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

Abstract

The olefin−hydride complexes Cp‘2Nb(H)(η2-RHCCH2) (Cp‘ = η5-C5H4SiMe3; R = H (3), C6H5 (4, endo isomer)) were prepared by the alkylation reactions of [Cp‘2NbCl]2 (1) with the appropriate Grignard reagents RMgX (R = CH2CH3, CH2CH2C6H5) followed by a stereoselective β-elimination from the intermediate alkyl complexes Cp‘2Nb(CH2CH2R). Complexes Cp‘2Nb(H)(η2-RHCCH2) (R = C6H4CH3 (5), C6H4OCH3 (6)) were prepared as a mixture of endo and exo isomers by reaction of Cp‘2NbH3 (2) with the corresponding olefin. Furthermore, reactions of CO with 3 and 4, and the reactions of CO2 with 3−6, afforded the alkylniobocene complexes Cp‘2Nb(CO)R (R = CH2CH3 (7), CH2CH2C6H5 (8)) Cp‘2Nb(η2-CO2)(R) (R = CH2CH3 (9)), CH2CH2C6H5 (10), CH2CH2C6H4CH3 (11), CH2CH2C6H4OCH3 (12)). The reactivity of 9 and 10 toward the strong Lewis acid B(C6F5)3 was also studied; in a first step the adducts Cp‘2Nb(η2-CO2−B(C6F5)3)(CH2CH2R) (R = H (13), C6H5 (14)) were formed and subsequently evolved to give the oxo−alkyl complexes Cp‘2Nb(O−B(C6F5)3)(CH2CH2R) (R = H (15), C6H5 (16)) with the loss of CO. Finally, reactions of 3 and 4 with CO under appropriate conditions gave the neutral acylniobocene complexes Cp‘2Nb(CO)(η1-C(O)CH2CH2R) (R = H (17), C6H5 (18)), which were alternatively prepared from the reactions of 7 and 8 with CO. The different complexes were characterized by spectroscopic methods.