ON THE ORIGIN OF HIGH IONOPHORICITY OF 1,3-ALTERNATE CALIX[4]ARENES - PI-DONOR PARTICIPATION IN COMPLEXATION OF CATIONS AND EVIDENCE FOR METAL-TUNNELING THROUGH THE CALIX[4]ARENE CAVITY

Nine conformationally-immobilized calix [4] arenes (including two doubly-bridged 1,3-alternate calix [4] arenes) and several reference calix[4] arenes with other conformations were synthesized. Two-phase solvent-extraction of and determination of association constants by H-1 NMR spectroscopy for alkali metal cations established that, surprisingly, 1,3-alternate and partial-cone conformers show an ion affinity higher than the corresponding cone conformers. Detailed examination with H-1 MMR spectroscopy presented unambiguous evidence that in the 1,3-alternate conformers the metal cation is bound asymmetrically to one of two metal-binding sites composed of two phenolic oxygens and two benzene rings, whereas in the partial-cone conformers the metal cation is bound to the upper rim composed of a phenolic oxygen in the inverted phenyl unit and two benzene rings in the proximal phenyl units. These metal-binding modes were rationalized in terms of the ''cation-pi interaction''. The contribution of the cation-pi interaction was further confirmed by the finding that these conformers all show high Ag+ affinity without exception. The X-ray crystallographic study of the partial-cone-2.Ag+ complex established that Ag+ is bound to a phenolic oxygen in the inverted phenyl unit and two benzene rings in the proximal phenyl units. Dynamic H-1 NMR spectroscopy at the low-temperature region showed that Ag+ alternates intramolecularly between the two binding sites through a pi-basic hole of 1,3-alternate calix[4]arenes. To the best of our knowledge, this is the first example for Ag+-tunneling across an aromatic cavity and has important implications with regard to the metal cation-pi interaction expected for metal transport through ion channels, metal inclusion in fullerenes, intercalation of metal cations into graphites, etc.