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

Abstract

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.