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

Protonation of the rhenium η 2 -alkyne complex C 5 Me 5 (CO) 2 Re(η 2 -MeC⋮CMe) ( 4 ) with HBF 4 at room temperature produced the η 3 -allyl complex C 5 Me 5 (CO) 2 Re(η 3 - exo, anti -MeHC−CH−CH 2 ) + BF 4 - ( 5 ). The protonation of 4 at −78 °C occurred at rhenium to give the rhenium alkyne hydride complex C 5 Me 5 (CO) 2 ReH(η 2 -MeC⋮CMe) + BF 4 - ( 6 ). At −16 °C, net proton migration from rhenium to the alkyne ligand of 6 occurred to produce the 1-metallacyclopropene complex C 5 Me 5 (CO) 2 Re(η 2 -CMeCHMe) + BF 4 - ( 7 ), which then rearranged to form the η 3 -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 η 2 -alkyne complex C 5 Me 5 (CO) 2 Re(η 2 -PhC⋮CPh) ( 10 ) with HBF 4 at −78 °C initially produced the rhenium alkyne hydride complex C 5 Me 5 (CO) 2 ReH(η 2 -PhC⋮CPh) + BF 4 - ( 11 ), which was observed spectroscopically. Upon warming to room temperature, 11 was converted to the stable 1-metallacyclopropene complex C 5 Me 5 (CO) 2 Re(η 2 -CPhCHPh) + BF 4 - ( 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 [C 5 H 5 (CO) 2 Re(η 2 -MeCCHMe)] + to compare η 2 -vinyl vs 1-metallacyclopropene formulations.