Iridium-Catalyzed Homogeneous Hydrogenation And Hydrosilylation Of Carbon Dioxide

The knowledge of the potential of transition metal-based complexes as catalysts for the reduction of CO2 has grown significantly over the last few decades. This chapter focuses on the progress made during recent years in the field of homogeneous iridium-catalyzed reduction of CO2 by using hydrogen and/or silicon hydrides as reducing agents, comparing them with homogeneous catalysts based on other transition metals.The reported studies on iridium-catalyzed CO2 reduction processes show that an important point to keep in mind when designing a catalyst is the nature of the reducing agent (hydrogen, hydrosilanes, and/or hydrosiloxanes). Thus, iridium(III) half-sandwich complexes with 4,4'-dihydroxy-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) ligands, and iridium(III)-PNP pincer complexes have proven to be excellent catalysts for the hydrogenation of CO2 to formic acid. However, Ir(III)-NSiNMe (NSiN = fac-bis-(4-methylpyridine-2-yloxy)methylsilyl) and Ir(III)-NSiMe (NSiMe = 4-methylpyridine-2-yloxydimethylsilyl) species are not stable under hydrogen atmosphere but are effective catalysts for the reduction of CO2 with hydrosiloxanes to silylformate under solvent-free conditions and moderate CO2 pressures and temperatures. Moreover, while using iridium(III)-DHBP half-sandwich complexes, high CO2 and H-2 pressures are required to achieve the catalytic CO2 hydrogenation to methanol; Ir-NSiMe species catalyze the reduction of CO2 to methoxysilane with hydrosiloxanes under low CO2 pressure.