Cooperative metal ion binding by metal-organized crown ether

Host molecules which satisfy both facile synthesis and strong as well as selective binding of metal ions have been developed by introducing an idea of metal-assisted organization of linear molecules into crown ethers. For this purpose, an oligo(oxyethylene) prehost having two beta-diketone units in both terminal positions was synthesized. Intramolecular coordination of the two beta-ketones to divalent transition metals, Cu, Zn, or Ni, organized the linear prehost into a macrocyclic pseudo crown structure. The crystal structure of the Cu-organized host and K+ picrate complex was analyzed by X-ray crystallography. The structure elucidated indicated that the intramolecular coordination of beta-ketoenolate oxygens to the transition metal furnished an organized cavity into which the potassium cation was entrapped. The metal-organized crowns showed a good size-fit selectivity, characteristic of macrocycles, and a superior extraction efficiency when compared with dibenzo-18-crown-6 as a reference compound for the picrate extraction of alkali and alkaline earth metal cations. These characteristics have been ascribed to the existence of polar bonds between negatively charged enolate oxygens and transition metals and illustrates the power of the self-organization method in preparing the host.