Molecular design of crown ethers. Part 9. Complexation thermodynamics of 1,4,7,10,13-pentaoxacyclohexadecane (16-crown-5) and its lariat derivative in methanol–water: solvent effect upon lariat complexation

Abstract

Calorimetric titrations have been performed in 60–100% methanol at 25 °C in order to obtain the complex stability constants (K_s) and the thermodynamic parameters for the stoichiometric 1 : 1 complexation of some alkali (Na⁺ and K⁺) and heavy metal (Ag⁺ and Tl⁺) ions with less symmetrical crown ethers, 1,4,7,10,13-pentaoxacyclohexadecane (16-crown-5)(1) and 15,15-dimethyl-16-crown-5 (2), and the lariat ether, 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (3). The thermodynamic quantities clearly indicate that the complex stability sequence is essentially entropy governed, although the complex formation itself is evidently enthalpy driven. The lower complex stabilities for 2 are attributed to the reduced enthalpic gains caused by the steric hindrance between the methyl substituents and the cation. This steric hindrance, however, helps to lock the ring conformation such that complexation can occur, as indicated by the less negative entropic changes for 2. The lariat effect is shown to be sensitive to solvent composition; the complex stability sequence for Na⁺ is 3 > 1 > 2 in 60–80% methanol but 1 > 3 > 2 in 100% methanol. Thermodynamically, the partial inversion of the stability sequence is attributed to the smaller entropic gain arising from the less effective desolvation by the lariat side arm in 100% methanol.