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.