Design and synthesis of diverse Cd2+/Zn2+/Cu2+ coordination polymers tuned by dicarboxylate and auxiliary 1,4-bis(pyridin-4-ylmethyl)piperazine ligands with luminescence and Hirshfeld surface analyses.

Four new coordination polymers, including 1D, 2D and 3D structures, were synthesized via a hydrothermal method using Cd²⁺/Zn²⁺/Cu²⁺ metal salts as nodes. These polymers were formed through self-assembly of four different dicarboxylic acid ligands, namely adamantane-1,3-dicarboxylic acid (H₂adc), glutaric acid (H₂glu), 5-hydroxyisophthalic acid (H₂hip) and fumaric acid (H₂fum), in conjunction with the auxiliary ligand [1,4-bis(pyridin-4-ylmethyl)piperazine (bpmp). The corresponding formulae are [Cd₃(adc)₂(bpmp)Cl₂(H₂O)₂]_n (1), {[Cd₂(glu)₂(bpmp)₂(H₂O)₂]·8H₂O·2CH₃OH}_n (2), [Zn(hip)(bpmp)(H₂O)]_n (3) and [Cu(fum)(bpmp)(H₂O)₂]_n (4). Single-crystal X-ray diffraction studies revealed that the Cd²⁺ centers in complex 1 all adopt a six-coordinate mode but two distinct {CdO₂N₂Cl₂} and {CuO₅Cl} units. The 3D network of complex 1 can be simplified to a binodal (4.6)-connected underlying net with the point symbol (3·4²·5·6²)⁴(3²·6²·7²·8⁸·10). Each Cd²⁺ cation in complex 2 adopts a seven-coordinate {CdO₅N₂} center, forming an asymmetric pentagonal bipyramidal coordination. Its stacking structure is formed by the interaction of hydrogen bonds between 2D supramolecular layers, with the adjacent layers exhibiting mirror symmetry. Each Zn²⁺ ion in complex 3 displays a {ZnO₃N} four-coordinate unit. Its stacking structure is formed by one-dimensional [Zn(hip)(bpmp)(H₂O)]_n chains connected through hydrogen bonds. On the other hand, complex 4 features a Jahn-Teller distorted {CuO₄N₂} octahedral coordination. Subsequently, the thermal stability of these complexes was investigated. The solid-state fluorescence spectroscopy was employed to analyze complexes 1, 2 and 3. Additionally, a Hirshfeld surface analysis was performed on complex 3.