Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation Catalyzed by Iridium Complexes Bearing Pyridyl-pyrazole Ligands: Effect of an Electron-donating Substituent on the Pyrazole Ring on the Catalytic Activity and Durability

Cp*Ir (Cp*=pentamethylcyclopentadienyl) complexes with an N,N-bidentate ligand such as 2,2 '-bipyridine serve as catalysts for both carbon dioxide (CO2) hydrogenation to formate and formic acid dehydrogenation in water. Previously, it was shown that the introduction of an electron-donating substituent on 2,2 '-bipyridine is an effective method to improve the catalytic activity. Especially, the highly electron-donating hydroxyl (OH) substituent performs much better than other substituents such as methyl or methoxy under basic conditions. However, the introduction of an OH substituent on the ligand has been limited to six-membered rings such as pyridine or pyrimidine. These results prompted us to develop a new ligand comprising a pyridyl-pyrazole with an OH group on the pyrazole moiety for Cp*Ir-catalyzed CO2 hydrogenation and formic acid dehydrogenation. The resultant catalyst showed high catalytic activity in CO2 hydrogenation and excellent robustness in formic acid dehydrogenation with a turnover number of 10 million.