Alkyne–alkenyl coupling at a diruthenium complex

Abstract

Dimetallic complexes are suitable platforms for the assembly of small molecular units, and the reactivity of bridging alkenyl ligands has been widely investigated to model C–C bond forming processes. Here, we report the unusual coupling of an alkenyl ligand, bridging coordinated on a diruthenium scaffold, with a series of alkynes, revealing two possible outcomes. The diruthenium complex [Ru₂Cp₂(Cl)(CO)(μ-CO){μ–η¹:η²-C(Ph)CH(Ph)}], 2, was prepared in two steps from [Ru₂Cp₂(CO)₂(μ-CO){μ–η¹:η²-C(Ph)CH(Ph)}]BF₄, [1]BF₄, in 69% yield. Then, the reaction of 2 with C₂(CO₂Me)₂, promoted by AgCF₃SO₃ in dichloromethane, afforded in 51% yield the complex [Ru₂Cp₂(CO)₂{μ–η³:η²-C(Ph)CH(Ph)C(CO₂Me)C(CO₂Me)}]CF₃SO₃, [3]CF₃SO₃, containing a ruthenacyclopentene-based hydrocarbyl ligand. On the other hand, 2 reacted with other alkynes and AgX salts to give the butadienyl complexes [Ru₂Cp₂(CO)₂{μ–η³:η²-C(R)CH(R′)C(Ph)C(Ph)}]X (R = R′ = H, [4]BF₄; R = R′ = Me, [5]CF₃SO₃; R = R′ = Ph, [6]CF₃SO₃; R = Ph, R′ = H, [7]CF₃SO₃), in 42–56% yields. All products were characterized by IR and NMR spectroscopy, and by single crystal X-ray diffraction in the cases of 2, [3]CF₃SO₃ and [6]BF₄. DFT calculations highlighted the higher stability of [4–7]⁺-like structures with respect to the corresponding [3]⁺-like isomers. It is presumable that [3]⁺-like isomers initially form as kinetic intermediates, then undergo H-migration which is disfavoured in the presence of carboxylato substituents on the alkyne. Such hypothesis was supported by the computational optimization of the transition states for H-migration in the cases of R = R′ = H and R = R′ = CO₂Me.