Direct Transformation of N2 to N-Containing Organic Compounds: A Review

Direct Transformation of N2 to N-Containing Organic Compounds

Jia-Peng Li, Jianhao Yin, Chao Yu, Wen-Xiong Zhang, and Zhenfeng Xi*

Acta Chim. Sinica 2017, 75, 733—743.

As a grand research area closely related to human civilization and living, the activation and transformation of dinitrogen (nitrogen fixation) under mild conditions used to be a central research theme worldwide in the 1970’s-1990’s. Nitrogen fixation is the process by which atmospheric nitrogen is directly converted to a bioavailable form. This basic chemical reaction process is essential to sustaining all life on this planet. However, due to great challenging of the nature of this research, slow progress and worldwide change of academic culture, the number of researchers engaged in this fundamental research area has been drastically reduced. Nevertheless, there is no doubt that realizing activation and transformation of dinitrogen under mild conditions is a grand scientific problem that people need to solve, required by sustainable development of human society. It is thus one of the most important missions of scientists, especially chemists. Three types of N-containing products can be obtained through direct transformation of dinitrogen. The most popular one is the formation of ammonia NH3 and NxHy. The industrial Haber-Bosch process, which requires harsh reaction conditions such as high temperature and pressure and uses at least 1-2% of the annual primary energy supply in the world, is still the main method to produce ammonia from molecular dinitrogen and dihydrogen gases. Inspired by the investigation of nitrogenase and the discovery of the first molecular nitrogen complex in 1965, chemists have paid more attention to achieving the reduction of dinitrogen to ammonia with transition metal complexes either as regents or as catalysts. Reports on the other two types of products, the N-E (E = P, Si) bonding compounds, and the N-C bonding compounds, are very rare. Compared with ammonia, nitrogen-containing organic compounds such as amines, amides, imides, amino acids and aza-heterocycles are also high-value products. This review mainly summarizes the progress in the field of direct transformation of molecular nitrogen to nitrogen-containing organic compounds by using transition metal complexes, as well as the elucidation of transformation mechanisms. The N-containing organic compounds thus formed include amines, amides, imides, nitriles, diazenes, azines, carbodiimides, isocyanates and heterocycles. Although some progress has been achieved, examples are still very much limited, efficiency is generally very low. Transition metal complex-catalyzed reaction process is in great demand. Synergetic strategy is considered to be one of the efficient ways to realize transition metal complex-catalyzed direct transformation of molecular nitrogen to nitrogen-containing organic compounds under mild conditions.

The formation of N-E (E = P, Si) bonding compounds and the reduction of dinitrogen to ammonia and other partially reduced or protonated products of dinitrogen are not covered here.