Surface Organometallic Chemistry on Oxides: Reaction of CO with Bis(allyl)rhodium Grafted onto Silica, Titania, and Alumina

Tris(allyl)rhodium(III) reacts with the hydroxyl-terminated surfaces of silica, titania, and alumina to form a surface bis(allyl)rhodium(III) fragment. In the presence of CO, this supported organometallic fragment undergoes a series of reactions which model elementary steps of heterogeneous C‒C and C‒O bond formation. On surfaces with low proton content (silica-400, alumina, titania), 1,5-hexadiene is produced quantitatively by reductive coupling of two allyl ligands, with simultaneous formation of surface-bound dicarbonylrhodium(I). In the presence of a high concentration of surface protons (silica-200), there are two reaction pathways: (i) formation of propene by reaction of an allyl ligand with a surface proton and (ii) insertion of CO into the metal‒carbon bond to give the acyl complex CH₂═CHCH₂C(O)Rh^III, detected by IR. The acyl ligand may undergo reductive elimination with an allyl ligand, giving the minor product 1,6-heptadien-4-one, or with a siloxy ligand, with transfer of 3-butenoate to the silica support (extracted as methyl-3 butenoate). The dicarbonylrhodium(I) product is mobile on the surface of silica: dimerization occurs spontaneously. Under H₂, reduction and aggregation of the dimers leads to small metal particles.