Hydrogen transfer processes mediated by supported iridium oxide nanoparticles

Homogeneous iridium catalysts have demonstrated exceptional catalytic activity for a number of hydrogen transfer reactions. Herein, we demonstrate the synthesis of a heterogeneous iridium catalyst supported on nanoparticulate cerium oxide and investigate its application for the aerobic oxidation of benzyl alcohol and the Meerwein-Ponndorf-Verley transfer hydrogenation of cyclohexanone. Along with the optimisation of the activity of the catalyst, the kinetic parameters have been examined to unravel the elementary reaction steps mediated by this catalyst and further rationalise the observed structure-activity relationships. Both spectroscopic and catalytic investigations suggest that iridium oxide nanoparticles, most likely Ir₂O₃, mediate these reactions via the formation of metal hydroxide species, which are subsequently reoxidised with either a molecular oxygen or a ketone. In contrast to many other metal- or metal oxide-based catalysts, this catalyst can perform the selective oxidation of alcohols in the absence of a base, at mild temperatures and at a low metal loading. An idea in transit: Iridium oxide, supported on nanoparticulate cerium oxide, is reported to catalyze the oxidative dehydrogenation of alcohols via β-hydride elimination. The catalyst surface can be reoxidized with either a molecular oxygen or a ketone, which completes the transfer hydrogenation cycle.