Mechanochemical nitrogen fixation catalysed by molybdenum complexes

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.