The cooperative “push-pull” effects of Re^I, Mo⁰, W⁰ complexes and borane Lewis acids on the dinitrogen bond are evaluated in molecular orbital diagrams: we extract electronic design principles in terms of orthogonal σ and π “push-pull” paths that may guide the design of complexes towards the desired thermal, electrochemical or photochemical reactivity of N₂.

Abstract

The 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-[Re^ICl(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.