Cation-binding hosts spherands

The cation-binding potential of ligands can be due to the relief of electron-electron repulsion upon complexation. The class names Spherands have been suggested for ligands of this type, e.g. (11) and metallospherium salts for their complexes with metal salts. Ideal hosts of this new type describe an oxygen shell whose cavity can be occupied only by spherical entities, such as single atoms or monoatomic ions, and not by parts of the hosts or by solvents. ... 

The hemispherands, spherands, calixarenes, and related derivatives. A number of hosts for which the pre-organization criterion is half met (the hemispherands) (Cram et al., 1982) or fully met (the spherands) (Cram, Kaneda, Helgeson Lein, 1979) have been synthesized. An example of each of these is given by (251) and (252), respectively. In (251), the three methoxyl groups are conformationally constrained whereas the remaining ether donors are not fixed but can either point in or out of the ring. This system binds well to alkali metal ions such as sodium and potassium as well as to alkylammonium ions. The crystal structure of the 1 1 adduct with the f-butyl ammonium cation indicates that two linear +N-H - 0... 

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+.

By way of comparison, acyclic podand hosts analogous to compound 3.33 have been produced in order to assess the importance of the rigid preorganisation afforded by a cyclic host. Comparison of the closely related podand 3.36 (which is estimated to possess over 10 000 possible conformations, only two of which can bind cations in a convergent manner) and 3.33 (which is locked in only one conformation) shows that the spherand binds Li+ more than 1012 times more effectively. This highlights the importance of preorganisation effects in host design. 

Spherands Hosts preorganised for binding cations , in Comprehensive Supramolecular... 

What are the consequences of this preorganization toward binding strength To answer this question. Cram and CO workers prepared compounds 49-53 which increase the number of m-anisyl units from 1 to 5 in a stepwise fashion relative to 18-crown-6 derivative 48. Table 1 shows the values of AG° (kcalmoP ) measured for these hosts toward the alkali-metal cations and NH4+ in CDCI3 saturated with water by the picrate extraction method. Several trends are noteworthy (i) Introduction of one or two anisyl units (49 and 50) lowers the overall binding affinity and selectivity toward the series of guests due to decreased preorganization relative to crown ether 48 (ii) For compounds with three, four, or five anisyl units (51-53), which substantially define the spherand cavity, the affinity and selectivity for the smaller Na+ and Li+ cations becomes more pronounced. 

E. Maverick and D. J. Cram, Spherands hosts preorganized for binding cations, in Comprehensive Supramolecu-lar Chemistry, ed. G. W. Gokel, Elsevier Science, Amster-dam/New York, 1996, vol. 1, pp. 213-244. 

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