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.