Tetrahedral recognition

Development of ditopic receptors 58 for both an anion and a cation is a logical extension [19b]. It should be stressed that the recognition by cyclodextrins, calixarenes, hemicarcerands, and many other systems does not involve specific recognition sites, whereas tetrahedral recognition by 53 and 54, the adenine 56 selective binding, and multiple recognition like the one shown in formula 59 [20] require the presence of specific sites in receptor capable of directional interactions. 

Recognition of a tetrahedral substrate geometry requires the construction of a receptor molecule with a tetrahedral recognition site. This may be realized by positioning four suitable binding sites at the corners of a tetrahedron and incorporating them into a bridged molecular framework. 

The spherical macrotricyclic ligands are also particularly suited for tetrahedral recognition by virtue of the location of the nitrogens. This aspect will be discussed in Section 21.3.8. 

Figure 3.63 Tetrahedral recognition of NH4+ by the soccer ball cryptand (3.29).

Figure 4.8 Tetrahedral recognition the soccer ball cryptand as a receptor for ammonium ion, water or chloride, depending on the pH of the medium.

Considering together the three cryptates [NH4 c (5)] (10), [H2O e (5)-2H ] (11) and [Cr cz (5)-4H ] (7), it may be concluded that the spherical macrotricycle (5) is a molecular receptor possessing a tetrahedral recognition site in which the substrates are bound in a tetrahedral array of hydrogen bonds. It represents a state of the art illustration of the molecular engineering involved in abiotic receptor chemistry. 

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