Challenges in Anion Receptor Chemistry

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 rnles 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 mnch more difficult because of some of the intrinsic properties of anions, listed below. 

In comparison to cations of similar size, anions have high free energies of solvation and hence anion hosts mnst compete more effectively with the snrronnding medium, e.g. AGhydration(F ) = 465 kJ mol  

In considering the differential or selective binding of one anion or another, the intrinsic properties of anions mean that we are not on a level playing field it is considerably easier to bind some anions than others. In general, in the absence of specific chemical recognition between anion and host, anion binding selectivity, particularly in solvent extraction experiments or in the detection of anions by membrane-based ion selective electrodes, follows the order of anion hydrophobicity. This order is termed the Hofmeister series, or lyotropic series and was first outlined in 1888 from experiments based on the 

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