Solvent Effects on Crown Ether Complexations¹

Abstract

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.

Bindungsmechanismen in supramolekularen Komplexen

“…[245] Abbildung S7 zeigt für den Komplex aus [18]Krone-6 mit Kaliumionen eine akzeptable Korrelation von lg K mit a) der freien Energie DG t des Transfers von K + von Wasser in verschiedene Solventien sowie b) mit Gutman-Donor-Zahlen des Mediums. [246] Schema 8. Ein auf Coulomb-Kräften beruhendes Rotaxan-Shuttle.…”

Polyoxaethylene polypodands - powerful reduction catalysts in solid-liquid and liquid-liquid phase transfer systems

“…This is a result of the charge density in the complex of podand ligands with Li + being higher than those in the complexes with other alkali metals. [18][19][20] As the reaction of benzaldehyde reduction with LiBH 4 under PTC conditions (with the use of the ligands described here as catalysts) runs in the presence of Lewis acids (AlCl 3 and ZrCl 4 were checked as an activators), it seems that the first explanation of the lithium borohydride/podand systems inactivity as reductors is correct ( Table 2). The addition of such an acid to the inactive system induces the coordination of AlCl 3 (or ZrCl 4 ) to the benzaldehyde oxygen atom (C¼OÁÁÁMCl x ) and activates the carbonyl group, which permits its reduction.…”

Steric and Stereoelectronic Effects in Aza Crown Ether Complexes[1]

“…C 64.8, H 9.2, N 8.0; found: C 64.6, 64.5; H 9.4, 9.3; N 7.9, 8.0 ,4,7,10, was prepared as described previously [18b] . 4,7,10,13, was obSynthesis of Ligands : 1,4,7,10,13, tained analogous to 2b from 2.0 g (7.6 mmol) of 3a, 0.54 g (18.2 was commercially available, and purified [5] before use. mmol) formaldehyde (as 30% aqueous solution) and 3.5 g (76.…”

“…Furthermore we wanted to analyse enthalpic vs. entropic binding differences by calorimetric tivarious solvents can be described essentially by the change of free solvation energy of the ion in these solvents. [5] How trations with several aza crown ether complexes. Until now there are only scattered reports on the thermodynamics of mismatch between ligand sites and the cation lowers the affinities has been analysed on the basis of computer aided azacrownether complexes [7] [8] most frequently with transition metal cations.…”