Cryptands ellipsoidal

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

Ellipsoidal cryptands can also be synthesized by direct alkylation procedures <77AG(E)720,80CB1487), obviating the need for a diborane or lithium aluminum hydride reduction step. In the case of [l.l.l]cryptand (15a) yields of the final amine alkylation step are enhanced by the amine proton itself acting as a template (81CC777). 

In a useful extension of the direct alkylation method, ellipsoidal cryptands have been prepared from dimethylated cyclic diamines by the stepwise quaternization-demethylation sequence shown in Scheme 4 <81 H.3039). 

As seen in Figure 14 the ellipsoidal cryptands may be prepared by the simultaneous formation of three bridges. This route has found limited use due to low yields but has been employed in the synthesis of some cryptands including the tris-pyridino system (36) (81TL3035) and bis-tren (18) (80UP52100). 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

Ellipsoidal cryptands of type (12) generally complex alkylammonium cations relatively weakly. However the spherical cryptand (13) contains four nitrogen atoms which define a tetrahedral cavity large enough to complex the tetrahedral cation. The crystal structure... 

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). 

In order to further develop the coordination chemistry of anions and to extend recognition of anionic substrates beyond the spherical halides, an ellipsoidal macro-bicyclic cryptand Bis-Tren (14) was designed, whose hexaprotonated form was expected to bind various anions [9, 10]. Indeed, potentiometric and spectroscopic measurements showed that (14)-6H complexes a number of monovalent and polyvalent anions. The strong and selective binding observed for the linear triatomic anion NJ may be attributed to its complementarity to the molecular cavity of (14)-6H . As confirmed by crystal structure determination, NJ forms the cryptate [N c (14)-6H ] (15), in which the substrate is bound inside the cavity by two pyramidal arrays of three hydrogen bonds, which hold the two terminal... 

In the supramolecular species, [Nj cz (14)-6H ] (15), the ellipsoidal cryptand is built from two protonated tripodal subunits of the tren type N(CH2CH2NH2)3 located at each pole of the molecule and linked by three bridges. The two subunits are situated at a distance such that each may hold one of the two terminal nitrogens of the NJ ion, so that they cooperate in substrate binding. Bis-Tren-6H may be considered as a ditopic coreceptor for linear triatomic substrates of a size compatible with the size of the molecular cavity. 

Figure 14 X-ray structure of 31 showing the inclusion of the linear triatomic Ns" ion in the ellipsoidal cavity of the hexaprotonated cryptand (red, oxygen blue, nitrogen gray, carbon). ...

---