Theoretical Studies on the Reduction of N₂ to NH₃/N₂H₄ Catalyzed by Chromium Complexes

Nitrogen fixation catalyzed by transition metal complexes provides an attractive alternative to the Haber-Bosch process and has received widespread attention. In this work, the reaction pathways of N₂ to NH₃/N₂H₄ catalyzed by dinuclear and mononuclear Cr-N₂ complexes bearing cyclopentadienyl-phosphine ligands and mediated by LutH⁺ (as proton donors) and CrCp*₂ (as electron donors) were investigated systematically using density functional theory calculations. The key step of the reactions was clarified as the first hydrogenation. The effect of different combinations of six proton sources (three pyridine acids and three anilino acids) and three electron sources (CrCp*₂, CoCp*₂, and CoCp₂) on the reduction of N₂ to NH₃/N₂H₄ was also explored by calculating the critical step of the reactions. Based on the calculations, the dinuclear Cr-N₂ complex is expected to be an effective catalyst for the reduction of N₂ to NH₃/N₂H₄ when using the combinations of CrCp*₂ with each of the six proton sources and of CoCp*₂ with anilino acids. Our work provides insights into understanding and optimizing Cr catalytic systems for efficient dinitrogen fixations.