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-κC,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.