Diloop crown ethers containing two crown ether rings linked by an aromatic spacer were synthesized, and their metal cation binding properties and complexation behavior were investigated. These compounds were designed based on prior studies of multi-loop crown ethers, which indicated that increasing aromatic spacers and extending methylene chain lengths can reduce metal ion binding affinity, and that the central ring in triloop systems plays a limited role in complexation.1)
The target diloop crown ethers were synthesized from chloromethylated benzo-15-crown-5 through reaction with 1,2-bis(2-hydroxyethoxy)benzene with oligoethylene glycols to produce derivatives with different crown ether ring sizes. The products were purified by column chromatography, and their structures were confirmed using NMR, IR, and mass spectrometry. Cation binding abilities were evaluated by solvent extraction experiments in a dichloromethane–water system at 25 °C, using picrate salts of alkali metal ions and heavy metal ions (Ag⁺ and Tl⁺). Extraction efficiencies were calculated to quantify binding performance. Complex formation and stoichiometry were further examined by electrospray ionization mass spectrometry (ESI-MS) using sodium and potassium perchlorates in acetonitrile.
ESI-MS analysis revealed the formation of both 1:1 and 1:2 (crown:metal) complexes for the synthesized compounds. In some cases, sandwich-type complexes, in which a single metal ion is coordinated by two crown ether molecules, were also detected. Solvent extraction results showed that diloop crown ethers generally exhibit lower alkali metal ion binding affinities than benzo-15-crown-5. However, marked differences in heavy metal ion selectivity were observed depending on crown ether ring size. Certain derivatives demonstrated enhanced affinity for Ag⁺, while others preferentially bound Tl⁺.
Reference
3670-3677 (1990)