Conversion of a Diruthenium μ-Methylene Complex, Cp2Ru2(μ-CH2)(μ-CO)(CO)2, into Methane through Reduction with Hydrosilane: Reactivity of Silyl−μ-Methylene Intermediates Cp2Ru2(μ-CH2)(SiR3)(X)(CO)2(X = H, SiR3) Relevant to Catalytic CO Hydrogenation and Activation of Si−H, C−H, and C−Si Bonds

Abstract

Thermolysis (170 °C, 3 days) of a diruthenium μ-methylene complex, Cp2Ru2(μ-CH2)(μ-CO)(CO)2 (1), in the presence of HSiMe3 produces methane along with methylsilane (SiMe4) and mononuclear organometallic products, CpRu(H)(SiR3)2(CO) (2) and CpRu(CO)2(SiMe3) (3). The reaction mechanism involving initial CO dissociation has been investigated by using a labile μ-methylene species, Cp2Ru2(μ-CH2)(μ-CO)(CO)(MeCN) (4), the MeCN adduct of the coordinatively unsaturated species resulting from decarbonylation of 1. Treatment of 4 with HSiR3 produces the hydrido−silyl−μ-methylene intermediate Cp2Ru2(μ-CH2)(H)(SiR3)(CO)2 (5) and the disilyl−μ-methylene complex Cp2Ru2(μ-CH2)(SiR3)2(CO)2 (6) successively. Further reaction of 5 and 6 with HSiR3 affords methane under milder conditions (120 °C, 12 h) compared to the methane formation from 1. Meanwhile complicated exchange processes are observed for the silylated μ-methylene species 5 and 6. The dynamic behavior of the hydrido−silyl species 5 giving a 1H-NMR spectrum consistent with an apparent Cs structure at ambient temperature has been analyzed in terms of a mechanism involving intramolecular H- and R3Si-group migration between the two ruthenium centers. It is also revealed that intramolecular exchange reaction of the hydride and μ-CH2 atoms in 5 proceeds via the coordinatively unsaturated methyl intermediate Cp2Ru2(CH3)(SiR3)(CO)2 (9). In addition to these intramolecular processes, the hydride, μ-CH2, and SiR3 groups in 5 and 6 exchange with external HSiR3 via replacement of the η2-bonded H2 or HSiR3 ligand in μ-methylene or μ-silylmethylene intermediates Cp2Ru2(μ-CHX)(μ-CO)(CO)(η2-H-Y) [X, Y = H, SiR3 (7), SiR3, H (18), SiR3, SiR3 (16)] as confirmed by trapping experiments of 7 with L (CO, PPh3) giving Cp2Ru2(μ-CHX)(μ-CO)(CO)(L) [X, L = H, CO (1), H, PPh3 (11), SiR3, CO (12), SiR3, PPh3 (13)]. Hydrostannanes (HSnR3) also react with 4, in a manner similar to the reaction with HSiR3, to give the hydrido−stannyl−μ-methylene intermediate Cp2Ru2(μ-CH2)(H)(SnR3)(CO)2 (20) and the distannyl−μ-methylene complex Cp2Ru2(μ-CH2)(SnR3)2(CO)2 (21) successively (the stannyl analogues of 5 and 6, respectively). The intramolecular exchange processes (H ↔ SnR3, H ↔ μ-CH2) are also observed for 20. But the HSnPh3 adduct 20c is further converted to a mixture containing the μ-η1:η2-phenyl complex Cp2Ru2(μ-Ph)(SnCH3Ph2)(CO)2 (22) and the bis(μ-stannylene) complex Cp2Ru2(μ-SnPh2)2(CO)2 (23) instead of 21c. The isolation of 22 supports viability of the methyl species (9). These results suggest that methane formation from 1 follows (i) CO dissociation, (ii) H−SiR3 oxidative addition giving the hydrido−silyl−μ-methylene intermediate 5, (iii) equilibrium with the methyl intermediate 9, (iv) a second oxidative addition of H−SiR3, and (v) elimination of methane repeating reductive elimination from mono- and dinuclear hydrido−methyl intermediates 27 and 28. The present reaction sequence can be viewed as a model system for methanation via the Fischer−Tropsch mechanism where hydrosilane behaves as a H2 equivalent (pseudo-hydrogen). The molecular structures of 6d,e, 12a, 13a, 21a, and 22 have been determined by X-ray crystallography.