Self assembly calixarene dimers

Figure 31.1 shows the structures of some of the calixarene and calixpyrrole derivatives that were found to self-assemble into dimeric capsules (see structures 2-5). The tetraureacalix[4]arenes (2) were found to from dimeric capsules with internal volumes estimated to be in the range of 160-200 [lib]. These dimeric capsules were found to encapsulate solvent molecules like chloroform and benzene and small ammonium salts [11]. In addition, it was demonstrated with the aid of diffusion NMR [16] that such dimers have a high affinity for charged systems, and it was found that tropylium cation and cobaltocenium cation have higher affinities than benzene and ferrocene, respectively, for the cavity of the dimer of 2 [17]. These and other studies demonstrated that indeed diffusion NMR is an excellent means to probe encapsulation [17, 18], a fact that was instrumental in the study of the large hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes as will be demonstrated below [18-20]. 

The inclusion of guests is not the only interesting aspect of the dimers formed by tetra-urea calixarenes. Numerous other self-assembled capsules are known, larger in volume, different in shape, and able to include more than one guest molecule, and various aspects of guest inclusion have been extensively studied [45]. 

The principal idea of this present essay was to show how the unique preorganization of functional groups in self-assembled dimers of tetra-urea calix[4]arenes can be used to prepare novel multi-rotaxanes and -catenanes or topologically even more complex molecules and supramolecular structures. We will conclude by summarizing some related studies in which calixarenes were used in a different way as building blocks for the construction of such structures or assemblies. 

In the specific case of calixarenes, there are very few studies related to metal-mediated self-assembly into cage molecules. Early examples reported by Shinkai and coworkers include bis(acac)-calix[4]arene dimer (1) held together by Cu2+ cations... 

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

Rebek and co-workers have studied the self-assembly of molecules containing substituted ureas.65 Dimerization of two calixarenes (where Bn is benzyl and Ar is 4-fluo-rophenyl ) (7.8) by hydrogen bonding produces a host that encapsulates benzene, fluorobenzene, p-difluorobenzene, and pyrazine, but not toluene. 

Not only the covalent combination of calixarenes with other building blocks leads to products with precisely defined molecular cavities. Julius Rebek and his group at Scripps were the first to explore the non-covalent assembly of calix[4] arenes to dimeric capsules in which small guest molecules can be complexed [27]. Together with the group of Volker Bohmer they have explored this self-assembly in great detail [28]. This chemistry is described by Aragay and Ballester in Chap. 32. 

Complementarity between different molecules has also been harnessed in the pursuit of capsular complexes. Schrader and Kraft have pioneered the design of complementary systems with three- and fourfold symmetry based on 1,3,5-tris(aminomethyl)benzene derivatives [18] and calix[4]arenes [19], respectively. This approach has also been taken by Reinhoudt whose vast output of calixarene chemistry includes work on calix[4]arene dimers held together by ionic interactions [20]. Other examples show how the presence of encapsulated guests templates the formation of self-complementary capsules [21], how two host molecules may be held together by metal coordination [22-24] or where hydrogen bonding by a second entity holds the assembly together [25,26],... 

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