title: CO2 Hydrogenation Catalyzed by Iridium Complexes with a Proton-Responsive Ligand.
abstract: 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.
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.|‡Japan Science and Technology Agency (JST), ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Naoya
authors.initials: N
authors.lastname: Onishi
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.|‡Japan Science and Technology Agency (JST), ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Shaoan
authors.initials: S
authors.lastname: Xu
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Manaka
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Suna
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Wan-Hui
authors.initials: WH
authors.lastname: Wang
authors.affiliation: ⊥Chemistry Department, Brookhaven National Laboratory (BNL), Upton, New York 11973, United States.
authors.forename: James T
authors.initials: JT
authors.lastname: Muckerman
authors.affiliation: ⊥Chemistry Department, Brookhaven National Laboratory (BNL), Upton, New York 11973, United States.
authors.forename: Etsuko
authors.initials: E
authors.lastname: Fujita
authors.affiliation: †National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.|‡Japan Science and Technology Agency (JST), ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Himeda