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

Thermolysis (170 °C, 3 days) of a diruthenium μ-methylene complex, Cp 2 Ru 2 (μ-CH 2 )(μ-CO)(CO) 2 ( 1 ), in the presence of HSiMe 3 produces methane along with methylsilane (SiMe 4 ) and mononuclear organometallic products, CpRu(H)(SiR 3 ) 2 (CO) ( 2 ) and CpRu(CO) 2 (SiMe 3 ) ( 3 ). The reaction mechanism involving initial CO dissociation has been investigated by using a labile μ-methylene species, Cp 2 Ru 2 (μ-CH 2 )(μ-CO)(CO)(MeCN) ( 4 ), the MeCN adduct of the coordinatively unsaturated species resulting from decarbonylation of 1 . Treatment of 4 with HSiR 3 produces the hydrido−silyl−μ-methylene intermediate Cp 2 Ru 2 (μ-CH 2 )(H)(SiR 3 )(CO) 2 ( 5 ) and the disilyl−μ-methylene complex Cp 2 Ru 2 (μ-CH 2 )(SiR 3 ) 2 (CO) 2 ( 6 ) successively. Further reaction of 5 and 6 with HSiR 3 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 1 H-NMR spectrum consistent with an apparent C s structure at ambient temperature has been analyzed in terms of a mechanism involving intramolecular H- and R 3 Si-group migration between the two ruthenium centers. It is also revealed that intramolecular exchange reaction of the hydride and μ-C H 2 atoms in 5 proceeds via the coordinatively unsaturated methyl intermediate Cp 2 Ru 2 (CH 3 )(SiR 3 )(CO) 2 ( 9 ). In addition to these intramolecular processes, the hydride, μ-C H 2, and SiR 3 groups in 5 and 6 exchange with external HSiR 3 via replacement of the η 2 -bonded H 2 or HSiR 3 ligand in μ-methylene or μ-silylmethylene intermediates Cp 2 Ru 2 (μ-CHX)(μ-CO)(CO)(η 2 -H-Y) [X, Y = H, SiR 3 ( 7 ), SiR 3, H ( 18 ), SiR 3, SiR 3 ( 16 )] as confirmed by trapping experiments of 7 with L (CO, PPh 3 ) giving Cp 2 Ru 2 (μ-CHX)(μ-CO)(CO)(L) [X, L = H, CO ( 1 ), H, PPh 3 ( 11 ), SiR 3, CO ( 12 ), SiR 3, PPh 3 ( 13 )]. Hydrostannanes (HSnR 3 ) also react with 4, in a manner similar to the reaction with HSiR 3, to give the hydrido−stannyl−μ-methylene intermediate Cp 2 Ru 2 (μ-CH 2 )(H)(SnR 3 )(CO) 2 ( 20 ) and the distannyl−μ-methylene complex Cp 2 Ru 2 (μ-CH 2 )(SnR 3 ) 2 (CO) 2 ( 21 ) successively (the stannyl analogues of 5 and 6, respectively). The intramolecular exchange processes (H ↔ SnR 3, H ↔ μ-C H 2 ) are also observed for 20 . But the HSnPh 3 adduct 20c is further converted to a mixture containing the μ-η 1:η 2 -phenyl complex Cp 2 Ru 2 (μ-Ph)(SnCH 3 Ph 2 )(CO) 2 ( 22 ) and the bis(μ-stannylene) complex Cp 2 Ru 2 (μ-SnPh 2 ) 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−SiR 3 oxidative addition giving the hydrido−silyl−μ-methylene intermediate 5, (iii) equilibrium with the methyl intermediate 9, (iv) a second oxidative addition of H−SiR 3, 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 H 2 equivalent (pseudo-hydrogen). The molecular structures of 6d, e, 12a, 13a, 21a, and 22 have been determined by X-ray crystallography.