Mechanism of Dinitrogen Photoactivation by P₂P^PhFe Complexes: Thermodynamic and Kinetic Computational Studies

The P₂P^PhFe(N₂)(H)₂ catalyst showed a significant ammonia yield under light irradiation. However, under thermal conditions, the hydrogen evolution reaction (HER) is favored over the nitrogen reduction reaction (N₂RR), making P₂P^PhFe(N₂)(H)₂ an ideal system for studying the competition between both reactions. In this study, we used a series of computational tools to elucidate the photochemical reaction mechanism for the N₂RR and thermal pathways leading to the HER with this catalyst. We calculated the energy profile for each transformation and estimated the rate constants for each step. Our results, which are consistent with experimental observations, indicate that photoinduced H₂ elimination from P₂P^PhFe(N₂)(H)₂ promotes the formation of P₂P^PhFe(N₂)₂, which is on-path for N₂RR. However, this elimination process is kinetically hindered due to high-energy barriers. Furthermore, our calculations reveal enhanced dinitrogen activation upon the conversion of P₂P^PhFe(N₂)(H)₂ to P₂P^PhFe(N₂)₂.