Hydrogenation of Carbon Dioxide with Organic Base by PCIIP-Ir Catalysts

Novel PCIIP-IrI monochloride complexes (1-Cl and 2-Cl) bearing a phosphine-carbene-phosphine pincer type ligand were synthesized. Reactions of 1-Cl with hexachloroethane, hydrogen chloride, and lithium triethylborohydride under a dihydrogen atmosphere afforded PCIIP-IrIII trichloride (1-Cl3), hydride dichloride (1-HCl2), and trihydride (1-H3) complexes, respectively. The strong electron-donating ability of carbene in PCIIP-Ir complexes was confirmed by X-ray crystallography and DFT calculations. Moreover, in complex 1-Cl, strong π back-donation from the iridium center to the carbene carbon was observed. Hydrogenation of CO2 with triethanolamine catalyzed by PCIIP-Ir complexes was investigated. The novel PCIIP-Ir complex 1-Cl exhibited a longer lifetime in comparison to the PNP-IrIII complex 3-H3: the turnover number of 1-Cl is significantly higher than that of 3-H3 (in 46 h, 1-Cl 230000 and 3-H3 54000).