Metal cations cryptates

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. 

Table 10 Rates and Selectivities of Alkali Metal Cation Transport via Cryptate Complexes...

As noted earlier, the similarities between H+ and alkali metal cations have led to the use of the former as a probe in biological studies, including studies with various macrocydic ligands, especially those with oxygen donor atoms. The thallium(I) cryptates behave kinetically like the potassium compounds, and the binding constants to 18-crown-6 have been measured by 205T1 NMR methods.347 Several Tl1 compounds with crown ethers (L) have been prepared in... 

Indeed, macrobicyclic ligands such as 7-9 form cryptates [Mn+ c cryptand], 10, by inclusion of a metal cation inside the molecule [1.26, 1.27, 2.17, 2.24-2.26]. The optimal cryptates of AC and AEC have stabilities several orders of magnitude higher than those of either the natural or synthetic macrocyclic ligands. They show pronounced selectivity as a function of the size complementarity between the cation... 

Inorganic cryptates in which a metal cation is enclosed in an inorganic cage structure have been reported [2.76,2.77]. 

In general, discrete Zintl anions, whether they obey the simple octet rule or are electronically more complex, can often be obtained intact from the initial Zintl phase by substituting tetraalkylammonium ions for the alkali metal cations or by encapsulating the latter in a cryptand such as cryptate-222 (see 1-XXII).25... 

Crown ethers and other cryptates form with several metal cations stable complexes, which generally are soluble in nonaqueous media, even in rather nonpolar solvents [394]. In some cases such complexes may be considered supporting electrolytes, especially when some of the special properties of a given cryptate are desirable also in other contexts. By complexing Ag with a cryptate, the Ag/AgCryp electrode becomes stable in DMF [201]. 

The anionic polymerization of cyclosiloxanes is a complex process. For the alkali metal silanolate catalysts the weight of experimental evidence supports a mechanism based on growth from the metal silanolate ion pair. The ion pair is in dynamic equilibrium with ion-pair dimers which, for the smaller alkali metal ions like lithium and sodium, are themselves in dynamic equilibrium with ion-pair dimer aggregates. The fractional order in catalyst which is observed is a direct result of the equilibria between ion pairs, ion-pair dimers and ion-pair dimer aggregates. Polar solvents break down the aggregates and increase the concentration of ion-pair dimers and hence the concentration of ion pairs. Species like crown ethers and the [2.1.1] cryptate which form strong complexes with the metal cation increase the dissociation of ion-pair dimers into ion pairs. In the case of the lithium [2.1.1] cryptate dissociation into ion pairs is complete and the order in catalyst is unity. 

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