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

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 (TOF_5min) 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.