A comprehensive study of complexation between potassium salts and 18-crown-6 in 14 different solvents shows stability constant K increases by >104 from water to pure methanol or to propylene carbonate (PC), with constants increasing in the order H2O < HMPT < DMSO < DMF < MeCHOHMe < MeCN < Me2CO < MeOH < PC. The mostly calorimetrically determined thermodynamic values of complexation between metal ion m and ligand l (ΔG ml, ΔH ml, ΔS ml) are compared with a large range of available solvent properties. Linear correlations (with coefficients R ≈ >0.95) are obtained for ΔG ml with standard Gibbs transfer energies ΔG°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 ΔG ml and even ΔH ml correlates well with the vol % of water, or with corresponding solvophobicity parameters S p (R = 0.97). The reaction enthalpies ΔH vary much more than ΔG, for instance from 12 kJ/mol (in MeCN) to 68 kJ/mol (in Me2CHOH), without meaningful correlations to known solvent properties, or between ΔG and ΔH. An exception is the correlation of ΔH 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.

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complexes with K ؉ . Formation of poly-nuclear (M ؉ ) n L complexes of tri-podands in solution has been confirmed by electro-spray mass spectrometry. At relatively small concentrations of the ligand (C 0 L ), P1 binds Na ؉ much better than Li ؉ , whereas P4 and P5 display a remarkable Li ؉ /Na ؉ selectivity; at large C 0 L the complexation selectivity decreases. X-Ray diffraction studies performed on monocrystals of complexes of NaNCS with tri-podands P2 and P3 show that Na ؉ is encapsulated inside a 'basket-like' pseudocavity, coordinating all donor atoms of the tri-podand. Molecular dynamics simulations on P2, P3 and P4 and on their 1 : 1 complexes with M ؉ in acetonitrile solution suggest that the structures of M ؉ L complexes in solution are similar to those found for P2 and P3 complexes in the solid state.