2.4 Effects of Complexation

The physicochemical properties of the supramolecular species resulting from the binding of a substrate to a receptor may differ drastically from those of either of the component species, with corresponding changes in electronic, ionic, optical, and conformational properties.

An example is the formation of AC and AEC crown ether complexes and cryptates. Three major effects occur: decreased cation/anion interaction, cation protection, and anion activation. Whereas in crown ether complexes, the bound cation is still accessible for ion pairing from the open "top" and "bottom" faces, this is not (or is less) the case for the cryptates, which surround the cation more completely. Cryptate formation transforms a small metal cation into a very large, spheroidal, charge-diffuse organic cation—a sort of superheavy AC or AEC (Lehn 1980). The stability of the cryptates and the large distance imposed by the organic ligand shell separating the enclosed cation from the environment have many physical and chemical consequences. Cryptation promotes the dissociation of ion pairs, resulting in strong anion activation. It markedly affects numerous reactions, such as those involving the generation of strong bases, nucleophilic substitutions, carbanion reactions, alkylations, rearrangements, and anionic polymerizations, to name a few. Equivalently, reactions involving the cation become inhibited due to cryptation. Crown ethers and cryptands, either alone or fixed on a polymer backbone, have been used in many processes, including selective extraction of metal ions, isotope separation (Heuman 1985; Chen and Echegoyen 1992), incorporation of radioactive or toxic metals, and cation-selective analytical methods.

Another example concerns the tetrahedral binding of ammonia, as mentioned earlier. The strong binding results in an effective pK_a for bound NH₄⁺ that is about six units higher than that of free NH⁺₄. (Graf et al. 1982). This is an example of how large the changes may be in substrate (or in receptor) properties brought about by binding. Similar changes may take place when substrates bind to enzyme-active sites and to biological receptors.