Metal-Free Activation of N₂ by Persistent Carbene Pairs: An Ab Initio Investigation

A metal-free activation process of dinitrogen is reported based on its reaction with two persistent (stable) carbenes. To analyze the reaction mechanism, we first employed methylene as the simplest representative. Potential energy profiles for every reaction step reveal that the reaction proceeds via the formation of an intermediate diazoalkane. The subsequent attack of second methylene facilitates the rupture of the two π-bonds of N₂ replacing them with π-bonds between carbon and nitrogen. This synergistic attack mimics the action of frustrated Lewis pairs with enhanced activity because of the dual chemical behavior of each carbene, being a Lewis acid and base at the same time. The reported energy landscape for a persistent ((CH₃)₃C)₂C carbene pair represents a model case, where the carbene dimerization is less favorable than N₂ activation. The present theoretical work introduces the notion of chemical bond activation by persistent carbene pairs. Future work will explore the possibility of activating other molecules with smaller energy barriers by tuning the electronic and geometric properties of the involved carbene.