Ring inversion kinetics of p-sulfonatocalix[4]arene and of its Ca(ii) and La(iii) complexes in water and water-acetone solutions

Abstract

The kinetics of the ring inversion of p-sulfonatocalix[4]arene and of its La(III) and Ca(II) 1 ∶ 1 complexes have been studied by ¹H NMR in D₂O and in a binary mixture 50 ∶ 50% w/w D₂O ∶ (CD₃)₂CO. The exchange between the axial and the equatorial methylene protons was followed. The ring inversion is slowed down in the binary mixture, and in the presence of Ca(II) and La(III) with a larger effect in the latter case. The results of this study are in agreement with a coordination of the sulfonate groups to the second sphere of coordination of the cations. The effect of the solvent and of the cations on the ring inversion rates is purely entropic, with entropies of activation ranging from 2 to 51 J K⁻¹ mol⁻¹, while the enthalpies of activation are constant in the limits of experimental errors, around 60 kJ mol⁻¹. It is proposed that the entropy control of the ring inversion results from the breaking of the structure of the water molecules forming the solvent cage around the calixarene.