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