Title of Paper:CO2 Hydrogenation Catalyzed by Iridium Complexes with a Proton-Responsive Ligand

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Date of Publication:2015-01-01

Journal:Inorganic Chemistry

Included Journals:SCIE

Volume:54

Issue:11

Page Number:5114-5123

Abstract:The catalytic cycle for the production of formic acid by CO<inf>2</inf> 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, CO<inf>2</inf> 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)<inf>2</inf>(TH2BPM)]Cl<inf>2</inf> (Cp? = pentamethylcyclopentadienyl; TH2BPM = 4,4  ,6,6  -tetrahydroxy-2,2  -bipyrimidine) enormously promotes the catalytic hydrogenation of CO<inf>2</inf> 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 CO<inf>2</inf> hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp?Ir(bpy)(OH<inf>2</inf>)]SO<inf>4</inf>. 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 CO<inf>2</inf>. ? 2015 American Chemical Society.