Surface organometallic chemistry on metals: Controlled hydrogenolysis of Me₄Sn, Me₃SnR, Me₂SnR₂, MeSnBu ₃ and SnBu₄ (R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto metallic rhodium supported on silica

The controlled hydrogenolysis of Me_xSnR_4-x (0 ≤ x ≤ 4; R = methyl, n-butyl, tert-butyl, neopentyl, cyclohexyl) onto Rh/SiO ₂ is followed by quantitative and qualitative analysis of evolved gases. Only MeH and RH are detected in the evolved gases. There is hydrogenolysis of the Sn-C bonds without any C-C bond hydrogenolysis, leading to formation of grafted organometallic fragments. Using various organotin compounds, Me_xSnR_4-x, it has been possible to determine the regioselectivity of the hydrogenolysis of the Sn-C bonds. The initial selectivity is inversely proportional to the steric bulk of the alkyl group: tBu < Np < Bu. The formation of a five-coordinate tetraalkyl tin on the surface, Rh-SnMe_xR_4-x (symmetry D_3h), in which the bulkiest group, e.g., R, is away from the surface could explain these results. This surface five-coordinate tin species could eliminate an alkyl group, generally a methyl group, thus decreasing the steric bulk around the tin, into the equatorial plane of D_3h, via a concerted hydrogen transfer-elimination mechanism to give Rh-SnMe_x-1R_4-x. Then, in the successive steps of the hydrogenolysis, the bulkiest group, R, would be eliminated.