摘要：

The stability constants of the [2]-cryptate inclusion complexes formed by the macrobicyclic ligands 1-6 with alkali and alkaline-earth cations have been measured. The origin of the stability sequences is discussed in terms of ligand structural features (topology, binding sites). The optimal alkali cryptates display much higher stability than any previously known complex. This high stability may be ascribed to the marobicyclic topology of the ligands; this cryptate effect is several orders of magnitude larger than the macrocyclic effect with respect to an open chain chelating ligand. The selectivity of the complexes is also remarkable. Optimal fit of the cation into the intramolecular cavity agrees with the selectivity of ugands [2.1.1], [2.2.1], and [2.2.2] for Li+, Na+, and K+, respectively. The smaller ligands display peak selectivity whereas the larger ones display plateau selectivity, with only small differences in stability for K+, Rb+, and Cs+. Unusual selectivities are observed for the alkaline-earth cryptates, e.g., the high Ca2+/Mg2+ and Sr2+, Ba2+/Ca2+ ratios of ligands [2.2.1] and [2.2.2], respectively. Furthermore, M2+/M+ selectivities are also of interest, especially the unique Li+ > Mg2+, Ca2+ selectivity of ligand [2.1.1]. Changing from water to methanol solution generally leads to a marked increase in cryptate stability and selectivity. The results described provide strategies for the rational design of other specific ligands for metal cations.

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