[Fe]-Hydrogenase-Inspired Proton-Shuttle Installation in a Molecular Cobalt Complex for High-Efficiency H<sub>2</sub> Evolution Reaction
Sanajit Kumar Mandal, Chandana Sunil, Joyanta Choudhury
Abstract
Developing efficient H 2 -evolution artificial electrocatalysts often takes inspiration from the active sites of [FeFe]/[NiFe]-hydrogenases to incorporate smartly designed amine/thiolato-based proton-shuttle motifs. Interestingly, [Fe]-hydrogenase (the third type of hydrogenases) consists of an Fe-bound adjacent pyridinol/pyridinone proton shuttle, which assists in reversible heterolytic H 2 -cleavage during its operation. However, so far, this type of proton-shuttle functionality has not been exploited to develop H 2 -evolution electrocatalysts. We reckoned that installing such a proton-shuttle motif in a suitable redox-active transition-metal-based complex can facilitate electrocatalytic H 2 generation via easy hydride/proton coupling, through proper positioning of the electrochemically generated metal-hydride and the “protonated” pyridinol form of the proton shuttle. To demonstrate this idea, a rationally designed Co-based complex Co– NHC U, containing an anionic uracil nucleobase, connected with a strong sigma-donor N-heterocyclic carbene (NHC) ligand, has been developed, wherein the anionic uracil motif offers a pyridinone-type proton-responsive site adjacent to the Co center. Gratifyingly, Co– NHC U acts as a high-efficiency H 2 -generation electrocatalyst with variable proton sources, achieving maximum turnover frequency (TOF max ) values in the range of 5000–13 700 s –1 (from cyclic voltammetry) or ∼2200–7360 s –1 (from controlled potential electrolysis) with Faradaic efficiencies of 93%–98%, at the overpotential range of 0.50–0.78 V. This system delivered the highest TOF max values achieved to date by any cobalt electrocatalyst in a nonaqueous medium: 10 200–13 700 s –1 with CF 3 COOH and Et 3 NHBF 4 as proton sources. This work could provide insights into the future design principles of homogeneous HER catalysts via capitalizing the power of pyridinone/pyridinol-based proton shuttle motifs coupled with Earth-abundant transition metals.