The reaction of 2,3-bis(2-hydroxybenzylideneimino)-2,3-butenedinitrile (H(2)L1) and CoCl2 center dot 6H(2)O affords a mixed-valent dinuclear Co-II-Co-I complex [(CoL)-L-I(py)(3)][(CoL2)-L-II] 1 (L = 2-(2-hydroxybenzylideneimino)-butenedinitrile ion), a new molecular electrocatalyst, which has been determined by X-ray crystallography and XPS spectrum. Electrochemical studies indicate that the catalyst is the first Co-II-Co-I complex species, that is among the most rapid homogeneous water-reduction catalysts, with a turnover frequency (TOF) of 2387 mol of hydrogen per mole of catalyst per hour at an overpotential of 787 mV vs SHE (pH 7.0). Sustained proton reduction catalysis occurs at glassy carbon (GC) to give H-2 over a 46 h electrolysis period with 97% Faradaic yield and no observable decomposition of the catalyst. (C) 2014 Elsevier B.V. All rights reserved.