Synthesis,Structure And Properties Of Pyridyl-based Metal-organic Framework Materials

Metal-organic frameworks（MOFs）are porous crystalline materials formed by self-assembly of metal central ions or metal clusters with organic ligands containing binding sites.In the construction process of MOFs,the introduction of conjugated organic ligands or metal clusters can form luminescent metal-organic frameworks（LMOFs）with different fluorescence properties,which have great potential in the field of fluorescence sensing.Therefore,in this paper,eight new metal-organic frameworks were successfully synthesized under hydrothermal or solvothermal conditions with transition metal ions as the metal center,pyridinyl ligand as the main ligand,carboxylic acid ligand with functionalized functional groups as the auxiliary ligand,and one of the metal-organic framework was modified after synthesis（PSM）to obtain a rare earth ion functionalized composite material.Their structure,stability,and fluorescence detection performance are then studied.The specific contents are as follows:（1）Three LMOFs with different dimensions were synthesized from transition metals（Zn and Cd）,pyridinyl ligands 3,5-bis（4-imidazol-1-yl）pyridine（Bip）and5-sulfonylisophthalic acid（H₂sip）:[Zn₃（Bip）₃（H₂sip）₂（H₂O）₄]·5H₂O（CUST-751）,[Cd₃（Bip）₂（H₂sip）₂（H₂O）₄]·2H₂O（CUST-752）and[Zn₂（Bip）₂（H₂sip）₂（H₂O）₄]·3H₂O（CUST-753）.Fluorescence sensing experiments have shown that both CUST-751 and CUST-752 can rapidly and sensitively detect Fe³⁺and Cr₂O₇^2-ions in organic solutions by fluorescence quenching,and the quenching constant(K_sv)can reach 10⁴M^-1.In addition,the luminescent CUST-751 can also detect 2,4,6-trinitrophenol（TNP）with a K_svof 3.44×10⁴M^-1.（2）With Bip as the main ligand,2-aminoterephthalic acid（NH₂-NDC）and2-bromoterephthalic acid（Br-BDC）as auxiliary ligands,and zinc ions as metal centers,two Zn-LMOFs with three-dimensional coordination networks were synthesized under solvothermal conditions:[Zn₂（Bip）₂（NH₂-NDC）₂]·H₂O（CUST-754）and[Zn₄（Bip）₄（Br-BDC）₆]·5DMF（CUST-755）.Subsequently,powder X-ray diffraction（PXRD）and thermogravimetric analysis（TGA）demonstrated their high phase purity and good thermal stability.Finally,luminescence experiments show that both MOFs can detect Fe³⁺,Cr⁶⁺,NFT,and NZF in aqueous environments with limits of detection（LOD）as low as 10^-6M.（3）A new metal-organic framework material[Ni₂（Bip）₂（H₂NDC）₂（H₂O）₆]（CUST-756）was successfully synthesized by solvothermal method with nickel（Ni）as the metal node and Bip and 1,4-naphthalic acid（H₂NDC）as ligands.By PSM method,rare earth Eu³⁺ions were loaded on CUST-756 with uncoordinated carboxyl sites to obtain Eu³⁺@CUST-756 luminescent composite.Through a series of basic characterizations such as infrared,PXRD,TGA,and X-ray photoelectron spectroscopy（XPS）,it is proved that Eu³⁺has coordination with carboxyl groups and the composite has good thermal stability.Fluorescence sensing experiments showed that Eu³⁺@CUST-756 material could detect Cr³⁺ions by fluorescence enhancement phenomena,with a LOD of 5.44μM.In addition,it can detect Fe³⁺ions by fluorescence quenching with a LOD of 7.51μM.（4）N,N-bis（pyridin-4-yl）oxalamide（4,4’-bpox）and H₂sip were used as ligands,and Co²⁺ions were used as metal centers;with Bip and terephthalic acid（BDC）as ligands and Ni²⁺ions as metal centers,two new MOFs:[Co₂（4,4’-bpox）（H₂sip）（DMF）₂]（CUST-757）and[Ni（Bip）（BDC）（H₂O）₂]（CUST-758）,were successfully synthesized under solvothermal conditions.They may act as excellent proton conductive materials.