Alkali metal cation-pi interactions stabilized solely by [M{N(SiMe3)(2)}(3)](-) anions (M = Mg or Zn): The competing influence of alkali metal center dot center dot center dot C(Me) agostic interactions

Forbes, G.C. and Kennedy, A.R. and Mulvey, R.E. and Roberts, B.A. and Rowlings, R.B. (2002) Alkali metal cation-pi interactions stabilized solely by [M{N(SiMe3)(2)}(3)](-) anions (M = Mg or Zn): The competing influence of alkali metal center dot center dot center dot C(Me) agostic interactions. Organometallics, 21 (23). pp. 5115-5121. ISSN 0276-7333

Abstract

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-pi interactions. It is shown that K-pi interactions and to a lesser extent Rb-pi 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.

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