Dissociation kinetics cryptates

Kinetics of H+-promoted dissociation of the Ni2+ complex of a tetra-dentate aza-oxa-cryptate derived from tren, conducted in acidic aqueous acetonitrile, indicate that its dissociation rate is smaller than that of [Ni(tren)(H20)2]2+, despite the much higher thermodynamic stability of the tren complex a kinetic cryptate effect is invoked to rationalize this (288). 

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 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. 

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. 

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... 

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... 

The kinetics of the release of Cs+ from its complexes with (3), (8), and two cryptands [(9) and (10)] have been measured in various solvents as a function of temperature by Cs n.m.r. spectroscopy. The activation enthalpy is substantially larger in the case of the cryptates than for the crown ether complexes (Table 6) but this is more than compensated by a favourable entropy of activation. Similar high values of A/T were obtained for the dissociation of Na+ from its complex with (10) in various solvents (Table 7) but in this case the sign of A5 was generally negative. 

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... 

Rare earth cryptates (table 5) are more stable than coronates and are highly kinetically inert towards dissociation in aqueous solutions. They can therefore be studied in water, but hydrolysis hinders their synthesis in this solvent. On the contrary, the preparation of unsolvated 1 1 complexes requires anhydrous conditions (Gansow and Triplett, 1981). The hydrated lanthanide salt solutions in... 

Ligand dissociation processes are usually slow, especially for cryptates which are highly inert complexes. They have been investigated for a few macrocyclic complexes either by NMR spectroscopy or by electrochemical methods. The Ln(III)/Ln(II) reduction kinetics is not discussed here, but will be reviewed in 4.1. 

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