Green Ammonia Generation with a Polypyridine Cobalt Complex and Visible Light

The use of a first-row transition metal complex and visible light was exploited to promote the photosynthesis of ammonia from nitrite ions. This is achieved in neutral aqueous solution by employing a cobalt polypyridine complex (CoL) within a light-driven catalytic scheme involving [Ru(bpy)3]2+ (where bpy = 2,2'-bipyridine) as the sensitizer and ascorbate as the electron donor. Under visible light irradiation, the resulting photosystem produces NH3 with a quantum yield up to 3% and a selectivity approaching 100%, achieving a maximum turnover number (TON) up to 2150. Transient absorption spectroscopy and density functional theory provided a robust mechanistic picture, showing that binding of the NO2- substrate occurs at the one-electron reduced metal complex, while also ascertaining the energetic feasibility of subsequent elementary reaction steps. All in all, this work uncovers the great potential of first-row transition metal polypyridine complexes toward the synthesis of chemicals of industrial relevance.