Assembly And Characterization Of Supramolecular Cages Based On Asymmetric Ligands

Supramolecular chemistry has become a powerful discipline that mimics the marvelous self-assembly behavior of nature.In recent decades,supramolecular chemists have used metal coordination to design and synthesize many complex and delicate supramolecular structures.Among them,most discrete metal complexes reported have been assembled based on symmetric ligands and metals,because symmetric ligand-based assemblies do not generate complex isomers,which are convenient for synthesis and characterization.However,asymmetry factors also play an important role in living systems.Increasing the asymmetry of the ligand can significantly change the structure and properties of the final assembly,and at the same time,it can better simulate the self-assembly behavior in the body.Therefore,the study of supramolecular self-assembly based on asymmetric ligands is of great significance.In this thesis,six kinds of 120-degree asymmetric monopyridine donor bidentate ligands,L1-L6,were designed and synthesized,and their assembly behaviors of Pd?II?salts were studied:1.The asymmetric ligands L1,L2,L3 and L4 were designed and synthesized.For L1,L2 and L3,the ligands are meta-linked pyridine on both sides.The lengths of the three ligands are the same on one side and increase in length on the other side,so that the geometrical constraints in the assembly are also increased in order.The smallest geometric difference between the two coordination sites in L1 led to insufficient geometrical constraint during assembly,resulting in a mixture of several isomer cages.The geometric limit of L2 in the assembly process is greater than that of L1,and finally a mixture of cis-Pd₂L2₄ and a small amount of by-products is formed.The geometric difference between the two coordination sites of L3 is the largest,and the geometric limitation is the strongest during the assembly process,which can generate a cis-Pd₂L3₄ molecular cage with higher purity,and it is proved by crystal that it contains two structurally similar isomers.In order to obtain a single-configuration assembly,the end-group convergence of the ligand was reduced,and an asymmetric ligand L4 with meta-pyridine on one side and para-pyridine on the other was designed and synthesized.Due to the special positions of the two coordination sites in the ligand,it brings about appropriate geometric constraint in the assembly.Assembly with Pd?II?salt can generate a single configuration of Pd₃L4₆ cage,and the crystal proved its structure.2.Reduce the end group convergence of ligand L4,design and synthesize asymmetric ligands L5 and L6 with para-pyridine on both sides of the ligand.The two can be assembled with Pd?II?salt to generate supramolecular cages Pd₁₂L5₂₄ and Pd₁₂L6₂₄.Tetrahydrofuran?THF?was slowly diffused into the DMSO solution of Pd₁₂L5₂₄ to obtain the Pd₁₂L5₂₄-THF crystal,and the irregular connection between the ligand and the metal ion was proved.According to the analysis of Pólya's theorem,the theoretical number of possible isomers of Pd₁₂L5₂₄-THF is 700,688.The lengths of the ligands L5 and L6 are the same on one side,and the length of the other side L6 is slightly larger than L5.Because the lateral length difference between the two ligands is not large enough,the ligand exchange phenomenon occured in the assembly of equivalent L5,L6 and Pd?II?.Finally,a series of supramolecular cage mixture Pd₁₂L5_XL6_24-X containing 24 ligands was obtained,and 24 ligands were composed of different numbers of ligands L5 and L6.