Synthesis and Catalytic Activity of Molybdenum Complexes Bearing Dihydroimidazole‐Based PCP‐Type Pincer Ligands Toward Nitrogen Fixation

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.