Cryptates cation exchange kinetics

The cation exchange kinetics of the cryptates [1.27] are several orders of magnitude slower than those of the macrocyclic complexes [2.20b] and usually decrease as the stability of the complex increases. 

Preliminary kinetic results on cation exchange in cryptates [ligand (21)] show some correlation with thermodynamic parameters, but only go a small way towards the complete elucidation of the mechanism. Metal replacement in complexes of macrocyclic ligands is also illustrated by studies of several metalloporphyrins. ... 

Complete kinetic determinations are apparently limited to the lipophilic [2.2.2, C14] cryptand 4b which behaves as a PTC catalyst. In this case as well, reactions follow a mechanism similar to that of quaternary salts the observed pseudo-first-order rate constants are linearly correlated with the concentration of the cryptate in the organic phase. Moreover, in a water-chlorobenzene system it is found entirely in the organic phase, as cryptand and/or cryptate. Anion exchange dora not require the simultaneous transfer from the aqueous phase of the cationic partner. 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

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