Formation of Tantalum “Tuck-in” Complexes by Activation of Methyl C−H Bonds in Pentamethylcyclopentadiene Groups by Carbazole Ligation

The reaction of [Cp*TaCl₄] with the potassium salt of carbazole (cbK, >3 equiv) in hydrocarbon solvents leads to the species [(C₅Me₄CH₂)Ta(cb)₂Cl] (1), in which one of the ring methyl C−H bonds of the Cp* ligand has been cleaved along with free cbH. Spectroscopic and structural studies of 1 show a lack of a plane of symmetry through the molecule with nonequivalent cb ligands. A minor component of the reaction mixture is believed to be the substitutional isomer 2, in which both carbazole ligands are equivalent. Alkylation of 1 with LiCH₂SiMe₃ or PhCH₂MgCl generates the corresponding monoalkyl derivatives [(C₅Me₄CH₂)Ta(cb)₂(R)] (7 or 8, respectively). Structural studies of 1, 7, and 8 support an η¹:η⁵-CH₂C₅Me₄ (σ:η⁵-CH₂C₅Me₄) description for the metalated ligands, with significant slippage toward an η¹:η³-CH₂C₅Me₄ resonance form. To compare the extent of π-bonding between cb and dialkylamido ligands, the complex [(cb)₂Ta(NMe₂)₃] (9) was synthesized and structurally characterized. The molecular structure of 9 consists of a tbp arrangement of nitrogen atoms with trans, axial cb ligands. The Ta−cb distances are 0.2 Å longer than the Ta−NMe₂ distances. An improved (safer) procedure for the synthesis of [Ta(NMe₂)₅] is presented.