Research Of Metal-Organic Frameworks Based On Semi-Rigid Carboxylate Ligand

Recently metal-organic frameworks (MOFs) have attracted much more interest because of their versatile structures and the potential applications in gas adsorption and separation, magnetism, catalysis, ions exchange, drug delivery, fluorescence. MOFs are usually composed by secondary building units (SBUs) and bridging ligands. As the nature of the central metal ions in the SBU is very important to the properties of MOFs, it is important to take an appropriate method for modifying the central metal ion (including adulteration, substitution). On the other hand, the topology of the organic ligands also affectsthe structures of MOFs materials. Rigid or flexible ligands with a triazine ring have been reported widely due to their ease of synthesis and rich functional groups. However, MOFs based on carboxylate triazine derivative containing both rigid and flexible coordination arms have not been reported. Accordingly, two semi-rigid ligands containing both rigid and flexible coordination arms have been synthesized in this paper. Some MOFs compounds based on these ligands were obtained. Meanwhile, their structures and properties were characterized:1. Two semi-rigid ligands with different coordination arms H6La and H5Lb (H6La=5,5’-(6-(bis-(carboxymethyl)-amino)-1,3,5-triazine-2,4-diyldiimino) diisophthalic acid; H5Lb=5,5’-(6-aceticacid-amino-1,3,5-triazine-2,4-diyldiimino) diisophthalic acid) were designed, synthesized and characterized.2. The reactions between H6La and Mn2+, Co2+, Cd2+ions led to three isostructural compounds:(Me2NH2)1.5[Mn2.25La(H2O)2]-2.5DMF1.5H2O (1),(Me2NH2)16[Co2.2La(H20)2]-2.5DMF-4.5H2O (2),(Me2NH2)2[Cd2La(H2O)2]-1.5DMF-12.5H2O (3). Structural analysis showed that they exhibit (4,4)-connected UMC topology. Rigid and flexible coordination arms of the ligand play different roles in the structure. The former is used to support the skeleton and the latter helps to stabilize the structure and capture the free metal ions. Thermal stability, IR, and fluorescence properties of the compounds were characterized.3. A series of heterometallic compounds which are isomorphic to1were directly synthesized through the reactions of H6La and mixed metal salts Mn2++M’(M’=Co2+, Ni2+Cu2+, Zn2+, Cd2+). One of them (Me2NH2)1.5[Co1.125Mn1.125La(H2O)2] DMF-8.5H2O (4) was characterized by single crystal structure analysis. The influence of the ratio of the starting material on the products was investigated. In addition, Post synthesis metathesis reactions of1 were also conducted by soaking crystals of1in solutions of different metal ions (Fe3+, Co2+, Ni2+, Cu2+and Zn2+). Either almost completely or partially transmetalated isostructural compounds can be obatined by controlling the reaction condition (different metal salts, soaking time, solvent and anion of metal salts). The two methods are compared4. A new MOFs compound (Me2NH2)2[Cd1.5Lb]-2.5DMF-28H2O (5) was prepared through the reaction of H5Lb and Cd2+ions. Topology calculations showed5exhibits3D (4,5,6)-connected framework with the correspounding point symbol is (45.63.82)2(45.6)(48.66.8). Thermal stability, IR, and fluorescence properties of5were characterized.