Molecular structures of cation...pi(arene) interactions for alkali metals with pi- and sigma-modalities

The monovalent cations of Na(+), K(+), Rb(+), and Cs(+) derived from the highly electropositive alkali metals represent prototypical charged spheres that are mainly subject to relatively simple electrostatic and solvation (hydration) forces. We now find that the largest of these Rb(+) and Cs(+) are involved in rather strong cation...pi(arene) interactions when they are suitably disposed with the ambifunctional hexasubstituted benzene C(6)E(6). The ether tentacles (E = methoxymethyl) allow these cations to effect eta(1)-bonding to the benzene center in a manner strongly reminiscent of the classical sigma-arene complexes with positively charged electrophiles where Z(+) = CH(3)(+), Br(+), Cl(+), Et(3)Si(+), etc. The somewhat smaller potassium cation is involved in a similar M(+)...pi(arene) interaction that leads to eta(2)-bonding with the aromatic center in the pi-mode previously defined in the well-known series of silver(I)/arene complexes. We can find no evidence for significant Na(+)... pi(arene) interaction under essentially the same conditions. As such, the sigma-structure of the Rb(+) and Cs(+) complexes and pi-structure of the K(+) complex are completely integrated into the continuum of sigma-pi bondings of various types of electrophilic (cationic) acceptors with arene donors that were initially identified by Mulliken as charge-transfer.