We have found molybdenum-catalyzed
ammonia formation using simple and commercially available monodentate
and bidentate phosphines as auxiliary ligands with a simple and convenient
procedure. Molybdenum complexes generated in situ from [MoI3(THF)3] and the corresponding phosphines such as PMePh2 and 1,5-bis(diphenylphosphino)pentane worked effectively
toward ammonia formation.
Dinitrogen is an abundant and promising material for valuable organonitrogen compounds containing carbon–nitrogen bonds. Direct synthetic methods for preparing organonitrogen compounds from dinitrogen as a starting reagent under mild reaction conditions give insight into the sustainable production of valuable organonitrogen compounds with reduced fossil fuel consumption. Here we report the catalytic reaction for the formation of cyanate anion (NCO−) from dinitrogen under ambient reaction conditions. A molybdenum–carbamate complex bearing a pyridine-based 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP)-pincer ligand is synthesized from the reaction of a molybdenum–nitride complex with phenyl chloroformate. The conversion between the molybdenum–carbamate complex and the molybdenum–nitride complex under ambient reaction conditions is achieved. The use of samarium diiodide (SmI2) as a reductant promotes the formation of NCO− from the molybdenum–carbamate complex as a key step. As a result, we demonstrate a synthetic cycle for NCO− from dinitrogen mediated by the molybdenum–PNP complexes in two steps. Based on this synthetic cycle, we achieve the catalytic synthesis of NCO− from dinitrogen under ambient reaction conditions.
Molybdenum–iodide complexes bearing a PCP[1] ligand have been found to work as excellent catalysts toward ammonia formation under ambient reaction conditions among dinitrogen‐bridged dimolybdenum complexes and other molybdenum complexes bearing PNP and PCP[2] ligands.
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.
A series of chromium‐halide, ‐nitride, and ‐dinitrogen complexes bearing carbene‐ and phosphine‐based PCP‐type pincer ligands has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen under atmospheric pressure, whereby up to 11.60 equiv. of ammonia and 2.52 equiv. of hydrazine (16.6 equiv. of fixed N atom) are produced based on the chromium atom. To the best of our knowledge, this is the first successful example of chromium‐catalyzed conversion of dinitrogen to ammonia and hydrazine under mild reaction conditions.
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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.
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