Molecular Structures of Cation···π(Arene) Interactions for Alkali Metals with π-and σ-Modalities

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Abstract
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···π(arene) interactions when they are suitably disposed with the ambifunctional hexasubstituted benzene C6E6. The ether tentacles (E = methoxymethyl) allow these cations to effect η1-bonding to the benzene center in a manner strongly reminiscent of the classical σ-arene complexes with positively charged electrophiles where Z+ = CH3+, Br+, Cl+, Et3Si+, etc. The somewhat smaller potassium cation is involved in a similar M+···π(arene) interaction that leads to η2-bonding with the aromatic center in the π-mode previously defined in the well-known series of silver(I)/arene complexes. We can find no evidence for significant Na+··· π(arene) interaction under essentially the same conditions. As such, the σ-structure of the Rb+ and Cs+ complexes and π-structure of the K+ complex are completely integrated into the continuum of σ−π bondings of various types of electrophilic (cationic) acceptors with arene donors that were initially identified by Mulliken as charge-transfer.