Zn(II) and Cd(II) metal–organic frameworks (MOFs) constructed from a symmetric triangular semirigid multicarboxylate ligand: Synthesis, structures and luminescent properties

Abstract

Three transition metal coordination polymers [Zn2(H2L)(2,2′-bpy)2(H2O)]n∙2nH2O (1), [Zn2(H2L)(2,2′-bpy)2]n (2), and [Cd2(H2L)(2,2′- bpy)2(H2O)2]n∙2nH2O (3), have been assembled from a semirigid triangular multicarboxylate ligand 3,3′,3″-(1,3,5-phenylenetri(oxy))triphthalic acid (H6L) with the help of 2,2′-bipyridine (2,2′-bpy) ligand. X-ray single crystal diffraction analysis reveals that complex 1 crystallizes in the space group of Pī and displays a one-dimensional (1D) ladder chain structure constructed from 2,2′-bpy ligand and H2L ligand, which stacks together in an -ABCABC- motif, featuring a mutually embedded chained structure. In complex 2, the H2L ligands bridge the adjacent Zn(II) atoms into a complicated ribbon chain along the b axis. There is π–π stacking interaction between the chains, which results in the formation of a 2D supramolecular structure. Complex 3 also exhibits a 1D ladder-like chain. The different molecular structures for complexes 1 and 2 formed from the same H6L and Zn(NO3)2∙6H2O in different metal-to-ligand ratios in the presence of NaOH, reveals the influence of metal–ligand ratio on the structure of the coordination polymer. In contrast, a series of same reaction using Cd(NO3)2∙4H2O as a starting material instead of Zn(NO3)2∙6H2O only led to the formation of 3, illustrating the fact organic ligands display different coordination preferences at different metal ions. In addition, the thermal and luminescent properties of complexes 1–3 were also investigated.