Mechanism\nof Dinitrogen Photoactivation by P<sub>2</sub>P<sup>Ph</sup>Fe Complexes:\nThermodynamic and Kinetic Computational\nStudies

The P<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)(H)<sub>2</sub> catalyst showed a\nsignificant ammonia yield under light irradiation.\nHowever, under thermal conditions, the hydrogen evolution reaction\n(HER) is favored over the nitrogen reduction reaction (N<sub>2</sub>RR), making P<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)(H)<sub>2</sub> an ideal system for studying the competition between both reactions.\nIn this study, we used a series of computational tools to elucidate\nthe photochemical reaction mechanism for the N<sub>2</sub>RR and thermal\npathways leading to the HER with this catalyst. We calculated the\nenergy profile for each transformation and estimated the rate constants\nfor each step. Our results, which are consistent with experimental\nobservations, indicate that photoinduced H<sub>2</sub> elimination\nfrom P<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)(H)<sub>2</sub> promotes\nthe formation of P<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)<sub>2</sub>, which is on-path for N<sub>2</sub>RR. However, this elimination\nprocess is kinetically hindered due to high-energy barriers. Furthermore,\nour calculations reveal enhanced dinitrogen activation upon the conversion\nof P<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)(H)<sub>2</sub> to\nP<sub>2</sub>P<sup>Ph</sup>Fe(N<sub>2</sub>)<sub>2</sub>.