Anion hosts

Kavallieratos, K., Moyer, B. A., Attenuation of Hofmeister bias in ion-pair extraction by a disulfonamide anion host used in strikingly effective synergistic combination with a calix-crown Cs+ host, Chem. Commun., 1620-1621, 2001. 

In comparison to cations of similar size, anions have high free energies of solvation and hence anion hosts are required to compete more effectively with the surrounding medium (e.g. AGp- 434.3 kJ mor , AG 337.2 kJ mor ). " ... 

Recently, our own group has taken a different approach to the use of calixarenes and related macrocycles as anion hosts. Direct attachment of cationic transition metal fragments to the outside arene faces of the calixarene bowl transforms the ostensibly electron-rich cavity into one which is amenable for the inclusion of anionic species. 

It is clear that, in spite of dramatic recent advances and the enormous variety of anion hosts synthesized to date, the problem of achieving strong, selective com-plexation still remains for a large majority of anionic guests and will doubtless be the subject of a great deal of rich and innovative chemistry in the future. 

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... 

Despite the early discovery of the katapinands, non-covalent anion coordination chemistry was relatively slow to develop in comparison with the development of hosts for cations and even neutral molecules. While it is generally true that anion hosts obey the same rules that govern the magnitude of binding constants and host selectivity in cation hosts (primarily based on preorganisation, complementarity, solvation and size and shape effects), their application is made much more difficult because of some of the intrinsic properties of anions, listed below. 

From Cation Hosts to Anion Hosts - a Simple Change in pH... 

Metal-based receptors provide excellent frameworks for anion host organisation and can act as reporter groups in electrochemical and luminescent anions sensing applications. 

Dual-host approaches have also been used to good effect for CsN03 extraction. Nitrate is a common anion found upon nitric acid extraction of 137Cs+ in the nuclear industry. Cs+ is conveniently com plexed by a large crown ether such as tetrabenzo[24] crown-8 while simple tripodal amide hosts of type 5.31 (where R is a long alkyl group to impart lipophilicity) are effective at binding N03 . Extraction efficiency of Cs+ from water into 1,2-dichloroethane was found to be enhanced by a factor of up to 4.4 in the presence of the anion host.28... 

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