Pronounced calcium binding selectivity with diamide derivatives of 12-N₂O₂ and the importance of desolvation

The complexation of alkali and alkaline-earth cations with two N-substituted amide derivatives of l,7-dioxa-4,10-diazacyclododecane has been studied in methanol and water by calorimetric and potentiometric methods. High stability (e.g. log K_CaL = 11.2: 298 K, MeOH) and selectivity (10^7.1 over Na ⁺) for calcium ions in methanol is shown by the secondary amide derivative, and although significantly higher enthalpies of complexation are found for the tertiary amide analog (resulting from lesser differential ligand solvation), accompanying lower entropies of complexation lead to somewhat lower binding free energies for 11a cations. In water, both the complex stability and the selectivity for the more charge dense ions is reduced.