Metal-to-Metal Silyl Migration and Silicon-Carbon Bond Cleavage/Re-formation Processes in the Methylene/Silyl

Ruthenium methylene/silyl complexes of stoichiometry Cp*2Ru2(μ-CH2)(SiR3)(μ-Cl), where SiR3 ) SiMe3 (1), SiEt3 (2), SiMe2Et (3), and SiMe2Ph (4), are produced when [Cp*RuCl]4 is treated with the appropriate dialkylmagnesium reagent, Mg(CH2SiR3)2. Each complex undergoes two fluxional processes as observed by variable-temperature 1H NMR spectroscopy. The low-temperature exchange process is migration of the SiR3 unit from one Ru center to the other, whereas the high-temperature process is the reversible re-formation of the C-Si bond between the silyl group and the bridging methylene unit. The activation parameters for the low-temperature exchange process in 1-4 are sensitive to the nature of the silyl group: ∆H becomes smaller and ∆S becomes more negative if the SiR3 group bears nonidentical or larger, more flexible substituents. This finding suggests that the transition state for this exchange process is crowded or characterized by different amounts of solvent reorganization depending on the SiR3 group involved. In contrast, the activation parameters for the high-temperature process in 1-4 are relatively independent of the nature of the silyl ligand. The small variation in the activation parameters for reformation of the C-Si bond is consistent with a noncrowded transition state in which the solvent reorganization is relatively independent of the nature of the SiR3 group.