MACROHETEROCYCLES .56. COMPLEX-FORMATION OF THE CRYPTANDS CONTAINING 2 PROPYLENE FRAGMENTS IN ONE OF THE BRIDGES WITH ALKALI AND ALKALINE-EARTH CATIONS

The stability constants of the complexes of the alkali and alkaline-earth cations with the cryptands containing two propylene (L-H) or 2-hydroxypropylene (L-OH) chains between the nitrogen bridgeheads and the adjacent oxygen atoms in one of the bridges were determined by pH-metry in 95% aqueous methanol. In most cases (L-H) and (L-OH) form less stable complexes as compared to classic cryptands (L) with the same number of donor atoms. Unlike the latter compounds the (L-H) and (L-OH) series do not show any regular alteration of complex stability dependent on the ratio of cavity and complexed cation sizes. All (L-H) and (L-OH) form the most stable complexes with Na+ and K+ in the series of alkali cations, and with Sr2+ and Ba2+ along the alkaline-earth cation series. Complexes of alkali cations with cryptands (L-H) are more stable than those with (L-OH). For the complexes of the alkaline-earth cations the opposite dependence is observed. Stability of alkaline-earth cryptates of (L-OH) increases with the increase of a ligand's conformational flexibility and decreases with the increasing radius of a cation. The cause of this is the possibility of endo-endo and exo-exo cryptates formation, differing by the orientation of hydroxyl groups toward the inside or the outside of the cryptands cavity. Conformational changes of an exo-exo form into endo-endo one lead to a decrease of the ligand's cavity size changing the number and nature of the binding sites. Preferable interaction of more charge-dense cations with the polar OH-groups in endo-endo cryptates results in unusually high M2+/M+ complexation selectivity of (L-OH). As opposed to (L-OH), (L-H) cryptands exhibit the unique for neutral ligands Na+/Ca2+ selectivity which is probably due to kinetic reasons. © 1990.