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

The reaction of [Cp*TaCl 4 ] with the potassium salt of carbazole (cbK, >3 equiv) in hydrocarbon solvents leads to the species [(C 5 Me 4 CH 2 )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 be the substitutional isomer 2, in which both carbazole ligands are equivalent. Alkylation of 1 with LiCH 2 SiMe 3 or PhCH 2 MgCl generates the corresponding monoalkyl derivatives [(C 5 Me 4 CH 2 )Ta(cb) 2 (R)] ( 7 or 8, respectively). Structural studies of 1, 7, and 8 support an η 1:η 5 -CH 2 C 5 Me 4 (σ:η 5 -CH 2 C 5 Me 4 ) description for the metalated ligands, with significant slippage toward an η 1:η 3 -CH 2 C 5 Me 4 resonance form. To compare the extent of π-bonding between cb and dialkylamido ligands, the complex [(cb) 2 Ta(NMe 2 ) 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−NMe 2 distances. An improved (safer) procedure for the synthesis of [Ta(NMe 2 ) 5 ] is presented.