Influence of solvent properties on cation-macrocycle complexation: Cesium cryptates

The chemical shift of the¹³³Cs resonance has been measured as a function of the mole ratio of cryptands C222, C222B, C221, and C211 to the Cs⁺ ion inN-methylformamide and dimethylacetamide solutions. The overall formation constants of the cesium cryptates were calculated from the NMR titration curves. From these and previously reported data it is clear that solvent properties such as dipole moment, donicity, dielectric constant and hydrogen bonding ability affect macrocyclic ligand-cation complexation equilibria through specific solvation of the cation, the ligand and the complex. A model of M⁺ · L bonding in solution, involving 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 partners, 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.This paper is dedicated to the memory of the late Dr C. J. Pedersen.Deceased.