Kazunari Yoshizawa

Ammonia is an essential chemical that underpins modern agriculture and industry, but the direct conversion of dinitrogen to ammonia under mild conditions remains highly challenging. To this end, molybdenum nitride complexes bearing 1,3-bis(di-tert-butylphosphinomethyl)benzimidazole-2-ylidene pincer ligands have been shown to catalyse ammonia formation from dinitrogen using samarium diiodide and water. However, the catalytic mechanism remains poorly understood owing to limited information on the key intermediates and the solution-state speciation of the samarium reductant. Here we show a full catalytic mechanism by combining mechanistic experiments and theoretical analyses. We isolate molybdenum methylimide and molybdenum methylamide complexes as stable analogues of catalytic intermediates, providing direct experimental evidence for the proposed catalytic pathway. Theoretical studies further suggest a plausible structure of the tetrahydrofuran-solvated samarium diiodide–water complex, and subsequent mechanistic analyses reveal that ammonia formation proceeds through proton-coupled electron transfer. Building on these experimental and theoretical findings, we identify the molybdenum–imide formation reaction as the most energy-demanding step. These findings provide a framework for understanding catalytic ammonia formation and inform future efforts to improve dinitrogen reduction catalysts.

New samarium complexes bearing a cyclopentadienyl–phenoxide ligand served as proton-coupled electron transfer (PCET) mediators in molybdenum-catalyzed ammonia formation, achieving over 130 catalytic cycles.

Aziridines are class of organic molecules that have a nitrogen‐containing three‐membered ring structure. These heterocyclic compounds have garnered recent interest owing to their bioactivity and intermediacy in the synthesis of value‐added products, such as pharmaceuticals. However, it is difficult to efficiently prepare aziridines that do not have N ‐substituents. This study employs a ruthenium(V)‐imido (Ru ^V ═NH) complex ( 2 ) with hydrogen‐bonding (HB) sites in the second coordination sphere (SCS) to selectively synthesize such aziridines. Notably, 2 oxidizes alkenes in water via nitrogen‐atom transfer to the alkene substrate, thereby producing aziridine derivatives without N ‐substituents in high yields. In contrast, an Ru ^V ═NH complex without HB sites in the SCS generates allylic alcohols via hydrogen‐atom transfer in water. The aziridination of alkenes by 2 is influenced by the electron density and steric hindrance of the C═C double bond in the substrate.

Aziridines are class of organic molecules that have a nitrogen‐containing three‐membered ring structure. These heterocyclic compounds have garnered recent interest owing to their bioactivity and intermediacy in the synthesis of value‐added products, such as pharmaceuticals. However, it is difficult to efficiently prepare aziridines that do not have N ‐substituents. This study employs a ruthenium(V)‐imido (Ru ^V ═NH) complex ( 2 ) with hydrogen‐bonding (HB) sites in the second coordination sphere (SCS) to selectively synthesize such aziridines. Notably, 2 oxidizes alkenes in water via nitrogen‐atom transfer to the alkene substrate, thereby producing aziridine derivatives without N ‐substituents in high yields. In contrast, an Ru ^V ═NH complex without HB sites in the SCS generates allylic alcohols via hydrogen‐atom transfer in water. The aziridination of alkenes by 2 is influenced by the electron density and steric hindrance of the C═C double bond in the substrate.

Epoxy resins, widely used as structural adhesives and protective coatings, are susceptible to degradation under environmental and chemical stress, which limits their long-term reliability. In this study, a comprehensive theoretical investigation of the degradation mechanisms of diglycidyl ether of bisphenol A cured with 4,4'-diaminodiphenylmethane was conducted using density functional theory, focusing on the cleavage of aromatic-alkyl ether C-O bond and C-N bond at the epoxy-amine linkages through homolysis, hydrolysis, and acidic hydrolysis. Homolytic cleavage of both C-O and C-N bonds is highly energy-demanding, with bond dissociation free energies (BDFEs) of 259.7 and 296.1 kJ mol^-1, respectively. In hydrolysis, the activation free energy for an ether C-O bond cleavage is calculated to be 244.1 kJ mol^-1, corresponding to a 100-fold increase in the reaction rate at 400 K, thereby accelerating degradation. Protonation alone only modestly reduces bond strengths, decreasing the C-O and C-N BDFEs to 236.0 and 248.7 kJ mol^-1, respectively, and thus provides limited bond weakening in isolation. Acidic hydrolysis was therefore examined to probe further the accelerated degradation pathway. Under weakly acidic conditions, both ether and amine sites are readily protonated, enhancing the bond polarization and electrophilicity at the reactive centers. Protonation reshapes the reaction landscapes by shifting the system onto distinct potential-energy surfaces, raising the surface associated with C-O bond cleavage and lowering that for C-N bond cleavage. Consequently, the activation energies for acidic hydrolysis decrease significantly to 100.0 and 139.2 kJ mol^-1, leading to a pronounced acceleration of reaction rates. In this context, even though the C-N pathway is thermodynamically favored, the C-O pathway becomes more accessible once protonation occurs at the ether site. Comparative energetics demonstrates that the degradation is strongly environment-dependent, with acidic hydrolysis emerging as the most accessible route.

