Nitrogen Fixation by MoSn₃ and MoSn₅ Clusters

The Haber-Bosch process (HBP) for the conversion of nitrogen into ammonia is arguably one of the most important chemical reactions in human history because it is the basis for the large-scale production of fertilizers and hence of global food production. Nevertheless, the HBP contributes in a decisive manner to the production of CO₂ and other global warming gases, since it is an extremely energy-intensive process. Consequently, the development of alternative conditions to reduce its negative environmental impact is of the highest priority. In this study, we investigate the detailed mechanism of nitrogen fixation by molybdenum-doped tin clusters MoSn₃ and MoSn₅, building upon previous work that demonstrated the potential of these complexes for activating the nitrogen molecule (Inorg. Chim. Acta 2025, 577, 122493). Using a combination of density functional theory and wave function analyses, we explored the stepwise reduction of N₂ into NH₃ on the surface of the above-mentioned clusters. Our results reveal the role of molybdenum as an excellent catalytic center for the HBP and the properties of the surrounding tin atoms as scaffolds for hydrogen adsorption. We identified the intermediates and transition states along six different reaction pathways, providing insights into the kinetic aspects of the nitrogen fixation process using MoSn₃ and MoSn₅ as catalysts. We also demonstrated that (i) the ability of these clusters to form hydrides mimics the biological activity of the nitrogenase enzyme and (ii) how the strong triple bond in dinitrogen, NN, is successively weakened in key steps of the reaction pathways. Overall, this work deepens our understanding of nitrogen fixation via heterogeneous catalysis, and it provides valuable insights for the rational design of more efficient and sustainable catalysts for ammonia synthesis.