Complex formation of alkali metal ions with 18-crown-6 and its derivatives in 1,2-dichloroethane Original Research Article

The ion-pair extraction of alkali metal ion (M+) complexes of the crown ethers (L), i.e., 18-crown-6 (18C6), benzo-18-crown-6 (B18C6) and dicyclohexano-18-crown-6 (DCH18C6), with picrate ion (Pic−) into 1,2-dichloroethane (1,2-DCE) has been studied at 25.0°C. From an analysis of the distribution results, the thermodynamic formation constants of ML+, Kcomp,o0, and ML2+, Kcomp(2),o0, complexes in 1,2-DCE were evaluated. The Kcomp,o0 value in 1,2-DCE, having very low solvating ability, is quite high as compared to that in solvating solvents and increases in the order dibenzo-18-crown-6 (DB18C6) < B18C6 < 18C6 < DCH18C6 for a given metal ion and Li+ < Na+ < K+ > Rb+ > Cs+ for any crown ether. Their differences both among crown ethers and among large metal ions, i.e., K+, Rb+ and Cs+, are large compared to those in solvating solvents. The Kcomp(2),o0 value in 1,2-DCE is in the order of Rb+ < Cs+ in contrast to Rb+ > Cs+ for Kcomp,o0 and increases in the order DCH18C6 < 18C6 < B18C6 < DB18C6 in contrast to a reverse sequence for Kcomp,o0.