Ammonia is an essential compound that can be used as an ingredient for various chemicals and a next‐generation energy carrier for a carbon‐neutral society. Although the Haber‐Bosch process is the widely used industrial production method for ammonia, this method consumes a large amount of energy to conduct the catalysis under high pressure and temperature; therefore, the development of efficient catalysts for ammonia production under ambient reaction conditions should be one of the most important research topics for attaining carbon neutrality. In this study, we have developed a new series of molybdenum complexes bearing dihydroimidazole‐based PCP‐type pincer ligands, which can catalyze ammonia production from dinitrogen and water using samarium diiodide as a reductant under ambient conditions. Although the nonsubstituted complex did not exhibit efficient catalytic activity, introducing phenyl groups onto the dihydroimidazole skeleton substantially enhanced its catalytic activity. According to our electrochemical studies and density functional theory calculations, the phenyl groups effectively increased the electron‐accepting ability of the molybdenum complex, resulting in an increase in the strength of NH bonds (i.e., bond dissociation free energy (BDFE _NH )) in the corresponding imide complex, which is an important intermediate in the catalytic cycle for ammonia production, and its catalytic activity.

Inorganic pyrophosphatases are essential metalloenzymes for phosphate metabolism. Bacterial Family II Inorganic pyrophosphatases utilize a trinuclear metal center and exhibit higher catalytic activity than binuclear counterparts. Here we show the mechanism underlying this enhanced hydrolytic efficiency in the enzyme from Shewanella sp. AS-11 using X-ray absorption spectroscopy, site-directed mutagenesis, and density functional theory calculations. We identify a catalytic μ ₃ -oxo nucleophile—generated by proton transfer from a bridging μ ₃ -hydroxide to Asp14—as the key reactive species for hydrolysis. Rotation of Asp14 drives this conversion and constitutes the rate-limiting step, with an activation barrier of 15.5 kcal mol ^-1 . The trinuclear metal center promotes hydrolysis by lowering the p K _a of the hydroxide to facilitate μ ₃ -oxo formation, stabilizing this intermediate, positioning the nucleophile for optimal in-line attack, and enhancing phosphorus electrophilicity. These findings highlight the importance of reactive species generation and illustrate how metalloenzymes exploit geometric and electronic tuning to achieve high catalytic reactivity.

Novel cobaltocenium compounds bearing a 4-(2,6-dimethylpyridin-4-yl)phenyl group are designed and synthesized for the use of proton-coupled electron transfer (PCET) reagents. Reduction and protonation of these compounds afford the corresponding cobaltocenes bearing a 4-(2,6-dimethylpyridin-1-ium-4-yl)phenyl moiety with bond dissociation free energies (BDFEs) of 34.4–43.0 kcal/mol. A superstoichiometric amount of ammonia is formed from the catalytic reduction of dinitrogen with these cobaltocenes in the presence of a molybdenum-nitride complex under ambient reaction conditions.

Osmium(III) complexes supported by N-methyl-di(pyridin-2-ylmethyl)amine (DPA), 4,4'-dimethyl-2,2'-bipyridyl (Me₂-bpy), and N¹,N¹,N³,N³-tetrakis(pyridin-2-ylmethyl)propane-1,3-diamine (TPPN) ligands were synthesized, and their reactivities toward a fluoride anion were examined. Crystal structures of the osmium(III) complexes showed that the metal centers adopted octahedral iso-structures with the osmium(II) centers of the one-electron reduced complexes. In the reaction with a fluoride anion, C(sp³)-H bonds of the pyridin-2-ylmethyl groups of TPPN and DPA ligands were fluorinated to give F-TPPN and F-DPA ligands containing a C(sp³)-F bond. In the case of the osmium(III) complex with the Me₂-bpy ligand, one of the methyl groups was fluorinated. The product complexes were characterized by ¹H NMR, ¹⁹F NMR, and mass spectra. From the osmium(III) complexes of TPPN and DPA ligands, two geometric isomers were formed by the fluorination reaction. One of the isomers has the fluorinated methylene group at the axial position whereas another isomer has the fluorinated methylene group at the equatorial position. There is no need to worry about the formation of such stereoisomers in the reaction with the osmium(III) complex of the Me₂-bpy ligand. The C(sp³)-H bond modification in the osmium(III) complex of the Me₂-bpy ligand also proceeded in the reactions with chloride, acetate, and benzoate anions. The reaction mechanism that involves a metallo-quinonoid canonical form comprising the osmium center and the deprotonated pyridin-2-ylmethyl or methylpyridine (picoline) moiety is proposed for the present C(sp³)-H bond modification reactions based on mechanistic studies including density functional theory calculations.

Development of molecular catalysts compatible with water is essential for achieving sustainable ammonia synthesis. In this context, met-al–oxo species have rarely been recognized as catalysts for nitrogen fixation, but rather regarded as dead-end species formed through the reaction of low-valent metal complexes with water. Herein, we report a series of cationic molybdenum–oxo complexes bearing NHC-based PCP-type pincer ligands and their utility as catalyst precursors for ammonia formation under ambient reaction conditions. The molybdenum–oxo complexes were prepared from reactions of the corresponding triiodide complexes with water in good yields. Reac-tions of an atmospheric pressure of N2 with SmI2 as a reductant and water as a proton source in the presence of the molybdenum–oxo complexes as catalysts at room temperature afforded up to 22,000 equiv of ammonia based on the molybdenum atom. The oxo com-plexes also served as effective catalysts for ammonia formation employing metallocenes as reductants and pyridinium salts as proton sources. Stoichiometric reactions, electrochemical measurements, and DFT calculations revealed that the oxo complexes are readily con-verted into the corresponding nitride complexes as catalytically active species. The oxo-to-nitride conversion is triggered by oxygen ab-straction with SmI2 or by protonation/reduction steps to form aqua complexes, followed by N≡N bond cleavage of the coordinated N2. We also revealed the reverse reaction from the nitride complex to the oxo complex in the presence of H2O under reductive conditions. Our study demonstrates that molybdenum–oxo complexes should not be merely regarded as dead-end species but as viable catalyst pre-cursors, offering new options for catalyst design toward sustainable nitrogen fixation.