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)­RuII(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.

本文对基于钌多吡啶配合物[(tpy)­(bpy)­RuII(NH3)]2+（a）及其NMe2取代衍生物（b）的电催化氨氧化为二氮的研究进行了全面的多重傅里叶变换（DFT）分析。电子转移（ET）和质子转移（PT）步骤及过渡态的热力学和动力学被计算得出。与化合物a相比，b中bpy的NMe2取代降低了平均8 kcal/mol的ET步骤。计算表明，通过亚胺中间体而非氮化物中间体，氨通过亲核攻击/氢转移形成N–N键。与第一行铁同系物Ru-b的吉布斯自由能谱对比显示，铁-b模型的热力学条件不如Ru-b模型，尤其是在ET步骤中。对于氨形成氮气的N–H键解离自由能（BDFEs）趋势如下：Ru-b < Ru-a < Fe-b，表明Ru-b复合物具有最低且最有利的BDFEs。