Catalytic Nitrogen Fixation Using Molybdenum–Oxo Complexes Bearing NHC-Based PCP-Type Pincer Ligands Via Oxo-to-Nitride Conversion

Development of molecular catalysts compatible with water is essential for achieving sustainable ammonia synthesis. In this context, metal–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. Reactions 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 complexes also served as effective catalysts for ammonia formation employing metallocenes and pyridinium salts. Stoichiometric reactions, electrochemical measurements, and DFT calculations revealed that the oxo complexes are readily converted into the corresponding nitride complexes as catalytically active species. The oxo-to-nitride conversion is triggered by oxygen abstraction 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 precursors, offering new options for catalyst design toward sustainable nitrogen fixation.