NMR spectroscopic and x-ray crystallographic studies of calix[4]arene·Ag ⁺ complexes. Influence of bound Ag ⁺ on C_2v-C_2v interconversion in cone-calix[4]arenes

The Ag⁺ complexes of conformationally immobilized tetra-O-propylcalix[4]arene with a cone or a partial-cone conformation (cone-2Prⁿ or partial-cone-2Prⁿ, respectively) have been successfully analysed by X-ray crystallography. In both complexes Ag⁺ was bound to the upper rim cavity, sandwiched by the two para carbons in the distal phenyl units. The findings provide clear evidence for π-base participation. In particular, the basic calix[4]arene skeleton in partial-cone-2Prⁿ·Ag⁺ is almost the same as that in partial-cone-2Prⁿ itself. This establishes that partial-cone-2R possesses two distal benzene rings ideally preorganized for Ag ⁺ -binding. ¹H NMR spectroscopic studies for the Ag⁺ complexes in solution indicated that Ag⁺ is bound to the same site as that in the solid state. In conformationally mobile 2Me, which exists in solution in equilibrium between cone and partial-cone, Ag⁺ induced a shift of the equilibrium to partial-cone to form the partial-cone-2Me·Ag ⁺ complex. This is ascribed to the ideal preorganization in partial-cone-2R for the Ag ⁺ -binding. These results are of great significance for an understanding of π-base participation in the metal-binding events and have important implications on the cation-π interaction in biological systems.