Abstract:

Achieving mild-condition ammonia synthesis from dinitrogen (N₂) reduction has been a longstanding challenge in heterogeneous catalysis, primarily due to the lack of catalysts capable of simultaneously breaking the N≡N bond and hydrogenating the atomic nitrogen with low energy barriers. Herein, we identify a fundamental trade-off between N≡N bond breaking and subsequent N-H bond formation steps across different molecular catalysts, which was not previously established in homogeneous catalysis. By balancing N≡N activation and N-H formation, our computational analysis not only effectively rationalizes experimentally observed activity trends among well-studied Mo-complexes but also offers a rationale for predicting new homogeneous catalysts. Based on this established theoretical structure-activity relationship, we further identified a 5,6-OCF₃-substituted tungsten (W) complex as a promising catalyst for ammonia synthesis, overperforming all available complexes in the literature under the same reaction conditions. This work not only explains the trend in ammonia synthesis activity of metal complexes in available experiments but also provides theoretical guidance for the rational design of next-generation molecular catalysts for ambient nitrogen fixation.

Key words: Nitrogen fixation, Molecular catalysis, Dissociative mechanism, Substituent effect, Density functional theory