A Sm(II)-based catalyst for the reduction of dinitrogen, nitrite, and nitrate to ammonia or urea

Industrial dinitrogen (N₂) reduction to ammonia in the Haber-Bosch synthesis is essential for producing fertilizers and, consequently, food. Methods wherein the energy for nitrogen activation is supplied by light could provide more sustainable alternatives to existing ones. The combination of a photosensitizer and a lanthanide catalyst is reported for an effective >2e⁻ reduction of N₂ in what is the first transition-metal-free molecular photocatalyst for ammonia synthesis. The lanthanide is Earth-abundant Sm. The reaction proceeds at ambient pressure and temperature, with high turnover numbers (up to 98), with visible light irradiation in aqueous solvent mixtures and even pure water, and it uses an environmentally benign non-metallic sacrificial reductant. Nitrite and nitrate were also efficiently reduced to ammonia. Thus, the first photocatalytic co-reduction of nitrite and bicarbonate to urea using an Sm-based photocatalyst was achieved.