Counteranion and solvent assistance in ruthenium-mediated alkyne to vinylidene isomerizations

The complex [Cp*RuCl(ⁱPr₂PNHPy)] (1) reacts with 1-alkynes HCî -CR (R = COOMe, C₆H₄CF ₃) in dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru =C =CHR(ⁱPr₂PNHPy)]Cl (R = COOMe (2a-Cl), C₆H₄CF₃ (2b-Cl)), whereas reaction of 1 with NaBPh₄ in MeOH followed by addition of HCî -CR (R = COOMe, C₆H₄CF₃) yields the metastable π-alkyne complexes [Cp*Ru(η²-HCî -CR)( ⁱPr₂PNHPy)][BPh₄] (R = COOMe (3a-BPh ₄), C₆H₄CF₃ (3b-BPh₄)). The transformation of 3a-BPh₄/3b-BPh₄ into their respective vinylidene isomers in dichloromethane is very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution. Reaction of 1 with HCî -CR (R = COOMe, C ₆H₄CF₃) in MeOH goes through the intermediacy of the π-alkyne complexes [Cp*Ru(η²-HCî -CR)( ⁱPr₂PNHPy)]Cl (R = COOMe (3a-Cl), C₆H ₄CF₃ (3b-Cl)), which rearrange to vinylidenes in minutes, i.e., much faster than their counterparts containing the [BPh₄] ^- anion. The kinetics of these isomerizations has been studied in solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanol-assisted hydrogen migrations. Calculations suggest participation of a hydrido-alkynyl intermediate in the process, in which the hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton shuttles.