Cryptate stability constants

The dissociation rates for a number of alkali metal cryptates have been obtained in methanol and the values combined with measured stability constants to yield the corresponding formation rates. The latter increase monotonically with increasing cation size (with cryptand selectivity for these ions being reflected entirely in the dissociation rates - see later) (Cox, Schneider Stroka, 1978). 

In water, the relatively low stability of the alkali metal and alkaline earth cryptates (except those for which there is a near-optimal fit of the cation in the intramolecular cavity) has resulted in difficulties in undertaking a wide-ranging kinetic study in this solvent. However, in non-aqueous media, the stability constants are larger and most of the studies have been performed in such media. 

Rate constants for reaction of Ca2+aq with macrocycles and with cryptands (281,282,291) reflect the need for conformational changes, considerably more difficult for cryptands than for crown ethers, which may be considerably slower than formation of the first Ca2+-ligand bond. Ca2+aq reacts with crown ethers such as 18-crown-6 with rate constants of the order of 5 x 107M 1 s, with diaza crown ethers more slowly (286,326). The more demanding cryptands complex Ca2+ more slowly than crown ethers (kfslow reaction for cryptands with benzene rings fused to the macrocycle. The dominance of kA over kt in determining stability constants is well illustrated by the cryptates included in Table X. Whereas for formation of the [2,1,1], [2,2,1], and [2,2,2] cryptates kf values increase in order smoothly and gently, the k( sequence Ca[2,l,l]2+ Ca[2,2,l]2+ Ca[2,2,2]2+ determines the very marked preference of Ca2+ for the cryptand [2,2,1] (290). 

The study of Lehn s cryptands has shown that a three-dimensional arrangement of binding sites leads to very stable inclusion complexes (cryptates) with many cations. For example, the stability constant for K+ in methanol/water (95/5) is five orders of magnitude higher with [2.2.2]-cryptand [37] (log K 9.75 Lehn and Sauvage, 1975) than with [2.2]-cryptand [38] (log... 

For potassium zeolites, cryptofix 222 and cryptofix 222BB, for example, can be used. The structures together with the stability constants Ks of the complexes (cryptates) of cryptofix 222 and cryptofix 222BB with potassium are shown in... 

Table 2 Structures of Potassium Selective Cryptands Cryptofix 222 and Cryptofix 222BB and the Stability Constants of the Matching Cryptates...

As the results make immediately clear (Tables 7—10), it is among ligands of the macrobicyclic type G (27—44), which give complexes of the [2]-cryptate type, that by far the highest stability constants may be found for any cation. In particular, these optimum Ks values are several decades higher than those of the most stable complexes formed by natural ligands,... 

Much more pronounced is the macrocyclic or [l]-cryptate effect found in 10 as compared with 2 the stability constant for K+ complexation increases by about 104 (in methanol) on ring formation. A similar increase has been observed between copper-(II) complexes of acyclic and macro-cyclic tetra-aza ligands (139). 

Table 4. Stability constants for the complexes of macroheterobicyclic ligands (cryptates) in aqueous solution (see Lehn et al. (42, 43))... 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

Table 8 Log Stability Constants of Some Cryptate Complexes in Water, Log Kl...

Figure 26 Stability constants (log Ka) of the alkali cryptates (left, in 95 5 methanol/water, M/W, or in pure methanol, M, at 25 °C) and of the alkaline earth cryptates (right, in water at 25 °C) (reproduced with permission from...

The results (Table 10) show that the cryptands could act to produce carrier-mediated facilitated diffusion and there was no transport in the absence of the carrier. The rate of transport depended upon the cation and carrier, and the transport selectivity differed widely. The rates were not proportional to complex stability. There was an optimal stability of the cryptate complex for efficient transport, logKs 5, and this value is similar to that for valinomycin (4.9 in methanol). [3.2.2] and [3.3.3] showed the same complexation selectivity for Na+ and K+ but opposing transport selectivities. The structural modification from [2.2.2] to [2.2.C8] led to an enhanced carriage of both Na+ and K+ but K+ was selected over Na+. The modification changes an ion receptor into an ion carrier, and indicates that median range stability constants are required for transport. Similar, but less decisive, results have been found in experiments using open-chain ligands and crown ethers.498... 

The stability and selectivity patterns of cryptands were found to be markedly solvent dependent and stability constants of Ag[2]cryptates in a range of solvents are presented in Table 62.474"478 Thermodynamic data for their formation in water are given in Table 63.476... 

Table 62 Stability Constants (Log K) for Silver (2]Cryptates in Various Solvents at 25 °C...

Fabbrizzi and co-workers have demonstrated the use of bis-copper(II) cryptates to sense ambidentate anions [49]. On titrating molecule 71 with NaN3 in aqueous solution, the colour changed from pale blue to bright green and an anion-metal LMCT absorption appeared at 400 nm. X-ray diffraction studies have shown that the azide anion is held colinear with the two Cu(II) centres, coordinated through the two terminal sp2-hybridised nitrogen atoms. Stability constants for 71 with a variety of anions in aqueous solution were calculated and the selectivity of this anion sensor was found to be controlled by the bite distance between the two copper atoms (Fig. 1). 

The rubidium and cesium complexes of [2.2.2] are isomorphous with approximate D3 symmetry and a crystallographic twofold rotation axis (41). While the rubidium cation is complexed almost without strain, the Cs+ is accommodated only by enlarging the cavity, increasing the mean C—C torsion angle to 71° (compared with 54° for the potassium cryptate). The ligand deformations required to complex Na+ and Cs + are reflected in their lower solution stability constants with respect to the K+ and Rb+ cryptates (see Section III,D). 

The stability constants for cryptate formation lEq. (3)] have been... 

Stability Constants of Cryptates and Macrocyclic Analogs in 95% MeOH-H20 at 298 K"... 

Stability constants for complexes between Ln(CF3SC>3)3 and various diazacrown ethers and cryptates in... 

Fig. 4.20. Stability constants of lanthanide cryptates in propylene carbonate at 298 K and p. = 0.1 M (NettCKL). From data reported by J.-C.G. Biinzli, in Handbook on the Physics and Chemistry of Rare Earths, eds K.A. Gschneidner, Jr., L. Eyring, Vol. 9, Ch. 60, North Holland, Amsterdam, 1987.

Figure 3-1 Stability constants for cryptate-221 and -222 versus alkali ion in MeOH/H20 (95 5).

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