Cyclic Amide-Anchored NHC-Based Cp*Ir Catalysts for Bidirectional Hydrogenation–Dehydrogenation with CO2/HCO2H Couple

Abstract

H2 storage in carbon dioxide (CO2) or bicarbonate (HCO3–) in the form of formic acid (HCO2H) or formate (HCO2–) and the reverse H2 liberation allows, in principle, to develop a rechargeable hydrogen carrier system along with a CO2-recycling mechanism. The key to such an alluring approach toward the realization of a carbon-neutral H2-based fuel option is the development of efficient bidirectional catalysts for CO2 (or HCO3–) hydrogenation and HCO2H (or HCO2–) dehydrogenation. With an aim toward (i) structurally robust catalysts under variable reaction conditions, (ii) metal–ligand bifunctionality-triggered heterolytic H2 splitting and H+/H– transfer during hydrogenation/dehydrogenation reactions, (iii) electron-rich catalytic metal center for facilitating hydride delivery, and (iv) water solubility of the catalysts via second coordination sphere interactions, herein, we applied a series of “cyclic amide–NHC” hybrid bidentate ligand-bound Cp*Ir(III) complexes (Ir-1–Ir-4) in bidirectional hydrogenation–dehydrogenation of CO2 (HCO3–)/HCO2H (HCO2–) couple in water as a “green” solvent without the use of organic additives/solvents. Notably, with the catalyst Ir-1, hydrogenation of CO2 achieving a turnover number (TON) of 16 680 at 60 °C in 6 h and dehydrogenation of formic acid with a turnover frequency (TOF5min) of 70 674 h–1 at 80 °C can be efficiently carried out. Key control and mechanistic studies emphasized the following aspects of the current system: (i) pH of the solution played a crucial role in controlling the rate of hydrogenation/dehydrogenation reactions, (ii) H2 was cleaved readily by the catalyst to form the iridium hydride intermediate, which could react with CO2 to furnish the formate product, (iii) pH (acid/base)-switchable on-demand formic acid dehydrogenation was devised, and (iv) the liberated H2 and CO2 gas from the Ir-1-catalyzed formic acid dehydrogenation reaction were reutilized in secondary reactions in a tandem fashion, signifying the suitability of the system to demonstrate the utility of formic acid as a typical H2/CO2 storage liquid, as it is advocated for.