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 N2 by transition‐metal complexes is—above all—the main instrument to progress in the mechanistic understanding of N2 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 N2 reduction. We mainly focus on multistep hydrogenation of N2 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. Artificial nitrogen fixation systems based on transition‐metal complexes are crucial for the mechanistic understanding of N2 splitting. The authors give an overview of molecular catalysts and illustrate alternative N2 reduction pathways in the search for the key factors of reactivity.