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

Abstract

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.

Catalytic nitrogen fixation using visible light energy

“…The generated Mo-imide complex should be transformed into the corresponding amide and ammonia complexes more smoothly, because the imide complex has the smallest bond dissociation free energy (BDFE) of the N-H bond of [Mo(NHx)I(PCP)] (x = 1-3), where 34 kcal/mol (x = 1), 53 kcal/mol (x = 2), and 41 kcal/mol (x = 3). 26 These calculated results support the view that the reduced photoredox catalyst [Ir] 3 and the radical cation acrH2 •+ act as electron and proton sources, respectively, in the PCET process to form the N-H bonds.…”

“…1e). We assume that the unique reactivity of acrH2 in our reaction system is due to the values of the BDFE of the N-H bond of the molybdenum-imide, molybdenum-amide and molybdenum-ammonia complexes (34, 53 and 41 kcal/mol, respectively) 26 being smaller than those of the corresponding manganese complexes (60, 84 and 85 kcal/mol, respectively), 22 which are large enough to drive the transfer of the hydrogen atom from acridine species like acrH2, acrH2 •+ and acrH • to the manganese nitrogenous complexes. 22 Finally, we evaluated the present reaction system from the viewpoint of thermodynamics.…”

“…The use as catalysts of other molybdenum complexes, such as a molybdenum trichloride complex bearing the PCP-type pincer ligand [MoCl3(PCP)] 24 (1b) and a molybdenum triiodide complex bearing a pyridine-based PNP-type pincer ligand [MoI3(PNP)] 25 (1c: PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine), afforded the production of only a small amount of ammonia (Table 1, Entries 2 and 3). By contrast, a molybdenum triiodide complex bearing a trifluoromethyl-substituted PCPtype pincer ligand [MoI3(CF3-PCP)] 26 (1d; see structure in Table 1) worked as a more effective catalyst than 1a; indeed, in the presence of this complex, up to 39.8 equiv. of ammonia were produced based on the number of catalyst-based molybdenum atoms (66% yield), together with 21.3 equiv.…”

Catalytic nitrogen fixation using visible light energy

“…3 and Table 1, Entries 2 and 3). By contrast, a molybdenum triiodide complex bearing a trifluoromethylsubstituted PCP-type pincer ligand [MoI 3 (CF 3 -PCP)] 29 (1d; see structure in Fig. 3) worked as a more effective catalyst than 1a; indeed, in the presence of this complex, up to 39.8 equiv.…”

“…5b). The generated Mo-imide complex should be transformed into the corresponding amide and ammonia complexes more smoothly, because the imide complex has the smallest bond dissociation free energy (BDFE) of the N-H bond of [Mo(NH x )I(PCP)] (x = 1-3), where 34 kcal/mol (x = 1), 53 kcal/mol (x = 2), and 41 kcal/mol (x = 3) 29 . These calculated results support the view that the reduced photoredox catalyst [Ir] 3 and the radical cation acrH 2…”