Synthesis Of Benzimidazolium And Perimidinium Based Receptors For Anion Recognition

The design and synthesis of novel artificial receptors that have strong affinity and selectivity for specific anions has received great attention due to their important role in a wide range of chemical, biological and environmental applications. The interaction between receptors and anions is noncovalent interactions which include electrostatic interactions, hydrogen bonding, hydrophobic interactions and so on. However, the development of anion receptors based on hydrogen bonding has been a hot theme in the field of anion recognition.Urea group which contains two acidic NHs is a good hydrogen bond donor. Benzimidazolium is also a good H-bond donor and can interact strongly with anions through a (C-H)+···X-type ionic hydrogen bond. However, receptors which contain both urea groups and benzimidazolium moieties have seldom been reported so far. Meanwhile, perimidinium motif as binding sites for anion recognition is never explored. As the same with imidazolium groups, perimidinium groups can be used as new anion binding hydrogen bond moieties by forming (C-H)+···X-type ionic hydrogen bonds between C(2)-H in perimidinium rings and the guest anions. In this dissertation, the following two series of novel anion receptors were investigated.Part I:Receptors based on bis-benzimidazolium bearing urea groups for recognition of anions. A series of receptors (1,2,3,4,5and6) based on bis-benzimidazolium bearing urea groups have been designed and synthesized. The recognition properties of these receptors for dicarboxylates (malonate, succinate, glutarate, adipate) were evaluated by1H NMR, UV-vis and fluorescence spectroscopies. Results demonstrated the fluorescent intensity of receptor1increases in the presence of the dicarboxylates. In contrast, a significant decrease in the emission of2,3,4,5and6is due to the photo-induced electron transfer (PET) process. The association constants of all receptors with adipate are higher as compared to other anions. Job plots indicated that1:1stoichiometry complex were formed between the receptors and dicarboxylates anions. In1H NMR titration with adipate, receptor1decomposed, the product and the cause of decomposition are in need of further research. However, under the identical condition, benzimidazolium C(2)-H and urea N-H of receptor5underwent downfield chemical shifts of0.84ppm and2.27ppm respectively, which revealed that the formation of conventional (NH···O) and unconventional hydrogen bonds(C+H···O) is the key interaction for recognition.Part Ⅱ:Novel perimidinium-based receptors for recognition of anions. Two novel perimidinium-based receptors (7,8) have been designed and synthesized. The recognition properties of them for various anions were investigated by1H NMR, UV-vis and fluorescence spectroscopies. The results of association constants from UV-vis and fluorescence titration both reveal that the affinity of receptor7and8for anions in DMSO is in the order of CH3COO-> F-> H2PO4->Cl-≈Br-≈I-≈HSO4-. Job plots showed that the receptors formed1:1complex with acetate. As showed in the1H NMR spectra, perimidinium C(2)-H of receptor7and8underwent downfield chemical shifts of0.49ppm and0.36ppm respectively, which can be attributed to the (C-H)+-X-hydrogen bonding between perimidinium moieties and acetate. Meanwhile, as acetate was added, new peaks began to show up in the area from4.5ppm to9.5ppm. The reason may be that the solution of receptor7and8react with acetate which possesses relatively stong basicity. However, the product and the cause of the reaction are under discussion.