Synthesis and Characterization of Novel Imidazole Crown Ethers for Host–Guest Interactions
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- Crown ethers exhibit important supramolecular behaviour due to their ability to coordinate positively charged species. Due to this property, they have a wide variety of applications. Since the initial crown ether discovery there have been efforts made to incorporate various functional groups within the crown ether scaffold. These modifications lead to novel properties and increasing the possibility for important applications, emphasizing the need to create crown ether scaffolds with diverse substituents. This project aims to synthesize a crown ether which incorporates an imidazole functional group spacing the ether functionalities. The incorporation of an imidazole can introduce novel properties to the crown ether, including tautomerization, pH responsiveness, and an N-atom in the cavity. In the literature, only a few imidazole-containing crown ethers have been reported. One of the most notable examples is the work of Zimmerman. However, the proposed synthetic route requires numerous steps and resulted in low overall yields. Here the concerted synthesis of a protected imidazole macrocycle is reported with an overall yield ranging between 2 to 13%. Following the synthesis, host-guest studies were performed via NMR titrations with KPF6 and NaPF6 to assess their binding properties. Demonstrating that the newly synthesized imidazole crown ethers generally have similar binding properties to classical crown ethers. However, subtle differences in geometry and binding strength due to the incorporation of the imidazole were also observed. The binding properties were further extended to pseudorotaxane formation through the complexation of secondary ammonium ions. Interestingly, the novel crown ether exhibited exceptional affinity to secondary ammonium ions.