Cobalt-dithiolene complexes for the photocatalytic and electrocatalytic reduction of protons in aqueous solutions

Artificial photosynthesis (AP) is a promising method of converting solar energy into fuel (H-2). Harnessing solar energy to generate H2 from H+ is a crucial process in systems for artificial photosynthesis. Widespread application of a device for AP would rely on the use of platinum-free catalysts due to the scarcity of noble metals. Here we report a series of cobalt dithiolene complexes that are exceptionally active for the catalytic reduction of protons in aqueous solvent mixtures. All catalysts perform visible-light-driven reduction of protons from water when paired with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial donor. Photocatalysts with electron withdrawing groups exhibit the highest activity with turnovers up to 9,000 with respect to catalyst. The same complexes are also active electrocatalysts in 1:1 acetonitrile/water. The electrocatalytic mechanism is proposed to be ECEC, where the Co dithiolene catalysts undergo rapid protonation once they are reduced to CoL22-. Subsequent reduction and reaction with H+ lead to H-2 formation. Cobalt dithiolene complexes thus represent a new group of active catalysts for the reduction of protons.