Studies on the Insertion Reactions of Activated Alkynes into Nb−H Bonds in Hydride−Niobocene Complexes. X-ray Crystal Structures of Nb(η⁵-C₅H₄SiMe₃)₂(H)[η²-RO₂C(H)C(H)CO₂R] (R = Me or ^<i>t</i>Bu)

Abstract

The hydride isocyanide complexes, Nb(η5-C5H4SiMe3)2(H)(CNR), R = xylyl = 2,6-dimethylphenyl; Cy (cyclohexyl); tBu; as well as the hydride carbonyl complex Nb(η5-C5H4SiMe3)2(H)(CO) react with several activated alkynes to afford the corresponding alkenyl isocyanide complexes Nb(η5-C5H4SiMe3)2(CNR)(C(R‘)CH(R‘‘)), R = xylyl, R‘ = R‘‘ = CO2Me 1; R = Xylyl, R‘ = R‘‘ = CO2tBu 2; R = Xylyl, R‘ = CO2Me, R‘‘ = H 3; R = Xylyl, R‘ = CO2Me, R‘‘ = Me 4; R = Cy, R‘ = R‘‘ = CO2Me 5; R = Cy, R‘ = R‘‘ = CO2tBu 6; R = Cy, R‘ = CO2Me, R‘‘ = Me 7; R = tBu, R‘ = R‘‘ = CO2Me 8; tBu, R‘ = R‘‘ = CO2tBu 9 and alkenyl-carbonyl complexes, Nb(η5-C5H4SiMe3)2(CO)(C(R‘)CH(R‘‘)), R‘ = R‘‘ = CO2Me 10; R‘ = R‘‘ = CO2tBu 11; R‘ = CO2Me, R‘‘ = H 12. The reaction of hydride isocyanide complexes with activated alkynes gives cis-insertion resulting in the formation of (E)-alkenyl products. However, the hydride carbonyl derivative undergoes insertion to give, under kinetic control, a mixture of (E)- and (Z)-alkenyl isomers. Finally, the trihydride niobocene complex Nb(η5-C5H4SiMe3)2(H)3, reacts with esters RO2CC⋮CCO2R‘ or MeO2CC⋮CH to give the hydride olefin derivatives, Nb(η5-C5H4SiMe3)2(H)[η2-RO2C(H)C(H)CO2R‘], R = R‘ = Me 13; R = R‘ = tBu 14; and Nb(η5-C5H4SiMe3)2(H)[η2-MeO2C(H)CH2] 15, probably as the result of a stereospecific trans-insertion. The different complexes have been characterized by spectroscopic methods. In addition, the structure of 13 and 14 were determined by single crystal X-ray diffraction.