A nickel pyrazinedithiolate ([Ni(dcpdt)2]2−; dcpdt=5,6-dicyanopyrazine-2,3-dithiolate), bearing a NiS4 core similar to the active center of [NiFe] hydrogenase, is shown to serve as an efficient molecular catalyst for the hydrogen evolution reaction (HER). This catalyst shows effectively low overpotentials for HER (330–400 mV at pH 4–6). Moreover, the turnover number of catalysis reaches 20 000 over the 24 h electrolysis with a high Faradaic efficiency, 92–100 %. The electrochemical and DFT studies reveal that diprotonated one-electron-reduced species (i.e., [NiII(dcpdt)(dcpdtH2)]− or [NiII(dcpdtH)2]−) forms at pH<6.4 via ligand-based proton-coupled electron-transfer (PCET) pathways, leading to electrocatalytic HER without applying the highly negative potential required to generate low-valent nickel intermediates. This is the first example of catalysts exhibiting such behavior.
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