Selectivities and thermodynamic parameters of alkali-metal and alkaline-earth-metal complexes of polyethylene glycol dimethyl ethers in methanol and acetonitrile

Abstract

Thermodynamic parameters (log K s, ΔH c, TΔS c) of alkali-metal and alkaline-earth-metal ion complexes of polyethylene glycol dimethyl ethers (glymes) have been determined in methanol and acetonitrile as solvents at 25 °C by using a modified calorimetric technique. It has been shown that, for a given number of donor sites, the stabilities of M + and M 2+ complexes by glymes are lower than those of the corresponding macrocyclic polyethers and open chain antibiotic anions of the Nigericin group by a factor of up to 10 5. The stability constants (log K s) of the cation-glyme complexes are higher in acetonitrile than in methanol. The selectivities towards alkali and alkaline-earth cations with similar ionic radii (e.g. K s(Ca 2+)/K s(Na +) and K s(Ba 2+)/K s(K +)K +) of the glymes are also low. This may result from less effective shielding of the cation from cation-solvent interactions in the glymes as compared to crown ethers and open-chain antibiotic anions. In both solvents, the stability constant of a given metal complex increases monotonically with increase of the glyme chain length. The stability of K + and Ba 2+ complexes by glymes is largely enthalpy dominated, accompanied by an unfavourable entropy change in both methanol and acetonitrile, in which the contribution of the enthalpy term to the stabilities of complexes has also been found to increase with increase of chain length.