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.

Biomimetic NAD(P)H-type organic hydride donors have recently been advocated as potential candidates to act as metal-free catalysts for fuel-forming reactions such as the reduction of CO2 to formic acid and methanol, similar to the natural photosynthesis process of fixing CO2 into carbohydrates. Although these artificial synthetic organic hydrides are extensively used in organic reduction chemistry in a stoichiometric manner, translating them into catalysts has been challenging due to problems associated with the regeneration of these hydride species under applied reaction conditions. A recent discovery of the possibility of their regeneration under electrochemical conditions via a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO2 reduction reactions (eCO2RR). However, success is yet to be realized to term them as “true” catalysts, as the typical turnover numbers (TONs) of the eCO2RR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10–3–10–2; thus, sub-stoichiometric only! Herein, we report a novel class of structurally engineered heterohelicene-based organic hydride donor with a proof-of-principle demonstration of catalytic electrochemical CO2 reduction reaction showing a significantly improved activity with more than stoichiometric turnover featuring a 100–1000-fold enhancement of the existing TON values. Mechanistic investigations suggested the critical role of the two cationic imidazolium motifs along with the extensive π-conjugation present in the backbone of the heterohelicene molecules in accessing and stabilizing various radical species involved in the generation and transfer of hydride, via multielectron-transfer steps in the electrochemical process.

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