Formation of a Boraruthenacyclopentenyl Skeleton via B–C Bond Formation across a Triruthenium Plane

The treatment of a trimetallic complex containing a μ3-borylene ligand, (Cp*Ru)3(μ3-BH)(μ-H)3 (6; Cp* = η5-C5Me5), with 1 atm of acetylene resulted in the incorporation of two molecules of acetylene on the opposite face of the μ3-borylene ligand to form a μ3-η2(∥)-ethyne μ-ethylidene complex, (Cp*Ru)3(μ3-BH)(μ3-η2(∥)-HCCH)(μ-CHCH3)(μ-H) (7a). Although an intermediate was not observed in the reaction of 6 with acetylene, the intermediary monoalkyne complex 8b was obtained by the reaction of 6 with phenylacetylene, which was converted into (Cp*Ru)3(μ3-BH)(μ3-η2(∥)-PhCCH)(μ-CHCH3)(μ-H) (7b) upon subsequent reaction with acetylene. The thermolysis of 7a provided a closo-2-boraruthenacyclopentenyl complex, {Cp*Ru(μ-H)}2{Cp*Ru(−BHCHCMeCH−)} (11), as a consequence of B–C bond formation across the Ru3 plane with the partial breakage of the cluster skeleton. An X-ray diffraction study for 11 clearly showed the formation of a boraruthenacyclopentenyl skeleton, which bisects the triruthenium plane. This transformation shows how important the flexibility of cluster skeleton is for the bond-forming reaction.