Stabilities in Polar Nonaqueous Solvents and Transfer Activity Coefficients from Water to Nonaqueous Solvents of 19-Crown-6-Alkali Metal Ion Complexes

Abstract

Formation constants of 1 : 1 19-crown-6(19C6) complexes with alkali metal ions weredetermined conductometrically at 25 °Cin acetonitrile(AN), propylene carbonate (PC), methanol, DMF, andDMSO. 19C6 always forms the most stable complex withK+. The selectivity order of 19C6 forheavy alkali metal ions isK+ > Rb+ > Cs+.The selectivity for Na+ varies withthe solvent; that for Li+ is the second lowest(AN, DMSO) or the lowest (PC). Transfer activity coefficients(SγH2 O) of19C6 from water to the nonaqueous solvents (S) weremeasured at 25 °C. The contributions of a methylenegroup and an ether oxygen atom to thelog SγH2 Ovalue of a crown ether wereobtained. The SγH2 Ovalues of the 19C6–alkali metal ion complexes(SγH2 O (ML+)) werecalculated, M+ and L denoting an alkali metal ionand a crown ether, respectively. For AN, PC, andCH3OH, although the M+ ion is more stronglysolvated by water than by AN, PC, or CH3OH, thelog SγH2 O (ML+) islarger than the correspondinglog SγH2 O (L)expect for the case of M+ = Li+.The higher lipophilicity of the19C6 complex ion is attributed to an enforcement ofthe hydrogen-bonded structure of water for the complexion caused by the greatly decreased hydrogen bondingbetween ether oxygen atoms and water uponcomplexation. For DMF and DMSO, thelog SγH2 O (ML+) is also greater thanthe correspondinglog SγH2 O (L).It was concluded from thisfinding that the unexpectedly lowest stability of the19C6 complex ion in water is due to the hydrogenbonding between 19C6 and water. The stabilities and thelog SγH2 Oof 19C6–alkali metal ion complexes were compared with those of 18C6complexes.