Exploiting the NADP⁺/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for...

Generation of clean energy in a viable manner demands efficient and sustainable catalysts. One prospective method of clean energy generation is the electrochemical hydrogen evolution reaction (HER). Over the years, various transition metal-based complexes/polymeric organic materials were utilized in HER. However, the use of a redox-active small organic molecule as a catalyst for HER has not been explored well. The requirements of a strongly acidic solution, very high overpotential, and stability under acidic conditions pose several challenges for applying organic electrocatalysts for HER. Considering these challenges, herein, we demonstrated an NADP⁺-like organic system (NADP⁺ = nicotinamide adenine dinucleotide phosphate), a bis-imidazolium-fused heterohelicene, which acts as a catalyst for HER with mild acid (acetic acid) as a proton source at moderate overpotential. The unique structural backbone of this dicationic heterohelicene allowed to exploit the NADP⁺/NADPH-type (NADPH = reduced nicotinamide adenine dinucleotide phosphate) hydride transfer-based redox cycle efficiently under the applied conditions, where the NADPH-like hydride intermediate transfers the hydride to the proton of the mild acid to generate H₂. The Faradaic efficiency and turnover number for the present HER were achieved up to 85 ± 5% and 50 ± 3, respectively. In addition, the maximum turnover frequency, TOF_max, value of 410 s^-1 was observed, which is around 400 times that obtained for the existing reported NADP⁺-like organic compounds used as catalysts for HER. Thorough mechanistic studies were conducted experimentally and computationally to establish a plausible catalytic cycle. This advancement could help in designing efficient organic electrocatalysts for HER from a mild proton source.