Alkali Metal Cation−π Interactions Stabilized Solely by [M{N(SiMe₃)₂}₃]^- Anions (M = Mg or Zn): The Competing Influence of Alkali Metal···C(Me) Agostic Interactions

A series of [K(ar)2]+ and [Rb(toluene)3]+ cations (ar = benzene, toluene, o-xylene, or p-xylene) has been synthesized and crystallized in the presence of [M{N(SiMe3)2}3]- anions (M = Mg or Zn). In the solid state all form either extended supermolecular, dimeric or supramolecular, polymeric structures. Only [K(toluene)2][Mg{N(SiMe3)2}3] was found to exist as both suprastructural isomers. Introducing cyclopentadienyl to the system gave a similar zincate with the unusual [K2Cp]+ cation. The absence of any traditional Lewis bases facilitates short metal-to-arene contact distances and makes these species excellent candidates for the study of alkali metal cation−π interactions. It is shown that K−π interactions and to a lesser extent Rb−π interactions are heavily influenced by the number and nature of agostic methyl interactions, especially when the electron-donating ability of these is maximized by adoption of near-linear geometries. These features combine so that the weakest potassium-to-arene interaction observed is that with benzene.