Cryptates, formation, dissociation

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... 

The dissociation rates for a number of alkali metal cryptates have been obtained in methanol and the values combined with measured stability constants to yield the corresponding formation rates. The latter increase monotonically with increasing cation size (with cryptand selectivity for these ions being reflected entirely in the dissociation rates - see later) (Cox, Schneider Stroka, 1978). 

The kinetics of formation and dissociation of the Ca2+, Sr2+ and Ba2+ complexes of the mono- and di-benzo-substituted forms of 2.2.2, namely (214) and (285), have been studied in water (Bemtgen et al., 1984). The introduction of the benzene rings causes a progressive drop in the formation rates the dissociation rate for the Ca2+ complex remains almost constant while those for the Sr2+ and Ba2+ complexes increase. All complexes undergo first-order, proton-catalyzed dissociation with 0bs — kd + /ch[H+]. The relative degree of acid catalysis increases in the order Ba2+ < Sr2+ < Ca2+ for a given ligand. The ability of the cryptate to achieve a conformation which is accessible to proton attack appears to be inversely proportional to the size of the complexed metal cation in these cases. 

Detailed information about the mechanism of carrier complex formation can be obtained by relaxation techniques (17) and NMR studies (80, 100—102). The rate constants of the formation reaction for monactin/Na+ (sound absorption) and valinomycin/Na+ (sound absorption, T-jump) in methanol are about 2 108 and 7 10 M-1 sec-1 respectively, and the corresponding rate constants of the dissociation reactions are 4 105 and 5 105 sec-1 (17). In contrast, the dissociation rate constant for some cryptates is much smaller (42, 103, 122). 

It should be mentioned that cation complexation by crown-type ligands can itself be solvent-dependent. For example, the dissociation rate of potassium [2.2.2]cryptate in EPD solvents increases with the donor number of the solvent [650]. Moreover, coronands themselves can interact with organic solvent molecules [651]. Such cation-solvent and ligand-solvent interactions can influence the formation of cation-ligand complexes. 

Dissociation (ki) and Formation (kf) Rate Constants for TF Cryptate Complexes, Eq. (5) ... 

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... 

Dissociation rates were obtained at high acidity where kn is fast and were corrected for ionic strength using a Debye-Huckel equation. The results show that the cryptate selectivity results mainly from kh and that the transition state for the reaction has little interaction between the metal and the cryptand to differentiate between metals. Rates kt increase with increasing cavity size. The thermodynamics of cryptand formation in water and methanol have also been used to calculate the free energy and enthalpy of transfer of the free ions between the two media. 

It is of interest now to consider the kinetics of complex formation (kf) and dissociation (kd) of chelate, macrocyclic, and cryptate complexes and how these are related to the type of ligand stmcture. Table 10 lists kf and kj valnes for chelating ( 1-3), macrocyclic ( 4 and 5), and cryptand ( 6-8) ligands and the appropriate reference compounds. For the chelate effect, the reactions of Ni " with py, bipy, and terpy show very little difference in kf values. On the other hand, the valnes of kj decrease by factors of 2 x 10 and 2 X 10, for bipy and terpy, respectively. A similar effect is seen in a comparison of the reactions of Cn + ion with the thioether macrocycle [14]aneS4 (ane denoting a saturated carbon macrocycle and S4 (he four sulfur donor atoms) and its acyclic... 

Table 10 Rate constants for the formation and dissociation of chelate, macrocyclic, and cryptate complexes along with the corresponding complexes of reference hgands"." - ...

The ultrasonic technique has been used to study the binding of calcium to sorbitol the interaction is weak and the formation rate constant (40 C) could only be approximately determined as (1-2) x 10 dm mol" s . The kinetics have also been studied of the dissociation of calcium cryptates in various solvents " (see Table 9.4) and of cryp tand exchange... 

Kinetics of the formation and dissociation of the cryptates Ag(2,2,2) and K(2,2,in acetonitrile/water mixtures have been reported. The... 

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