Design and synthesis of diverse Cd 2+ /Zn 2+ /Cu 2+ 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 Cd ²⁺ /Zn ²⁺ /Cu ²⁺ metal salts as nodes. These polymers were formed through self-assembly of four different dicarboxylic acid ligands, namely adamantane-1,3-dicarboxylic acid (H ₂ adc), glutaric acid (H ₂ glu), 5-hydroxyisophthalic acid (H ₂ hip) and fumaric acid (H ₂ fum), in conjunction with the auxiliary ligand [1,4-bis(pyridin-4-ylmethyl)piperazine (bpmp). The corresponding formulae are [Cd ₃ (adc) ₂ (bpmp)Cl ₂ (H ₂ O) ₂ ] _n (1), {[Cd ₂ (glu) ₂ (bpmp) ₂ (H ₂ O) ₂ ]·8H ₂ O·2CH ₃ OH} _n (2), [Zn(hip)(bpmp)(H ₂ O)] _n (3) and [Cu(fum)(bpmp)(H ₂ O) ₂ ] _n (4). Single-crystal X-ray diffraction studies revealed that the Cd ²⁺ centers in complex 1 all adopt a six-coordinate mode but two distinct {CdO ₂ N ₂ Cl ₂ } and {CuO ₅ Cl} units. The 3D network of complex 1 can be simplified to a binodal (4.6)-connected underlying net with the point symbol (3·4 ² ·5·6 ² ) ⁴ (3 ² ·6 ² ·7 ² ·8 ⁸ ·10). Each Cd ²⁺ cation in complex 2 adopts a seven-coordinate {CdO ₅ N ₂ } 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 Zn ²⁺ ion in complex 3 displays a {ZnO ₃ N} four-coordinate unit. Its stacking structure is formed by one-dimensional [Zn(hip)(bpmp)(H ₂ O)] _n chains connected through hydrogen bonds. On the other hand, complex 4 features a Jahn-Teller distorted {CuO ₄ N ₂ } 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.