Synthesis And Properties Of Metal-Organic Frameworks(MOFs) Based On 2-(4-pyridyl) Terephthalic Acid Ligands

Metal-organic frameworks(MOFs),also known as coordination polymers,represent an interesting type of solid crystalline materials that can be straightforwardly self-assembled through the coordination of metal ions/clusters with organic linkers.In recent decades,MOFs have attracted tremendous interest because of their structure variety,low solubility,and their enormously high surface areas with tailorable pore size and surface functionality.Although various diverse MOFs structures have been reported and studied up to now,controllable design and construction of coordination polymers is still challenging.Taking these into consideration,we choose a versatile pyridyl-decorated dicarboxylic acid ligand and different metal ions to build MOFs with fascinating structures and interesting properties through the reasonable control of reaction conditions under the guidance of coordination chemistry and crystal engineering principle.Firstly,the reactions of Ln? ions with a versatile pyridyl-decorated dicarboxylic acid ligand lead to the formation of a series of novel three-dimensional(3D)Ln-MOFs,[Ln3(pta)4(Hpta)(H2O)]·xH2O(Ln=Dy(1),Eu(2),Gd(3),Tb(4),H2pta=2-(4-pyridyl)terephthalic acid,x=6 for 1,2.5 for 2,1.5 for 3 and 2 for 4).Single-crystal X-ray crystallography studies reveal that complexes 1-4 have isostructural trinuclear structures and the Ln3+ions act as nine-coordinated muffin spheres,linking to each other to generate trinuclear {Ln3(OOC)6N2} SBUs,which are further extended to be interesting 3D topological architectures.To the best of our knowledge,the Dy-MOF exhibits zero-field single-molecule magnet(SMM)behaviour with the largest effective energy barrier of 126.0 K among the previously reported 3D MOF-based Dy-SMMs.The combined analyses of a diluted sample(1@Y)and ab initio calculations demonstrate that the thermally assisted slow relaxation is mainly attributed to the single-ion magnetism.Furthermore,based on high red emission intensity of Eu-MOF and high green emission intensity of Tb-MOF under UV light,we conducted the fluorescence sensing experiments on them respectively.The results of fluorescence sensing measurements show that 2 and 4 have a good selectively sensing for Fe3+ and Cr2O72-.Finally,it may be attributed to the competitive adsorption of the excitation wavelength energy between Eu-MOF,Tb-MOF and Fe3+/Cr2O72-due to the UV-vis absorption spectra of Fe3+/Cr2O72-in water,and the excitation spectrum of Eu-MOF and Tb-MOF showed a moderate overlap.Secondly,use of unsymmetric ligand 2-(4-pyridyl)-terephthalic acid(H2pta)and controlling the reaction pH value to constract three-dimensional In3+-based metal-organic framework,H3O[In(pta)2](5)and[In(pta)(Hpta)]·2H2O(6),with both thermostability and framework flexibility has been synthesized.Structural analysis of complexes 5 and 6 shows that each In3+ is a seven-connected node linked by one nitrogen atoms and six oxygen atoms from different H2pta ligands,giving a mononuclear[InO6N]unit.They contains one-dimensional open channels and possess a new(3,5)connected topology.Considering the open channels of 5 and 6,the porosity was evaluated by N2 adsorption at 77 K and the CO2 adsorption at different temperature.The fluorescent properties reveal the efficient probing performance of 5 and 6 for Fe3+/CrO42-/Cr2O72-in water.In summary,the present work presents a strategy to the synthesis of metal-organic frameworks with pyridine carboxylate ligand through select different metal ions,which provides theoretical foundation for the directed synthesis of functionalized coordination polymers.