Cryptands cryptate effect

CRYOENZYMOLOGY CRYPTANDS CRYPTATE EFFECT CRYPTIC CATALYSIS CRYPTIC STEREOCHEMISTRY CRYSTAL FIELD SPLITTING LIGAND FIELD SPLITTING CRYSTAL FIELD THEORY Crystal growth,... 

The stability of cryptate complexes. The cage topology of the cryptands results in them yielding complexes with considerably enhanced stabilities relative to the corresponding crown species. Thus the K+ complex of 2.2.2 is 105 times more stable than the complex of the corresponding diaza-crown derivative - such enhancement has been designated by Lehn to reflect the operation of the cryptate effect this effect may be considered to be a special case of the macrocyclic effect mentioned previously. 

Macropolycyclic ligands containing intramolecular cavities of a three-dimensional nature are referred to as cryptands. The bicyclic cryptands (73) exist in three conformations with respect to the terminal nitrogen atoms, exo-exo, endo-exo and endo-endo 6 these forms can rapidly interconvert via nitrogen inversion but only the endo-endo form has been found in the crystal structures of a variety of complexes372 and for the free ligand ([2.2.2], 73, m = n = / = l).449 In their complexes with alkali and alkaline earth cations, the cryptands exhibit an enhanced stability over the crown ethers and coronands dufe to the macrobicyclic, or cryptate, effect.33 202... 

Cryptands 7-9 thus function as receptors for spherical cations. Their special com-plexation properties result from their macropolycyclic nature and define a cryptate effect characterized by high stability and selectivity, slow exchange rates, and efficient shielding of the bound ion from the environment [2.17,2.27]. 

Fig. 4 Complex formation of podands (a)-(c)log Kj of the Na and complexes of (a) pentaglyme, (b) 18-crovvn-6. and (c) [2.2.2] cryptand demonstrating the macrocyclic and cryptate effects (d)-(f) Rb" coordination spheres in the solid-state. structures of the Rbl complexes of bisquinolino podands of different chain length (cf Fig. 2j) (g)-(i) podand anion complexation (g) podand HPO4" complex, and podands for (h) halide (Cl , Br ) and (i) carboxylate (benzene-1.3.5-tricarboxylate) complexation and (j,k) complexes of podands with uncharged molecules involving (j) thiourea and (k) adenine.

Macropolycyclic ligands, commonly referred to as cryptands, contain intramolecular cavities of three-dimensional shape ( crypts"). In their complexes Q cryptates") with alkali and alkaline earth cations, they display considerably enhanced stabilities with respect to crown ethers " cryptate effect") Thus, the K complex of... 

For [2.2.2]open the only data available are for Na+ and K+ in 95 5 MAV, and the cryptate effects (Alog Xcryp) are 3.86 and 4.95 for Na+ and K+, respectively, similar to those found with the [2.2]N(CH3)2, but 3-18 times lower than those of 2.2-BHF. It thus appears that [2.2]BHE is the closest corollary in terms of binding affinities compared to the cryptand [2.2.2], although in most cases, a significant cryptate effect is seen in progressing from the macrocycle to the cryptand. In summary, while affinities do depend... 

Interestingly, the K+ complex of 2.2.2 is approximately 10 more stable than the corresponding diazacrown complex. Such enhanced stabihty has been termed by Lehn the cryptate effect (which encompasses both enhanced kinetic as well as thermodynamic stability). The cryptate effect may be considered to be a special case of the well-known macrocyclic effect. In this context, it needs to be noted, however, that a number of complexes exist in which a metal ion is coordinated exo to the cryptand s cavity (or which only partially occupy the cavity) and, of course, the cryptate effect will be absent or greatly reduced in such cases. Similarly, complexes of 2 1 (metal cryptand) are known. For example, cryptand 2.1.1 forms a complex of type [Pb2(2.1.1)]" + for which it was postulated that one or both metals are bound exo to the cavity as clearly both lead ions are unable to simultaneously occupy the cryptand s central cavity. ... 

The effect of cryptands on the reduction of ketones and aldehydes by metal hydrides has also been studied by Loupy et al. (1976). Their results showed that, whereas cryptating the lithium cation in LiAlH4 completely inhibited the reduction of isobutyraldehyde, it merely reduced the rate of reduction of aromatic aldehydes and ketones. The authors rationalized the difference between the results obtained with aliphatic and aromatic compounds in terms of frontier orbital theory, which gave the following reactivity sequence Li+-co-ordinated aliphatic C=0 x Li+-co-ordinated aromatic C=0 > non-co-ordinated aromatic C=0 > non-co-ordinated aliphatic C=0. By increasing the reaction time, Loupy and Seyden-Penne (1978) showed that cyclohexenone [197] was reduced by LiAlH4 and LiBH4, even in the presence of [2.1.1]-cryptand, albeit much more slowly. In diethyl ether in the absence of... 

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... 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

Cryptand was obtained as the corresponding sodium cryptate 20 in 27% yield. The cation-free cryptand was isolated by passing an acidic solution of the complex through cation- and anion-exchangers. However, the overall yield was not reported. When sodium carbonate was replaced by potassium carbonate, no detectable amount of cryptate K+ c [2.2.2] was observed this confirms the involvent of a template effect, although this method is rather limited to simple mononucleating cryptands. 

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