Cooperative Dinitrogen Activation: Identifying the Push‐Pull Effects of Transition Metals and Lewis Acids in Molecular Orbital Diagrams

Abstract

AbstractThe sustainable fixation of atmospheric N2 and its conversion into industrially relevant molecules is one of the major current challenges in chemistry. Besides nitrogen activation with transition metal complexes, a “push‐pull” approach that fine‐tunes electron density along the N−N bond has shown success recently. The “pushing” is performed by an electron rich entity such as a transition metal complex, and the “pulling” is achieved with an electron acceptor such as a Lewis acid. In this contribution, we explore the electronic structure implications of this approach using the complex trans‐[ReICl(N2)(PMe2Ph)4] as a starting point. We show that borane Lewis acids exert a pull‐effect of increasing strength with increased Lewis acidity via a π‐pathway. Furthermore, the ligand trans to dinitrogen can weaken the dinitrogen bond via a σ‐pathway. Binding a strong Lewis acid is found to have electronic structure effects potentially relevant for electrochemistry: dinitrogen‐dominated molecular orbitals are shifted into advantageous energetic positions for redox activation of the dinitrogen bond. We show how these electronic structure design principles are rooted in cooperative effects of a transition metal complex and a Lewis acid, and that they can be exploited to tailor a complex towards the desired thermal, electrochemical or photochemical reactivity.