Coordination Polymers Assembled from Flexible Tetracarboxylate Ligands: Hydrothermal Syntheses, Structural Diversity, and Catalytic Application

In this work, a novel family of one-, two-, and three-dimensional coordination polymers (CPs) was synthesized via hydrothermal reaction between various metal(II) chloride precursors and the underutilized, conformationally adaptable tetracarboxylate ligand 5,5′-(pyridine-3,5-diyl)diisophthalic acid (H4pdda). To facilitate crystal growth and structural diversification, a range of aromatic N,N-donor coligands were incorporated, including 1,4-bis(4-pyridyl)benzene (bpb), 1,10-phenanthroline (phen), 2,2′-bipyridine (2,2′-bipy), 4,4′-bipyridine (4,4′-bipy), 2,2′-biimidazole (H2biim), and 1,2-di(4-pyridyl)ethylene (dpe). All resulting compounds were thoroughly characterized and designated as [Ni3(μ3-Hpdda)2(μ-bpb)3(H2O)4]n·4nH2O (1), [Mn2(μ6-pdda)(phen)2]n·3nH2O (2), [Ni2(μ4-pdda)(phen)2(H2O)4]n·4nH2O (3), [Co2(μ7-pdda)(2,2′-bipy)(H2O)]n (4), [Ni2(μ4-pdda)(μ-4,4′-bipy)(H2O)4]n·2nH2O (5), [Zn2(μ4-pdda)(μ-4,4′-bipy)(H2O)4]n (6), [Co(H2biim)2(H2O)2]n[Co(μ4-pdda)]n·2nH2O (7), and [Co2(μ7-pdda)(dpe)(H2O)2]n·3nH2O (8). Their structural and topological features were also explored, allowing to identify a diversity of 1D, 2D and 3D coordination networks. Remarkably, Zn-based coordination polymer 6 revealed high catalytic activity and reusability in the Knoevenagel condensation and cyanosilylation of benzaldehydes under optimized conditions. The study also investigated their structural architectures and topological arrangements, uncovering a broad spectrum of coordination frameworks─ranging from one-dimensional (1D) to two-dimensional (2D) and three-dimensional (3D) structures. Notably, zinc-centered coordination polymer 6 demonstrated exceptional catalytic performance and excellent recyclability in both Knoevenagel condensation and cyanosilylation reactions involving benzaldehyde derivatives, when conducted under carefully optimized reaction conditions. This study expands the structural diversity of coordination polymers (CPs) and metal–organic frameworks (MOFs) constructed using flexible polycarboxylate ligands, while also demonstrating their potential as efficient heterogeneous catalysts.