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.