Association of alkali and alkaline earth cations with benzo-18-crown-6 and its neutral and negatively charged acyclic analogues in methanol solution

The effect of added alkali (Li, Na, K, Rb, and Cs) and alkaline earth (Ca, Sr, and Ba) metal bromides on the acid dissociation of o-hydroxyphenyl 3,6,9,12-tetraoxatridecyl ether (ArOH) in MeOH at 25°C has been investigated spectrophotometrically over a wide range of salt concentration. Except for the Li ion, all the investigated salts increased significantly the apparent acidity of ArOH in a way that could be accounted for by the formation of 1:1 addition compounds of the metal cations with both ArOH and its conjugate base ArO-. A proper analysis of the experimental data yielded the equilibrium constants for the formation of the associated species. Complexation of the given cations by benzo-18-crown-6 (C) was also studied spectrophotometrically under the same conditions. The set of equilibrium constants here reported constitute a proper basis for a systematic discussion of the ligation properties of sexadentate ligands with oxygen donors belonging to different structural types. The cyclic ligand C binds the cations more strongly than does its open-chain analogue ArOH. The magnitude of the macrocyclic effect is found to be mainly determined by the size of the cation, rather than by its charge. On the other hand, the advantage of the anionic ligand ArO- over ArOH is highly sensitive to the charge of the cation with little or no dependence upon its size. This suggests that direct contact between the oxide ion and the cation is lacking in the ArO-Mz+ species. © 1981 American Chemical Society.