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)(2)Cl] (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 he 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 eta(1):eta(5)-CH2C5Me4 (sigma:eta(5)-CH2C5Me4) description for the metalated ligands, with significant slippage toward an eta(1):eta(3)-CH2C5Me4 resonance form. To compare the extent of pi-bonding between cb and dialkylamido ligands, the complex [(cb)(2)Ta(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 Angstrom longer than the Ta-NMe2 distances. An improved (safer) procedure for the synthesis of [Ta(NMe2)(5)] is presented.