Cryptates dissociation constants

The dissociation constant of ion-pairs of the cryptated sodium salt is relatively large, 10 5 M at —98 °C, hence the k value of the free polymethyl methacrylate anion is most reliable, when computed from kinetic results of its propagation. 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Figure 2. The dissociation constants for cryptates depend on the relative size of the metal ion and the cavity in the cryptand.

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). 

These complexes, unlike the crown ether complexes but similar to the aza-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid catalysis.504 Association constant values for lanthanide cryptates have been determined.505,506 A study in dimethyl sulfoxide solution by visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Yb are higher than those of the other lanthanides). The values are set out in Table 9.507... 

The dissociation reactions of TE cryptates in water (and other solvents) have been followed conductimetrically in stopped-flow experiments (67). An equilibrium mixture of TE cryptate, with a concentration ratio of TE to cryptate > 1, was mixed with an excess of acid (HCIO4, CF3SO3H). The pseudo-first-order rate constant, k, was found to be linearly dependent on the acid concentration ... 

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... 

Table 1.6 Rate constants for dissociation of cryptates in various solvents.

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... 

Data, which refer to 25 "C, are from ref. 13. The dissociation rate constant kt corresponding to the addition of excess acid to the cryptate is used to define ka and kti+ ke=ka... 

Rate constants and activation parameters for the dissociation of Eu and Yb cryptates at 298 K. ... 

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