Alternative Mechanisms of the Alkyne to Vinylidene Isomerization Promoted by Half-Sandwich Ruthenium Complexes. X-ray Crystal Structures of [Cp*RuCCHCOOMe(dippe)][BPh₄] and [Cp*RuH(C⋮CCOOMe)(dippe)][BPh₄] (dippe = 1,2-bis(diisopropylphosphino)ethane; Cp* = C₅Me₅)

Abstract

The complex [Cp*RuCl(dippe)] reacts with 1-alkynes in MeOH in the presence of NaBPh4, yielding the metastable hydrido−alkynyl derivatives [Cp*Ru(H)(C⋮CR)(dippe)][BPh4] (R = COOMe, Ph or SiMe3), intermediates in the formation of the corresponding vinylidene complexes, to which these compounds rearrange both in solution and in the solid state. The X-ray crystal structures of the isomers [Cp*RuCCHCOOMe(dippe)][BPh4] and [Cp*RuH(C⋮CCOOMe)(dippe)][BPh4] have been determined. Kinetic studies show that the mechanism for this isomerization process seems to be dissociative and that it is inhibited in solution by strong acids. In contrast with this, no hydrido−alkynyl complex has been observed in the course of the reaction of 1-alkynes with [CpRuCl(dippe)]. Instead, the alkyne adducts have been detected, and isolated in some cases. Such species have only been observed in the Cp*Ru system for acetylene, and [Cp*Ru(η2-HC⋮CH)(dippe)]+ seems to be in equilibrium with the corresponding hydrido−alkynyl complex [Cp*RuH(C⋮CH)(dippe)]+, which isomerizes to [Cp*RuCCH2(dippe)]+ via the formation of the π-alkyne complex. The effects on these tautomerization processes of the phosphine, Cp*, and Cp ligands, as well as the R group of the alkyne, are discussed.