Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP + Cofactor Analogue for Electrocatalytic CO 2 Reduction.

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 CO 2 to formic acid and methanol, similar to the natural photosynthesis process of fixing CO 2 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 CO 2 reduction reactions (eCO 2 RR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCO 2 RR 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 CO 2 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.