Coordinated carbenes from electron-rich olefins on RuHCl(PPr3i)2

Abstract

Dehydrohalogenation of RuH₂Cl₂L₂ (L=PPr₃ⁱ) gives (RuHClL₂)₂, shown to be a halide-bridged dimer by X-ray crystallography; the fluoride analog is also a dimer. (RuHClL₂)₂ reacts with N₂, pyridine and C₂H₄ (L′) to give RuHClL′L₂, but with vinyl ether and vinyl amides, H₂CCH(E) [E=OR, NRC(O)R′] such olefin binding is followed by isomerization to the heteroatom-substituted carbene complex L₂HClRuC(CH₃)(E). The reaction mechanism for such rearrangement is established by DFT(B3PW91) computations, for C₂H₄ as olefin (where it is found to be endothermic), and the structures of intermediates are calculated for H₂CC(H)(OCH₃) and for cyclic and acyclic amide-substituted olefins. It is found, both experimentally and computationally, that the amide oxygen is bonded to Ru, with a calculated bond energy of approximately 9 kcal mol⁻¹ for an acyclic model. Less electron-rich vinyl amides or amines form η²-olefin complexes, but do not isomerize to carbene complexes. Calculated ΔE values for selected ‘‘ competition’’ reactions reveal that donation by both Ru and the heteroatom-substituted X are necessary to make the carbene complex L₂HClRuC(X)(CH₃) more stable than the olefin complex L₂HClRu(η²-H₂CCHX). This originates in part from a diminished endothermicity of the olefin→carbene transformation when the sp² carbon bears a π-donor substituent. The importance of a hydride on Ru in furnishing a mechanism for this isomerization is discussed. The compositional characteristics of Schrock and Fischer carbenes are detailed, it is suggested that reactivity will not be uniquely determined by these characteristics, and these new carbenes RuHCl[C(X)CH₃]L₂ are contrasted to Schrock and Fischer carbenes.