Computational insights into Ir-catalyzed transfer hydrogenation of CO2 to formate: critical roles of abnormal NHC ligands and hydrogen donors

A computational study was conducted to elucidate the mechanism of Ir-catalyzed transfer hydrogenation of CO₂ to formate, with a particular focus on investigating how abnormal N-heterocyclic carbene (aNHC) ligands and hydrogen donors influence reaction efficiency. The results reveal that β-H elimination serves as the rate-determining step in the catalytic cycle. The dissociation of the Ir–N bond is a prerequisite for the β-H elimination step, which is crucial for the formation of iridium hydride species. Compared to isopropanol, the higher reactivity observed with glycerol is attributed to H-bonding interactions between the dissociated anionic N ligand and the hydroxyl groups of glycerol. The rotational flexibility of aNHC ligands is critical for enhancing the reactivity of β-H elimination, as evidenced by the strong linear correlation between the dihedral angles of free aNHC ligands and the barriers of β-H elimination.