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-κ_C,O)] (6). DFT calculations confirm the formation of 5 and 6 as reversibly formed side products, and suggest an η¹-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.