Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe–NxHyIntermediates Be Active Under Turnover Conditions?

Nitrogen reduction reaction (N 2 RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber-Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N 2 RR intermediates in EP 3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N 2 RR elementary steps. We find that, in contrast to previous suggestions, early N 2 RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N 2 RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N 2 RR intermediates as proposed earlier to design more efficient biomimetic catalysts.