Double cavity calixarenes

Steric effects are shown to affect the outcome of the aroylation reaction as illustrated, for example, by the reaction with 3,5-dinitrobenzoyl chloride which affords an excellent yield of a di-aroylate and provides a starting material for the synthesis of a double cavity calixarene, as illustrated in Figure 4. 

Fig. 9. Complexation of a double cavity calixarene with carboxylic acids.

Fig. 12. Ti values for guest molecules, free and complexed with a double cavity calixarene.

One of the important applications of butylcalix[4]arenes arises from their ability to trap alkali metal ions. In particular, Cs+-calixarene complexes have received much attention because of the need to remove the Cs radionucleotide from nuclear wastes. Benevelli et al. have used one-pulse solid state NMR experiments to directly observe Li, Na and Cs ions in the host cavity [52]. More advanced experiments, which allow the investigation of metal lattice interactions were also reported. Rotational-echo double resonance (REDOR) NMR is a useful tool for obtaining structural details of butylcalix[4]arene [53]. Gullion and coworkers used REDOR to determine the position of the... 

The fullerene-rich calix[6]arene complexes. (C60 or C7o)2calix[6]arene, were structurally authenticated as isomorphous complexes,showing the calixarene in the double-cone conformation with a fullerene perched in each of the shallow cavities, resembling the jaws of a pincer acting on two adjacent cage molecules the... 

The four-fold symmetry of calixarenes and resorcinar-enes relates to complexation of the siloxane. 8, with 50 in toluene. " Here the fullerenes are arranged in double columnar arrays shrouded by edge on siloxanes (Fig. 2f), the siloxanes interlocking by one of the phenyl groups of one molecule residing in the cavity of another. 

For comparison, in the strucmre of [(calix[6]arene)(C7o)2] the C70 molecules are bound noncovalently to the pseudo-cavities of calix[6]arene in the double cone conformation [9]. The overall structure is complicated and is isostructural with [(calix[6]arene)(C6o)2]. despite the anisotropic shape of C70, The calixarenes are in the double-cone conformation, and each of the associated shallow cavities is occupied by a fuUerene. The overall arrangement according to the authors resembles the jaws of apincer acting on two adjacent ellipsoids , and the principal axis of the ellipsoidal C70 molecule is not directed toward the calixarene cavity (Fig. 33.4). 

Calix[6]arenes in partial double cone and inverted partial double cone conformation may adopt in each pseudocalix[4]arene a guest molecule [49]. But it is also observed that the guest molecule is located in one cavity while the second one is occupied by a one of the substituents of neighboring calixarene, similarly to inclusion dimer formation in calix[4]- and calix[5]arenes. The examples are shown in Fig. 38.23. 

There is only eight crystal structures of calix[7]arenes deposited in CSD. The reason for this is much more difficult synthesis of seven-membered calixarenes and/or difficulties in obtaining monocrystals of this compounds and their complexes/co-crystals. Like for smaller calixarenes discussed above also here the self-inclusion of one of the substituent may occur like in para-t-hvXy -carboxymethoxycalix[7]arene/deuterochloroform clathrate (Fig. 38.25a) [52] or inclusion complexes are obtained. For the later, due to a great degree of flexibility, many conformations of calixarene host molecule is possible but the most observed is a double partial cone, e.g. 1 2 para-t-butyl-calix[7]arene/pyridine inclusion complex where one guest molecule is located in each partial cone cavity (Fig. 38.25b) [53]. In one case the flattened cone conformation was found where the (Fig. 38.25c). In this crystal structure one toluene and one disordered benzene/ toluene molecule are involved in inclusion complex formation with one para-t-butyl-calix[7]arene molecule [54]. 

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