Making a Splash in Homogeneous CO₂ Hydrogenation: Elucidating the Impact of Solvent on Catalytic Mechanisms

Abstract

Molecular catalysts for hydrogenation of CO are widely studied as a means of chemical hydrogen storage. Catalysts are traditionally designed from the perspective of controlling the ligands bound to the metal. In recent years, studies have shown that the solvent can also play a key role in the mechanism of CO hydrogenation. A prominent example is the impact of the solvent on the thermodynamic hydride donor ability, or hydricity, of metal hydride complexes relative to the hydride acceptor ability of CO . In some cases, simply changing from an organic solvent to water can reverse the direction of hydride transfer between a metal hydride and CO . Additionally, the solvent can impact catalysis by converting CO into carbonate species, as well as activate intermediate products for hydrogenation to more reduced products. By understanding the substrate and product speciation, as well as the reactivity of the catalyst towards the substrate, the solvent can be used as a central design component for the rational development of new catalytic systems.

Kinetics of Hydride Transfer from Metal-Free Hydride Donors to CO2

“…An increase in the temperature lead to an increase in formate yields ( Figure S1) consistent with the large kinetic barrier associated with HT. The increased polarity of the medium, either by solvents with higher dielectric constant 30,31 (SI, Figures S2 and S3) or by adding alkali and alkaline salts ( Figure S4) leads to an increased in formate yields, as expected for a reaction that produced charged products from neutral reactants. 26,32,33 In all reactions, the loss of organic hydride reactants was higher than the growth of formate product (blue and red traces in Figures S1-S5), indicating additional reaction pathways that organic hydrides undergo.…”

Emerging Implications of the Concept of Hydricity in Energy‐Relevant Catalytic Processes

“…Other complexes considered in Figure 3a lso show as imilars ignature for the solvent dependency of the hydricity parameter.Some recent reviews covered the role of somek ey variable characteristics, such as dielectric constant (e), hydrogen bonding, and acceptorn umber (AN) of solvents in understanding the solvent-dependent change of hydricity parameters. [3,11] Furthermore, to expand the scope of solvent-dependent hydricity values, solvents other than acetonitrile and water were used to calculate the hydricity of af ew complexes.F or example, Saouma and co-Abhishek Kumar obtained his BSc degree from the Universityo fD elhi in 2015. After that, he joined IISER Bhopal as an integrated-PhD student and, in 2017, he started his doctoral research in the group of Dr.J oyanta Choudhury in the area of organometallic chemistry.H is work is based on developing efficientt ransition-metal-NHC-basedh omo-and heterogeneous catalysts for CO 2 -conversion processes by using hydrogenation and transfer hydrogenation approaches.…”

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

“…Thus, the sustainability of the direct CO 2 -route is strictly dependent on the use of green hydrogen produced without contemporary CO 2 release in the atmosphere, as well as catalytic systems operating efficiently under milder reaction conditions. A variety of homogeneous catalytic systems has been employed for the direct hydrogenation of CO 2 into green fuels [213,214,517,[525][526][527][528][529][530][531][532][533][534][535], including first-row metal complexes [536][537][538][539][540][541][542][543][544]. Pincer-type ligation shows again very encouraging features in terms of stability and mild reaction conditions, with promising possibility of further optimization.…”