Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

We have now found that molybdenumiodide complexes bearing a PNP-pincer ligand have a higher catalytic activity than the so far reported molybdenumdinitrogen complexes for ammonia formation from nitrogen gas under ambient reaction conditions, up to 830 equiv being produced based on a dinitrogen-bridged dimolybdenum complex (415 equiv of ammonia based on the molybdenum atom). This remarkable catalytic activity is induced by a novel reaction pathway, where the generation of a dinitrogen-bridged dimolybdenumiodide complex is a key point to promote direct cleavage of the nitrogennitrogen triple bond of the bridging dinitrogen ligand in the MoN¸NMo core.

A series of dinitrogen-bridged dimolybdenum-dinitrogen complexes bearing 4-substituted PNP-pincer ligands are synthesized by the reduction of the corresponding molybdenum trichloride complexes under 1 atm of molecular dinitrogen. In accordance with a theoretical study, the catalytic activity is enhanced by the introduction of an electron-donating group to the pyridine ring of PNP-pincer ligand, and the complex bearing 4-methoxy-substituted PNP-pincer ligands is found to work as the most effective catalyst, where 52 equiv of ammonia are produced based on the catalyst (26 equiv of ammonia based on each molybdenum atom of the catalyst), together with molecular dihydrogen as a side-product. Time profiles for the catalytic reactions indicate that the rates of the formation of ammonia and molecular dihydrogen depend on the nature of the substituent on the PNP-pincer ligand of the complexes. The formation of ammonia and molecular dihydrogen is complementary in the reaction system.

Newly designed and prepared molybdenum-nitride complexes bearing a mer-tridentate triphosphine as a ligand have been found to work as the most effective catalysts toward the catalytic reduction of dinitrogen to ammonia under ambient conditions, where up to 63 equiv of ammonia based on the Mo atom of the catalyst were produced.

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP-type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).