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)-NSiN^Me (NSiN = fac-bis-(4-methylpyridine-2-yloxy)methylsilyl) and Ir(III)-NSi^Me (NSi^Me = 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 H₂ pressures are required to achieve the catalytic CO₂ hydrogenation to methanol; Ir-NSi^Me species catalyze the reduction of CO₂ to methoxysilane with hydrosiloxanes under low CO₂ pressure.