Solvent effects on complexation of crown ethers with LiClO4, NaClO4 and KClO4 in methanol and acetonitrile

The thermodynamic formation constants K_f for complexation of Li⁺, Na⁺ and K⁺ with the crown ethers 12C4 and 15C5 have been determined in methanol and acetonitrile at 25°C using precision conductivity data. The method permits evaluation of very small K_f values (e.g., K_f=6.98 mol^–1-dm³ for LiClO₄+12C4 in methanol) as well as fairly large values (e.g., K_f=2.73×10⁴ mol^–1-dm³ for NaClO₄+15C5 in acetonitrile). The determination of K_f values from conductivity data takes into consideration the often neglected ion pair formation of both the uncomplexed and the complexed cations. Our results for K_f are generally consistent with previously reported values based on potentiometry, calorimetry and polarography, but there are significant differences in several cases which we attribute to neglect of ion association both for the uncomplexed or free cation K_a and the macrocyclic complexed cation K_a2. Our results are also consistent with the well known concepts relating the magnitude of K_f to both the cavity diameter and ion-solvent interactions. Limiting molar conductivities ₂ ⁰ for the complex salt (M-crown ether) (ClO₄) in both solvents were generally found to be smaller or very close to the corresponding quantity ₁ ⁰ for the binary MClO₄-solvent system. However, in methanol, single ion limiting molar conductivities for the cationic complexes ₂ ⁰ exhibit anomalous behavior which is attributed to solvation differences between free cations and complexed cations.