Selective Synthesis of a Triruthenium Pentahydrido Complex with Mixed-Cp Ligands (C₅^tBu₃H₂ and C₅Me₅) and Its Transformation into Face-Capping Benzene Complexes: Fluxionality of a Face-Capping Benzene Ligand Induced by Oxidation

A mixed-ligand triruthenium μ-chloro complex, [(Cp^‡Ru)­(Cp*Ru)₂(μ-H)₄(μ-Cl)] (7; Cp^‡ = η⁵-C₅^tBu₃H₂, Cp* = η⁵-C₅Me₅) was synthesized by the reaction of [{Cp*Ru­(μ-H)₂}₂] (5a) with [{Cp^‡Ru­(μ-Cl)}₂] (6). The subsequent treatment of 7 with 2-propanol and a base resulted in the replacement of the μ-Cl ligand by a hydride and afforded a mixed-ligand triruthenium pentahydrido complex, [(Cp^‡Ru)­(Cp*Ru)₂(μ-H)₃(μ₃-H)₂] (1b), which is the analogue of the D_3h symmetrical [{Cp*Ru­(μ-H)}₃(μ₃-H)₂] (1a). The pseudo C_s symmetrical Ru₃ skeletons of 1b and 7 supported by one Cp^‡ and two Cp* were unambiguously confirmed by X-ray diffraction. The Ru₃ skeleton of 1b promoted the evaluation of the mobility of a face-capping benzene ligand in [(Cp^‡Ru)­(Cp*Ru)₂(μ-H)₃(μ₃-η²:η²:η²-C₆H₆)] (2b), which was prepared by the reaction of 1b with 1,3-cyclohexadiene or benzene. Unlike previously reported face-capping benzene complexes, in which the μ₃-η²:η²:η²-C₆H₆ ligand rotates rapidly on the trinuclear plane, the variable-temperature NMR study showed that the face-capping benzene in 2b does not rotate on the Ru₃ plane even at 80 °C. This was likely due to the electron-rich nature of the Ru₃ site, which strongly binds the benzene ligand. In contrast, the μ₃-η³:η³-C₆H₆ ligand in [(Cp^‡Ru)­(Cp*Ru)₂(μ-H)₃(μ₃-η³:η³-C₆H₆)]²⁺ (4b), obtained by the two-electron oxidation of 2b, rotated rapidly due to the reduced back-donation from the dicationic Ru₃ site.