Guest Inclusion by Hydrogen Bonding

Most without pobr or hydrogen bonding site 

Host with polar or hydrogen bonding sites Guest with polar or hydrogen bonding site 

Benniston, A. C., Cydophanes endoaddic, endobasic and endolipophilic cavities , in Encyclopedia of Supramolecular Chemistry, Atwood, J. L. and Steed J. W. (eds), Dekker New York, 2004, vol. 1, pp. 424-431. 

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Cyclotriveratrylene/1180-60-5] (34) is a cyclocondensation product of veratrole with formaldehyde. It possesses a stable trigonal crown conformation and forms crystalline inclusion compounds with benzene, chlorobenzene, toluene, thiophene, decalin, chloroform, acetone, carbon disulfide, acetic acid, and butyric acid (101). Their structures consist of columns of cyclotriveratrylene molecules that are not amenable to close packing and provide channels into which the guests are accommodated. A number of modified host structures, derived from prototype (34) have been prepared. The hexaphenol analogue cyclotricatechylene also yields well-defined channel inclusions (101). They involve mosdy polar guests and the structures are held by hydrogen bonding. 

Chiral discrimination of guests by inclusion in hydrogen-bonded capsules of tetraureas is a highly interesting and challenging problem. The analysis of such systems by NMR spectroscopy is complicated by two facts ... 

The most common hosts for inclusion polymerization are urea, thiourea, perhydrotriphenylene (PHTP), deoxycholic acid (DCA), apocholic acid (ACA) and tris(o-phenylenedioxy)cyclotriphosphazene (TPP)(Fig. 2). They have the common feature of forming channel-like clathrates, but differ in many specific properties. For instance, urea and thiourea have a rigid structure in which the host molecules are connected by hydrogen bonds and possess a high selectivity towards the guests. In urea channels are rather narrow whereas in thiourea they are wider as a consequence, linear molecules include only in urea and branched or cyclic molecules in thiourea. On the contrary, chainnels existing in PHTP clathrates are very flexible and can accomodate linear, branched and cyclic molecules. 

Figure 35. Diagrammatic (projection) representation of two different modes of channel inclusion involving hydrogen bonding interactions (indicated by broken lines) (a) guest molecules retained by steric barriers formed by the hydrogen-bonded host lattice (b) hydrogen bonds also exist between host and guest components.

A clathrate hydrate is a crystalline inclusion compound in which small guest molecules, usually hydrophobic, are trapped in polyhedral cages formed by hydrogen-bonded water molecules. True clathrates are formed by guests that interact with the hydrate lattice only by weak, nondirectional forces. In such cases, the water molecules form a completely hydrogen-bonded network, and the inaterials effectively are ices. A number of structures are known for true clathrate hydrates, including the three major families of clathrate hydrate structures that will be discussed later. 

By the way, the sizes and shapes of both canals and guest molecules v ich can be accommodated depend on the mutual positions and on the separation of two adjacent bilayers formed by hydrogen bonds among host molecules [8]. This suggests that monomers with different sizes and shapes can be included into the canals in different ways, which may bring different microstructures of the polymers obtained by inclusion polymerization in the canals. 

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