Anion receptors, supramolecular neutral hosts

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

In summary, our approach of using cyclopeptides with natural amino acids and 3-aminobenzoic acid subunits for the development of macrocydic receptors has afforded remarkably efficient hosts. The cation affinity of 4b, for example, exceeds that of many calixarene derivatives. Even more interesting is the high anion affinity of 5 in aqueous solution. By introdudng additional functional groups such as car-boxylates to the periphery of the cavity, we recently also obtained cydopeptides that interact with neutral substrates, for example, carbohydrates [25]. Our peptides therefore represent a versatile dass of artificial receptor that should prove useful in supramolecular and bioorganic chemistry. 

Ferrocene has been by far the most used redox-active group in a supramolecular sensor due its stability, its ease of functionalization and its well-understood and reversible redox chemistry. A host of feiTOcene-contain-ing compounds are now known that bind cations anions, and neutral molecules. It can be argued that the first examples of redox-active receptors, for which the complexation of other species was demonstrated, were ferrocenyl crown ether compounds, as reported in the late 1970s and in 1980. " However, it was not until... 

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