Preorganisation and Complementarity

9 Cram, D. J., Preorganisation - from solvents to spherands , Angew. Chem., Int. Ed. Engl. 1986,25, 1039-1134. 

Thermodynamic measurements on the analogous (unmethylated) Zn + complexes reveal that the stabilisation by macrocyclic preorganisation has both enthalpic and entropic contributions (Table 1.4). 

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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+.

Perhaps the most important concepts in supramolecular host design and preorganisation and complementarity, which encapsulate more traditional concepts such as the macrocyclic effect. 

Make brief explanatory notes on the following concepts. Source material may be found in both Chapters 1 and 3 (a) the template effect (b) the chelate and macrocyclic effects (c) the high dilution technique in macrocyclic synthesis and (d) preorganisation and complementarity. 

In order to design a host that will selectively bind a particular guest, we make use of the chelate and macrocyclic effects as well as the concept of complementarity (matching of host and guest steric and electronic requirements) and, crucially, host preorganisation. 

Despite the early discovery of the katapinands, non-covalent anion coordination chemistry was relatively slow to develop in comparison with the development of hosts for cations and even neutral molecules. While it is generally true that anion hosts obey the same rules that govern the magnitude of binding constants and host selectivity in cation hosts (primarily based on preorganisation, complementarity, solvation and size and shape effects), their application is made much more difficult because of some of the intrinsic properties of anions, listed below. 

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