Behavior of Benzo-18-crown-6 Complexes with Alkali Metal Ions in Various Nonaqueous Solvents

The formation constants for 1 : 1 complexes of benzo-18-crown-6 (B18C6) with alkali metal ions and the solubility of B18C6 in various solvents were determined at 25 °C by conductometry and spectrophotometry, respectively. In general, the selectivity of B18C6 for alkali metal ions is governed primarily by the size relationship between cations and the B18C6 cavity. For a given alkali metal ion, the solvation ability of the solvent plays the most important role in determining stability sequences of the B18C6 complex in the solvents. Transfer activity coefficients of B18C6-alkali metal ion complexes were calculated. The transfer activity coefficient of the B18C6 complex from methanol to aprotic solvents somewhat varies with alkali metal ions in the B18C6 cavity. The magnitudes of the transfer activity coefficients of the B18C6 complex from acetonitrile and propylene carbonate to DMF and DMSO are determined mainly by the relative strength of the solvation ability of the former group and the latter group for the alkali metal ion in the B18C6 cavity. In methanol, due to the hydrogen bonds between ether oxygen atoms of B18C6 and methanol, the B18C6 complex is more strongly solvated than the corresponding alkali metal ion, compared to the case of the aprotic solvents.