Surface Organometallic Chemistry: Reaction of Tris-Allyl Rhodium with Surfaces of Silica, Alumina, Titania and Magnesia

The reaction of Rh(η³-C₃H₅)₃,1, with silica, alumina, titania, and magnesia has been followed by infrared spectroscopy, isotopic labeling experiments, quantitative analysis of gaseous products, and elemental analysis. Complex 1 reacts rapidly with alumina pretreated to 350 °C and with silica and titania to form a bis-allylic rhodium species grafted to the surface, but no immediate reaction was observed between 1 and alumina pretreated to 200 °C or magnesia. This reaction likely proceeds via electrophilic cleavage of Rh-C bonds of 1 by a surface OH group. On silica, the results of IR experiments after labeling the surface with ¹⁶O-D and ¹⁸O-H and structural arguments suggest that two 18-electron species are possible on the surface: one for which the rhodium is coordinated both to an oxygen and a hydroxyl group linked to the same silicon atom and another for which the oxygen atom and the OH group are linked to two different silicon atoms. Molecular mechanics modeling of silica, alumina, and titania was used as a complement to the experimental work to test the plausibility of various modes of attachment of the rhodium allyl complex.