Supported Clusters and Heterogeneous Catalysis Surface Organometallic Chemistry

SUPPORTED CLUSTERS AND HETEROGENEOUS CATALYSIS SURFACE ORGANOMETALLIC CHEMISTRY 

Thermal decomposition of the grafted cluster (H)Os3(CO)io(OSi ) results in a total destruction of the molecular cluster frame. This destruction probably results from a multiple oxidative addition of silanol groups on 3 metal atoms with formation of (H)Os(CO)3(OSi ) which can be reversibly decarbonylated to HOs(CO)2(OSi ). These mononuclear osmium hydrido species are also catalysts for hydroformylation. 

It is only by treatment under vacuum above 250 C that small metal particles of osmium are formed. Those particles have a narrow size 

Above 250 °C the equilibrium could be shifted towards the left whereas below 250 °C this equihbrium could be shifted to the right and lead to an aggregation phenomenon. Those metallic particles of Os are active in the reaction of methanisation of carbon monoxide. 

The reaction of Os3(CO)j2 with the alumina surface is different from that observed with the silica surface. The first step is the oxidative addition of anA1—OH group into the Os—Os bond which gives the species (H)Os3(CO)io(OA1c ). However it is much less stable than on silica it decomposes above 150 C to give mononuclear Os(II) species with liberation of 3 moles of H2. In contrast to what is observed on silica, these surface species are linked to silica by two covalent bonds Os(OAlC)2. Those osmium(II) carbonyl complexes are not easily reduced by H2 to metallic osmium. In contrast to (H)Os(CO)3(OSi ), the Os(CO)3(OAl C) species is not a catalyst for olefin hydroformylation. In order to achieve a reduction to Os metal, it is necessary to reduce Os(II) with H2 at 400 for 20 hours. The metallic particles which are thus obtained are much smaller than on silica since their size is situated around 8 A. 

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