Contributions of Solute-Solvent Interactions to Ion Selectivity of Macrocyclic Ligands

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Stability constants of 1:1 complexes of dibenzo-3m-crown-m(m=6,7,8,10) with alkali metal ions (Na^+,K^+,Rb^+,Cs^+) have been determined at 25℃ in water, nitromethane, and a few nonaqueous solvents by capillary electrophoresis or conductometry. The selectivities of these crown ethers in acetonitrile, propylene carbonate, methanol, N,N-dimethylformamide and water often obey cavity-size/ion-size relationship. On the other hand, the stability constants in nitromethane, which has the weakest solvation power for the metal ions, decrease with increasing ionic size (Na^+>K^+>Rb^+>Cs^+), reflecting the gas-phase selectivity governed by the electrostatic interaction between the ether oxygen atoms and the metal ion. It follows from this that the selectivities of these crown ethers in the solvents other than nitromethane are largely due to the solute-solvent interactions. The transfer activity coefficients of the crown ethers and their alkali metal ion complexes from nitromethane to the other polar solvents have been determined to evaluate the solvation of these solutes. It is shown that the crown ethers shields the alkali metal ions effectively from the solvents; the shielding is generally more effective for the larger crown ether. The solvent effects on the selectivities of the crown ethers are attributed to the solvation of the free metal ions.