Solvent Effects on Crown Ether Complexations1

A comprehensive study of complexation between potassium salts and 18-crown-6 in 14 different solvents shows stability constant K increases by >10 4 from water to pure methanol or to propylene carbonate (PC), with constants increasing in the order H 2 O < HMPT < DMSO < DMF < MeCHOHMe < MeCN < Me 2 CO < MeOH < PC. The mostly calorimetrically determined thermodynamic values of complexation between metal ion m and ligand l (Δ G ml, Δ H ml, Δ S ml ) are compared with a large range of available solvent properties. Linear correlations (with coefficients R ≈ >0.95) are obtained for Δ G ml with standard Gibbs transfer energies Δ G ° t of the metal ion from water to the given solvent. Analyses of literature data with some other cations and ligands, including the [222] cryptand, also revealed, that the complexation constant changes are essentially a linear function of the cation desolvation free energies. Less meaningful correlations (R ≈ <0.9) are obtained with values characterizing the electron donor capacity of the solvent. Parameters characterizing the solvent polarity, such as E T, are extremely poor descriptors ( R = 0.3) of the medium effects. In binary dioxane−water mixtures Δ G ml and even Δ H ml correlates well with the vol % of water, or with corresponding solvophobicity parameters S p ( R = 0.97). The reaction enthalpies Δ H vary much more than Δ G, for instance from 12 kJ/mol (in MeCN) to 68 kJ/mol (in Me 2 CHOH), without meaningful correlations to known solvent properties, or between Δ G and Δ H . An exception is the correlation of Δ H with the solvent polarity index π* (with R = 0.996, if MeCN is excluded). Solvent effects on the ligands are studied by NMR in view of the possible geometry changes of crown ethers from oxygen-in (with aprotic solvents) to oxygen-out (with water) conformations of the macrocycles. Preliminary NMR results, however, point to similar oxygen-in conformations in water as well as in chloroform, in line with molecular mechanics calculations.