COMPLEXATION OF ALKALINE-EARTH (GROUP-2A) CATIONS BY NONCYCLIC, MACROCYCLIC, AND MACROBICYCLIC LIGANDS IN PROPYLENE CARBONATE SOLUTIONS

Stability constants and thermodynamic parameters for the reaction of non-cyclic polyethers, crown ethers and cryptands with alkaline earth cations in propylene carbonate were measured by means of calorimetric, potentiometric and conductometric titrations. A linear correlation between the number of oxygen donor atoms of the non-cyclic polyethers and the values of the reaction enthalpies is found. From these data, mean interaction energies between one cation and one donor atom can be calculated. These values can be used to predict the values of the reaction enthalpies of crown ethers with these cations in propylene carbonate. If the cation dimensions are smaller than or equal to the cavity dimensions, the agreement between the calculated and experimentally measured reaction enthalpies is very good, indicating that the macrocyclic effect for these ligands and cations is caused only by entropic contributions.