A cobalt complex, a highly efficient catalyst for electro- and photochemical driven hydrogen generation in purely aqueous media

The reaction of 2,2-bipyridine (bpy), Co(NO3)(2)center dot 6H(2)O and KCN produces a new cobalt complex, [(bpy)(2)Co (CN)(2)]center dot NO3 1. Under photoirradiation with blue light (lambda(max) = 469 nm) in air, together with [Ru(bpy)(3)]Cl-2 and ascorbic acid in a pH 5.0 aqueous solution, this cobalt complex possesses extremely high activity for photocatalytic hydrogen generation with a remarkable initial turnover rate of 1.56 x 10(5) mol H-2/mol catalyst/h. This photocatalytic activity can be sustained for at least 18 h with a turnover number (TON) of 7.3 x 10(5) moles of H-2 per mole of catalyst. This performance is significantly better than any molecular catalysts reported in the literature for photochemical hydrogen production. The result can be attributed to the fact that the reductive quenching to [Ru(bpy)(3)]Cl-2 by ascorbic acid (k(q) = 1.55 x 10(10) M-1 s(-1)) dominates over the oxidative quenching by 1 (k(q) = 1.027 x 10(10) M-1 s(-1)). Additionally, 1 can electrocatalyze hydrogen generation from a neutral water with a turnover frequency (TOF) of 462 mol of hydrogen per mole of catalyst per hour at an overpotential of 638 mV. This finding affords a new method for water reduction by using transition metal complexes as molecular catalysts in purely aqueous media. (C) 2018 Elsevier Ltd. All rights reserved.