Catalytic production of ammonia from dinitrogen employing molybdenum complexes bearing N-heterocyclic carbene-based PCP-type pincer ligands

Here, we established a mechanistic insight into the catalytic production of ammonia from dinitrogen via the combination of samarium diiodide (SmI2) and water in the presence of molybdenum complexes bearing PCP-type pincer ligands as the catalysts. The experimental and theoretical studies revealed that the rate-determining step was the proton-coupled electron transfer (PCET) during the formation of the N–H bond on the molybdenum imide complex at high catalyst concentrations. Additionally, we confirmed that the concentration of the catalyst affected the rate-determining step and the dimerisation step of the catalyst became the rate-determining step at a low catalyst concentration. Thus, we designed PCP-type pincer ligands in which various substituents were introduced at the positions 5 and/or 6, to accelerate the rate-determining PCET reaction and observed that the introduction of electron-withdrawing groups promoted the reaction rate, as predicted by density-functional theory calculations. Finally, the molybdenum trichloride complex bearing a trifluoromethyl group containing PCP-type pincer ligand functioned as the most effective catalyst for producing up to 60,000 equivalents of ammonia based on the molybdenum atom of the catalyst, with a turnover frequency of up to 800 equivalents/Mo·min−1. The amount of ammonia produced via this reaction, as well as its production rate, were approximately one order of magnitude larger than those obtained under the previous reaction conditions. The findings reported herein can contribute to the development of an environmentally friendly next-generation nitrogen fixation system.