Coordination Polymers Assembled from Flexible Tricarboxylate Linkers: Hydrothermal Synthesis, Structural Diversity, and Catalytic Features.

The molecular design of coordination polymers (CPs) and metal-organic frameworks (MOFs) has attracted increasing attention in the areas of inorganic chemistry and functional materials. In this study, a new series of 2D CPs and 3D MOFs was hydrothermally assembled from metal(II) chlorides and 2,2'-((4-carboxy-1,2-phenylene)bis(oxy))diacetic acid (H₃cpbda) as a flexible and little-explored tricarboxylate linker. Additionally, several types of aromatic N,N-donor auxiliary ligands were used to promote crystallization, namely, 1,10-phenanthroline (phen), 4,4'-bipyridine (bipy), bis(4-pyridyl)amine (bpa), 1,2-di(4-pyridly)ethylene (dpey), or 1,2-di(4-pyridly)ethane (dpea). The obtained products were fully characterized and identified as [M₃(μ₆-cpbda)₂(phen)₂]n·4nH₂O (M = Zn (1), Cd (2)), [Co₃(μ₅-cpbda)₂(μ-bipy)₂]n·2nH₂O (3), [Zn₃(μ₅-cpbda)₂(μ-bipy)₂]n (4), [Zn(μ₃-cpbda)(Hbpa)]n·4nH₂O (5), [Zn₄(μ₃-cpbda)₂(μ-OH)₂(μ-dpey)₃(H₂O)₂]n·2nH₂O (6), [Co₃(μ₄-cpbda)₂(μ-dpey)₃]n·2nH₂O (7), and [Ni₃(μ₄-cpbda)₂(μ-dpea)₃]n·2nH₂O (8). Their structural and topological features were also explored, allowing us to identify a diversity of 2D and 3D coordination networks. Remarkably, Zn-based coordination polymers 5 and 6 revealed a high catalytic activity and reusability in the condensation reaction between benzaldehyde and malononitrile (or ethyl cyanoacetate), leading to almost quantitative product yields (99%) under optimized conditions. The present work contributes to widening the family of CPs/MOFs assembled from flexible polycarboxylate linkers and highlights a promising application of these compounds in heterogeneous catalysis.