Labile Imidazolium Cyclopentadienides

A series of imidazolium-based synthetic routes to imidazolium cyclopentadienides are reported. The 2:1 reaction of imidazolium (1) with [Cp2TiIIICl]2 gives [NHCDippH]+[Cp2TiIIICl2]− (NHCDipp = 2,6-diisopropylphenyl-substituted N-heterocyclic carbene) (2). The “inverse sandwich” imidazolium cyclopentadienide (3) is obtained as a crystalline solid via reaction of 1 with both [Cp2TiIIICl]2 and cyclopentadienyllithium (CpLi) (in a molar ratio of 2:1:1) in THF/toluene at elevated temperature. Notably, the 1:1 reaction of 1 with CpLi leads to the isolation of “multidecker” 4 in the solid state, while the corresponding 1:2 reaction of 1 with CpLi gives the crystals of CpLi-bound cyclopentadienyl-imidazolium complex 5. While compounds 3, 4, and 5 have been structurally characterized using single-crystal X-ray diffraction, in solution these complexes dissociate due to the conversion of [NHCDippH]+[Cp]− to both cyclopentadiene and NHCDipp through intramolecular proton transfer. The equilibrium of the acid–base conversion between [NHCDippH]+Cp– and the 1:1 mixture of NHCDipp and cyclopentadiene is probed by theoretical methods.