Catalytic conversion of CO2 via “transfer hydrogenation” using renewable non-H2 compounds (such as biomass-derived (poly)­alcohols) to produce valuable energy-relevant chemicals, is a promising alternative strategy to the traditionally employed “hydrogenation” of CO2 with gaseous H2. However, the CO2-transfer hydrogenation has been explored exceptionally less, and limited but encouraging success has been achieved in recent time by applying high pressure (up to 50 atm) of CO2 gas. For safe and simple operation, ambient-pressure protocols are desirable, and toward this end, suitable catalysts are required. Aiming to this goal, herein we report an efficient Ir–aNHC catalyst (aNHC = an abnormal NHC ligand) to achieve ambient-pressure CO2-transfer hydrogenation catalysis for generating formate salt (HCO2 –) at the turnover frequency (TOF) value of 90 h–1 in 12 h of reaction at 150 °C using glycerol as hydrogen source.

The “hydricity” of a species refers to its hydride‐donor ability. Similar to how the pKa is useful for determining the extent of dissociation of an acid, the hydricity plays a vital role in understanding hydride‐transfer reactions. A large number of transition‐metal‐catalyzed processes involve the hydride‐transfer reaction as a key step. Among these, two key reactions—proton reduction to evolve H2 and hydride transfer to CO2 to generate formate/formic acid—represent a promising solution to build a sustainable and fossil‐fuel‐free energy economy. Therefore, it is imperative to develop an in‐depth relationship between the hydricity of transition‐metal hydrides and its influencing factors, so that efficient and suitable hydride‐transfer catalysts can be designed. Moreover, such profound knowledge can also help in improving existing catalysts, in terms of their efficiency and working mechanism. With this broad aim in mind, some important research has been explored in this area in recent times. This Minireview emphasizes the conceptual approaches developed thus far, to tune and apply the hydricity parameter of transition‐metal hydrides for efficient H2 evolution and CO2 reduction/hydrogenation catalysis focusing on the guiding principles for future research in this direction.

Numerous strategies have been developed for the reduction of highly challenging CO 2 gas and its conversion into useful feedstock chemicals. Among all of the developed protocols, the traditional approach where H 2 gas is used as a reductant has been dominantly exploited. During the past decade, enormous efforts have been made in tackling the challenge by keeping sustainability as a major goal. As an alternative option, the adoption of a "transfer hydrogenation" strategy has received attention for the CO 2 reduction process. The utilization of biomass-derived alcohols as hydride donors promises to make the process viable and advantageous over the hydrogenation process. The survival of homogeneous transition-metal-based catalysts used in these processes under the harsh reaction conditions (elevated temperature and highly basic reaction medium) is a considerable challenge. Hence, the development of efficient and robust homogeneous catalysts for the CO 2 -transfer hydrogenation process is highly important. In this Perspective, we highlight the overall evolution of the transfer hydrogenation strategy for the reduction of CO 2 gas (and its derivatives) to hydrogenrich useful products achieved during the past decade. The role of tuning the ligand backbone to make the process kinetically more favorable is discussed in detail. The available reports in the field emphasized the advantages of using biomass-derived alcohols as hydride donors in place of nonrenewable H 2 gas. Potential benefits and opportunities of the CO 2 -transfer hydrogenation process over the traditional hydrogenation are critically presented to encourage further intense research in the field.

H 2 storage in carbon dioxide (CO 2 ) or bicarbonate (HCO 3 − ) in the form of formic acid (HCO 2 H) or formate (HCO 2 − ) and the reverse H 2 liberation allows, in principle, to develop a rechargeable hydrogen carrier system along with a CO 2 -recycling mechanism. The key to such an alluring approach toward the realization of a carbonneutral H 2 -based fuel option is the development of efficient bidirectional catalysts for CO 2 (or HCO 3 − ) hydrogenation and HCO 2 H (or HCO 2 − ) dehydrogenation. With an aim toward (i) structurally robust catalysts under variable reaction conditions, (ii) metal− ligand bifunctionality-triggered heterolytic H 2 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 CO 2 (HCO 3 − )/HCO 2 H (HCO 2 − ) couple in water as a "green" solvent without the use of organic additives/solvents. Notably, with the catalyst Ir-1, hydrogenation of CO 2 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) H 2 was cleaved readily by the catalyst to form the iridium hydride intermediate, which could react with CO 2 to furnish the formate product, (iii) pH (acid/base)-switchable on-demand formic acid dehydrogenation was devised, and (iv) the liberated H 2 and CO 2 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 H 2 /CO 2 storage liquid, as it is advocated for.

An imidazolylidene-based abnormal NHC ligand partnering with a proton-responsive benzimidazolato motif renders an Ir-catalyst highly efficient in both ambient-pressure CO2-hydrogenation and low-temperature HCO2H-dehydrogenation pertinent to hydrogen storage/delivery processes.