Electrochemical proton reduction is a promising energy storage method because H 2 molecule has a simple structure with a relatively low potential energy. Current interest in hydrogen catalysts has increased research efforts on synthetic analogs of hydrogenase active sites. In this study, we demonstrated the electrochemical H 2 evolution reactivity of [NNN R -Co(CH 3 CN) 3 ] 2+ (R CH 2 (1b), NCH 3 (2b)) complexes and examined a proton-relay process in the H 2 evolution reaction (HER). Upon one-electron reduction, the Co(II) ion center in a high-spin state dissociated a CH 3 CN ligand, while opening a reaction site. Cyclic voltammograms of the Co complexes indicated quasi-reversible Co(II/I) redox behaviors, and both complexes 1b and 2b showed catalytic H 2 evolution activity. Interestingly, 2b, assisted by a protonrelaying NCH 3 group, exhibited more efficient catalytic activity than 1b.

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