Complex formation cryptands

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. 

An investigation of the kinetics of formation of the Li+ and Ca2+ complexes of cryptand 2.1.1 using stopped-flow calorimetry suggests that complexation occurs initially at one face of the cryptand such that the metal is only partially enclosed (to yield an exclusive complex). Then follows rearrangement of this species to yield the more stable product, containing the metal ion inside the cryptand (the inclusive product) (Liesegang, 1981). X-ray diffraction studies have indeed demonstrated that exclusive complexes are able to be isolated for systems in which the metal is too large to readily occupy the cryptand cavity (Lincoln et al., 1986). 

The kinetics and thermodynamics of complex formation in methanol for the interaction of cryptands 2.1.1,2.2.1 and 2.2.2 with the alkali metal... 

Rates of decomplexation (kJ2) of cation complexes can also be determined by nmr spectroscopy on the cation. Rates of complex formation are then calculated from kn and the binding constant. The results for several ligands, cations, and solvents are given in Table 20. Despite the wide variations, the rates of complex formation are all in the range 2 x 107 to 8 x 10 M 1 s 1. In contrast, rates of decomplexation for crown-ether complexes span a much broader range 6.1 x 102 to 2 x 105 s 1. Comparison of crown-ether data with data for [2.2.2]-cryptand [37] and the linear polyether [92] also shows that the... 

The addition of a cryptand to some polyelectrolytes leads to significant increases in conductivity and in some cases IR and Raman spectroscopy demonstrate that the cryptand breaks up the ion-ion interactions (Chen, Doan, Ganapathiappan, Ratner and Shriver, 1991 Doan, Ratner and Shriver, 1991). Apparently the reduction of ion association more than offsets the reduction in mobility of the cation-crypt complex, which has a larger effective radius than the simple cation. It is also possible that the cryptand-ion complex is rendered more mobile by the reduction of polymer-cation complex formation, but this point has not been investigated in any detail. 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

The complexation of metal ions with neutral molecules has also been studied extensively. We consider here the complexes of metal ions with macrocyclic ligands such as crown ethers and cryptands [33]. The 1 1 complexation of a metal ion Mn+ with macrocyclic ligand L is expressed by Eqs (2.8) and (2.9), where K is the complex formation constant ... 

The number of donor atoms can be influential in complex formation. Ideally, as the incoming ligand is comparable to an inner solvation sphere, the number of donor atoms should match the preferred coordination number of the cation. An example of this factor comes from a comparison of two similarly sized ligands, [2.2.2]cryptand and [2.2.C8]cryptand, the latter having one dioxoether chain replaced by a C8 unit.478 A reversal of the Ba2+/K+ selectivity of the order of 106 is found, the ratio being 104 for [2.2.2]cryptand and <10 2 for [2.2.C8]cryptand the preferred coordination numbers are six for potassium and eight for barium. 

Table 9 Overall Rates for Complex Formation Between Cryptands and Alkali Metal Cations (MeOH, 25 °C)...

The larger-ringed macrocycles of (53a-d) form binuclear complexes with alkali metal, alkaline earth, silver(I) and lead(II) cations.68,192 The two 18-membered rings are large enough to allow for cations as large as Rb+ to be incorporated within their cavity, with a net result of increasing the metal-metal separation. Thus, crystal structure data for the disodium complex of (53a) indicate the sodium ions to be 6.40 A apart,193,194 compared to a 3.88 A separation found for the aforementioned disilver complex of (52a). Heteronuclear complex formation has also been observed, e.g. with both Ag+ and Pb2+ incorporated in the same cryptand.192... 

Buschmann, H. J., and Schollmeyer, E. (2000) The interpretation of thermodynamic data for the complex formation of cations with crown ethers and cryptands. Part I The reaction entropy, J. Incl. Phenom. Macrocycl. Chem. 38, 85-97. 

Pioneering work in this area was carried out by the groups of Lehn and Martell [34, 35]. One example of a metal containing cryptand is dicopper(II) complex 14 which was shown to interact with various anions such as N3 , OCX. SCN. SO)2. HCOCT, CH3COO, HC03-, and 03 [36]. Complex formation can easily be detected by the color change of an aqueous solution of the receptor from blue in the absence of suitable anionic substrates to green in their presence. 

If complex formation takes place in one phase of a heterogeneous system, e.g. when crown ethers or cryptands (ligand L) are present, Eq. (10) has to be modified ... 

The results for the isotopic separation of Na/ Na which were obtained by Knochel and Wilken in the system Dowex 50/aqueous or methanolic solution of cryptands are summarized in Table 13 (explanation for Krl and Kr see Chap. 4.3.1.2). To reach a high total enrichment compared with one equilibrium stage, the batch experiments were carried out as a cascade (Chap. 2.5.2). Then Eq. (20) was used for the calculation of a-values. To determine the isotopic separation factor Mr for the complex formation as well, the Kr-vuIucs were analyzed in the same system without cryptands " .iss) see Chap. 4.3.1.2). In all experiments 30 mg cation exchanger resin (Li - or Cs -form) were equilibrated with a 10M Na -solution where a lithium or cesium salt, which corresponds to the counterion of the resin, was added up to a total cation concentration of 10 M. If one has used a complexing ageftt, the initial cryptand concentration has been established to be 10 M (pH = 8). For most of the systems, the standard deviations given in Table 13 correspond to seven parallel experiments. The measurement of the radionuclides Na and Na was carried out as described in Chap. 4.2.4. 

Table 13. Stability constants of complex formation of cryptands with alkali cations...

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- ). 

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