Cryptands exclusive complex

An investigation of the kinetics of formation of the Li+ and Ca2+ complexes of cryptand 2.1.1 using stopped-flow calorimetry suggests that complexation occurs initially at one face of the cryptand such that the metal is only partially enclosed (to yield an exclusive complex). Then follows rearrangement of this species to yield the more stable product, containing the metal ion inside the cryptand (the inclusive product) (Liesegang, 1981). X-ray diffraction studies have indeed demonstrated that exclusive complexes are able to be isolated for systems in which the metal is too large to readily occupy the cryptand cavity (Lincoln et al., 1986). 

An exclusion complex of this type has been identified by X-ray crystallography for the KNCS complex of the smaller [2.2.1]cryptand,454 ([2.2.1]cryptand = 73, n = 1, m = 1, / = 0). The cation occupies a site in the 18-membered ring provided by the ligand rather than in the central cavity and so resembles the coordination of potassium by 18-crown-6 (Figure 21a). In contrast, the smaller Na+ cation is of optimal size for the [2.2.1]cryptand and so an inclusion complex occurs with the cation inside the cage (Figure 21b).454 Binuclear inclusion complexes have not yet been isolated for bicyclic cryptands. 

An interesting result reported recently by the same group of workers (70) relates to the discovery of two types of 1 1 caesium cryptates for cryptand C222. The possibility for the existence of an exclusive complex... 

An exclusive complex of this type was actually found also in the crystalline state for the KNCS complex of the smaller [2.2.1] cryptand From molecular models, the cavity radius of this ligand was estimated to be 1.1 A which is too small for potassium to enter. As a result, occupies a site in the 18-membered ring rather than in the central cavity (see Fig. 38), thus resembling the coordination of K" by [18]crown-6. Additionally, the potassium ion is bonded to the isocyanate anion whereas anions are generally not coordinating to alkali metal ions in inclusive cryptates. 

Because of the particular properties of crown ethers and cryptands in forming complexes with alkali and alkaline earth elements, it is understandable that these elements have been exclusively Investigated in respect to isotopic separations. Among these elements the enrichment of heavy calcium isotopes for medical investi-... 

Recently, two cryptands and their lanthanide complexes have been synthesized which include either a bipyridyl (L ) or pyridyl (L ) chromophore 89), These have proved effective at populating the lanthanide excited states. Aqueous luminescence lifetimes of up to 1.12 ms and quantum yields of up to 0.02 (europium) and 0.25 (terbium) are reported. The better luminescence from complexes of ligand is due to the better exclusion of water from the lanthanide center. 

In these solid inclusion componnds, the association of host and guest components is strictly a solid state phenomenon, and the examples covered in this chapter focns almost entirely on solid inclnsion componnds of this type. However, it is relevant to note that there is also another class of inclnsion compound in which the host is a molecnle containing an appropriate cavity or binding site that can inclnde gnest molecules. Such molecular host-guest complexes generally exist as associated entities both in the solntion state and in the crystalline state, and thns the incln-sion phenomenon is not exclusively a property of the solid state. Examples of host molecules of this type are crown ethers, cyclodextrins, cryptands, and calixarenes. 

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