Caffeinated-N-Heterocyclic Carbene Ligand at Play in a Molecular H2-Evolving Cobalt Electrocatalyst

In this work, we implemented a proton-responsive caffeine wingtip at the strong sigma-donor N-heterocyclic carbene (NHC) ligand in a new cobalt complex C-2 for the hydrogen evolution reaction (HER) with various proton sources. The NHC ligand is expected to boost the hydricity of the Co-H bond for the reaction with H+. In parallel, the proton-responsive functionality of the caffeine wingtip should help in shuttling protons to the catalytic center to facilitate the formation of the Co-H bond and H2 and thereby should improve the rate of the HER. Cyclic voltammetry (CV) and controlled potential electrolysis (CPE) experiments using CH3CO2H, Et3NHBF4, and CF3CO2H as proton sources showed that C-2 was an efficient HER catalyst, achieving maximum turnover frequency (TOFmax) values in the range of 2592-13,270 s-1, Faradaic efficiency (FE) values of 75-90% at the overpotential range of 0.40-0.73 V, and turnover number (TON) values of 81-216. Detailed CV analyses highlighted some key mechanistic insights into the electron- and proton-transfer steps involved in the C-2-based catalytic cycles with acid substrates. Overall, the work supports the possibilities of HER catalyst design using biosourced ligand motifs such as caffeine and many other nucleobases with Earth-abundant transition metals.