Cation binding by crown ethers

For an explanation of these preorganisation effects, we must break down the enthalpic and entropic contributions to the stability of complexes such as [K c 3.11] and [K+ c 3.6]. At 298 K, we find the parameters given in Table 3.4. From these data, it is clear that both enthalpic and entropic contributions favour the stability of the macrocyclic complex. In this case the dominant effect is the enthalpy, although this is not true in every system. 

Cation Diameter (A) Crown ether Cavity diameter (A) 

If this were true of all supramolecular systems, the prospects for designed selectivity would be bleak 

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In conclusion, the major achievement resulting from the gas-phase data is a much more profound understanding of the role of solvation in cation binding by crown ethers. 

Cation Binding by Crown Ethers The Hole-Size Relationship ... 

Y. Inoue and G. W. Gokel, Cation Binding by Macrocycles. Complexation of Cationic Species by Crown Ethers, Marcel Dekker, New York and Basel (1990). 

Inoue. Y. Hakushi, T. Liu, Y. Thermodynamics of Cation-Macrocycle Complexation Lnthalpy-Lntropy Compensation. In Cation Binding by Macrocycles Complexation of Cationic Species by Crown Ethers Inoue, Y., Gokel, G.W., Lds. Marcel Dekker. Inc. New York, 1990 1-110. 

In Pedersen s early experiments, the relative binding of cations by crown ethers was assessed by extraction of alkali metal picrates into an organic phase. In these experiments, the crown ether served to draw into the organic phase a colored molecule which was ordinarily insoluble in this medium. An extension and elaboration of this notion has been developed by Dix and Vdgtle and Nakamura, Takagi, and Ueno In efforts by both of these groups, crown ether molecules were appended to chromophoric or colored residues. Ion-selective extraction and interaction with the crown and/or chromophore could produce changes in the absorption spectrum. Examples of molecules so constructed are illustrated below as 7 7 and 18 from refs. 32 and 131, respectively. 

Studies of cation binding by poly(crown ethers) have revealed very interesting features, but a detailed discussion of this subject would be beyond the scope of this review. For the pertinent literature the reader is referred to recent articles by Smid (1976), Wong and Smid (1977) and references cited therein. 

The complexation of protonated amines, RNHJ, by crown ethers differs in many aspects from the complexation of metal cations. Whereas complexes with metal cations derive their binding energy mainly from electrostatic forces, complexes with ammonium ions are likely also to be stabilized by hydrogen... 

Figure 14, Schematic drawings of conformational changes upon cation binding by glymes, crown ethers, and cryptands (D denotes donor atom) also shown are the slopes (a) and intercepts TASo oi AH-TAS plots as measures of conformational change and desolvation.

Pedersen and Frensdorff studied the binding and extraction of the cation by crown ethers, which require coextraction of an anion. Without the presence of an anion host, solvation directs selectivity, giving rise to Hofmeister bias selectivity favoring low anion charge density. Anions initially in aqueous solution must be dehydrated (at least partially) and are then resolvated in the solvent phase. Empirically, the HBD ability of the solvent medium is the single most important determinant of the solvation of small, inorganic anions.100... 

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