The Bis(η⁶ -benzene)lithium Cation: A Fundamental Main-Group Organometallic Species

The synthesis and characterization of the bis(η⁶ -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl₂ [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η⁶ -benzene)Li]⁺ cation with the [{FmesB(-C≡C-Fmxyl)₃ }₂ Li]^- anion. This parent [(arene)₂ Li]⁺ cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η⁶ -toluene)₂ Li]⁺ and [(η⁶ -durene)₂ Li]⁺ containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state ⁷ Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations.