Capsule-like dimers

Calix[5]arenes have also been demonstrated to form crystalline capsule-like dimers. Specifically, a tetra-substituted calix[5]arene with four methyl and iodo groups attached at opposite rings along the upper rim formed a dimer that encapsulated buckminsterfullerene, or C60 (Fig. 18).36 Raston has also described dimers based on tetrabenzyl-derivatized calix[3]- and calix[5]arenes that encapsulate C60 in the solid state.37,38 The components assembled via van der Waals forces. 

Fig. 45 X-ray crystal structure of the carcerand-like dimeric capsule 55 with pyrazine as a guest...

An early example of a self-assembled calix[4]pyrrole system was reported in 1996 by Sessler and coworkers [79]. Solid state analysis of the calix[4]pyrrole monoacid 38 revealed that it forms a homoditopic complex both in the solid state (cf. Fig. 12.24) and in organic media upon deprotonation. This dimer could be broken (i.e., disassembled) by exposure to a source of fluoride anions. However, it is only in recent years that calix[4]pyrroles have been explored for the creation of capsule-like systems that can be responsive to a specific stimulus [79]. 

Crossley (89) described the self-assembly of a spherical cage-like structure made up of two zinc porphyrin dimers bound to a tetramine ligand. The 2 1 complex is stable at 10-6m concentrations, but addition of excess of ligand causes dissociation of the capsule with formation of a 1 1 complex. 

Rebek and co-workers have also produced self-assembling capsules where the monomer possesses C3 symmetry (as opposed to C2), by incorporating multiple glycoluril moieties into a flattened aromatic surface. For example, jelly donut dimer 50 forms in CDCI3 from monomer 49. This capsule features a flattened spherical interior cavity that can accommodate disk-shaped molecules like benzene and eyclohexane (Scheme 4-11) [43]. 

All force-distance experiments were conducted in mesitylene (1,3,5-trimethylbenzene) solution. Mesitylene is a good solvent for calixarenes and has a high boiling point, which is desirable for single-molecule experiments. The dimeric calixarene capsules have a cavity size suitable for the inclusion of solvent molecules such as mesitylene. Because hydrogen bridges inside the capsules can be influenced by the dielectric permittivity of the guest molecules, it is important to test whether impurities of the solvent may affect the experimental result. Toluene is probably the most likely contamination. Control experiments were conducted with 5 vol% toluene as an... 

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