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 [Ru 2 Cp 2 (Cl)(CO)(μ-CO){μ-η 1 :η 2 -C(Ph)CH(Ph)}], 2, was prepared in two steps from [Ru 2 Cp 2 (CO) 2 (μ-CO){μ-η 1 :η 2 -C(Ph)CH(Ph)}]BF 4 , [1]BF4, in 69% yield. Then, the reaction of 2 with C 2 (CO 2 Me) 2 , promoted by AgCF 3 SO 3 in dichloromethane, afforded in 51% yield the complex [Ru 2 Cp 2 (CO) 2 {μ-η 3 :η 2 -C(Ph)CH(Ph)C(CO 2 Me)C(CO 2 Me)}]CF 3 SO 3 , [3]CF 3 SO 3 , containing a ruthenacyclopentene-based hydrocarbyl ligand. On the other hand, 2 reacted with other alkynes and AgX salts to give the butadienyl complexes [Ru 2 Cp 2 (CO) 2 {μ-η 3 :η 2 -C(R)CH(R')C(Ph)C(Ph)}]X (R = R' = H, [4]BF 4 ; R = R' = Me, [5]CF 3 SO 3 ; R = R' = Ph, [6]CF 3 SO 3 ; R = Ph, R' = H, [7]CF 3 SO 3 ), 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]CF 3 SO 3 and [6]BF 4 . 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' = CO 2 Me.