Insertion of Isocyanide into Metal−Carbon Bonds of Alkylchloro(pentamethylcyclopentadienyl)niobium- and -tantalum Complexes − X-ray Structure of [TaCp*Cl2(CH2CMe2Ph){η2-C(CH2CMe2Ph)=N(2,6-Me2C6H3)}] and Unexpected Decomposition of Alkyldichloro(η2-iminoacyl) Complexes of Tantalum

Methylation of NbCp*Cl2Me2 using excess ZnMe2 gives NbCp*ClMe3 (1) which has been found to exhibit a Berry pseudorotation process on the NMR time scale (log A = 12.2 ± 0.3, Ea = 12.2 ± 0.4 kcal·mol−1, ΔH≠ = 11.6 ± 0.4 kcal·mol−1, ΔS≠ = −4.4 ± 1.3 e.u., ΔG≠298K = 12.9 kcal·mol−1). Alternatively, lithium dimethylamide reacts with NbCp*Cl2Me2 to form NbCp*Me2(NMe2)2 (2) which decomposes in solution under the elimination of methane to give the (dimethylamido)methylazaniobacyclopropane derivative NbCp*Me(NMe2)(η2-CH2NMe) (3). Reaction of NbCp*Cl2Me2 with 1 equiv. of 2,6-Me2C6H3NC results in a double methyl group migration to give the dichloroazaniobacyclopropane complex [NbCp*Cl2{η2-CMe2N(2,6-Me2C6H3)}] (4). Dialkyldichloro complexes TaCp*Cl2R2 [Cp* = η5-C5Me5; R = CH2SiMe3 (5), CH2CMe2Ph (6), CH2CMe3 (7), CH2C6H5 (8)] were obtained by treating TaCp*Cl4 with the requisite amounts of the appropriate alkylating agents. Reactions of the dialkyldichloro complexes TaCp*Cl2R2 (5−8) with 1 equiv. of 2,6-Me2C6H3NC resulted in migration of only one of the two alkyl groups to give (alkyl)dichloro(η2-iminoacyl) complexes [TaCp*Cl2R{η2-C(R)=NAr}] [Ar = 2,6-Me2C6H3; R = CH2SiMe3 (9), CH2CMe2Ph (10), CH2CMe3 (11), CH2C6H5 (12)]. The molecular structure of complex 10 has been determined by X-ray diffraction analysis. The η2-iminoacyl complexes 9−12 decompose in [D6]benzene or n-hexane solutions to give [TaCp*Cl2{N(2,6-Me2C6H3)}] and the corresponding trans or cis olefins R′−CH=CH−CH2−R′ [R′ = SiMe3 (9o), CMe2Ph (10o), CMe3 (11o), C6H5 (12o)]. A mechanism for this reaction is proposed. All the new compounds have been characterized by IR spectrophotometry, 1H- and 13C{1H}-NMR spectroscopy, and elemental analysis.