A round-trip journey of electrons: Electron catalysis enables direct fixation of N2 to azo compounds

Nitrogen gas (N₂) is one of the most abundant yet highly stable gases in Earth's atmosphere. Its N≡N triple bond has an extremely high bond dissociation energy (~940.95 kJ mol⁻¹), making its activation and conversion under conventional conditions very challenging. Although the Haber–Bosch process can convert N₂ to ammonia (NH₃), it requires high temperatures (350–550 °C) and pressures (150–350 atm), leading to significant energy consumption. The synthesis of azo compounds (R₁-N=N-R₂) poses an even greater challenge. Traditional methods involve multiple steps—oxidation of ammonia, nitrite preparation, and subsequent azo coupling—requiring multiple redox transitions, bond breaking and reformation, and substantial energy input. Developing a method for the direct, efficient conversion of N₂ to azo compounds under mild conditions remains a critical challenge in chemistry.

Recently, the research team led by Professors Zidong Wei and Cunpu Li from Chongqing University (China) proposed an innovative electron catalysis strategy. By controlling electron flow, this strategy achieves efficient activation and direct transformation of N₂ under mild conditions to synthesize azo compounds in a single step, offering a new approach to green nitrogen-based compound synthesis. Unlike traditional complex azo synthesis routes, this strategy cleverly uses electrons as catalysts—they actively participate in the reaction without being consumed or regenerated—circumventing the limitations of energy-intensive “N₂ → NH₃ → nitrite” pathways, which suffer from high energy consumption and low atomic efficiency. The results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(24)60179-8).

The key breakthrough lies in the matching of N₂’s π* antibonding orbitals, enabling selective bond activation. The high bond energy of N₂ makes its π orbitals challenging to activate directly. The research team introduced an aromatic system, where electrons are injected into aryl compounds, forming aryl radicals (Ar^●). Since the antibonding orbital of Ar^● closely matches the π* orbital of N₂ in both energy and symmetry, electrons can be efficiently transferred to N₂'s π* orbital, successfully activating it and leading to the formation of diazo radical intermediate ([Ar-N₂]^●). Moreover, this strategy controls the push and pull of electrons electrochemically. The diazo radical intermediate ([Ar-N₂]^●) can be further oxidized, removing an electron to form a relatively stable diazonium salt ([Ar-N₂]⁺), which readily reacts with phenols or other nucleophiles to generate the desired azo compounds. Throughout the entire process, electrons act as a “catalyst” shuttling between electrodes, neither consumed nor altering the overall Gibbs free energy—thus establishing a revolutionary electron catalysis reaction model.

Computational results demonstrate that the electron catalysis strategy significantly lowers the activation energy for converting N₂ to azo compounds. Compared to the non-catalyzed reaction, which requires 3.44 eV (making it nearly impossible under normal conditions), the electron-catalyzed pathway reduces the activation energy to just 0.14 eV, making the reaction kinetically feasible. Furthermore, this strategy exhibits broad applicability, extending beyond azo synthesis to various aryl halides and nucleophilic aromatic compounds, offering an efficient approach for synthesizing high-value-added chemicals.

This study presents a novel electron catalysis-based approach to direct N₂ fixation, leveraging electrochemical control to regulate electron flow. This enables efficient and selective activation of N₂ under mild conditions, leading to one-step azo compound synthesis. Compared to traditional synthesis methods, this strategy reduces energy consumption, simplifies the synthesis route, and enhances overall efficiency. Additionally, this research establishes a brand-new catalytic reaction mechanism, providing fresh insights into future nitrogen-containing compound synthesis.

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About the Journal

Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top one journals in Applied Chemistry with a current SCI impact factor of 15.7. The Editors-in-Chief are Profs. Can Li and Tao Zhang.

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