Dissociation kinetics crown ether complexes

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 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. 

NMR spectral techniques are widely used to study complexation of macrocycles as well as the thermodynamic and kinetic quantities that describe host-guest interactions. Measurements of the free energy of activation for complex dissociation (AG. ) at the H NMR coalescence temperature, have been done using a variable temperature H NMR procedure . The method is advantageous for ammonium cations because the dissociation rate for most crown ether-ammonium cation complexes is within the H NMR time scale and only a small amount of sample (3-5 mg) is required. 

Finally, we quote the work of Chang et al. (1988) who have reported on the dissociation kinetics of lanthanide (La, Eu, Lu) diaza-crown ether carboxylate complexes, K22MA, K22MP, K22DP (general structure below)... 

N -diacetic acid (DACDA) in conjunction with TT A (chloroform). Ligands of this type offer interesting possibilities as they combine the characteristics of crown ethers with those of aminopolycarboxylic acid complexants. At pH 5, only R(TTA)3 is extracted, while a 1 1 1 complex R(DAPDA,DACDA)TTA is extracted at pH7.5-8.0. The extraction kinetics at pH 5 are slow, governed by the rate of dissociation of aqueous metal-crown complexes, but are relatively fast and independent of the aqueous macrocycle complex dissociation rate in the high-pH regime. These kinetic results suggest the possibility of interlanthanide separations based on differences in com-plexation rates. 

The mechanism of the reaction with various alkyl halides in NMP is believed to involve nucleophilic attack by both the solvent-separated ion-pairs Na2[Fe(CO)J (kj) and [Na(Fe(CO)J (k j) (where k 2 = O.lSdm mol s and Atj < t 2) evidenced by the non-linear dependence of the observed rate constant on the concentration of complex. In THF the dissociation of the ions is less extensive, and the rate of reaction with alkyl halides is correspondingly slower. However, the addition of polar solvents or crown ethers increases the rate of the reaction, and this has been ascribed to the kinetically more reactive, more dissociated species. 

When the organic solvent containing crown ether is used in the form of a liquid membrane between two aqueous solutions (see Section 5.4.4), an alkali metal salt can be selectively transferred through the organic liquid membrane from one aqueous feed solution to an aqueous strip solution. Illustrative treatment of the kinetics and mechanisms of formation and dissociation of the metal complexes with crown ethers is available in Burgess... 

The kinetic deuterium solvent isotope effects on the acid-catalyzed dissociation of Li(2,l,l) and Ag(2,2,2) have been reported, as have the exchange kinetics " of cryptand (2,2,1) on Tl(2,2,2) and T1(2b,2,2) . The kinetics of complex formation between Li and 18-crown-6 ether in 1,3-dioxalane and 1,2-dimethoxyethane solvents have also been reported. ... 

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