Zn(ii) metal–organic architectures from ether-bridged tetracarboxylate linkers: assembly, structural variety and catalytic features

Abstract

Semi-flexible aromatic polycarboxylic acids are gaining impetus in crystal engineering of functional coordination polymers (CPs). This work opens up the use of two ether-bridged tetracarboxylic acids, 3-(2,5-dicarboxyphenoxy)phthalic acid (H₄dpa) and 5-(2,5-dicarboxyphenoxy)isophthalic acid (H₄dia), as versatile and still unexplored linkers for the synthesis of new Zn(II) coordination polymers, formulated as [Zn(μ-H₂dpa)(bipy)(H₂O)]_2n·nH₂O (1), [Zn₂(μ₄-dpa)(bipy)₂(H₂O)]_n (2), [Zn₂(μ₃-dpa)(μ-bpa)₂]_n·nH₂O (3), [Zn₂(μ₄-dia)(bipy)₂]_n (4), and [Zn₂(μ₇-dia)(py)₂]_n (5). These compounds were prepared via a facile hydrothermal procedure using ZnCl₂, H₄dpa or H₄dia linkers, and supporting N-donor ligands (2,2′-bipyridine, bipy; bis(4-pyridyl)amine, bpa; or pyridine, py) acting as crystallization mediators. Compounds 1–5 were fully characterized and their X-ray crystal structures were established, disclosing the metal–organic architectures that range from 1D chains (1, 2, 4) to 2D layers (3) and 3D framework (5). Both the structural and topological features of 1–5 were underlined, indicating their dependence on the type of linker and/or mediator of crystallization. Thermal, emission, and catalytic properties of 1–5 were also investigated. In particular, the catalytic potential of the obtained CPs in the Knoevenagel condensation of benzaldehydes with propanedinitrile was evaluated, disclosing an excellent performance of several heterogeneous catalysts with up to 99% product yields. All the CPs obtained herein are the first examples of structurally characterized coordination compounds assembled from H₄dpa and H₄dia. The present study thus opens up the application of these ligands as novel ether-bridged tetracarboxylate linkers in designing functional crystalline materials.