Tracing the Route to Ammonia: A Theoretical Study on the Possible Pathways for Dinitrogen Reduction with Tripodal Iron Complexes

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The chemistry of nitrogen fixation has been the subject of considerable research with a view to gaining a proper understanding of the mechanistic details. In this article, density functional calculations are performed on all the mechanistic possibilities for dinitrogen reduction mediated by the tripodal iron complexes [(SiP(Me)3)Fe(I)] ([Fe(Si)]) and [(BP(Me)3)Fe(0)] ([Fe(B)]). Dinitrogen addition to the neutral bare complex is found to be thermodynamically more favorable than that to the anionic one. Both symmetric and asymmetric pathways, along with the possible intermediates and transition states, are considered in this study. For both systems, the symmetric path is found to be more likely, although the prospect of the asymmetric path cannot be ignored. Moreover, interconversions between these two pathways are found to be less likely. This study corroborates most experimental observations and provides theoretical insight into the possible existence of some hitherto unknown intermediates such as multiple-bonded Fe-N species in a trigonal bipyramidal geometry. Furthermore, in agreement with experimental observations, this study also highlights the possibility of hydrogen and hydrazine evolution during the complete reduction of dinitrogen.