A Reversible Silicon‒Carbon Bond Cleavage Process. Dynamics and Reactivity of Cp*₂Ru₂(μ-CH₂)(SiMe₃)(μ-Cl)

The methylene/silyl complex Cp*₂Ru₂(μ-CH₂)(SiMe₃) (μ-Cl), where Cp* = C₅Me₅, has been prepared by alkylation of [Cp*RuCl]₄ with 1 equiv of Mg(CH₂SiMe₃)₂; the product is formed via the unusual cleavage of the α-C—Si bond of the alkyl group. The presence of a bridging methylene ligand and a terminal trimethylsilyl ligand has been demonstrated by ¹H and ¹³C NMR spectroscopy and confirmed by an X-ray single crystal structure determination. Variable-temperature NMR spectroscopy shows that this molecule undergoes two dynamic processes: the lower temperature process (ΔH^≠ = 9.0 ± 0.2 kcal mol⁻¹, ΔS^≠ = 0.5 ± 0.8 cal mol⁻¹ K⁻¹) involves hopping of the silyl ligand between the two ruthenium centers, whereas the higher temperature process (ΔH^≠ = 12.0 ± 0.3 kcal mol⁻¹, ΔS^≠ = −7 ± 1 cal mol⁻¹ K⁻¹) involves the reversible reformation of the C—Si bond between the silyl and methylene ligands. Further evidence that the C—Si bond can be re-formed rests on the observation that treatment of Cp*₂Ru₂(μ-CH₂)(SiMe₃)(μ-Cl) with PMe₃ or CO instantly gives the mononuclear Ru^II products Cp*Ru(CH₂-SiMe₃)L₂ (2) and Cp*RuClL₂ (3), in essentially quantitative yield. Protonation of Cp*₂Ru₂(μ-CH₂)(SiMe₃)(μ-Cl) with HO₂CCF₃ at low temperature gives the new alkylidene complex Cp*₂Ru₂(μ-CH₂)(μ-O₂CCF₃) (μ-Cl) (4), while protonation of 1 with HO₂CCF₃ at room temperature gives a mixture of 4 and the different alkylidene complex Cp*₂Ru₂(μ-CHSiMe₃)(μ-O₂CCF₃)(μ-Cl) (5), which evidently results from C-Si bond re-formation in 1. Deuterium labeling studies show that protonation of 1 occurs at a ruthenium center and not at the methylene carbon. Treatment of 4 and 5 with PMe₃ yields the cationic alkylidene complexes [Cp*₂Ru₂(μ-CH₂)(PMe₃)(μ-Cl)][O₂CCF₃] (6) and [Cp*₂Ru₂(μ-CHSiMe₃)(PMe₃)(μ-Cl)][O₂CCF₃] (7), respectively. Crystal data for 1: monoclinic, space group P2₁/c, with a = 10.373(3) Å, b = 14.896(4) Å, c = 17.508(9) Å, β = 104.36(4)°, V = 2621(3) ų, Z = 4, R_F = 0.044, and R_wF = 0.049 for 254 variables and 2707 data with I > 2.58σ(I).