Synthetic and Mechanistic Studies into the Activation of Terminal Alkynes by a Ruthenium Acetate Complex

A variety of novel organometallic complexes have been easily prepared from the ruthenium precursor complex 1 (cis-[Ru(κ2-OAc)2(PPh3)2]. This complex itself is synthesised from readily available starting materials. The family of organometallic complexes derived from complex 1 include carbonyl, nitrosyl, carbene, vinylidene and acetylide derivatives. A number of these complexes share a common structure, as evidenced by X-Ray crystallographic studies, as well as several spectroscopic features. The differing donor/acceptor properties of these ancillary ligands may be detected by a comparison of these common structural and spectroscopic features.

The addition of terminal alkynes (HC≡CR) to complex 1 results in the rapid formation of vinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(=C=CHR)(PPh3)2], whilst the addition of propargylic alcohols (HC≡C{R}{R’}OH) results in the equally rapid formation of hydroxy-vinylidene complexes [Ru(κ1-OAc)(κ2-OAc)(=C=CHC{R}{R’}OH)(PPh3)2]. The rapid formation of these vinylidene complexes has been attributed to the assistance provided by a coordinated acetate ligand. A comprehensive DFT study has shown how the acetate ligand is able to behave as both acid and base, acting as a proton shuttle, to facilitate the alkyne-to-vinylidene tautomerisation. This process has been termed a Ligand-Assisted Proton Shuttle (LAPS), which is related to the AMLA/CMD process that also facilitates the formation of acetylide complexes

The novel hydroxy-vinylidene complexes are not observed to undergo the typical dehydration process to form an allenylidene derivative. Instead, a conversion to a carbonyl complex with the concomitant formation of an alkene is observed. An experimental investigation using 18O-labelling, kinetic and stoichiometric studies into the mechanism of this conversion has once more demonstrated the chemically non-innocent behaviour of an acetate ligand.