Synthesis, structure, and property of a triruthenium cluster having a μ-alkyl ligand: Transformation of a μ3(⊥)-alkyne ligand into a μ-alkyl ligand via a μ3-vinylidene complex

A triruthenium μ-alkyl complex, (Cp∗Ru)3(μ-η2-HCHCH2R)(μ-CO)2(μ3- CO) (2a, R = Ph; 2b, R = tBu, Cp∗ = η5-C5Me5), which contains a two-electron and three-center interaction among Ru, C, and H atoms, has been synthesized by the reaction of a perpendicularly coordinated 1-alkyne complex, {Cp∗Ru(μ-H)}3(μ3-η2:η2(⊥)-RCCH) (1a; R = Ph, 1b; R = tBu), with carbon monoxide. A diffraction study for 2b clearly represented the bridging neohexyl group on one Ru–Ru edge. This μ-alkyl group exhibited dynamic behavior resulting in site-exchange of the α-hydrogen atoms between the terminal and bridging positions, which was synchronized with the migration of the μ-alkyl groups between the two ruthenium atoms. The agostic C–H bond was readily cleaved upon pyrolysis. Whereas the μ-phenethylidene intermediate resulting from the σ-C–H bond cleavage has never been observed, a μ3-phenethylidyne complex, {Cp∗Ru(μ-CO)}3(μ3-CCH2Ph) (7a), and a μ3-methylidyne complex, {Cp∗Ru(μ-CO)}3(μ3-CH) (8), were obtained by the successive C–H/C–H and C–H/C–C bond cleavages at the μ-alkyl moiety, respectively.