Synthesis And Characterization Of Metallo-Supramolecular Macrocycles Based On Dissymmetrical Terpyridine Ligands

In nature and life systems,asymmetrical and dissymmetrical structures are wide spread and play a critical role.The delicate and complex structures of DNA,viral coat,and protein based on dissymmetrical units are constructed by multiple non-covalent interactions.Inspired by self-assembly of natural molecules,supramolecular scientists are devoted to mimic the nature and design structures with specific functionalities.Due to the highly directional and predictable nature of the metal-ligand coordination mode,coordination-driven self-assembly have been applied in construction of supramolecular structures.Up to date,in the field of metallo-supramolecular assemblies,it is still in its infancy for constructing artificial architectures using dissymmetrical building blocks.In this dissertation,we designed discrete supramolecular structures by dissymmetrical ligands based on 2,2′:6′,2′′-terpyridine coordination with metal ions.Introduced by steric hindrance or thiophene group,the precise and controllable supramolecular structures can be obtained.There are four chapters in this dissertation as shown below.Chapter 1 Coordination-driven supramolecular self-assembly was introduced,including the self-assembly of metallo-supramolecules based on symmetrical ligands,self-assembly of metallo-supramolecules based on dissymmetrical ligands,and three methods of supramolecular constructions based on 2,2′:6′,2′′-terpyridine.The projects are proposed at the end of this chapter.Chapter 2 Two double-layer dissymmetrical tetratopic ligands LA and LB based on 2,2′:6′,2′′-terpyridine were designed and synthesized,and the self-assembly behavior of the ligand coordinated with Zn²⁺was studied.During self-assembly progress of tetratopic ligands,there should be four coordination modes,corresponding to four isomers.In the past decades,conventional characterization techniques like NMR,electrospray ionization-mass spectrometry（ESI-MS）,traveling wave ion mobility-mass spectrometry（TWIM-MS）,and single-crystal X-ray diffraction have been well established for characterizing supramolecules.However,these techniques become less effective in investigating supramolecules assembled by dissymmetrical ligands.For instance,the increased chemical environments caused by the low symmetry would significantly broaden the NMR signals and make the spectrum obscure.Furthermore,if all the isomeric structures with the same charges have similar structural collision cross sections,TWIM-MS is unable to distinguish those isomers.In crystallographic analysis,the isomers constructed by dissymmetrical ligands usually lead to uncertain and irregular packing of supramolecules.As such,only one or few isomers could crystallize out of a mixture of isomers in the solutions.With the aid of ultra-high-vacuum,low-temperature scanning tunneling microscopy（UHV-LT-STM）,we were able to investigate four isomeric structures corresponding to four types of binding modes of ligand LA with two major conformations of complexes A.The distribution of isomers measured by STM and total binding energy of each isomer obtained by density functional theory（DFT）calculations suggested that the most abundant isomer A1 could be the most stable one with highest total binding energy.Finally,through shortening the linker between inner and outer layers and the length of arms,the arrangement of dissymmetrical ligand LB could be controlled within one binding mode corresponding to the single conformation for complexes B.Moreover,the hierarchical self-assembly behaviors of supramolecules A and B with long alkyl chains in different solvents were investigated.The result of hierarchical self-assembly of complexes A is nano-fiber structures.Different with the hierarchical self-assembly behavior of A,spherical aggregates with diameters about 50-250 nm were observed in B system.The different hierarchical self-assembly behaviors could be attributed to the following two aspects:（i）the reduced distance between inner and outer layers inhibitsπ-πstacking owing to the pseudo-octahedral configuration of tpy with transition metal ions;and（ii）the interior angles of heptamer and octamer are different from the designed 120°on the backbone,which may lead to nonplanar structures and hampered the hierarchical packing.Chapter 3 We designed and synthesized compound 2 based on 2,2′:6′,2′′-terpyridine with thiophen groups at 6,6′′positions.And through NMR and ESI-MS analysis self-assembly behavior of compound 2 and Zn²⁺,the coordination behavior is different with 2,2′:6′,2′′-terpyridine without modification.It can only form the homoleptic structures with Zn²⁺,and the chemical shift is significantly different from the terpyridine without modification.The single-crystal structure of complex S2displayed a twisted structure with five coordination points andπ-πstacking existed between thiophen and terpyridine.This verified the unnormal phenomenon in NMR spectrum.Interestingly,compound 2 can selectively coordinate with some metal ions,such as Zn²⁺,Cd²⁺,Ni²⁺,Cu²⁺,Co²⁺,and Mn²⁺.However,for the self-assembly of 2with Fe²⁺or Ru²⁺,only the peaks of ligand were seen in ESI-MS spectra,which indicated no complexes were formed.For the same one metal ion,different counterions had no effect on the self-assembly result.Based on the above result,two unsymmetrical ligands L1 and L2 based on 2,2′:6′,2′′-terpyridine with modification and without modification were designed.With the combinations of Fe²⁺+Zn²⁺or Ru²⁺+Cd²⁺,ligand L1 could self-assembly to form tetramers and hexamers with controllable distribution of metal ions by step-by-step synthesis method.And the supramolecules with controllable distribution of metal ions could also be obtained by ligand L1 with the combinations of Fe²⁺+Zn²⁺by one-pot reaction.To obtain pure product,rigid ligand L2was designed.However,there are only intermediates（Fe L22 and Ru L22）and no macrocycles in the result of self-assembly.We speculated,during self-assembly process,ligand L2 could not twist to counteract the tension produced by the twisted coordination mode of thiophene modified terpyridine.Chapter 4 Based on the study of chapter 3,two double-layer（LA and LB）and one three-layer（LC）ligands were designed by 2,2′:6′,2′′-terpyridine modified by thiophen groups at 6,6′′positions and without modification.The inner layer of LA is 2,2′:6′,2′′-terpyridine with modification at 6,6′′positions.For LB,the modified 2,2′:6′,2′′-terpyridine with thiophen groups is designed at the outer layer of ligand.LB is the isomer of LA.Discrete heptamer could be obtained by the self-assembly of ligand LA and Cd²⁺,but LB could not self-assembly to discrete structures.We speculated that the terpyridine units in the inner layer can’t be bent to adapt the angle of the outer layer.For three-layer ligand LC,no discrete structure was obtained.The possible reason is that it is difficult to bend the outer two-layer terpyridines to adjust twisted coordination mode of the inner layer of the terpyridine.