Formation kinetics cryptates

Under mild hydrothermal conditions, around 200°C in the presence of a liquid phase, an increase in pressure of up to 10 bar does not seem to have a marked effect on the nature of the products obtained. On the other hand, thermal effects, which act by decreasing kinetic barriers, mostly influence the physicochemical properties of the solvent, favoring electrostatic interaction and the formation of hydrogen bonds. These interactions, that play a major role in the molecular recognitibn that characterizes the formation of cryptates [46,47], are probably the cause of the very surprising stnjctures observed under those conditions (see Chapter 4, Section 4.3.1). However, the state of current knowledge of these reactions requires some caution in the inteipretalion of the mechanisms involved under such conditions. 

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. 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

Use of cryptates leads to the formation of a simple equilibrium between cryptated alkoxlde Ion pairs and free Ions. The Kj) has been measured directly for K + [222] t Kp 3x10 . This value Is In good agreement with that determined from two sets of kinetic experiments made with and without 4 + (222]... 

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 anionic polymerization of cyclosiloxanes was examined In benzene and toluene with lithium cryptates as counterions. Only one type of active species Is observed In the case of LI + [211] thus, the kinetics of the propagation and of the by-product cyclosiloxanes formation can be studied In detail for the first time. The reactivity of cryptated sllanolate Ion pairs toward the ring opening of D3 Is greatly enhanced compared to that of other systems. 

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... 

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... 

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... 

Several determinations of stability constants have been performed in methanol (table 15) and water (table 16). The lanthanide cryptates have a slow kinetics of formation so that measurements are difficult to perform precisely. For the La(III) ion, Anderegg (1981) has found that one month is necessary to reach equilibrium... 

---