NMR spectroscopic and X-ray crystallographic studies of calix[4]arene·Ag+complexes. Influence of bound Ag+on C2v–C2vinterconversion in cone-calix[4]arenes

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Abstract
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-2Prnitself. 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.