Ammonia Formation Catalyzed by Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia in the reaction with KC8 as a reductant and [HPCy3]BArF4 (Cy = cyclohexyl, ArF = 3,5-(CF3)2C6H3) as a proton source at –78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC8 as a reductant and Me3SiCl as a silylating reagent under ambient reaction conditions to afford 11.3 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions by using rhenium-dinitrogen complexes as catalysts

Catalytic Borylation of Dinitrogen into Borylamines and Borylhydrazines Using Rhenium Complexes under Ambient Reaction Conditions

In the presence of a catalytic amount of a dinitrogen-bridged dirhenium complex bearing PNP-type pincer ligands, an atmospheric pressure of dinitrogen reacted with potassium as a reductant and dicyclohexylchloroborane as a borylating reagent at room temperature to give 14.4 equiv of ammonia and 3.2 equiv of hydrazine based on the rhenium atom of the catalyst upon hydrolysis (20.8 equiv of fixed N atom). This result demonstrates the first successful example of the borylation of dinitrogen into ammonia and hydrazine equivalents under ambient conditions with rhenium complexes as catalysts

Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine Using Chromium Complexes Bearing PCP-Type Pincer Ligand

A series of chromium–halide, –nitride, and –dinitrogen com-plexes bearing a carbene- and phosphine-based PCP-type pin-cer ligand is newly prepared and some of them are found to work as effective catalysts to reduce dinitrogen under atmos-pheric pressure, whereby up to 8.40 equiv of ammonia and 2.46 equiv of hydrazine (13.32 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

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

Direct synthesis of cyanate anion from dinitrogen catalysed by molybdenum complexes bearing pincer-type ligand

Molybdenum–carbamate complex bearing a pyridine-based 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP)-pincer ligand is synthesised 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 cyanate anion (NCO−) from the molybdenum–carbamate complex as a key step. Based on the stoichiometric reactions, we have demonstrated a synthetic cycle for NCO− from dinitrogen mediated by the molybdenum–PNP complexes in two steps. Based on this synthetic cycle, we have achieved the catalytic synthesis of NCO− from dinitrogen under ambient reaction conditions. We believe that these results described in this manuscript provide valuable information to achieve the catalytic transformations of dinitrogen into valuable organonitrogen compounds under ambient reaction conditions