Fine-Tuning The Spring-Like Motion Of An Anion-Based Triple Helicate

Recently,we have developed a new strategy,anion-coordination-driven assembly（ACDA）,for construction of discrete supramolecules,and various well-defined structures.In this paper,the p-xylylene-spaced bis-bis（urea）ligand L was then used to build the“caged”triple helicate because this moderately flexible spacer can allow conformational modulation of the ligand strands.The triple phosphate helicate can undergo a reversible extension–contraction motion,and the pitch of the helicate can be fine-tuned by encapsulation of tetraalkylammonium cations of different shapes and sizes.This paper is divided into three chapter,which mainly introduce the following contents.The first chapter is the introduction,which briefly introduces the development of supramolecular self-assembly structure,mainly introduces the dynamic response helical structure of metal coordination,the development of supramolecular springs,and the development of anionic urea-containing helicate structure.Based on the above content,we have the ideas for this thesis and set the aim and significance for this topic.In the second chapter,we synthesize a C₂-symmetric bis-bis（urea）ligand L with the p-xylylene linker,which assembles with phosphate anion to form the A₂L₃“caged”triple helicate structure.When adding 1 equiv of TMA⁺cations to H-[K（18-C-6）],the structure contracted and the helical pitch is reduced.Then,when a tetrahedral cage that exhibits a stronger binding affinity to TMA⁺is introduced into it,the TMA⁺cations can be removed and the contracted helicate returns to the previous extended state.The two-state tirple helicate structures are supported by crystal,NMR and MS.In the third chapter,we are inspired by previous work,further studied the fine regulation of the triple helicate molecular spring.we introduced a series of tetraalkylammonium cations with varying size and shape,N₁₁₁₂,N₁₁₁₃,N₁₁₁₄,N₁₁₂₂,N₁₂₂₂ and N₁₁₃₃.By separately adding these guest cations to H-[K（18-C-6）],we have obtained crystal structures of all the host-guest complexes,which reveal the inclusion of a guest cation within the central cavity.Moreover,the helicate host can adjust its conformation to adapt to different cations,with reducing PO₄^3-???PO₄^3-distance in the following order:N₁₂₂₂>N₁₁₁₂>N₁₁₁₄>N₁₁₁₁（TMA）>N₁₁₁₃>N₁₁₂₂>N₁₁₃₃,thus achieving fine-tuning of the extension?contraction motion of the helicate spring.Next,we tried three other lager tetrahedral countercations,TEA⁺（tetraethylammonium）,TPA⁺（tetrapropylammonium）and TBA⁺（tetrabutylammonium）,considering the“external template”effect of the cations,the ligands are C-shaped and form a meso triple-helical structure.Futhermore,the meso-helicate can be reversibly converted with the triple helicate upon TMA⁺binding and release.