Synthesis And Properties Of Metal-organic Frameworks Based On Tetrazolbenzoic Acid

Over the past two decades,metal-organic frameworks（MOFs）have attracted great attention from scientists not only because of their wide diverse,beautiful and interesting geometries,but also due to their versatility in gas storage/separation,catalysis,biomedicine and sensor materials and other potential applications.Research found that the geometry of organic linkers predominates in MOFs dimension formation and performance.Compared with the pure carboxylic acid ligands,the bifunctional ligands containing carboxyl and nitrogen-containing heterocycles（pyrazole,imidazole,tetrazole and so on）ligands can be combined with the flexible diversity of carboxyl coordinations and the structural stability of azole coordinations,which groups can be assembled with a variety of metal ions to form a diversified framework structure.There are few reports about MOFs formed by azo-carboxylic acid bifunctional coordination motifs compared to the widely reported MOFs based on carboxylic acid ligands or azole ligands.It is of great significance to carry out the synthesis of relevant MOFs and to analyze in-depth their structural features and functional studies.In this paper,the author selected bifunctional ligands containing tetrazole and carboxy4-（1H-tetrazol-5-yl）benzoic acid（H₂tzba）and 2-F-4-（1H-tetrazol-5-yl）benzoic acid（F-H₂tzba）ligands as organic host ligands,combined with 2,2’-bipyridine（2,2’-bpy）and4,4’-bipyridine（4,4’-bpy）auxiliary ligands,the four MOFs were synthesized by the solvothermalreaction:[Zn₃（μ₃-OH）（tzba）₂（2,2’-bpy）（H₂O）]OH·DMF（1）、[Co₃（tzba）₂（4,4’-bpy）₃（F）₂]·DMF·2H₂O·C₂H₅OH（2）、[（CH₃）₂NH₂]_0.5[Zn_1.5（μ₃-O）_0.5（tzba）_1.25（4,4’-bpy）_0.25（μ₂-H₂O）_0.5]·DMF（3）、[（CH₃）₂NH₂]_0.5[Zn_1.5（μ₃-O）_0.5（F-tzba）_1.25（4,4’-bpy）_0.25（μ₂-H₂O）_0.5]·DMF（4）.The structures of complexes were determined by X-ray single crystal diffraction and focusing on the properties of adsorption,separation and luminescent.Among them,Complex 1 contains the novel hexanuclear clusters Zn₆（μ₃-OH）₂（μ₂-H₂O）₂（COO）₄（N₄C）₂ and further forms agrid layer due to the termination of chelating 2,2’-bpy ligands,the layers stacked into a 3D supramolecular framework which leads to a significant selective adsorption of CO₂ over CH₄.At 100 kPa,298 K,the CO₂/CH₄ selectivities for the mixture with a 50%CO₂ component is 7.9.Complex2 is an uncommon flexible interpenetrated pillar-layered stable framework,assembled from two similar linear trinuclear metal-azolate-halide clusters[Co₃（CN₄）₂（F₂）]and 4,4’-bpy as pillars.The framework reveals a reversible dynamic flexibility related to the guest molecules,which exhibits an interesting gate opening-closing phenomenon and S-shaped isotherms for the adsorption of CO₂.The selective adsorption of CO₂ over CH₄ as well as the adsorption mechanism for CO₂ were also discussed deeply.Complexes 3 and 4 are anionic frameworks based on crowned-like hexanuclear cluster[Zn₆（μ₃-O）₂（COO）₅（N₄C）₅（μ₂-H₂O）₂]and contain octahedral cage structure with a large specific surface area,which show selective adsorption separation for CO₂/CH₄ and organic dye MO^-/MB⁺,MR^-/MB⁺and AO^-/MB⁺.All above complexes have been characterized by powder diffraction（PXRD）,infrared spectrum（IR）and thermogravimetry（TGA）,respectively.The adsorption and fluorescence properties of some complexes have been mainly studied.