Insights into the intramolecular acetate-mediated formation of ruthenium vinylidene complexes: a ligand-assisted proton shuttle (LAPS) mechanism

Abstract

The ruthenium bis-acetate complex Ru(κ²-OAc)₂(PPh₃)₂ reacts with HCCPh to afford the vinylidene-containing species Ru(κ¹-OAc)(κ²-OAc)(CCHPh)(PPh₃)₂. An experimental study has demonstrated that this reaction occurs under very mild conditions, with significant conversion being observed at 255 K. At lower temperatures, evidence for a transient metallo-enol ester species Ru(κ¹-OAc)(OC{Me}O–CCHPh)(PPh₃)₂ was obtained. A comprehensive theoretical study to probe the nature of the alkyne/vinylidene tautomerisation has been undertaken using Density Functional Theory. Calculations based on a number of isomers of the model system Ru(κ¹-OAc)(κ²-OAc)(CCHMe)(PH₃)₂ demonstrate that both the η²(CC) alkyne complex Ru(κ¹-OAc)(κ²-OAc)(η²-HCCMe)(PH₃)₂ and the C–H agostic σ-complex Ru(κ¹-OAc)(κ²-OAc)(η²{CH}-HCCMe)(PH₃)₂ are minima on the potential energy surface. The lowest energy pathway for the formation of the vinylidene complex involves the intramolecular deprotonation of the σ-complex by an acetate ligand followed by reprotonation of the subsequently formed alkynyl ligand. This process is thus termed a Ligand-Assisted Proton Shuttle (LAPS). Calculations performed on the full experimental system Ru(κ¹-OAc)(κ²-OAc)(CCHPh)(PPh₃)₂ reinforce the notion that lowest energy pathway involves the deprotonation/reprotonation of the alkyne by an acetate ligand. Inclusion of the full ligand substituents in the calculations are necessary to reproduce the experimental observation of Ru(κ¹-OAc)(κ²-OAc)(CCHPh)(PPh₃)₂ as the thermodynamic product.