SUPRAMOLECULAR CHEMISTRY .60. SOLVENT EFFECTS ON CROWN-ETHER COMPLEXATIONS

A comprehensive study of complexation between potassium salts and 18-c rown-6 in 14 different solvents shows stability constant K increases b y > 10(4) from water to pure methanol or to propylene carbonate (PC), with constants increasing in the order H2O < HMPT < DMSO < DMF < MeCHO HMe < MeCN < Me(2)CO < MeOH < PC. The mostly calorimetrically determin ed thermodynamic values of complexation between metal ion m and ligand 1 (Delta G(ml), Delta H-ml, Delta S-ml) are compared with a large ran ge of available solvent properties. Linear correlations (with coeffici ents R approximate to >0.95) are obtained for Delta G(ml) with standar d Gibbs transfer energies Delta G(t) degrees of the metal ion from wat er to the given solvent. Analyses of literature data with some other c ations 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 approximate to <0.9) are obtained with values characterizing the elec tron donor capacity of the solvent. Parameters characterizing the solv ent polarity, such as E(T), are extremely poor descriptors (R = 0.3) o f the medium effects. In binary dioxane-water mixtures Delta G(ml) and even Delta H-ml correlates well with the vol % of water, or with corr esponding solvophobicity parameters S-p (R = 0.97). The reaction entha lpies Delta H vary much more than Delta G, for instance from 12 kJ/mol (in MeCN) to 68 kJ/mol (in Me(2)CHOH), without meaningful correlation s to known solvent properties, or between Delta G and Delta H. An exce ption is the correlation of Delta H with the solvent polarity index pi (with R = 0.996, if MeCN is excluded). Solvent effects on the ligand s are studied by NMR in view of the possible geometry changes of crown ethers from oxygen-in (with aprotic solvents) to oxygen-out (with wat er) conformations of the macrocycles. Preliminary NMR results, however , point to similar oxygen-in conformations in water as well as in chlo roform, in line with molecular mechanics calculations.