Molecular Catalysts for N 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 by transition-metal complexes is-above all-the main instrument to progress in the mechanistic understanding of N 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 reduction. We mainly focus on multistep hydrogenation of N 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.