Metal–Organic Frameworks Based on a Bent Triazole Dicarboxylic Acid: Magnetic Behaviors and Selective Luminescence Sensing Properties

Four new metal–organic frameworks (MOFs) constructed from the bent triazole dicarboxylic acid ligand 3,5-bis­(4′-carboxyphenyl)-1,2,4-triazole (H2bct) have been discovered by a solvothermal method. These MOFs, [M5(μ2-H2O)2­(μ3-OH)2­(bct)4­(H2O)8]­·4H2O} n (1, M = Co; 2, M = Ni), {[Cd2(bct)2­(bipy)]­·5H2O} n (3) (bipy = 4,4′-bipyridine), and {[Cd­(bct)­(tib)]­·H2O·DMF} n (4) (tib = 1,3,5-tris­(1-imidazolyl)­benzene), exhibit diverse coordination modes and fascinating crystal structures. The crystallographic structures, thermal stabilities, photoluminescence, and magnetic properties have been studied. 1 and 2 share the identical structural features based on planar pentanuclear clusters except for the central ions, where the one-dimensional chain structures are constructed from the pentanuclear secondary building units linked with bct2– ligands, while 3 and 4 have similar layered structures. Magnetic analyses of 1 and 2 reveal that an overall antiferromagnetic coupling within the pentanuclear Co­(II) cluster nodes are observed, which can be switched to strong ferromagnetic coupling when using Ni­(II) to replace the Co­(II) sites. Both 3 and 4 show intense blue emission under ultraviolet excitation. Moreover, 4 can be applied as a selective sensor for quantitatively testing the concentration of acetone in water based on the luminescence quenching effect, with a quenching constant (K sv) value of 12.89 L mol–1 in water.