Anion receptors, supramolecular hosts

Figure 1.9 Supramolecular host-guest complexation stabilised by positive cooperativity between binding sites Ag+ binding by 1.12, a host for citrate anion (1.13) and a drug-receptor complex formed by vancomycin (1.14).

Keywords Anion-n interactions Anion coordination Anion receptors Anions Host-guest systems Supramolecular chemistry... 

Reviews Dietrich, B. (1993) Design of anion receptors Applications, Ptire and Appl. Chem. 65, 1457 Bianchi, A., Bowman-James, K. and Garcia-Espana, E. (1997) Supramolecular Chemistry of Anions, Wiley-VCH, New York Schmidtchen, F. P. and Berger, M. (1997) Artificial Organic Host Molecules for Anions, Chem. Rev. 97,1609-1646. 

These problems have been addressed in a wide variety of imaginative and novel ways and progress in anion complexation has been rapid in recent years " to the extent that it has now been described by Lehn as a full member of the field of supramolecular chemistry. It is important to note that the search for anion-selective receptors has not been limited to the mimicry of Mother Nature s approach. Indeed, a great number of the hosts developed are far from being biocompatible as the tools of the chemist are not limited to the building blocks of natural systems. This aspect of anion recognition chemistry lies at the heart of supramolecular chemistry, the interface between chemistry and biology. ... 

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... 

Binding of hard anions occurs strongly at the hard Lewis acidic uranyl center, whereas cation- tt interactions are established between the aromatic side arms and the cation counterpart of the ion pair. Thus, complexation of alkali metal salts (MX) such as CsCl and RbCl with 15 resulted in the formation of isomorphous supramolecular assemblies in the solid state. In the dimeric [15-CsCl], each cation is coordinated to six oxygens, three from each receptor, thus creating a pseudo-crown-ether-Uke environment for the cation. Additionally, each metal ion in the dimeric unit is coordinated to both halide ions and, most importantly, to two aromatic side arms, one from each of the receptors giving decacoordination for the cation. The closest metal ion-aromatic carbon distances of 3.44(1) A for CsCl, 3.34-3.38(1) A for RbCl, and 3.58(1) A for CsF are observed in the respective alkali halide complexes [15 MX] indicating the conformational flexibility of the side arms and adaptability of the receptors 15 and 16 to form multiple cation- rt interactions with the hosted cations. 

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