On the Origin of High Ionophoricity of 1, 3-Alternate Calix[4]arenes: π-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 ¹H 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 ¹H NMR 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-ir interaction”. The contribution of the cation-π 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 ¹H NMR spectroscopy at the low-temperature region showed that Ag⁺ alternates intramolecularly between the two binding sites through a π-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-π interaction expected for metal transport through ion channels, metal inclusion in fullerenes, intercalation of metal cations into graphites, etc.