Reductive Cleavage of CO₂ by Metal-Ligand-Cooperation Mediated by an Iridium Pincer Complex

A unique mode of stoichiometric CO₂ activation and reductive splitting based on metal-ligand-cooperation is described. The novel Ir hydride complexes [(^tBu-PNP)Ir(H)₂] (2) (^tBu-PNP, deprotonated ^tBu-PNP ligand) and [(^tBu-PNP)Ir(H)] (3) react with CO₂ to give the dearomatized complex [(^tBu-PNP)Ir(CO)] (4) and water. Mechanistic studies have identified an adduct in which CO₂ is bound to the ligand and metal, [(^tBu-PNP-COO)Ir(H)₂] (5), and a di-CO₂ iridacycle [(^tBu-PNP)Ir(H)(C₂O₄-_C,O)] (6). DFT calculations confirm the formation of 5 and 6 as reversibly formed side products, and suggest an ¹-CO₂ intermediate leading to the thermodynamic product 4. The calculations support a metal-ligand-cooperation pathway in which an internal deprotonation of the benzylic position by the η¹-CO₂ ligand leads to a carboxylate intermediate, which further reacts with the hydride ligand to give complex 4 and water.