Synthesis,Structure And Characterization Of Metal-pyridylimidazoledicarbozylic Frameworks

Materials science, information technology and energy are widely recognized as three of the main pillars of modern high-tech industries. The sustainable development and application of functional materials is therefore extremely important for the development of human society. Metal-organic frameworks (MOFs) have recently emerged as a novel class of porous materials that are highly modular, highly crystalline and amenable to design, and exhibit controllable pore structure. Many MOFs have been shown to exhibit permanent porosity with large surface areas whereby the framework maintains the stability after the removal of guest molecules. The unique characteristics of MOFs render them more suitable in a more diverse range of applications as compared to other porous materials such as activated carbon and zeolites. Such applications include gas storage, gas selective adsorption, and drug storage or delivery. Accordingly, MOFs have attracted great attention, from both academics and industry, and have been developed into one of the hottest topics in materials research. Rational control over the design and synthesis of targeted structures often leads to predisgned structures. However, unprecedented or unexpected results are frequently obtained even for the assembly of the simplest molecular building blocks. It therefore remains an ongoing challenge to absolutely predict the outcome each time. It is therefore critical at this stage to accumulate a large database of synthetic data in order to explore the diverse synthetic conditions that yield specific structures and learn more about the synergy between the synthesis and structure. This will ultimately aid us to better direct the rational design and controllable synthesis of functional MOFs and will have a far-reaching significance.With these principles in mind, we modified an organic linker, i.e. 4,5-imidazoledicarboxylate ligand (HalmDC), by appending a pyridyl moiety at the 2-position of the imidazole ring. The resultant ligand, pyridine-4,5-imidazole dicarboxylic acid (H3PylmDC), permits control of the N atom positions on the pyridyl group and carboxylate atoms on the imidazole ring to yield a ligand capable of T-shape coordination modes. Because of the introduction of pyridine groups, a new coordination point N atom has been added, and the charge distribution of organic ligand also changed, resulting in a certain space effect, which will be favorable for constructing a number of novel MOFs. Therefore, reacting two types of H3PyImDC ligand with transition or lanthanide metal ions,respectively, we get the following three aspects results.1. The assembly of H3PyImDC and copper ions yields two chiral MOFs with bmn and lcy-a topology 1 and 2.The copper ion in compound 1 is divalent; whereas,it exists in two mixed valence states in compound 2,i.e. CuII and CuI, with a ratio of 2:1. The coordination modes of the ligand are not the same in compounds 1 and 2; that is, the former assumes a T-shape coordination mode while the latter exhibits two coordination modes of both T-shape and linear. Two independent and opposite ehiral helical channels also exist and are formed by the cross-winding spiral chains. The source of the metal ion, solvent, template, and chiral induction agent used in the synthesis for 1 and 2 were systematically investigated. It was determined that the synthesis conditions had indeed no effect on the generation of the chiral frameworks. Structural analysis reveals that the N atom on the pyridyl ring coordinates with the metal ions in a flexible manner and therefore it permits a certain amount of distortion in the T-shape coordination mode. This feature facilities the formation of two chiral structures. In the view of energy, the T-shape distortion effect reduces the activation energy of the system so as to meet the minimum of energy requirements. Two compounds can successfully exchange with guest molecules, and they also have fluorescence properties, which will provide a foundation for their applications for further research.2. The molecular building blocks (MBBs) approach was employed in this study to further establish a relationship between targeted MBBs in MOFs and the topologies observed in traditional inorganic zeolites. Accordingly, we chose to react H3PyImDC with transition metal ions, through the fine regulation of the synthesis conditions, to obtain rigid tetrahedral MBBs, whose self-assembly yielded a class of MOFs 3-5b, with 3D netswork bbm, and zeolite-like dft and gis topology, respectively. Each transition metal ion assumes an octahedral MN3O3 coordination mode and connects to four independent metal centers through the organic ligands to form a 4-connected MBB which translates into a tetrahedral (a metal atom occupies the center of the tetrahedron, the other four metal atoms occupy four vertices of tetrahedral) secondary building units (SBUs). The deprotonated HpylmDC2- ligand coordinates to the metal ions in a T-shape mode, with three N atoms and two O atoms of the carboxylic acid groups chelating to the metal centers. Accordingly, this coordination mode fully utilizes coordinating atoms, and makes the three metal ions in a plane, which is more conducive to generate tetrahedral SBUs；and is more convenient for constructing the zeolite-like MOFs as well. Comparison of the pore structure of compounds 3-5b by analyzing the structures in Materials Studio software, found that they all have large surface area and open pore structure, providing a basis for the potential application in gas adsorption and storage. The fluorescent and magnetic experiments conducted on these materials reveal that compounds 3 and 4 exhibit a strong excitation effect at room temperature, while compound 5a has an anti-ferromagnetic property in the 1000Oe field.3. The assembly of two different H3PylmDC ligand with rare earth and transition metal centers, through theπ-πand intermolecular hydrogen bonds interactions to yield a series of novel three-dimensional (3D) supramolecular compounds 6a-6d and 9, and a mineral-like ant and a four cones sqp topology structure MOFs 7a-7d and 8a-8d. Comparison of the coordination modes of two organic ligands show that when not fully coordinated with the metal ions, it is more likely to form ID chain structures; when fully coordinated, it is tending to generate 3D network structures. Therefore, the structures often develop from OD to 3D with the increasing coordination atoms. We also predict that the assembly of two different organic ligands with metal center will produce more types of MBBs, and when they are assembled with metal centers, more new structures will generated. The magnetic and fluorescent properties study on compounds 6-9 were carried out by which we found that the different structures lead to the different fluorescence phenomenon, and the reason for the difference, which provide a certain amount of experimental data and theoretical basis for the potential applications of such materials. Magnetic measurements performed on compounds 6 and 7 reveal they display obvious anti-ferromagnetic properties.The results described herein summarize the synthesis, topological analysis and fluorescence properties of an assortment of MOFs constructed using metal-KbPylmDC as organic linkers. A general relationship between the product and synthesis method has been established and we hope that this accumulation of experimental data will be valuable for the continuous development of functional MOFs.