Macrobicyclic cryptate complex stability

Macrobicyclic cryptate cation selectivity, 27 16-17 complex stability, 27 14-15 kinetic studies, 27 13-14... 

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). 

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,... 

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... 

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. 

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