Molecular Catalysts for N₂ Reduction: State of the Art, Mechanism, and Challenges

Fixation of atmospheric nitrogen is central for the production of ammonia, which is the source of nitrogen fertilizers and is also emerging as a promising renewable fuel. While the development of efficient molecular‐based artificial nitrogen fixation systems working under mild conditions is probably a Holy Grail, the catalytic reduction of N 2 by transition‐metal complexes is—above all—the main instrument to progress in the mechanistic understanding of N 2 splitting. In this Minireview we first give an overview of molecular‐based catalytic systems, including recent breakthroughs, and then we illustrate the alternative pathways for N 2 reduction. We mainly focus on multistep hydrogenation of N 2 by separated proton and electron sources, with a particular attention for the possibility of proton‐coupled electron transfer events. Finally, we try to identify the key factors to achieve catalytic reduction of dinitrogen by metal complexes and to enhance their efficiency.