Amide derivatives of tantalum and a niobium-promoted ring opening of 3,5-lutidine

Various tantalum amide compounds were prepared in an attempt to observe C-N bond activation. Addition of lithium amides to THF solutions of (Silox)(2)TaCl3 (3) in the presence of trace amounts of the corresponding amine afforded (Silox)(2)TaCl2NR2 (NR2 = NMe2, 4-NMe2; N(CH2)(3)CH2, 4-pyrr; N(CH2)(4)CH2, 4-pip). Treatment of 4-pyrr with EtMgCl and 4-pip with MeMgBr provided (silox)(2)TaEt2(N(CH2)(3)CH2) (5-pyrr-Et-2) and (silox)(2)TaMe2(N(CH2)(4)CH2) (5-pip-Me-2). Thermolysis of 5-pip-Me-2 generated CH4 and eta(2)-imine complex (Silox)(2)Ta(CH3)(eta(2)-NC5H9) (6). In contrast, 5-pyrr-Et-2 thermally degraded to the ethylene adduct, (Silox)(2)(C4H8N)Ta(eta(2)-C2H4) (7-pyrr) and C2H6. Na/Hg reductions of 4-pyrr and 4-pip in the presence of 1 equiv. C2H4 generated 7-pyrr and ((Bu3SiO)-Bu-1)(2)(C5H10N)Ta(eta(2)-C2H4) (7-pip); under an excess of C2H4, tantallacyclopentane (silox)(2)(C4H8N)TaCH2(CH2)(2)C H-2 (8-pyrr) was prepared. With pyridine present, Na/Hg reduction of 4-pyrr and 4-pip provided eta(2)-pyridine complexes (Silox)(2)(C4H8N)Ta(eta(2)-(N,C)-NC5H5) (9-pyrr) and (Silox)(2)(C5H10N)Ta(eta(2)-(N,C)-NC5H5) (9-pip). Treatment of 9-pyrr and 9-pip with 1 equiv. CH3NH2 in C6D6 revealed the formation of tantalum imido derivatives (Silox)(2)(C4H8N)Ta=NMe (10-pyrr) and (Silox)(2)(C5H10N)Ta=NMe (10-pip), respectively, via the alpha-pyridyl-methylimido complex, (silox)(2)Ta(NMe)(NC5H4) (11). Na/Hg reduction of (silox)(3)NbCl2 (13) in the presence of excess 3,5-lutidine afforded (silox)(3)Nb(eta(2)-3,5-Me-2-NC5H3) (2-3,5-Lut), which was heated for 4 days at 120 degreesC to give (silox)(2)NbN-cis-CH=CMe-cis-CH=CMe-trans-CH=(CHCMe2SiBu2O)-Bu-t (14), whose structure was confirmed by X-ray crystallography. The direct participation of a silox group in the ring opening is a rare occurrence for this typically ancillary ligand, and provides another pathway for C-N bond activation relevant to hydrodenitrogenation (HDN). (C) 2002 Elsevier Science B.V. All rights reserved.