Synthesis and properties of metal organic frameworks with novel metal-imidazolate secondary building units

Abstract

Metal-organic frameworks (MOFs) are a class of inorganic-organic hybrid materials and have potential applications in many fields such as gas adsorption and storage, catalysis, ion exchange and separation, chemical sensing and so on. It has attracted a great deal of chemists attention. Moreover, the development of new secondary building units (SBUs) is of great importance for the synthesis of MOFs. In this work, we choose 2-(1-hydroxy-ethyl)-3H-benzo- imidazole-5-carboxylic acid (Hebimc) as organic ligand, which is a trifunctional imidazolate derivative comprising metal coordination groups and diverse functional sites. In this ligand, the imidazolate unit can provide μ2-bridging coordination mode to form zeolitic MOFs, the hydroxy unit could offer active site for further reactions, the benzene ring may offer π-π interaction with guest molecules for fascinating structures, and the carboxy unit also could coordinate to metal center. So far, MOFs assembled by Hebimc ligand and auxiliary dicarboxylic ligands have not been reported yet. By employing the Hebimc ligand and introduce auxiliary dicarboxylic ligands to assemble with different transition metal salts, respectively, four MOFs from one-dimensional (1D) to three-dimensional (3D), namely, Zn2(Hebimc)4 ( 1 ), Zn2(Hebimc)2(BDC)∙ x G( 2 ,BDC=terephthalic acid, G=guest molecule), Zn2 (Hebimc)2 (NDC)∙ x G ( 3, NDC=2,6-naphthalene diacid), Zn8(Hebimc)6(DOBDC)(OH)2(H2O)2∙ x G( 4 ,DOBDC=2,5-dih-ydroxyterephthalic acid), have been synthesized under different reaction conditions. 1 was synthesized by the reaction of Hebimc and ZnSiF6·6H2O in H2O at 100°C for 3 d (yield: 40% based on Hebimc); 2 was prepared by the reaction of Hebimc and BDC and ZnSiF6·6H2O in isopropanol/water (6:3, v:v) at 100°C for 3 d (yield: 65% based on Hebimc); 3 was prepared by the reaction of Hebimc, NDC, pyrazine, tetraethylammonium bromide (TEAB) and Zn(NO3)2·6H2O in N , N -dimethyl formamide (DMF)/water (3:1.5, v:v) at 120°C for 3 d (yield: 78% based on Hebimc); 4 was prepared by the reaction of Hebimc, DOBDC, pyrazine, triphenylphosphine and Zn(NO3)2·6H2O in DMF/water (6:1.5, v:v) at 120°C for 3 d (yield: 50% based on Hebimc). Single crystal X-ray diffraction was used to characterize their structures. It is noted that Hebimc ligands in this structures are chelate coordinate to metal centers to form Zn2(Hebimc)2 SBUs, which is unusual in MOFs. 1 crystallized in orthorhombic P2 1 2 1 2 1 space group and exhibited 1D chain connected by N–H…O hydrogen bonds into a supermolecular structure. By introducing auxiliary dicarboxylic ligands, two 2D layers 2 and 3 are obtained. 2 crystallized in monoclinic P2 1/ c space group and exhibited 2D (4,4)- sql layer. There is no obvious interaction between the layers, packing a 3D accumulation structure in the ABAB way. 3 crystallized in triclinic P-1 space group and exhibited an 2D (4,4)- sql layer, packing a 3D accumulation structure in the AA way. Under the inducement of the ligand DOBDC, a pillar-layer 3D structure 4 is obtained. 4 crystallized in monoclinic C2 / c space group and can be simplified as a (4,5)-connected network with the point symbol (42.53.65)2(52.64). Besides, hydroxy-functional neutral compound 4 shows an obvious degradation effect on photocatalysis of crystal violet aquous after loading Ag nanoparticles (NPs). These results demonstrated that the mixed ligand approach is a successful route for the construction of MOFs.