Unusual Mechanism for the Reaction of a Niobocene Hydride Complex with Activated Alkynes. Experimental and DFT Studies

The reactivity of the activated alkynes
RCCR (R = CO2Me, CO2tBu)
with the Nb–H bond of the 18e– niobocene
hydride trimethyl phosphite complex [Nb­(η5-C5H4SiMe3)2(H)­(P­(OMe)3)] (1) has been studied, and the new (E)-niobocene alkenyl trimethyl phosphite complexes [Nb­(η5-C5H4SiMe3)2(C­(CO2R)C­(CO2R)­H)­(P­(OMe)3)] (R = Me
(2a), tBu (2b)) have been obtained.
DFT studies were carried out to identify the lowest energy mechanism
and the possible intermediates in the process. An unusual nucleophilic
d2 Nb­(III) center was identified as a σ-donor entity
responsible for the nucleophilic attack on a carbon atom of the activated
(electrophilic) alkyne molecules. A subsequent hydrogen transfer to
the other carbon atom produces the final (E)-niobocene
alkenyl trimethyl phosphite complexes, the stereochemistries of which
were elucidated by NMR spectroscopy.