Synthesis,Characterization And Properties Of Copper(Ⅱ) Metal-organic Cages With A Truxene-based Trispyridine Ligand

Metal-organic cages（MOCs）are supramolecular cages with specific shapes and cavities,which have shown great application prospects in many fields.Among them,chiral MOCs have attracted extensive attention due to their unique supramolecular chirality and properties.However,during MOCs self-assembly process,metal ions and ligands are in dynamic equilibrium of coordination and dissociation,which is easy to racemize or isomerize.Therefore,the accurate construction of chiral MOCs has been a hot topic in supramolecular chemistry.In this thesis,an achiral trispyridine ligand based on truxene L were designed and prepared,and a series of octahedral supramolecular cages were obtained by self-assembly of L with copper ions sites.These corresponding chiral cages were obtained through chiral self-resolution without using any chiral sources,and their structures and properties were also investigated.1.A new trispyridine ligand based on a truxene L was designed and synthesized.Using truxene and ethyl bromide as starting materials,truxene-based pyridyl ligands L was prepared by substitution,bromination,and Suzuki coupling reaction.Then,Cu₆L₈-type octahedral supramolecular cages 1-3 were constructed by ligand L with three different Cu（Ⅱ）salts（BF₄^-,Cl O₄^-,OTf^-）in acetonitrile solution,and characterized by NMR and MS.Computational simulation shows that these cages have three levels of chirality,including truxene’s planar chirality（C/A）,axial chirality between pyridine and truxene（R/S）,and propeller chirality of pyridine and metal coordination sites（Λ/Δ）.In theory,there should be22 stereoisomers,and two homochiral structures as a pair of enantiomers(C₈R₂₄Λ₆and A₈S₂₄Δ₆)were most stable.2.The detailed crystal structures of cages 1-3 were confirmed by X-ray single crystal diffraction.Crystallization of the cages with BF₄^-as counterions afforded racemate crystals,while crystallizations of cages with Cl O₄^-and OTf^-as counterions resulted in a pair of enantiomers with spontaneous resolution.Based on detailed structural analysis,we speculated that anion may be the source of the regulation of chiral self-resolution,and the cluster structure formed by anions and lattice water,which is the key to the realization of chiral self-resolution of supramolecules.Then,the standard concentration curves of cages 2-3 were obtained by ultraviolet（UV）measurement,and their molar ellipticities were determined by circular dichroism（CD）.Circular dichroism spectra and theoretical calculation showed that the chirality of metal cage is mainly contributed by the planar chirality of truxene group.In addition,the stability of cages 2-3 was investigated by thermogravimetric analysis（TG）and MS.Racemic dynamic mechanism show the chiral cages were very slow racemization even at low concentration(6.6×10^-6mol L^-1),suggesting their high stability in solution.