Structures and energetics of the cation-pi interactions of Li+, Na+, and K+ with cup-shaped molecules: effect of ring addition to benzene and cavity selectivity.

The interactions of alkali metal cations (Li (+), Na (+), and K (+)) with the cup-shaped molecules, tris(bicyclo[2.2.1]hepteno)benzene and tris(7-azabicyclo[2.2.1]hepteno)benzene have been investigated using MP2(FULL)/6-311+G(d,p)//MP2/6-31G(d) level of theory. The geometries and interaction energies obtained for the metal ion complexation with the cup-shaped systems trindene and benzotripyrrole are compared with the results for benzene-metal ion complexes to examine the effect of ring addition to the benzene on structural and binding affinities. The cup-shaped molecules exhibit two faces or cavities (top and bottom). Except for one of the conformers of tris(7-azabicyclo[2.2.1]hepteno)benzene), the metal ions prefer to bind with the top face over bottom face of the cup-shaped molecules. The selectivity of the top face is due to strong interaction of the cation with the pi cloud not only from the central six-membered ring but also from the pi electrons of rim C=C bonds. In contrast, the metal ions under study exhibit preference to bind with the bottom face rather than top face of tris(7-azabicyclo[2.2.1]hepteno)benzene) when the lone pair of electrons of three nitrogen atoms participates in binding with metal ions. This bottom face selectivity could be ascribed to the combined effect of the cation-pi and strong cation-lone pair interactions. As evidenced from the values of pyramidalization angles, the host molecule becomes deeper bowl when the lone pair of electrons of nitrogen atoms participates in binding with cation. Molecular electrostatic potential surfaces nicely explain the cavity selectivity in the cup-shaped systems and the variation of interaction energies for different ligands. Vibrational frequency analysis is useful in characterizing different metal ion complexes and to distinguish top and bottom face complexes of metal ions with the cup-shaped molecules.