Calixarenes hydrogen bonded assemblies

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

The relatively low thermodynamic stability of complexes of hemicarcerands or other container-type hosts is a direct consequence of structural aspects of the walls that make up the inner surface of such compounds. These walls are lined by aromatic subunits while free electron pairs of heteroatoms such as those of the ether oxygen atoms are preferentially oriented to the outside. Complexes are therefore enthalpically stabilized only by weak dispersive interactions. In the case of positively charged guests cation-re interactions can contribute to binding enthalpy as in a self-assembled calixarene-derived capsule [9], but directed interactions such as hydrogen-bonding interactions are usually absent. 

In fact, as expected, the interaction of these calixarene derivatives with barbiturates e.g. 136 (or cyanurates 138) in the ratio 1 2 results in the formation of box-like assemblies consisting of nine particles (3 x 135 and 6 X 136) held together by 36 hydrogen bonds. These aggregates are stable on the NMR timescale at ambient temperature in apolar solvents such as CDC13 or toluene-d6. However,... 

Three equivalents of a calix[4]arene derivative, diametrically substituted at its upper rim with two melamine units, have also been demonstrated to yield well-defined box-like assemblies on interaction with six equivalents of 5,5-diethylbar-bituric acid. The product, which is based on an extended rosette motif, consists of nine different components held together by 36 hydrogen bonds. It is stable in apolar solvents and remains so even at very low concentrations. Further, the structure remains intact in the solid state, as confirmed by an X-ray diffraction study. In an extension of this study, three enantio-pure calix[4]arene-dimelamine derivatives were employed for a parallel study. In this instance, the chiral information in the individual calixarene derivatives proved successful in controlling the conformation of the final assembly - a homochiral supramolecular entity was the result. 

Bohmer has reported the ability of a bowl-shaped molecule known as a calixarene to self-assemble in the solid state to form a homodimer.35 A calix[4]arene with four urea groups attached to the upper rim self-assembled via 16 hydrogen bonds to give a cavity with an approximate volume of 200 A3 (Fig. 17). The cavity hosted a highly... 

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). 

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],... 

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. 

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