Complexation organic guests

It should be noted that molecular complexes of the cyclodextrins may be isolated as crystalline solids for example, a crystalline complex is obtained with iodine (which resembles the well known blue complex between iodine and starch) as well as with a large number of other inorganic and organic guests. 

A number of capped cyclodextrins which are able to bind a metal ion at one end of their cavity, together with an organic guest in the cavity, have been synthesized. Such species parallel in several respects the family of completely synthetic vaulted transition-metal complexes prepared by Busch and coworkers and already discussed in Chapter 3 (section 3.5). 

In view of the compensatory enthalpy-entropy relationship observed for a wide variety of ionophore types, we may conclude that the cation-binding behavior, where the weak ion-dipole and dipole-dipole interaction is the major driving force for complexation, can be quantitatively analyzed and characterized by the slope and intercept of the AH-TAS plot without any exception. In this context, it is stimulating to extend the scope of this theory to the inclusion complexation of organic guests with molecular hosts. 

Figure 23. AH-TAS plot for inclusion complexation of organic guests by a-( ), P-(o), and y-cyclodextrins (ffl).

Cyclophanes consist of a class of artificial hosts featured with well-defined hydrophobic cavities constructed by aromatic rings incorporated in their macrocy-clic structures, and also with high design versatility because they are totally synthetic.The first direct evidence of the formation of an inclusion complex with an organic guest was obtained for tetraazacyclophane 62, the cavity of which is constructed with diphenylmethane units bridged by tetramethylene chains. 

Empty ft- (11) and y-cyclodextrins (12) also take normal torus shapes, as revealed by X-ray crystallography, and no significant deformations are observed for these two cyclodextrins when they bind guest molecules. It is noteworthy that all empty cyclodextrins include water molecules in their cavities as shown in Table IV and Fig. 4. Since no water molecules were observed in inclusion complexes of cyclodextrins with organic guest molecules, it is evident that the expulsion of these water molecules in the cyclodextrin cavities is one of the important factors for formation of the inclusion complexes. 

---