Cation Binding Properties of Benzo-type Crown Ethers. 2. Effect of Large Ring Size upon Complexation Phenomena of Crown-7 and Crown-8 Derivatives

The cation-binding by two large ring-sized benzo-21-crown-7 and benzo-24-crown-8 has been performed by the solvent extraction of monovalent metal picrates (Na^+, K^+, Rb^+, Cs^+, Ag^+, and Tl^+) between water and dichlolomethane. The effect of ring size and benzo group upon the cation binding ability is discussed. The results are compared with benzo-18-crown-6 and simple unsubstituted crown ethers (18-crown-6, 21-crown-7 and 24-crown-8). It was found that large ring benzo-crown ethers have lower cation binding ability as the ring size increases compared with simple unsubstituted crown ethers while cation selectivity for large cations (Cs^+ and Tl^+) has been observed.