Synthesis and reactivity of new oxo alkyl or oxo acyl niobocene complexes and crystal structure of Cp′2Nb(O)(OC(O)CF3) (Cp′=η5-C5H4SiMe3)

New alkyl and acyl oxo niobocenes Cp′2Nb(O)(R) [Cp′=η5-C5H4SiMe3; R=CH2CH3 (3), CH2CH2Ph (4), COCH2CH3 (7), COCH2CH2Ph (8)] have been synthesized. The oxo alkyl derivatives were obtained by reaction of olefin-hydride complexes Cp′2Nb(H)(η2-CH2CHR) [R=H (1), Ph (2)] with oxygen, whereas the oxo acyl complexes were prepared by reaction of the niobium(III) carbonyl derivatives Cp′2Nb(CO)(COCH2CH2R) [R=H (5), Ph (6)] with oxygen. We have also studied the reaction of Cp′2Nb(O)(CH2CH2R) [R=H (3), Ph (4)] with HBF4 and CF3COOH. These reactions afforded Cp′2NbF(O→BF3) (9) and Cp′2Nb(O)[η1-(O)OCOCF3)] (10), respectively. When protonation of oxo acyl niobocenes Cp′2Nb(O)(COCH2CH2R) [R=H (7), Ph (8)] with CF3COOH was carried out, complex 10 was obtained. Furthermore, carbon dioxide–niobocene derivatives Cp′2Nb(CO2)(CH2CH2R) [R=H (11), Ph (12)] react with BF3·Et2O to yield the fluoro niobocene complex 9. The structure of complex 10 was confirmed by X-ray diffraction. Complex 10 crystallizes in the monoclinic system, in the centrosymmetric P21/c space group, with unit cell parameters: a=9.671(1) Å; b=12.935(2) Å, c=18.43(1) Å; β=99.03(2)°; V=2277(1) Å3; δ(Calc.)=1.448 mg m−3 and R=0.062.