Coordination Polymers Based On Biphenyl-2,5,2',5'-tetracarboxylic Acid: Preparation, Crystal Structures And Properties

In recent years, the coordination polymers have been the keystone of the coordination chemistry and the crystal engineering because of their intriguing and novel architectures as well as their tremendous potential applications for magetism, gas storage, catalysis, nonlinear optics, biology, and so on. Many scientists are devoting to this field and innumerable coordination compounds have been synthesized. Among these, preparing coordination polymers through the reaction of the aromatic carboxylic acid ligand with the metal ions has been the focus of research in the coordination chemistry field, the carboxyl groups have strong bridging ability and rich coordination modes, aromatic carboxylic acid is conducive to the existence of the largeÏ€electron system, which all provide excellent conditions for synthesizing coordination polymers based on the aromatic carboxylic acid ligands.In this paper, we choose biphenyl-2,5,2',5'-tetracarboxylic acid as the main ligand and rigid N-donor ligands as the auxiliary ligand with different transition metals to generate eight compounds. The biphenyl-2,5,2',5'-tetracarboxylic acid ligand possesses the below properties: it has four carboxyl groups that may be completely or partially deprotonated to generate bptc^4-, Hbptc^3-, H₂bptc^2- or H₃bptc- anionic depending on the pH value; in addition, the torsion of two benzene rings around C–C single bond and the twisted configuration of the carboxylate groups provide the potential capability to form helical structures. Meanwhile, biphenyl-2,5,2',5'-tetracarboxylic acid is rarely used in preparing coordination polymers.Eight complexes have been synthesized under hydrothermal method by changing the molar ratio of reactants, pH value, temperature and reaction time and so on. The crystal structures have been characterized by single crystal X-ray diffraction analysis, elemental analysis, IR spectra, powder X-ray diffraction, and qualitative analyzed by the thermogravimetric analysis, fluorescence properties as well as the magnetic properties. The experiment results as follows:1. Five new compounds have been synthesized by biphenyl-2,5,2',5'-tetracarboxylic acid as main ligand and 2,2'-bipyridine as auxiliary ligand with Mn^â…¡, Ni^â…¡, Zn^â…¡, Cu^â…¡ ions under hydrothermal method, namely, [Mn(H₂bptc)(2,2'-bpy)₂]·2H₂O (1), [Mn₃(Hbptc)₂(2,2'-bpy)3(H₂O)8]·2H₂O (2), [Ni₂(bptc)(2,2'-bpy)₂(H₂O)₆]·2H₂O (3), [Zn₂(bptc)(2,2'-bpy)₂(H₂O)₆]·3H₂O (4) and [Cu₂(bptc)(2,2'-bpy)₂(H₂O)]·H₂O (5). Compounds 1–4 are 0D structures. Compound 5 exhibits 1D wavelike [Cu₂(bptc)]_n chains. Compound 4 is isostructural with 3. Interestingly, the non-covalent interactions (O–H···O, C–H···O, C–H···πandπ···πinteractions) extend compounds 1–5 into 3D supramolecular networks. In addition, magnetic properties for 1, 2 and 5 and fluorescence properties in solid state for 3, 4 have been investigated, respectively. Complexes 1, 2 and 5 show antiferromagnetic behaviour. Compared with the emission spectrum of H4bptc, the emission band of 3 shows red shift and that of 4 shows blue shift.2. Three new compounds based on biphenyl-2,5,2',5'-tetracarboxylic acid have been constructed by using 4,4'-bipyridine as the second ligand with Cu^â…¡, Ni^â…¡, Co^â…¡ ions under hydrothermal methods. Namely, {[Cu₂(bptc)(H₂O)₆]·11H₂O}_n (6), {[Ni₂(bptc)(H₂O)₆]·11H₂O}_n (7) and {[Co₂(H₂bptc)₂(4,4'-bpy)₂(H₂O)₂]·4H₂O}_n (8). The three compounds all shows 2D layer structures. Compounds 6 and 7 are isomer. Compounds 6–8 are linked into 3D supramolecular networks by the non-covalent interactions. Meanwhile, we discuss the magnetism for 6, 8 and fluorescence properties in solid state for 6–8, compounds 6 and 8 all exhibit antiferromagnetic behaviour. Compared with the emission spectrum of H4bptc, red shifts of emission bands for 6'8 have been observed.