Cyclopropyllithium aggaregation and 199 Hg NMR as a tool for exchange studies with organo-mercury compounds

Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two-Coordinate Lithium Cations and CC Agostic Interactions in Cyclopropyllithium Derivatives

Understanding and controlling the aggregation state is germane to alkyllithium chemistry. Lewis base-free alkyllithium compounds normally form tetrahedral tetramers or octahedral hexamers in the solid state with the lithium cations being three-coordinate. We report that the unsupported cyclopropyl derivative 1-(trimethylsilyl)cyclopropyllithium [{μ-c-C(SiMe₃ )C₂ H₄ }Li]₄ (1), synthesized by the reduction of 1-(phenylthio)-1-(trimethylsilyl)cyclopropane, crystallizes as a tetramer in the space group I-4 with the two-coordinate lithium atoms forming a square. CC agostic interactions complete the coordination sphere around each lithium. The aggregate is preserved in hydrocarbon solution. Furthermore, CC agostic interactions compete intra- and intermolecularly with Lewis base donation as in the unsaturated dimer of 1-(phenylthio)cyclopropyllithium [Li(thf)₂ {μ-c-C(SPh)C₂ H₄ }₂ Li (thf)] (3) which is also fully characterized.