Protonation of Rhenium Alkyne Complexes Produces η³-Allyl Rhenium Complexes via Observable 1-Metallacyclopropene Intermediates

Protonation of the rhenium η²-alkyne complex C₅Me₅(CO)₂Re(η²-MeC⋮CMe) (4) with HBF₄ at room temperature produced the η³-allyl complex C₅Me₅(CO)₂Re(η³-exo,anti-MeHC−CH−CH₂)⁺BF₄^- (5). The protonation of 4 at −78 °C occurred at rhenium to give the rhenium alkyne hydride complex C₅Me₅(CO)₂ReH(η²-MeC⋮CMe)⁺BF₄^- (6). At −16 °C, net proton migration from rhenium to the alkyne ligand of 6 occurred to produce the 1-metallacyclopropene complex C₅Me₅(CO)₂Re(η²-CMeCHMe)⁺BF₄^- (7), which then rearranged to form the η³-allyl complex 5. The degenerate rearrangement of 7 by hydride migration between the two metallacyclopropene carbons was demonstrated by deuterium labeling. Protonation of the rhenium η²-alkyne complex C₅Me₅(CO)₂Re(η²-PhC⋮CPh) (10) with HBF₄ at −78 °C initially produced the rhenium alkyne hydride complex C₅Me₅(CO)₂ReH(η²-PhC⋮CPh)⁺BF₄^- (11), which was observed spectroscopically. Upon warming to room temperature, 11 was converted to the stable 1-metallacyclopropene complex C₅Me₅(CO)₂Re(η²-CPhCHPh)⁺BF₄^- (12), which was characterized by X-ray crystallography. Hybrid density functional theory calculations and natural bond orbital analysis were performed on the 1-metallacyclopropene cation [C₅H₅(CO)₂Re(η²-MeCCHMe)]⁺ to compare η²-vinyl vs 1-metallacyclopropene formulations.