Supramolecular Chemistry. 60. Solvent Effects on Crown-Ether Complexationsстатья

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[1] Supramolecular chemistry. 60. solvent effects on crown-ether complexations / V. P. Solov’ev, N. N. Strakhova, O. A. Raevsky et al. // Journal of Organic Chemistry. — 1996. — Vol. 61. — P. 5221–5226. 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 (ΔGml, ΔHml, ΔSml) are compared with a large range of available solvent properties. Linear correlations (with coefficients R ≈ >0.95) are obtained for ΔGml with standard Gibbs transfer energies ΔGot 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 ET, are extremely poor descriptors (R = 0.3) of the medium effects. In binary dioxane−water mixtures ΔGml and even ΔHml correlates well with the vol % of water, or with corresponding solvophobicity parameters Sp (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. [ DOI ]

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