Other Hydrogen Bonded Capsules

Figure 3. Capsules based on other hydrogen-bonding atoms omitted) Tetraurea calixarene dimer 16-16,...

Hydrogen-bonded capsules and other higher architectures... 

Co-encapsulation of [2,2]-paracyclophane with other guests shown in Scheme 3.103 and its mechanically regulated rotation within the cavity of the hydrogen-bonded capsule 429 has been studied in [105] as follows from H NMR data, these co-guests are able to affect rotational freedom of the caged paracyclophane. 

The design and synthesis of supramolecular architectures with parallel control over shape and dimensions is a challenging task in current organic chemistry [13, 14], The information stored at a molecular level plays a key role in the process of self-assembly. Recent examples of nanoscopic supramolecular complexes from outside the dendrimer held include hydrogen-bonded rosettes [15,16], polymers [17], sandwiches [18, 19] and other complexes [20-22], helicates [23], grids [24], mushrooms [25], capsules [26] and spheres [27]. 

A molecular capsule can be composed of more than two units also a sphere can be obtained assembling together a generic number n of slices. Rebek and collaborators, in their extensive exploration of the properties and features of self-assembled capsules [36], developed a L-shaped molecule characterized by the presence at one end of the glycoluril hydrogen bond system, and at the other end, separated by an aromatic ring, a sulfamide moiety (Fig. 14). 

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. 

Other types of molecular capsules with included anions have been described, but they do not involve directed hydrogen-bonding interactions between functional groups on the inside of the capsule and the guests [78, 79]. 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

Self-assembly has also been applied to numerous different classes of complexes in supramolecular chemistry [26]. Since we cannot discuss them all here, Fig. 1.3 shows only one example of a capsule reversibly formed from two identical self-complementary monomers which are bound to each other by hydrogen bonding... 

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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