Cryptate effects

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. 

Kinetics of H+-promoted dissociation of the Ni2+ complex of a tetra-dentate aza-oxa-cryptate derived from tren, conducted in acidic aqueous acetonitrile, indicate that its dissociation rate is smaller than that of [Ni(tren)(H20)2]2+, despite the much higher thermodynamic stability of the tren complex a kinetic cryptate effect is invoked to rationalize this (288). 

Cryptate effect, 7 576 Cryptates, 7 576-577 14 161 Cryptococcus, 26 446, 475 76 Cryptococcus noeformans, genome of, 26 450t... 

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

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). 

Finally, a macrobicyclic or [2]-cryptate effect is found by comparing the stability of the K+ complex of 30 with that of 22, where the solvation shell is completed by solvent molecules (a better model would be a ligand of type 22 bearing a — CH2CH2OCH2CH2OCH3 chain on one nitrogen) a stability increase of more than 105 (in methanol/water, 95/5) is found on introduction of the third bridge. 

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... 

Macrocyclic and maeropolycyclic (cryptate) effects designate the greater thermodynamic stability of macrocyclic ligand complexes compared to nonmacrocyclic analogs. 

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]. 

Since binding in solution results from a compromise between interaction with the ligand and solvation, new insights into the origin of the cation recognition process and of the macrocyclic and cryptate effects can be gained from experimental gas phase studies [2.34, 2.35] as well as from computer modelling calculations in vacuo or in a solvent [1.35b, 1.42, 1.43, 1.45, 2.36, 2.37, A.37]. In particular, molecular dynamics calculations indicate that complementarity is reflected in restricted motion of the ion in the cavity [1.45, 2.36]. 

A cryptate effect is observed for anion complexes as is the case for cation complexes (Section 2.3). In general, an increase in cyclic order from acyclic to macro-cyclic to macrobicyclic significantly increases the stability and selectivity of the anion complexes formed by polyammonium ligands. 

Thus, certain structural features, in particular incorporation of the central atom into a five-membered ring, chelate, macrocyclic and cryptate effects may stabilize the geometry of penta- and hexa-coordinate silicon species. Much of the insight as to which structural features might be expected to stabilize this species comes from studies of species involving higher valent states of other nonmetallic elements of the third row of the periodic table os-so )... 

Finally, Sargeson and co-workers have prepared a series of thioether-containing cage ligands, typically using template reactions. Transition metal complexes of these compounds exhibit interesting properties and the extremely robust nature of the complexes is attributed to the cryptate effect. 50,51... 

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.

Martell, A.E. Hancock, R.D. The chelate, macrocyclic, and cryptate effects. In Coordination Chemistry A Century of Progress Kaufman, G.B., Ed. American Chemical Society Washington, 1994, 241-254. 

Figure 7. Topological constraints in enhanced receptor/receptee binding chelate effect, macrocyclic effect, and cryptate effect...

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