Macrotricycles spherical

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

The macrotricyclic ligands (240) and (241) may be synthesized by multistep high-dilution procedures (Graf Lehn, 1975). They contain spherical cavities which are able to accommodate suitable guests whether they be cationic, neutral, or anionic. 

Thus the spherical macrotricyclic molecule represented below gives a tetraprotonated species that has very unusual binding properties. It binds very strongly the spherical chloride anion, holding it in an ideal tetrahedral arrangement of N+—H Cl- hydrogen bonds. This complex is much more stable and the Cl /Br selectivity is much higher than any other known to date. These properties may be attributed to the presence... 

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. 

Fig. 2. Some macropolycyclic structures (a) macrocyclic, (b) macrobicyclic, (c) cylindrical macrotricyclic, and (d) spherical macrotricyclic. L-j- = 5, 8, 8 and 9, respectively [1.27].

Spherical recognition of halide ions is displayed by protonated macropolycyclic polyamines. Thus, macrobicyclic diamines yield katapinates [3.9]. Anion cryptates are formed by the protonated macrobicyclic 16-6H+ [2.52] and macrotricyclic 21-4H+ [2.97] polyamines, with preferential binding of F and Cl- respectively in an octahedral and in a tetrahedral array of hydrogen bonds. 

H+ binds Cl- very strongly and very selectively compared with Br- and other types of anions, giving the [Cl- c (21-4H+)] cryptate 26. Quaternary ammonium derivatives of oxygen free macrotricycles of type 21 also bind spherical [3.10a] and other [3.10b] anions. 

Macrotricyclic cryptates may have either spherical or cylindrical topology (90). The cylindrical ligands, such as 5, are formed by linking together two macrocycles and define three cavities, two lateral and one central cavity inside the macrotricycle (18,19,91, 92). The macrotricyclic ligands of spherical topology are Particularly well suited for com-plexation of alkali cations as they define a spherical cavity (93, 94). Ligand 10, for example, contains a spherical cavity (diameter 3.4 A)... 

By replacement of just one oxygen atom in 5 by a methylene group, affinity of the corresponding cryptand 15 to NH4+ decreases by a factor of ca. 100 (NH4+ c 15 log /Ca = 4.3). The macrobicycle 16 has almost completely lost the complexation ability and the selectivity of 5 (NH4+ c 16 log /Ca = 1.7). This dramatic effect results from the removal of one bridge of 5, i.e., from a decrease in cyclic order from the tricyclic to the bicyclic ring system, demonstrating the importance of the spherical macrotricyclic structure for the binding properties of 5. 

The high-dilution method (for technical details see for example Ref. 21) has been used not only to form simple cryptands, but also in the syntheses of more complex systems — cylindrical macrotricyclic molecules24), lateral macrobicycles 25), spherical macrotetracyclic systems 26), and speleands 27). Nevertheless, the important drawback of the method is the last step, e.g. reduction with diborane. That means only compounds which do not interfere with this reagent can be formed in this manner. Also, even a small contamination of the solvents and reactants with water has a significant influence on the yield. 

Synthesis of diaza-polyoxa-macrobicyclic compounds (cryptands) and spherical macrotricycles ligands (supercryptands) (see 1st edition). 

The above-described spherical macrotricyclic cryptand 13 was designed for the ammonium cation, whereas the series of cylindrical macrotricycles 15 (Fig. 15) was... 

Anion cryptates are formed by macrotricycles like (5) in their tetraprotonated state with the spherical halide anions [8]. (5)-4H binds the chloride ion very strongly and very selectively, giving the [Cl" c (5)-4H J cryptate (7), but does not complex other types of anions. These properties are unique at present with respect to both synthetic and natural halide binding sites, very little being known about the latter. Non-complementarity between an ellipsoidal cryptand and the spherical halides results in appreciable ligand distortions in the cryptates formed and in lower binding constants [9, 10] (see also below). 

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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