Cyclic ligands and metal-ion selectivity

Finally, a discussion of the kinetic features of the macrocyclic effect (the kinetic macrocyclic effect ) mentioned in Chapter 1 is deferred until the next chapter. 

As the flexibility of the macrocycle increases, then mismatch hole-size effects are expected to be moderated. In any case, as discussed in Chapter 1, a metal ion which is too large for the cavity may be associated with folding of a flexible macrocycle thereby allowing normal metal-ligand bond distances to be achieved. However, this is not always the case, and a number of examples of unfolded macrocyclic complexes containing compressed metal-donor distances are known (Henrick, Tasker Lin-doy, 1985). 

The binding constants are also influenced by the number of ion-dipole interactions present involving the metal and the ether oxygen donors. As the ring size increases, so will the number of available dipoles this effect will be superimposed (and may dominate) the metal-ion binding pattern along a series of crowns. 

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

In contrast to the peak selectivity just discussed, there is evidence that the larger, more flexible, ligands tend to exhibit plateau selectivity - a reflection that a number of the larger metal ions are accommodated by the cryptand without major variation in binding energy. 

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Certain other metal ions also exhibit catalysis in aqueous solution. Two important criteria are rate of ligand exchange and the acidity of the metal hydrate. Metal hydrates that are too acidic lead to hydrolysis of the silyl enol ether, whereas slow exchange limits the ability of catalysis to compete with other processes. Indium(III) chloride is a borderline catalysts by these criteria, but nevertheless is effective. The optimum solvent is 95 5 isopropanol-water. Under these conditions, the reaction is syn selective, suggesting a cyclic TS.63... 

Another important type of complex is formed between metal ions and cyclic organic compounds, known as macrocycles. These molecules contain nine or more atoms in the cycle and include at least three heteroatoms, usually oxygen, nitrogen, or sulfur. Crown ethers such as 18-crown-6 and dibenzo-18-crown-6 are examples of organic macrocycles. Some macrocyclic compounds form three-dimensional cavities that can just accommodate appropriately sized metal ions. Ligands known as cryptands are examples. Selectivity occurs to a large extent because of the size and shape of the cycle or cavity relative to that of the metal, although the nature of the heteroatoms and their electron densities, the compatibility of the donor atoms with the metal, and several other factors also play important roles. 

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