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

Hydrolysis of [NbCp'Cl-4] (Cp' = eta(5)-C5H4SiMe3) with the water adduct H2O center dot B(C6F5)(3) afforded the oxo-borane compound [NbCp'Cl-2{O center dot B(C6F5)(3)}] (2a). This compound reacted with [MgBz(2)(THF)(2)] giving [NbCp'Bz(2){O center dot B(C6F5)(3)}] (2b), whereas [NbCp'Me-2{O center dot B(C6F5)(3)}] (2c) was obtained from the reaction of [NbCp'Me-4] with H2O center dot B(C6F5)(3). Addition of Al(C6F5)(3) to solutions containing the oxo-borane compounds [(MCpX2)-X-R{O center dot B(C6F5)(3)}] (M = Ta, Cp-R = eta(5)-C5Me5(Cp*), X = Cl 1a, Bz 1b, Me 1c; M = Nb, Cp-R = Cp', X = Cl 2a) afforded the oxo-alane complexes [(MCpX2)-X-R{O center dot Al(C6F5)(3)}] (M = Ta, Cp-R = Cp*, X = Cl 3a, Bz 3b, Me 3c; M = Nb, Cp-R = Cp', X = Cl 4a), releasing B(C6F5)(3). Compound 3a was also obtained by addition of Al(C6F5)(3) to the dinuclear mu-oxo compound [TaCp*Cl-2(mu-O)](2), meanwhile addition of the water adduct H2O center dot Al(C6F5)(3) to [TaCp*Me-4] 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.