Solvents regulate the packing porosity of a bilayer metal-organic cage

Metal-organic cages (MOCs) are an emerging class of porous materials with promising applications. However, controlling the configuration of the cage packing, which can influence the overall porosity of the materials, remains a difficulty, as many factors can influence the cage assembly and stacking. Herein, we report a solvent strategy to fine-tune the packing configuration of a bilayer MOC, a small triangular prism cage (six Cu ions act as vertices, three nitrate ions act as pillars, and six nitrate ions act as caps) incorporated into a large triangular prism cage (another six Cu ions act as vertices, a couple of oxygen atoms act as pillars and six ligands (L1: 3,5-bis(pyridine-3-yl)-4H-1,2,4-triazole) act as a jointed cap) by the coordination between the triazole nitrogen from L1 and the inner vertex Cu ions. The involved solvents water, acetonitrile (MeCN) and N,N′-dimethylformamide (DMF) form hydrogen bonds with this bilayer MOC, resulting in three different types of packing associated with systemically tuned porosity (NTU-93: 12.2%, NTU-94: 19.3%, and NTU-95: 42.1%). Gas adsorption and breakthrough tests demonstrate that NTU-95 has potential ability for C₂H₂/C₂H₄ separation. This work not only shows a case of finely tuned packing of coordination cages, but also provides a powerful tool that may be extended to other cage families.