Reactions of the cyclopentadienyl ruthenium complexes (C5R5)Ru(cod)Cl and [(C5R5)Ru(MeCN)3]+ (R = H, Me) with phenylacetylene and acetic acid: Unexpected difference in reactivity of CpRu and Cp*Ru complexes

Reaction of CpRu(cod)Cl with phenylacetylene and AcOH gives an unusual binuclear ruthenium complex CpRu(μ-σ,η3:η3,σ-C6H3Ph3)RuCp (5, 58% yield) with a bridging acyclic flyover ligand C6H3Ph3. Under similar conditions [CpRu(MeCN)3]+ undergoes an unexpected cleavage of Cp ligand giving 4,7-diphenyl-3a,7a-dihydroindene (6, 65% yield). In sharp contrast, the pentamethylated congeners Cp*Ru(cod)Cl and [Cp*Ru(MeCN)3]+ react with phenylacetylene and AcOH in catalytic fashion giving 1,4-diphenyl-1-acetoxy-1,3-butadiene. The structures of 5 and 6 were established by X-ray diffraction. The mechanism of Cp ligand cleavage was proposed on the basis of DFT calculations.