Surface-supported metal cluster carbonyls. Chemisorption, decomposition and reactivity of Os₃(CO)₁₂, H₂Os₃(CO)₁₀ and Os₆(CO)₁₈ supported on silica and alumina and the investigation of the fischer-tropsch catalysis with these systems

On silica and alumina, Os₃(CO)₁₂, H₂Os₃(CO)₁₀ and Os₆(CO)₁₈ are physisorbed (or weakly adsorbed) at room temperature. When the physisorbed cluster Os₃(CO)₁₂ is thermally decomposed at ca. 150°C, there is an oxidative addition of a surface MOH group (M Al???, Si???) to the OsOs bond of Os₃(CO)₁₂ with formation of the surface species Os₃(H)(CO)₁₀(OM) (M Al???, Si???) in which the cluster is covalently bonded to the surface by MOOs₃bonds.Such a grafted cluster was also obtained was also obtained by bringing Os₃(CO)₁₀(CH₃CN)₂ or H₂Os₃(CO)₁₀ into reaction with the surface of silica and alumina. On silica the grafted cluster, when treated with CO + H₂O, can regenerate the starting Os₃(CO)₁₂ cluster. The structure of the covalently bonded cluster was also confirmed by the synthesis of the model compound Os₃(H)(CO)₁₀(OSi(Ph)₃). Such covalent attachment of a cluster to a surface can be regarded as a model for the metal-support interaction which is frequently involved in heterogeneous catalysis. When the physisorbed clusters Os₃(CO)₁₂, H₂Os₃(CO)₁₀, Os₆(CO)₁₈ or the chemisorbed cluster Os₃(H)(CO)₁₀(OM), (M Al???, Si???), are heated at about 200°C, there is a breakdown of the cluster cage with simultaneous oxidation of the osmium to two osmium(II) carbonyl species by the surface proton with simultaneous release of hydrogen. The [Os^II(CO)₃X₂]₂ and [Os^II(CO)₂X₂]_n surface species in which X is a surface oxygen atom can be interconverted by a reversible carbonylation-decarbonylation process at 200°C. These two species can be also obtained by decomposition of [Os(CO)₃X₂]₂, (X Cl, Br) onto silica or alumina surface or by CO reduction at 200°C of OsX₃ adsorbed on silica or alumina. The structure of one these surface compounds was confirmed by synthesis of the model compound [Os(CO)₃(OSiPh₃)₂]_m. These surface osmium carbonyl species exhibit a rather high thermal and chemical stability. They appear to be reduced by H₂ to metallic osmium only at 400°C. The thermal decomposition of the supported clusters is followed by a stoichiometric water gas shift reaction as well as a stoichiometric formation of methane. Under CO + H₂, a Fischer-Tropsch catalyst, which exhibits a high selectivity for methane, is obtained. From the range of temperatures over which those stoichiometric and catalytic reactions are observed it seems reasonable to assume that they involve the Os^II carbonyl species rather than metallic osmium.