Transition metal-catalysed mechanochemical reactions using ball milling have emerged as important tools to realize unique organic transformations. Mechanochemical reactions have advantages over conventional homogeneous reactions, such as using a small amount of organic solvent, having a broad substrate scope and being generally fast and selective reactions. An increasing number of mechanochemical reactions between solid substrates have been examined, but only a few examples of mechanochemical reactions involving gaseous substrates, such as nitrogen fixation to convert dinitrogen into ammonia, have been reported until now. Here we develop catalytic mechanochemical nitrogen fixation using molybdenum complexes as molecular catalysts. An atmospheric pressure of dinitrogen was reacted with samarium diiodide as a reductant and water or alcohols as proton sources in the presence of the molybdenum catalysts, using ball milling under solvent-free and near-ambient reaction conditions to afford up to 860 equivalents of ammonia based on the catalyst. In addition, we demonstrated that even insoluble cellulose can be applied as the proton source. Further, we revealed that the molybdenum-catalysed mechanochemical nitrogen fixation proceeds via nitrogen–nitrogen bond cleavage at the gas–solid interface and nitrogen–hydrogen bond formation in the solid phase. The use of large amounts of solvents is an obstacle to the practical application of nitrogen fixation using transition metal complexes. Here catalytic ammonia synthesis is achieved by reacting dinitrogen (1 atm) with samarium diiodide and proton sources, including cellulose, in the presence of molybdenum catalysts under solvent-free ball-milling conditions.

en clear that the ammonia is indeed electrocatalytically formed. [7] Systems with very high reported ammonia yields were even shown to be irreproducible. [8] Photocatalysis suffers from the drawback of only producing trace amounts of ammonia. [9] Efforts in the area of molecular catalysts for nitrogen activation—as interesting as they are—can rather be considered as model studies for the natural enzymatic way of nitrogen fixation, but not as a competitive alternative method for ammonia synthesis, due to their prevalent use of special proton donating as well as reducing agents. [10] Promising results were achieved in the catalytic conversion of nitrogen in non‐thermal plasma processes, but plasma‐catalytic processes do not meet the required targets of ammonia output and energy input. [11] In recent years, mechanochemistry has evolved as a promising alternative strategy for the activation and transformation of molecules. [12] Also heterogeneously catalyzed gas‐phase reactions can be mechanically activated, resulting in significant enhancements of activity. [13] Due to the high importance, mechanocatalytic ammonia synthesis has also been attempted, albeit in less than a handful of reports with partly unclear validity of the claims. The first claimed mechanochemical ammonia syntheses date back to 1961 and 1974. [14] However, details are only sketchily described, ammonia concentrations were very low in the ppm range, and formation of ammonia by alternative pathways (activation of impurities) could not be excluded, so that these reports are at least questionable. In 2020, Tricker et al. claimed a mechanocatalytic synthesis of ammonia over TiN in a common laboratory shaker mill. [15] However, during milling no ammonia was observed in the gas phase, and ammonia was determined by washing the milled solid with an aqueous solution, so that hydrolytic ammonia formation from a nitride seems possible, even probable. A true step towards mechanochemical ammonia formation was taken