Spherands binding constants

Figure 1.12 Comparison of the effects of preorganisation and complementarity on the magnitudes of the binding constant of polyether hosts for alkali metal cations. The figure for Li+ is given for the highly preorganised spherand-6 since it is too small to accommodate K+.

Fig. 1,1, Preorganization does matter. A comparison of 18-crown-6 and the spherand on the right with respect to alkali metal ion binding reveals that the spherand has an up to 10 orders of magnitude higher binding constant.

X 10 forNa+,2.8 X 10 for, and 1.3 x 10 for Li These constants were of a magnitude that persuaded the authors to liken the binding properties of these calixar-enes to cryptands and spherands. 

These macrocycles show the highest stability constants known for certain alkali metal ions, and remarkable selectivity. For example, when the ionic diameter (1) for the guest metal cations becomes too large for the preformed cavity no complexation occurs. The three spherands (84)-(86) bind only Na" and Li no binding to could be detected (81). This behaviour is in contrast to that of crowns and cryptands in which size-related peak selectivity is found. 

---