Synthesis, Characterization, and Reactivity of Niobium and Tantalum Complexes Bearing Metal−Nitrogen Bonds. X-ray Molecular Structure of [Nb(C₅H₄SiMe₃){NH(CH₂)₂-η-NH₂}Cl₃] and the Novel Tetranuclear Niobium Oxo Derivative [{Nb(C₅H₄SiMe₃)Cl(μ₂-O)}₄(Cl)₂(μ₃-O)]

The reaction of the cyclopentadienyl-silyl-amido titanium compound [Ti{η⁵-C₅H₄SiMe₂-η-N(CH₂)₂-η-NH₂}Cl₂] with group 5 metal monocyclopentadienyl complexes [MCp^RCl₄] (M = Nb, Ta; Cp^R = C₅H₄SiMe₃ (Cp‘), C₅Me₅ (Cp*)) afforded the heterobimetallic complexes [TiCl₂{η⁵-C₅H₄SiMe₂-η-N(CH₂)₂-κ-NH₂}MCp^RCl₄] (M = Nb, Cp^R = Cp‘, 2a; Cp*, 2b; M = Ta, Cp^R = Cp‘, 3a; Cp = Cp*, 3b). Compounds 2 evolve at room temperature to give a three-component mixture, the chlorosilyl-substituted cyclopentadienyl titanium compound [Ti(η⁵-C₅H₄SiMe₂Cl)Cl₃], the corresponding mononuclear amido-amino, [NbCp^R{NH(CH₂)₂-η-NH₂}Cl₃] (Cp^R = Cp‘, 4a; Cp*, 4b), and the dinuclear imido niobium complexes [{NbCp^RCl₂}₂(μ-N(CH₂)₂-η-N)] (Cp^R = Cp‘, 5a; Cp*, 5b). In contrast, the analogue tantalum complexes thermally degraded, when Cp^R = Cp‘ at normal temperature, whereas when Cp^R = Cp* at temperatures higher than 50 °C, they gave a unique tantalum complex, the corresponding mononuclear amido-amino derivative [TaCp^R{NH(CH₂)₂-η-NH₂}Cl₃] (Cp^R = Cp‘, 6a; Cp*, 6b). Alternatively, these group 5 complexes were prepared by direct reaction of the monocyclopentadienyl derivatives with the corresponding organic diamine, in the appropriate proportion and reaction conditions. Tantalum amino adducts were observed as intermediate species in the course of such reactions, and even the dinuclear derivative [{TaCp*Cl₄}₂(μ-NH₂(CH₂)₂NH₂)] (8) could be isolated. Hydrolysis of the dinuclear imido complex 5a yielded the tricyclic tetranuclear niobium oxo derivative [{NbCp‘Cl(μ₂-O)}₄(Cl)₂(μ₃-O)] (9), which displays an asymmetrical structure as a consequence of the triply connected oxygen site. These compounds were characterized by elemental analysis and NMR spectroscopy, and the crystal structures of 4a and 9 were determined by X-ray diffraction methods.