INFLUENCE OF SOLVENT PROPERTIES ON CATION-MACROCYCLE COMPLEXATION - CESIUM CRYPTATES

The chemical shift of the Cs-133 resonance has been measured as a func tion of the mole ratio of cryptands C222, C222B, C221, and C211 to the Cs+ ion in N-methylformamide and dimethylacetamide solutions. The ove rall formation constants of the cesium cryptates were calculated from the NMR titration curves. From these and previously reported data it i s clear that solvent properties such as dipole moment, donicity, diele ctric constant and hydrogen bonding ability affect macrocyclic ligand- cation complexation equilibria through specific solvation of the catio n, the ligand and the complex. A model of M+ .L bonding in solution, i nvolving a rather intimate electrostatic attraction between the cation and the local electron distribution of the macrocycle accompanied by little perturbation of the charge distributions of the separated partn ers, is presented. In addition to the specific interactions previously noted, the presence of the solvent mitigates the ion-multipole forces which would otherwise govern the long-range attraction.