Synthesis, structures, and physical properties of metal flexible dicarboxylate frameworks with dipyridyl coligand

Five new metal-organic frameworks [Zn(bct)(4,4′-bipy)] (1) (H2bct = 2,5-bis(carboxymethylmercapto)-1,3,4-thiadiazole acid; 4,4′-bipy = 4,4′-bipyridine) [Cd(bct)(H2O)(4,4′-bipy)0.5] (2), as well as three isostructural compounds, [M(bct)(H2O)(4,4′-bipy)] [M = Zn (3), Mn (4), Co (5)], were obtained by reactions of the corresponding metal salts with a flexible dicarboxylic acid, H2bct and 4,4′-bipy, and their structures were determined by single-crystal X-ray diffraction. The results reveal that 1 features a two-dimensional (2-D) layer structure consisting of one-dimensional (1-D) Zn-bct chain and 4,4′-bipy, while 2 has 2-D rectangular-grid layers with the bct carboxylate groups adopting similar coordination mode in 1 but different conformations in ligating Cd(II) ions, which is inserted into 4,4′-bipy. Additionally, there exist π-π stacking and intermolecular hydrogen-bonding interactions in 1 and 2, respectively, forming 3-D supramolecular structures. In 3-5, undulating 2-D metal-bct layer structures are formed with another coordination mode of bct carboxylate groups, which are further extended by 4,4′-bipy into 3-D structures. The luminescence properties of 1-3 were investigated. Low-temperature magnetic data indicate weakly antiferromagnetic exchange in 4 and 5.