Internal alkyne isomerization to vinylidene versus stable π-Alkyne: Theoretical and experimental study on the divergence of analogous Cp*Ru and TpRu systems

The activation of internal alkynes by Cp*Ru and TpRu complexes gives respectively π-alkyne and disubstituted vinylidene as stable species, even though both systems bear identical pyridylphosphine ligands (κ 2P,N- iPr2PXPy, X = NH, CH2, S). The activation of the alkynones PhC≡CCOR (R = Me, Ph) by [TpRuCl( iPr2PXPy)] complexes allowed us to isolate and characterize metastable η1-O=C(R)C≡CPh adducts. These complexes isomerize spontaneously to vinylidene both in solution and in the solid state. Kinetic studies have been carried out in solution by 31P{1H} NMR and in the solid state by IR spectroscopy, providing the Eyring and Avrami-Erofeev parameters, respectively. The activation of internal alkynes without ketone groups provided vinylidene species as well, but without isolable intermediates. In contrast with the TpRu system, the activation of alkynones by [Cp*RuCl(iPr2PXPy)] always results in stable π-alkyne species. Representatives of both Cp*Ru - π-alkyne and TpRu-vinylidene compounds have been characterized by X-ray diffraction. DFT calculations have been carried out with the actual experimental complexes, including solvent effects, in order to analyze the mechanism of the π-alkyne to vinylidene isomerization of internal alkynes and to explain the divergent results obtained for Tp and Cp*. © 2011 American Chemical Society.