Coordination Polymers from 2-Chloroterephthalate Linkers: Synthesis, Structural Diversity, and Catalytic CO2 Fixation

Abstract

In contrast to the broad use of terephthalic acid as a building block for generating metal-organic architectures, functionalized terephthalate ligands remain significantly less investigated. In the present study, 2-chloroterephthalic acid (CltaH2) was used as an unexplored linker for assembling a new series of metal(II) coordination polymers (CPs) formulated as {[Cu2(μ-Clta)(μ4-Clta)(bipy)2]·2H2O}n (1), {[Cu2(μ-Clta)(μ4-Clta)(phen)2]·2H2O}n (2), [Co3(μ4-Clta)3(phen)2]n (3), {[Zn4(μ3-Clta)2(μ4-Clta)(μ3-OH)2(phen)2]·2H2O}n (4), {[Cd(μ3-Clta)(phen)]·H2O}n (5), [Co2(μ-Clta)(μ4-Clta)(μ-bpa)2]n (6), [Zn2(μ4-Clta)(μ4-biim)]n (7), and [Cd(μ-Clta)(H2biim)2]n (8). These compounds were prepared hydrothermally from the respective metal(II) chlorides, CltaH2, and different N-donor supporting ligands acting as mediators of crystallization (i.e., bipy, 2,2′-bipyridine; phen, 1,10-phenanthroline; bpa, bis(4-pyridyl)amine; H2biim, 2,2′-biimidazole). The structures of the coordination polymers 1-8 range from one-dimensional chains (5 and 8) and two-dimensional layers (1-3) to three-dimensional (3D) frameworks (4, 6, and 7); the latter also include 3D + 3D interpenetrated examples (4, 6). Apart from different dimensionalities, the compounds also feature distinct topologies, namely, 3,4L13 in 1 and 2, 3,4,6L12 in 3, pcu in 4, SP 1-periodic net (4,4)(0,2) in 5, sqc65 in 6, 2C1 in 8, and a new kir1 topology in 7. Given the similarity of the reaction conditions, the obtained wide diversity of structures is primarily determined by the type of metal(II) node and/or crystallization mediator. All the CPs were also screened as heterogeneous catalysts in a model CO2 fixation reaction, namely, in the coupling of epoxide (epichlorohydrin) with carbon dioxide, under atmospheric pressure, and in the absence of added solvent, to produce the corresponding cyclic carbonate. The catalytic reactions were optimized for CP 4, which revealed the best activity (up to 71% product yields), good stability, and recyclability for up to four reaction steps. Finally, CPs 3-8 are the first examples of cobalt, zinc, and cadmium coordination compounds assembled from 2-chloroterephthalic acid, thus widening its very limited application in the design of functional metal-organic architectures. ©