Selective Synthesis of a Triruthenium Pentahydrido Complex with Mixed-Cp Ligands (C5 t Bu3H2 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 t Bu3H2, 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.