Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP Cofactor Analogue for Electrocatalytic CO 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 to formic acid and methanol, similar to the natural photosynthesis process of fixing CO 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 a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO reduction reactions (eCORR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCORR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10-10; 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 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, multielectron-transfer steps in the electrochemical process.