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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)2TaCl3 (3) in the presence of trace amounts of the corresponding amine afforded (silox)2TaCl2NR2 (NR2=NMe2, 4-NMe2; N(CH2)3CH2, 4-pyrr; N(CH2)4CH2, 4-pip). Treatment of 4-pyrr with EtMgCl and 4-pip with MeMgBr provided (silox)2TaEt2(N(CH2)3CH 2) (5-pyrr-Et2) and (silox)2TaMe2(N(CH2)4CH 2) (5-pip-Me2). Thermolysis of 5-pip-Me2 generated CH4 and η2-imine complex (silox)2Ta(CH3)(η2-NC5H 9) (6). In contrast, 5-pyrr-Et2 thermally degraded to the ethylene adduct, (silox)2(C4H8N)Ta(η2-C 2H4) (7-pyrr) and C2H6. Na/Hg reductions of 4-pyrr and 4-pip in the presence of 1 equiv. C2H4 generated 7-pyrr and (tBu3SiO)2(C5H10N) Ta(η2-C2H4) (7-pip); under an excess of C2H4, tantallacyclopentane (silox)2(C4H8N)TaCH2(CH 2)2CH2 (8-pyrr) was prepared. With pyridine present, Na/Hg reduction of 4-pyrr and 4-pip provided η2-pyridine complexes (silox)2(C4H8N)Ta(η2-(N,C)- NC5H5) (9-pyrr) and (silox)2(C5H10N)Ta(η2-(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 α-pyridyl-methylimido complex, (silox)2Ta(NMe)(NC5H4) (11). Na/Hg reduction of (silox)3NbCl2 (13) in the presence of excess 3,5-lutidine afforded (silox)3Nb(η2-3,5-Me2-NC5H 3) (2-3,5-Lut), which was heated for 4 days at 120°C to give (silox)2NbN-cis-CH=CMe-cis-CH=CMe-trans-CH=CHCMe2Si tBu2O (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).