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)R-u(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 N-2 show the following trend: Ru-b < Ru-a < Fe-b, indicating the lowest and most favorable BDFEs for Ru-b complex.