Alkyne–alkenyl coupling at a diruthenium complex

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)(mu-CO) {mu-eta(1):eta(2)-C(Ph)=CH(Ph)}], 2, was prepared in two steps from [Ru2Cp2(CO)(2)(mu-CO){mu-eta(1):eta(2)-C(Ph)=CH (Ph)}]BF4, [1]BF4, in 69% yield. Then, the reaction of 2 with C-2(CO2Me)(2), promoted by AgCF3SO3 in dichloromethane, afforded in 51% yield the complex [Ru2Cp2(CO)(2){mu-eta(3):eta(2)-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){mu-eta(3):eta(2)-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.