Study On The Structure And Properties Of Metal-Organic Double Cage With Palladium Medium

Metal-organic cages are supramolecular compounds driven by coordination bonds.They have been widely used in drug delivery,molecular recognition and catalysis,and have become a promising supramolecular material.Amongst the many discrete metallo-cage structures,Pd（Ⅱ）-based organic cage is the most versatile supramolecular cage model,which has the advantages of easy formation,clear metal ligand ratio and predictable spatial structure.Although a lot of progress have been made in exploring the structure and properties of metal cages,most of these studies are focused on rigid cages.However,there is a lack of in-depth understanding of the possible special properties of relatively flexible cages.Based on this,we use ligands with amide bond to construct flexible metal-organic cages,and study their properties of helix chirality and molecular recognition.The following results are obtained.（1）Firstly,monocages Pd₂（L1）₄was constructed by the coordination of banana-shaped bis（pyridyl）ligand L1 containing amide bond with metal Pd^Ⅱ.Due to the suitable length and curvature of the ligand skeleton,the two single cages can further interpenetrate with each other to form a Pd₄（L1）₈-type double cage.The double cage has three cavities,and the two outer cavities can be adjusted by anionic packing,so that the structure of double cage will undergo a certain tensile compression conformation change.When the two outer cavities were filled with chloride ions,we found that the double cage can recognize the guest molecules with polyhydroxy functional groups,such as monosaccharide,and thus realize the recognition function of the host and guest.（2）Secondly,we found that the cages based on amide ligands have special helix chirality.Due to the distortion of the amide bond,a single cage will have three helix chiral conformations of PP,MM and PM.However,after the formation of double cages,the dimer structure has certain selectivity for helix chirality and resulte in only four isomers of PPPP,MMMM,PMMP and MPPM,due to the limited space of amide bond twist.Under the introduction of chloride ions,the double cage compress and the packing between ligands becomes more compact.At this time,there were only two stable isomers PPPP and MMMM in the double cage.When the chiral ligand L2 were used,the chiral transfer from the side chain to the center can be realized,and the spiral chiral signal can be regulated and amplified step by step.（3）Finally,a double shell cage-in-cage structure Pd₂（L4）₄（?）Pd₂（L3）₄was successfully constructed by the utilization of the twist behavior of amide bond.Two kinds of single cage structures with different sizes were designed and synthesized,in which the large cage Pd₂（L3）₄was composed of longer rigid ligands L3 and the small cage Pd₂（L4）₄was composed of shorter flexible amide ligands L4.When the two cages were mixed,the small cage Pd₂（L4）₄is encapsulated in the cavity of the cage Pd₂（L3）₄,and plays a certain role in protection.As a control,we prepared a small rigid cage with the same length as the cage Pd₂（L4）₄,and found that they could not be encapsulated by large cages Pd₂（L3）₄,so we verified the particularity of amide flexible ligands in the formation of cage structure.In other words,the deflection and distortion of amide bond can make the inner and outer cages match and combine better.At last,the cage-in-cage structure was used to perform the classification process among different small cages,which is similar to the phagocytosis of cells.This kind of separable double shell cage structure provides a new way to construct completely controllable molecular cage system.