CO₂ hydrogenation catalyzed by iridium complexes with a proton-responsive ligand

The catalytic cycle for the production of formic acid by CO₂ hydrogenation and the reverse reaction have received renewed attention because they are viewed as offering a viable scheme for hydrogen storage and release. In this Forum Article, CO₂ hydrogenation catalyzed by iridium complexes bearing sophisticated N^N-bidentate ligands is reported. We describe how a ligand containing hydroxy groups as proton-responsive substituents enhances the catalytic performance by an electronic effect of the oxyanions and a pendent-base effect through secondary coordination sphere interactions. In particular, [(Cp∗IrCl)₂(TH2BPM)]Cl₂ (Cp∗ = pentamethylcyclopentadienyl; TH2BPM = 4,4′,6,6′-tetrahydroxy-2,2′-bipyrimidine) enormously promotes the catalytic hydrogenation of CO₂ in basic water by these synergistic effects under atmospheric pressure and at room temperature. Additionally, newly designed complexes with azole-type ligands were applied to CO₂ hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp∗Ir(bpy)(OH₂)]SO₄. Furthermore, the introduction of one or more hydroxy groups into ligands such as 2-pyrazolyl-6-hydroxypyridine, 2-pyrazolyl-4,6-dihydroxypyrimidine, and 4-pyrazolyl-2,6-dihydroxypyrimidine enhanced the catalytic activity. It is clear that the incorporation of additional electron-donating functionalities into proton-responsive azole-type ligands is effective for promoting further enhanced hydrogenation of CO₂.