The Ag+ complexes of conformationally immobilized tetra-O-propylcalixarene 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 calixarene 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.
|Number of pages||8|
|Journal||Journal of the Chemical Society, Perkin Transactions 2|
|Publication status||Published - 1994|
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