Transition metal-catalyzed mechanochemical reactions, often employing ball milling, have gained prominence as a tool for unique organic transformations. These reactions offer advantages over conventional homogeneous reactions, including reduced solvent use, broader substrate scope, faster reaction times, and higher selectivity. While numerous mechanochemical reactions between solid substrates have been studied, fewer examples involve gaseous reactants. This research focuses on catalytic mechanochemical nitrogen fixation, a process of paramount importance due to the increasing demand for ammonia as a feedstock and clean energy carrier. Industrial ammonia production relies heavily on the energy-intensive and carbon-emitting Haber-Bosch process. Recent advancements have demonstrated ammonia formation under mechanochemical conditions using iron or titanium metals, but these reactions are not catalytic. The authors' group and others have developed catalytic nitrogen fixation using transition metal complexes under homogeneous conditions, achieving high turnover numbers. However, the use of large amounts of organic solvents hinders practical application. This study aims to overcome these limitations by developing a catalytic mechanochemical nitrogen fixation process using molybdenum complexes, samarium diiodide as a reductant, and various proton sources under solvent-free and mild conditions, leveraging the gas-solid interface for reactivity.

The literature review highlights the advantages of mechanochemical reactions over conventional homogeneous reactions in transition metal catalysis. It emphasizes the scarcity of mechanochemical reactions involving gaseous reactants, particularly in nitrogen fixation. The Haber-Bosch process, while industrially important, suffers from high energy consumption and CO2 emissions. Existing mechanochemical nitrogen fixation methods using iron or titanium metals lack catalytic efficiency. The authors' previous work on homogeneous catalytic nitrogen fixation using molybdenum complexes, samarium diiodide, and water in tetrahydrofuran (THF) is presented, noting the high turnover numbers achieved but acknowledging the limitations posed by the use of large amounts of organic solvents. This sets the stage for exploring a solvent-free mechanochemical approach.

The researchers employed a Retsch MM400 shaker mill with a 5 ml stainless-steel milling jar and a 10 mm stainless-steel ball for mechanochemical reactions. Initial experiments used a molybdenum triiodide complex (1a) with samarium diiodide THF adduct (SmI2(THF)2) and water. The reaction conditions (30 Hz for 1 h) yielded ammonia and hydrogen. Control experiments confirmed the necessity of all reactants and ball milling. The effect of different proton sources was investigated, including methanol, ethanol, ethylene glycol, and solid sources like pentaerythritol and D-glucose. Cellulose, an insoluble biopolymer, surprisingly proved effective as a proton source under mechanochemical conditions, contrasting with its ineffectiveness in homogeneous reactions in THF. Time profile studies compared mechanochemical and homogeneous reactions using cellulose, demonstrating significantly faster ammonia formation under mechanochemical conditions. An induction period was observed with cellulose, hypothesized to be due to the slow formation of active species between SmI2(THF)2 and cellulose. Diffuse-reflectance ultraviolet-visible (UV-vis) spectroscopy analyzed the active species formed during reactions with SmI2(THF)2 and various proton sources. A two-step procedure using solid KOH was developed to selectively generate gaseous ammonia without solvents. Stoichiometric reactions, NMR, mass spectrometry, and X-ray absorption spectroscopy were used to probe the reaction mechanism, revealing the formation of a molybdenum-nitride complex (2) as an intermediate. Further experiments investigated proton-coupled electron transfer (PCET) processes using anthracene and trans-stilbene, showcasing the applicability of the SmI2(THF)2-pentaerythritol combination to other PCET reactions in the solid phase. Isotope labeling experiments were conducted to confirm the PCET mechanism.

The study successfully developed a catalytic mechanochemical nitrogen fixation method using molybdenum complexes, achieving up to 860 ammonia equivalents per catalyst. The use of insoluble cellulose as a proton source significantly improved the efficiency, highlighting the advantages of mechanochemical conditions over homogeneous methods. The reaction was remarkably faster using cellulose under mechanochemical conditions compared to its homogeneous counterpart in THF. The formation of a molybdenum-nitride complex (2) as a key intermediate in the nitrogen-nitrogen bond cleavage was confirmed through various spectroscopic techniques. The nitrogen-hydrogen bond formation was shown to proceed via PCET, as evidenced by the successful mechanochemical reduction of anthracene and trans-stilbene. The two-step procedure enabled selective generation of gaseous ammonia without the use of solvents. The superior performance of the mechanochemical method over the homogeneous reaction in THF was underscored by the substantial difference in the cost of solvent required. The cost of THF required for the homogeneous method is estimated at 4,000 USD per mole of NH3 produced, demonstrating the economic benefits of the mechanochemical approach.

The findings address the research question by demonstrating the feasibility and efficiency of catalytic mechanochemical nitrogen fixation. The results highlight the advantages of mechanochemical methods for handling insoluble substrates (like cellulose) and generating gaseous ammonia selectively. The elucidation of the reaction mechanism, including nitrogen-nitrogen bond cleavage at the gas-solid interface and subsequent PCET for nitrogen-hydrogen bond formation, provides valuable insights into the unique reactivity of mechanochemical gas-solid reactions. The observed high turnover numbers and the significant cost reduction compared to homogeneous methods showcase the potential of this approach for practical applications in ammonia synthesis.

This research successfully established a catalytic mechanochemical nitrogen fixation method using molybdenum complexes under solvent-free conditions, demonstrating high efficiency and the ability to utilize insoluble proton sources. The mechanistic insights gained reveal a unique reaction pathway involving gas-solid interface reactions and solid-phase PCET. Future research could explore the optimization of catalyst design, further investigation of substrate scope, and the scaling up of the process for industrial applications.

While the study demonstrates significant progress, limitations exist. The induction period observed with cellulose warrants further investigation to fully optimize the reaction conditions. Further optimization of the two-step procedure for selective gas-phase NH3 generation could improve the yield. The study is limited to specific molybdenum complexes and substrates; further research will expand its applicability and explores the influence of different milling parameters on the reaction efficiency.