Alkali and Alkaline Earth Metal Cryptates

Alkali and alkaline earth metal cryptates. D. Parker, Adv. Inorg. Chem. Radiochem., 1983, 27, 1-26 (150). 

As well as an acid-independent pathway, an acid-dependent pathway also occurs for the dissociation of a number of the alkali and alkaline earth metal cryptates (Table 7.1). Similar behaviour to that just discussed for methanol also occurs for reactions in a range of other non-aqueous solvents (Cox, Truong Schneider, 1984). 

Alkali and Alkaline Earth Metal Cryptates David Parker... 

Extensive thermodynamic and kinetic data have been collected concerning interactions between macrocyclic ligands and cations especially alkali and alkaline-earth metal ions p4. The formation rates of cryptates of alkali and alkaline earth metal cations have generally been estimated by combining observed rates for the dissociation reaction with the independently measured formation constants 3S. Thus if C = cryptand... 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor... 

Crown and Ciyptate Complexes.—Interest in crown, cryptate, and related complexes of alkali-metal (and alkaline-earth-metal) ions has been maintained during the period of this Report. A new class of ether-ester ligands (4) and (5) has been developed.Whereas (4) forms complexes with Mg, Ca, Sr, and Ba, (5) forms complexes with Ca ", Sr ", and Ba only. The complex between (4) and Mg is the first reported Mg crown compound although the Mg " is probably bound within the cavity, it is possible that it may be bound to the externally directed l,3-dicarboxy-group. " ... 

In water, the relatively low stability of the alkali metal and alkaline earth cryptates (except those for which there is a near-optimal fit of the cation in the intramolecular cavity) has resulted in difficulties in undertaking a wide-ranging kinetic study in this solvent. However, in non-aqueous media, the stability constants are larger and most of the studies have been performed in such media. 

The alkaline-earth metal salts form complex cryptate-type compounds analogous to those formed by the alkali-metal salts (see Chapter 1 complexes of alkali metals). The bivalent metal ions, however, can take both water molecules and anions into the cryptate cage. Two compounds containing Ba and one containing Ca have been reported. The structures of these compounds are detailed in Table 2. In (8) and (9), the cryptate molecule is of the... 

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