A Nickel Dithiolate Water Reduction Catalyst Providing Ligand-Based Proton-Coupled Electron-Transfer Pathways.

A nickel pyrazinedithiolate ([Ni(dcpdt) ] ; dcpdt=5,6-dicyanopyrazine-2,3-dithiolate), bearing a NiS 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., [Ni (dcpdt)(dcpdtH )] or [Ni (dcpdtH) ] ) 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.