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

Abstract

Mechanistic insight into the catalytic production of ammonia from dinitrogen is needed to improve the synthesis of this vital molecule. Here we study the use of samarium diiodide (SmI2) and water in the presence of molybdenum complexes that bear PCP-type pincer ligands to synthesize ammonia. The proton-coupled electron transfer during the formation of a N–H bond on the molybdenum imide complex was found to be the rate-determining step at high catalyst concentrations. Additionally, the dimerization step of the catalyst became the rate-determining step at low catalyst concentrations. We designed PCP-type pincer ligands with various substituents at the 5- and 6-positions and observed that electron-withdrawing groups promoted the reaction rate, as predicted by density functional theory calculations. A molybdenum trichloride complex that bears a trifluoromethyl group functioned as the most effective catalyst and produced up to 60,000 equiv. ammonia based on the molybdenum atom of the catalyst, with a molybdenum turnover frequency of up to 800 equiv. min−1. The findings reported here can contribute to the development of an environmentally friendly next-generation nitrogen-fixation system.

Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and pKa

“…As the milder SmI 2 /HMPA n reductants do not show appreciable binding to Fe N 2 – following electron transfer, this analysis of BDFE limits is not complicated by the effect of Lewis acid coordination. However, the proposed role of Sm coordination in the turnover-limiting step of the well-defined Sm II system described here prompts us to comment on such interactions in the context of the recent emergence of SmI 2 /ROH as highly effective H • sources in N 2 R with molecular catalysts. − Sm coordination is predicted to increase BDFE N–H for key M–N x H y intermediates; for example, the [ Fe –N 2 H- -Sm III ] n + species is expected to be more stable than the corresponding [ Fe –N 2 H] ( n −1)+ , enabling N–H bond formation that might otherwise be expected to be too uphill. In addition to this thermodynamic effect, coordination of both MN x H y intermediates and ROH to a coordinatively unsaturated Sm III (or Sm II ) center should enhance the kinetics of proton (or alternatively H • ) transfer .…”

Direct Synthesis of Organonitrogen Compounds from Dinitrogen Using Transition Metal Complexes: Leap from Stoichiometric Reactions to Catalytic Reactions

“…min À 1 . [8,68] Based on the highly efficient catalytic cycle for the formation of ammonia from N 2 , we have envisaged the formation of organonitrogen compounds from N 2 when carbon-centered electrophiles were used instead of proton source under the same reaction conditions. In order to achieve novel catalytic reactions for the formation of organonitrogen compounds, we first investigated stoichiometric reactions of molybdenum-nitride complexes in detail and then we tried to achieve the catalytic reactions based on information of the stoichiometric reactions.…”

“…of ammonia were produced based on the molybdenum atom of the catalysts with a molybdenum turnover frequency of up to 800 equiv. min −1 [8,68] . Based on the highly efficient catalytic cycle for the formation of ammonia from N 2 , we have envisaged the formation of organonitrogen compounds from N 2 when carbon‐centered electrophiles were used instead of proton source under the same reaction conditions.…”

“…Since then, stoichiometric or catalytic formation of nitrogen-hydrogen bonds to afford NH 3 using transition metal complexes with appropriate proton sources and reductants has been developed, [7] and turnover number of 60,000 has been achieved quite recently by the state-of-the-art catalyst. [8] There has been also high motivation to directly convert N 2 into nitrogen-containing compounds other than NH 3 by forming nitrogen-non-hydrogen bonds, instead of nitrogen-hydrogen bonds, on transition metal complexes under mild reaction conditions, considering that a large part of produced NH 3 by the Haber-Bosch process is finally converted to nitrogencontaining compounds. Among them, the formation of nitrogen-carbon bonds to produce organonitrogen compounds is the most attractive approach because of their diversity and vital importance to our lives as biomolecules, medicines, and chemical fibers.…”

“…The discovery of coordinated N 2 on transition metal complexes derived from hydrazine [5] or N 2 gas [6] demonstrated the potential of transition metal complexes to capture and activate N 2 . Since then, stoichiometric or catalytic formation of nitrogen–hydrogen bonds to afford NH 3 using transition metal complexes with appropriate proton sources and reductants has been developed, [7] and turnover number of 60,000 has been achieved quite recently by the state‐of‐the‐art catalyst [8] …”