In the presence of Et3N, the reaction of CoCl2 center dot 6H(2)O and 1-[(2-methoxy)benzene]-3-[benzothiazole]triazene (HL) provides a dinuclear cobalt(II) complex [CO2L4] 1, a molecular catalyst. 1 can electrocatalyze hydrogen evolution both from acetic acid and purely aqueous media (pH 7.0), with a turnover frequency (TOF) of 32.1 and 137 mol of hydrogen per mole of catalyst per hour at an overpotential of 942 mV and 789 my, respectively. Sustained proton reduction catalysis occurs at glassy carbon (GC) to give H-2 over a 40 h electrolysis period with 75% Faradaic yield and no observable decomposition of the catalyst. Photocatalytic experiments indicate that complex 1 in pH 4.0 aqueous solution under air, together with [Ru(bpy)(3)]Cl-2 and ascorbic acid, in the presence of blue light (lambda(max) = 469 nm) can produce hydrogen with a turnover number (TON) = 4367 mol of H-2 (mol of cat)(-1) under air, which is significantly larger than any other reported molecular cobalt catalysts for photochemical hydrogen production. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.