Synthesis and Characterization of New Alkyl−Carbon Dioxide Compounds and the First Neutral Acyl−Carbonyl Niobocene Complexes

The olefin−hydride complexes Cp‘ 2 Nb(H)(η 2 -RHC CH 2 ) (Cp‘ = η 5 -C 5 H 4 SiMe 3; R = H ( 3 ), C 6 H 5 ( 4, endo isomer)) were prepared by the alkylation reactions of [Cp‘ 2 NbCl] 2 ( 1 ) with the appropriate Grignard reagents RMgX (R = CH 2 CH 3, CH 2 CH 2 C 6 H 5 ) followed by a stereoselective β-elimination from the intermediate alkyl complexes Cp‘ 2 Nb(CH 2 CH 2 R). Complexes Cp‘ 2 Nb(H)(η 2 -RHC CH 2 ) (R = C 6 H 4 CH 3 ( 5 ), C 6 H 4 OCH 3 ( 6 )) were prepared as a mixture of endo and exo isomers by reaction of Cp‘ 2 NbH 3 ( 2 ) with the corresponding olefin. Furthermore, reactions of CO with 3 and 4, and the reactions of CO 2 with 3 − 6, afforded the alkylniobocene complexes Cp‘ 2 Nb(CO)R (R = CH 2 CH 3 ( 7 ), CH 2 CH 2 C 6 H 5 ( 8 )) Cp‘ 2 Nb(η 2 -CO 2 )(R) (R = CH 2 CH 3 ( 9 )), CH 2 CH 2 C 6 H 5 ( 10 ), CH 2 CH 2 C 6 H 4 CH 3 ( 11 ), CH 2 CH 2 C 6 H 4 OCH 3 ( 12 )). The reactivity of 9 and 10 toward the strong Lewis acid B(C 6 F 5 ) 3 was also studied; in a first step the adducts Cp‘ 2 Nb(η 2 -CO 2 −B(C 6 F 5 ) 3 )(CH 2 CH 2 R) (R = H ( 13 ), C 6 H 5 ( 14 )) were formed and subsequently evolved to give the oxo−alkyl complexes Cp‘ 2 Nb(O−B(C 6 F 5 ) 3 )(CH 2 CH 2 R) (R = H ( 15 ), C 6 H 5 ( 16 )) with the loss of CO. Finally, reactions of 3 and 4 with CO under appropriate conditions gave the neutral acylniobocene complexes Cp‘ 2 Nb(CO)(η 1 -C(O)CH 2 CH 2 R) (R = H ( 17 ), C 6 H 5 ( 18 )), which were alternatively prepared from the reactions of 7 and 8 with CO. The different complexes were characterized by spectroscopic methods.