Precise Control Of Metal-organic Cage Configurations Based On Dissymmetrical Pyridyl Ligands

Metal-organic cages（MOCs）possess diverse structures and tunable functionalities,and have been widely applied in fields such as molecular recognition,biomedicine,catalysis,and gas storage.However,most reported MOCs exhibit high symmetry,and studies on MOCs with low symmetry are relatively scarce.The construction of low-symmetry MOCs can be achieved by reducing the symmetry of ligands or by assembling multiple ligands.These cages possess unique molecular recognition and separation properties,and are easily functionalized.Despite the importance of studying low-symmetry MOCs,research in this area is limited,mainly due to the difficulty of constructing such cages using both strategies:firstly,using dissymmetrical ligands to assemble low-symmetry MOCs may generate multiple isomers with similar thermodynamic stability;secondly,when using multiple ligands to construct multi-component cages,it may generate homogeneous compounds of single components or other expected structures.Therefore,a universally applicable strategy for constructing low-symmetry MOCs is of great significance to the field.In this paper,various dissymmetrical ligands based on mono-pyridine were designed and synthesized,and different construction strategies were used to assemble them with Pd（Ⅱ）to form various low-symmetry MOCs with controllable configurations.The results are obtained in this thesis as follows:1、The configuration control of low-symmetry metal-organic cages assembled based on dissymmetrical ligands is achieved through three strategies.Six dissymmetrical ligands,L1-L6,were designed and synthesized and used to assemble a series of low-symmetry metal-organic cages with Pd（Ⅱ）metal ions.The structures of these assemblies were characterized by NMR,electrospray ionization mass spectrometry,single-crystal X-ray diffraction,and density functional theory（DFT）calculations.Analysis showed that during the assembly of L1 and L5 with Pd（Ⅱ）metal ions,insufficient spatial constraints resulted in the formation of multiple isomers.By increasing the length ratio of the dissymmetrical ligand on both sides of L1,single-configuration tetramer cis-Pd₂L2₄ and cis-Pd₂L3₄ were obtained with L2and L3,respectively.By changing the coordination position of L1 and increasing the angle of two N coordination points,L4 obtained appropriate spatial constraints during the assembly process,forming a single configuration hexamer cage.Ligand L6 was obtained by modifying the pyridine group on the long end of L5 with a methyl group.L6 with a steric hindrance group was assembled with Pd（Ⅱ）to selectively generate a single configuration trans-Pd₁₂L6₂₄ from over 700,000 possible isomers.2、The strategy of steric hindrance control was introduced into the multi-ligand co-assembly system,achieving the configuration control of multi-component metal-organic cages based on dissymmetrical ligands.Based on previous work,ligand L7was designed and synthesized to coordinate with dissymmetrical ligands L5、L6 and Pd（Ⅱ）metal ions,resulting in multi-component metal-organic cages Pd₆L5₆L7₆ and Pd₆L6₆L7₆.Due to ligand L7 has solvent effects during assembly,co-assembly in DMSO cannot be achieved and needs to be completed in DMF.The crystal characterization of Pd₆L5₆L7₆ showed that the dissymmetrical ligand L5 was randomly distributed in the assembled structure,leading to the existence of multiple isomers.Conversely,co-assembly of L7 with Pd（Ⅱ）and L6,which contained steric hindrance modifications,in DMF produced a oriented structure of Pd₆L6₆L7₆.3、A universal construction strategy for low-symmetry molecular cages was proposed,and a series of low-symmetry metal-organic cages were constructed by introducing bulky steric hindrance groups on dissymmetrical ligands.Specifically,six dissymmetrical ligands,L8-L13,are connected by acetylene bonds and include benzene,naphthalene,2-anthracene,triphenylamine,azobenzene,and 9-anthracene.When L8-L12 were assembled with Pd（Ⅱ）,they formed low-symmetry hexameric molecular cages,Pd₃L8₆-Pd₃L12₆,with three different chemical environments.L13can assemble with Pd（Ⅱ）to form an octameric molecular cage,Pd₄L13₈,which has two distinct chemical environments.It is noteworthy that among the corresponding symmetrical ligands,only L14,which is decorated with a smaller bulky group acetylene-benzene,can form discrete hexameric molecular cage Pd₃L14₆ upon assembly with Pd（Ⅱ）.For larger bulky groups on L15-L19,however,they could not form discrete complexes with Pd（Ⅱ）due to the excessive steric hindrance during the assembly process.This demonstrates the more flexible tuning ability of dissymmetrical ligands based on bulky group modification in assembly.