Tetrahydroquinolinyl Amido and Indolinyl Amido Complexes of Tantalum as Models for Substrate−Catalyst Adducts in Hydrodenitrogenation Catalysis

The reactions of TaCl₅ with Me₃SiNC₉H₁₀ or LiNC₉H₁₀, where [NC₉H₁₀]^- = tetrahydroquinolinyl (the amido anion of tetrahydroquinoline), afford selective preparative routes to the complete series of amido halide complexes of tantalum(V) Ta(NC₉H₁₀)_nCl_5-n for n = 1−5 (compounds 1−5, respectively). The monokis(tetrahydroquinolinyl) complex is isolated as an ether adduct Ta(NC₉H₁₀)Cl₄(OEt₂) while the complexes Ta(NC₉H₁₀)_nCl_5-n (n = 2−5) are found to be base-free, monomeric species. The related complexes of indolinyl [NC₈H₈]^- (the amido anion of indoline), Ta(NC₈H₈)_nCl_5-n(THF) for n = 1 (6) or 2 (7), have been prepared from TaCl₅, Me₃SiNC₈H₈, and THF. An X-ray structural determination of Ta(NC₉H₁₀)₂Cl₃ (2) reveals that it adopts a trigonal bipyramidal geometry with equatorial amido ligands that are closer to lying parallel (within) than perpendicular to the TBP equatorial plane. Routes to mixed-ligand aryloxide−amide complexes have been developed from either aryloxide or amido precursors but not from both. Thus, Ta(NC₉H₁₀)(OAr)Cl₃(OEt₂) (8), where Ar = 2,6-C₆H₃ⁱPr₂, and Ta(NC₈H₈)(OAr)Cl₃(OEt₂) (9) are available from reacting Ta(OAr)Cl₄(OEt₂) with Me₃SiNC₉H₁₀ and Me₃SiNC₈H₈, respectively, while Ta(NC₉H₁₀)₂(OAr)₂Cl (10) is available from Ta(NC₉H₁₀)₂Cl₃ (2) and excess LiOAr·OEt₂. The alkyl derivatives Ta(NC₉H₁₀)(OAr)Me₂Cl (11), Ta(NC₉H₁₀)(OAr)Et₂Cl (12), Ta(NC₉H₁₀)₂(OAr)₂Me (13), and Ta(NC₉H₁₀)Me₂Cl₂ (14) are prepared from AlR₃ or ZnR₂ reagents and the appropriate precursor. Thermolyzing compounds 4, 5, and 11−14 in solution afforded no evidence for the formation of any η²(N,C)-heterocyclic complexes arising from metalation of a NC₉H₁₀ ligand.