridging DMF or DEF molecules pointing into the cavity. Upon exposure to solvent vapors, both display a reversible flexibility, as characterized by single-crystal to single-crystal phase transitions in 1. The O2 adsorption isotherms for the compounds show a two-step adsorption behavior associated with a permanent microporosity and a pore-opening process. In the case of N2 adsorption, only 1 exhibits a two-step adsorption isotherm, whereas 2 does not present any pore opening, demonstrating that design of a flexible framework cavity can control the pore opening and thereby possibly enhance O2/N2 separation.