Conversion of a diruthenium mu-methylene complex, Cp(2)Ru(2)(mu-CH2)(mu-CO)(CO)(2), into methane through reduction with hydrosilane: Reactivity of silyl-mu-methylene intermediates Cp(2)Ru(2)(mu-CH2)(SiR(3))(X)(CO)(2) (X=H, SiR(3)) relevant to catalytic CO hydrogenation and activation of Si-H, C-H, and C-Si bonds

Thermolysis (170 degrees C, 3 days) of a diruthenium mu-methylene complex, Cp(2)Ru(2)(mu-CH2)(mu-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 mu-methylene species, Cp(2)Ru(2)(mu-CH2)(mu-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-mu-methylene intermediate Cp(2)Ru(2)(mu-CH2)(H)(SiR(3))(CO)(2) (5) and the dislyl-mu-methylene complex Cp(2)Ru(2)(mu-CH2)(SiR(3))(2)(CO)(2) (6) successively. Further reaction of 5 and 6 with HSiR(3) affords methane under milder conditions (120 degrees C, 12 h) compared to the methane formation from 1. Meanwhile complicated exchange processes are observed for the silylated mu-methylene species 5 and 6. The dynamic behavior of the hydrido-silyl species 5 giving a H-1-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 mu-CH2 atoms in 5 proceeds via the coordinatively unsaturated methyl intermediate Cp(2)Ru(2)(CH3)(SiR(3))(CO)(2) (9). In addition to these intramolecular processes, the hydride, mu-CH2, and SiR(3) groups in 5 and 6 exchange with external HSiR(3) via replacement of the eta(2)-bonded H-2 or HSiR(3) ligand in mu-methylene or mu-silylmethylene intermediates Cp(2)Ru(2)(mu-CHX)(mu-CO)(CO)(eta(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)(mu-CHX)(mu-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-mu-methylene intermediate Cp(2)Ru(2)(mu-CH2)(H)(SnR(3))(CO)(2) (20) and the distannyl-mu-methylene complex Cp(2)Ru(2)(mu-CH2)(SnR(3))(2)(CO)(2) (21) successively (the stannyl analogues of 5 and 6, respectively). The intramolecular exchange processes (H <-> SnR(3), H <-> mu-CH2) are also observed for 20. But the HSnPh(3) adduct 20c is further converted to a mixture containing the mu-eta(1):eta(2)-phenyl complex Cp(2)Ru(2)(mu-Ph)(SnCH(3)Ph(2))(CO)(2) (22) and the bis(mu-stannylene) complex Cp(2)Ru(2)(mu-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-mu-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 hydride-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.