Stabilities in water of 1:1 lariat 16-crown-5 complexes with metal ions: transfer activity coefficients from protic to aprotic solvents of 1:1 complexes of lariat and substituted 16-crown-5 derivatives with metal ions

Abstract

Stability constants (K_ML) of 1:1 complexes of 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (L 16C5) with metal ions have been determined in water at 25 °C by conductometry and potentiometry with ion-selective electrodes. The selectivity order of L 16C5 to monovalent and bivalent metal ions are Ag⁺ > Na⁺ > Tl⁺ > K⁺ and Sr²⁺ > Ba²⁺ > Pb²⁺, respectively. The selectivity is not altered by the side chains. The stability of the L 16C5–metal ion complex in water is much lower than might be expected on the basis chains of the solvation power of water for the metal ion (i.e. the relative solubility of the metal ion). Transfer activity coefficients (γ) of L 16C5–Na⁺ and –K⁺ complexes from water to acetonitrile, propylene carbonate and methanol [^sγ^H₂O(ML⁺)] were calculated from the ^sγ^H₂O(L 16C5) and K_ML values determined in this study and the literature values; s, M⁺ and L denote a solvent, a monovalent metal ion, and a crown ether, respectively. ^sγ^H₂O(ML⁺) of L 16C5 is larger than ^sγ^H₂O(L 16C5) and the corresponding ^sγ^H₂O(M⁺) values. The unexpectedly low stabilities of the L 16C5–Na⁺ and –K⁺ complexes in water are due to strong hydrogen bonding between the uncomplexed L 16C5 and water.