Surface organometallic chemistry on oxides: Reaction of hydrogen with bis(allyl)rhodium grafted to silica, titania and alumina

Rh(η³-C₃H₅)₃, 1, reacts at room temperature with hydroxyl groups present on the surfaces of silica, titania and partially dehydroxylated alumina, leading to the formation of a supported bis(allyl)rhodium complex and a molecule of propene. Exposure of these grafted complexes (respectively 2, 3 and 4 to molecular hydrogen has been studied 'in situ' using qualitative and quantitative analytical techniques such as GPC, MS, volumetry, electron microscopy and IR spectroscopy. 2 reacts with H₂ or D₂ to give metallic particles of rhodium. Most of the allylic fragments are eliminated as propane and hexanes. A band of weak intensity at ca. 2048 cm^-1 has been unambiguously attributed (labelling experiments using ¹⁸O, D₂O, C¹⁶O, etc.) to a small amount of CO coordinated to a metallic particle of zero-valent rhodium and not to a hydride coordinated to a mononuclear monoallylic fragment of rhodium. The oxygen of the CO probably arises from the water or hydroxyl groups of the support. These particles exhibit activity similar to that observed for 'conventional' Rh/SiO₂ catalysts synthesized by impregnation with metal salts. Similar observations are obtained when 3 and 4 are exposed to molecular H₂.