Cryptands alkali metal complexes

Cryptands, 7, 731-761 alkali metal complexes NMR, 7, 740 reactivity, 7, 743-744 alkaline earth complexes reactivity, 7, 743-744 anion complexes, 7, 747-748 applications, 7, 753-761 as biological models, 7, 753-754 bis-tren... 

Table 7.1. Formation (kf) and dissociation (kd) rate constants for the alkali metal complexes of the cryptands of type (213) in methanol at 25 °C (Cox, Schneider Stroka, 1978).

Table 1 Structural Features of Alkali Metal Complexes of [2.2.2]Cryptand (12) <73AX(B)383, 69ax...

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... 

Fig. 18 Top-. Structures of alkali metal complexes of 2.2.2-crypt, 9 (hydrogen atoms omitted for clarity) 139—42. Bottom Torsion angles defined by the two triangles obtained by linking the two sets of three oxygen atoms. The smaller the torsion angle, the higher the distance between the two pivot nitrogen atoms (and the larger the cavity size of the cryptand)...

Techniques for the study of alkali metal complex formation involve devices to measure fast reactions. There are only few exceptions, namely very stable cryptand complexes where the binding of metal ions might be elucidated using classical methods. Any of the recording instruments mentioned in Section 2.2 should be suitable to follow the kinetics of the reactions, possibly in combination with a flow apparatus. Processes with half times as low as 1 ms could easily be investigated in this way. (For technical detail cf.46- ). 

Figure 10.8c shows the structure of the cryptand ligand 4,7,13,16,21,24-hexaoxa-l,10-diazabicyclo[8.8.8]hexacosane, commonly called cryptand-222 or crypt-222, where the 222 notation gives the number of O-donor atoms in each of the three chains. Cryptand-222 is an example of a bicyclic ligand which can encapsulate an alkali metal ion. Cryptands protect the complexed metal cation even more effectively than do crown ethers. They show selective coordination behaviour cryptands-211, -221 and -222 with cavity radii of 80, 110 and 140 pm, respectively, form their most stable alkali metal complexes with Li, Na+ and K+ respectively (see Table 10.1 for r on). 

Key words Cryptand, alkali metal cation complexation, picrate extraction. 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

In the case of Kryptofix 221D, a cryptand able to complex the alkali metal cations [141-143], it has been observed that it is solubilized mainly in the palisade layer of the AOT-reversed micelles. And from an analysis of the enthalpy of transfer of this solubilizate from the organic to the micellar phase it has been established that the driving force of the solubilization is the complexation of the sodium counterion. In addition, the enthalpy... 

Crown ethers (Fig. 3.57) and cryptands (Fig. 3.58) can solubilize organic and inorganic alkali metal salts even in nonpolar organic solvents they form a complex with the cation (see Fig. 3.57c), and thus act as an organic mask (Gates, 1992). 

Novel anions stabilized by alkali-polyether cations The ability of a crown (such as 18-crown-6) or a cryptand (such as 2.2.2) to shield an alkali cation by complex formation has enabled the synthesis of a range of novel compounds containing an alkali metal cation coordinated to a crown or cryptand for which the anion is either a negatively charged alkali metal ion or a single electron (Dye Ellaboudy, 1984 Dye, 1984). Such unusual compounds are called alkalides and electrides , respectively. 

Polyether complexation. The solution of the above problem is to add a suitable crown ether or cryptand to the alkali metal solution. This results in complexation of the alkali cation and apparently engenders sufficient stabilization of the M+ cation for alkalide salts of type M+L.M" (L = crown or cryptand) to form as solids. Thus the existence of such compounds appears to reflect, in part, the ability of the polyether ligands to isolate the positively charged cation from the remainder of the ion pair. 

Table 6.2. Thermodynamic data for complexation of alkali metal ions by cryptands in water (Lehn Sauvage, 1975 Kauffmann, Lehn Sauvage, 1976).

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

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