Stabilities in water and Gibbs free energies of transfer from water to nonaqueous solvents of alkali metal ion complexes with 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (acyclic polyether)

Abstract

Stability constants of 1:1 complexes, ML⁺ (M⁺ and L being a univalent metal ion and a ligand, respectively), of Na⁺ and K⁺ with 1,2-bis[2-(2-methoxyethoxy)ethoxy]benzene (AC · B18C6), a linear counterpart of benzo-18-crown-6 (B18C6), were determined in water at 25°C by conductometry. The stability of ML⁺ is lower for AC · B18C6 than for B18C6 because of the macrocyclic effect, and AC · B18C6 is selective for Na⁺ in contrast to B18C6. Also determined were Gibbs free energies of transfer (ΔG_H2O→S^°) from water to nonaqueous solvent S (S=acetonitrile, propylene carbonate and methanol) of AC · B18C6 and B18C6 at 25°C through measurements of the distribution constants between tetradecane and the polar solvent. From these data as well as the literature values of K_ML in the nonaqueous solvents and of ΔG_H2O→S^° of M⁺, the ΔG_H2O→S^° values of ML⁺ complexes of the alkali-metal ions with AC · B18C6 and B18C6 were calculated. The ΔG_H2O→S^° values of M⁺ are positive but those of ML⁺ and L are negative in all the systems. This indicates that, although the M⁺ ions are more soluble in water than in the nonaqueous solvents, when the M⁺ forms a complex with the lipophilic L, the complex becomes more soluble in the nonaqueous solvent than in water. The lipophilicity of AC · B18C6 is higher than that of B18C6, but the reverse holds for the ML⁺ complexes. It was concluded that AC · B18C6 shields the M⁺ ions less effectively in the complexes from the surrounding solvents than B18C6.