Synthesis And Properties Of Coordination Compounds Based On Pyridine Carboxylic Zwitterionic Ligands

Currently,there is increasing demand for energy due to the rapid development of the human society.However,fossil fuels,the amount of which is decreasing every year,are not considered as a clean energy source since they will bring serious pollution to the environment.Therefore,nuclear energy,as a relatively clean,reliable,effective and safe energy,has attracted increasing attention.One of the most important factor for the development of the nuclear industry is the safe disposal of the spent nuclear fuel,whose long-term radioactivity majorly attributed from two types of long-lived radionuclides.The first type contains the cationic nuclides,²³⁵U、²³⁸U、²³⁹Pu、²³⁷Np、²⁴¹Am、²⁴³Am、²⁴⁴Cm、⁹⁰Sr,¹³⁷Cs,and etc.The second type is consisted of anionic nuclides ⁹⁹Tc and ⁷⁹Se,which mostly exist in the form of TcO₄^-、SeO₃^2-and SeO₄^2-.The disadvantages of the traditional anionic adsorbents include slow adsorption kinetics,poor selectivity and low adsorption capacity.So noval materials that are suitable for the separation and enrichment of such nuclides are desired.MOF materials have been applied in the fields of catalysis,adsorption,separation and biology,owing to its large surface area,porosity and convience to modification.In particular,pyridine carboxylic ligand-based MOF materials have attracted increasing attention because they contain the especial structure,pyridine-carboxylic-metal（positive-negative-positive electric charge）which brings special magnetic properties and electric fields of the MOF materials.In this paper,a series of cationic MOFs material were synthesized via solvothermal method.Uranyl cation(UO₂²⁺)and lanthanide ions(Ce³⁺)were used as metal centers and the zwitteronic ligands with pyridine and carboxylate were carefully designed and chosen as linker ligands.The structural diversity and potential applications of synthesized materials were systematically invesitigated.The results are as follows:（1）Under solvothermal conditions,a neutral 2D MOF material with uranyl tetranucleus was synthesized,which was consisted of uranyl ion and the ligand H₃TTTPCBr₃(H₃TTTPCBr₃=1,1’,1"-（2,4,6-trimethylbenzene-1,3,5-triyltris（methylene）tris-（4-carboxypyridinium）chlorine）.Then the amount of water in solvothermal reaction system was reduced to control the hydrolysis of uranyl unit,which resulted in the reduction of the the number of the coordinated OH^-and O^2-,and a new compound was obtained.In this new compound,the secondary buliding unit is a dinuclear uranyl,which is then linked by the ligand to become a 2D structure.Because of the presence of pyridine cations,the overall framework is positively charged.Thus,this compound is the first uranyl cationic organic framework.（2）In this part,compound 3 and 4 were synthesized using one lanthanide metal ions(Ce³⁺)and 4-carboxyl-4-carboxybenzyl)pyridine ammonium chloride.Compound 3 was synthesized in a mixed DMF and H₂O solution（DMF:H₂O=4:1）.Compound 3 is a 3D cationic organic framework,which can be used to remove chromate in DMF solution.Due to the strong affinity of water molecules to lanthanide metal ions,distortion of ligand,and instability of cationic MOFs,compound 3 is not stable in aqueous solution and can transform to a one-dimensional neutral framework（compound 4）.There are three equivalent water molecules coordinated to each metal ion,and free water molecules in the space in compound 4.Comparing to compound 3,compound 4 contains more coordinated and free water molecules,therefore,the proton conductivity of 4 is two orders of magnitude higher than compound 3.