Trapping Unstable Terminal M−O Multiple Bonds of Monocyclopentadienyl Niobium and Tantalum Complexes with Lewis Acids

Hydrolysis of [NbCp′Cl4] (Cp′ = η5-C5H4SiMe3) with the water adduct H2O·B(C6F5)3 afforded the oxo-borane compound [NbCp′Cl2{O·B(C6F5)3}] (2a). This compound reacted with [MgBz2(THF)2] giving [NbCp′Bz2{O·B(C6F5)3}] (2b), whereas [NbCp′Me2{O·B(C6F5)3}] (2c) was obtained from the reaction of [NbCp′Me4] with H2O·B(C6F5)3. Addition of Al(C6F5)3 to solutions containing the oxo-borane compounds [MCpRX2{O·B(C6F5)3}] (M = Ta, CpR = η5-C5Me5 (Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, CpR = Cp′, X = Cl 2a) afforded the oxo-alane complexes [MCpRX2{O·Al(C6F5)3}] (M = Ta, CpR = Cp*, X = Cl 3a, Bz 3b, Me 3c; M = Nb, CpR = Cp′, X = Cl 4a), releasing B(C6F5)3. Compound 3a was also obtained by addition of Al(C6F5)3 to the dinuclear μ-oxo compound [TaCp*Cl2(μ-O)]2, meanwhile addition of the water adduct H2O·Al(C6F5)3 to [TaCp*Me4] gave complex 3c. The structure of 2a and 3a was obtained by X-ray diffraction studies. Density functional theory (DFT) calculations were carried out to further understand these types of oxo compounds.