Iron-Nitride Isomerization as a Deactivation Pathway in Ammonia Oxidation: A Computational Study of Sequential HAT...

Ammonia oxidation remains a kinetic bottleneck in utilizing ammonia as a carbon-free energy carrier. Iron-based complexes offer a cost-effective alternative to noble-metal catalysts, but use is hampered by low turnover numbers. Here, density functional theory is used to investigate the catalytic activity and deactivation pathways of Fe2+/Fe3+ complexes bearing cis-ammonia ligands. We confirm the ground-state electronic structures of these complexes, then establish an accurate SMD/DRACO protocol─using a van der Waals cavity with Gaussian-smeared surface charge discretization─that predicts redox potentials with mean error below 0.013 V versus experiment. Thermodynamic evaluation of sequential hydrogen atom transfer (HAT) steps reveals a "peaked" bond dissociation free energy profile. Crucially, the exergonic third HAT step yields not the anticipated nitrido species, but a novel isomer in which the nitride inserts into the Fe-C bond. This Fe-N-C isomer is a deep thermodynamic sink, trapping the catalyst in an inactive state. A similarly low-energy Fe-N-C isomer exists for the imido intermediate but is kinetically inaccessible. These results indicate that Fe-C-containing complexes are unlikely to serve as efficient AO catalysts, as a high-valent iron center with a multiply bonded nitrogen ligand renders the Fe-C bond prone to alkyl migration, a facile deactivation pathway.