Cryptates formation constants

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

The formation constants for cryptates, which show greatly enhanced stability and selectivity compared to crown ethers, can be —lO6 times greater than those of the ethers. As for Na+ and K+, there are both synthetic and natural ligands with selective affinity for Ca2+, the latter exercising a controlling effect in Ca2+ metabolism. One example is the antibiotic A, a monocarboxylic acid that binds and transports Ca2+ across membranes. It is tridentate and forms a 7-coordinate complex Ca(N,0,0)2(H20). 

Not only does chelation make the complex more stable, but it also forces the donor atoms to take up adjacent or cis sites in the resulting complex. Equation (2) shows how displacement of a chelating carbonate ion gives the unusual cis dichloride product instead of the thermodynamically more stable trans dichloride. Polydentate chelating ligands with three or more donor atoms also exist. Macrocyclic ligands, such as (4) and (5), confer an additional increment in the formation constant (the macrocyclic effect) they have been given trivial names, such as cryptates (4) and sepulchrates (5).i... 

Extensive thermodynamic and kinetic data have been collected concerning interactions between macrocyclic ligands and cations especially alkali and alkaline-earth metal ions p4. The formation rates of cryptates of alkali and alkaline earth metal cations have generally been estimated by combining observed rates for the dissociation reaction with the independently measured formation constants 3S. Thus if C = cryptand... 

Motekaitis RJ, Martell AE, Lehn JM, Watanabe E (1982) Bis(2,2, 2"-triaminotriethylamine) cryptates of cobalt(II), nickel(II), copper(II), and zinc(II). Protonation constants, formation constants, and hydroxo bridging. Inorg Chem 21 4253 257... 

Motekaitis R, Martell AE, Murase I, Lehn JM, Hosseini MW (1988) Comparative study of the copper(II) cryptates of C-BISTREN and 0-BISTREN. Protonation constants, formation constants, and secondary anion bridging by fluoride and hydroxide. Inorg Chem 27 3630-3636... 

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). 

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. 

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). 

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). 

Detailed information about the mechanism of carrier complex formation can be obtained by relaxation techniques (17) and NMR studies (80, 100—102). The rate constants of the formation reaction for monactin/Na+ (sound absorption) and valinomycin/Na+ (sound absorption, T-jump) in methanol are about 2 108 and 7 10 M-1 sec-1 respectively, and the corresponding rate constants of the dissociation reactions are 4 105 and 5 105 sec-1 (17). In contrast, the dissociation rate constant for some cryptates is much smaller (42, 103, 122). 

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... 

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... 

Dissociation (ki) and Formation (kf) Rate Constants for TF Cryptate Complexes, Eq. (5) ... 

A similar effect occurs with the cryptates. Comparing the stability constants of c 2.2.2] and its one-strand opened analogue (54) (known as a lariat ether , see later) the stability increases by a factor of 10 (Figure 2.16), and this is known as the cryptate effect or as the macrobicyclic cryptate effect (47). After comparing the thermodynamic quantities associated with formation of the [K c 2.2.2] complex to those for the K -dicyclohexyl-18-crown-6 (2) complex, Kauffmann and co-workers concluded that the cryptate effect is of enthalpic origin (52). 

Table 10 Rate constants for the formation and dissociation of chelate, macrocyclic, and cryptate complexes along with the corresponding complexes of reference hgands"." - ...

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