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-2Pr(n) or partial-cone-2Pr(n), 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 nn-base participation. In particular, the basic calix[4]arene skeleton in partial-cone-2Pr(n).Ag+ is almost the same as that in partial-cone-2Pr(n) itself. This establishes that partial-cone-2R possesses two distal benzene rings ideally preorganized for Ag+-binding. H-1 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 pi-base participation in the metal-binding events and have important implications on the cation-pi interaction in biological systems.