Computational Mechanistic Study of Electro-Oxidation of Ammonia to N₂ by Homogenous Ruthenium and Iron Complexes

A comprehensive DFT study of the electrocatalytic oxidation of ammonia to dinitrogen by a ruthenium polypyridyl complex, [(tpy)­(bpy)­Ru^II(NH₃)]²⁺ (a), and its NMe₂-substituted derivative (b) is presented. The thermodynamics and kinetics of electron (ET) and proton transfer (PT) steps and transition states are calculated. NMe₂ substitution on bpy reduces the ET steps on average 8 kcal/mol for complex b as compared to a. The calculations indicate that N–N formation occurs by ammonia nucleophilic attack/H-transfer via a nitrene intermediate rather than a nitride intermediate. Comparison of the free energy profiles of Ru-b with its first-row Fe congener reveals that the thermodynamics are less favorable for the Fe-b model, especially for ET steps. The N–H bond dissociation free energies (BDFEs) for NH₃ to form N₂ show the following trend: Ru-b < Ru-a < Fe-b, indicating the lowest and most favorable BDFEs for Ru-b complex.