Complexation kinetics cryptands

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). 

For a more extensive discussion of the kinetics of complexation with cryptands and natural antibiotics, the reader is referred to a recent review of Liesegang and Eyring (1978). 

Polyether complexation. The kinetics of formation of polyether crown, cryptand and related complexes have received considerable attention. Since formation rates are often quite fast, techniques such as temperature-jump, ultrasonic resonance, and nmr have typically been used for such studies. 

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... 

Thermodynamic behaviour of the complexes should be differentiated from their kinetic behaviour. For instance, conformationally most rigid cryptands form their complexes very slowly. 

The complex Me3SiCH2SiMe20Li (2) (7Li shift 0.74 ppm) is the only product identified in the reaction of 1 (7Li shift 2.5 ppm) with D3 or D4 at 20 °C in toluene even in the presence of excess siloxane, as shown in equation 1. Addition of the cryptand [211] shifted the Li resonance to —0.93 ppm in agreement with other lithium cryptand [211] complexes. This lithium silanolate was then shown to initiate polymerization of D3, D4, Dg and functional cyclics such as (SiMe(HC=CH2)0)4 and (SiMe(CH2CH2CF3)0)3. Kinetic measurements using this initiator show a reactivity order of D3 D4 >Ds >Dg and the results are in good agreement with those previously reported for anionic polymerization under similar conditions. Co-polymerization reaction involving vinyl dimethyl cyclics... 

The pioneering work of Lehn established the fundamental strategy of using preformed three dimensional cage ligands, dubbed cryptands, to ensure enhanced thermodynamic and kinetic stability for cation complexation [1], Positively charged cryptands, typically the N-donor azacryptands, can also serve to encapsulate anions. 

Tummler, B. Maass, G. Weber, E. Wehner, W., Voegtle, F., (1977) Noncyclic crown-type polyethers, pyridinophane cryptands, and their alkali metal ion complexes Synthesis, complex stability and kinetics J. Am. Chem. Soc. 99, 4683-4689. 

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... 

The kinetics of the complexation reaction of Lil with a cryptand in an imidazo-lium-Tf2N IL and one with a functionalized ammonium cation has been studied by Shirai and Ikeda [71]. Here, they found no difference in kinetics for the two ILs, ruling out strong cation participation in the mechanism. 

Usually, the ion exchange rate between a solvated metal ion and the ion fixed in the complex compoimd is very fast. This is the reason why most of the cation complexes with cyclic polyethers have high exchange rates which are in the order of 10 s >20) Although the exchange rate for metal ions with a bicyclic cryptand is significantly lower than with monocyclic crown ethers the kinetic isotopic... 

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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