Isocyanide Ligation Enables Electrochemical Ammonia Formation in a Synthetic Cycle for N2 Fixation

Abstract

Transition-metal-mediated splitting of N₂ to form metal nitride complexes could constitute a key step in electrocatalytic nitrogen fixation, if these nitrides can be electrochemically reduced to ammonia under mild conditions. The envisioned nitrogen fixation cycle involves several steps: N₂ binding to form a dinuclear end-on bridging complex with appropriate electronic structure to cleave the N₂ bridge followed by proton/electron transfer to release ammonia and bind another molecule of N₂. The nitride reduction and N₂ splitting steps in this cycle have differing electronic demands that a catalyst must satisfy. Rhenium systems have had limited success in meeting these demands, and studying them offers an opportunity to learn strategies for modulating reactivity. Here, we report a rhenium system in which the pincer supporting ligand is supplemented by an isocyanide ligand that can accept electron density, facilitating reduction and enabling the protonation/reduction of the nitride to ammonia under mild electrochemical conditions. The incorporation of isocyanide raises the N–H bond dissociation free energy of the first N–H bond by 10 kcal/mol, breaking the usual compensation between pK_a and redox potential; this is attributed to the separation of the protonation site (nitride) and the reduction site (delocalized between Re and isocyanide). Ammonia evolution is accompanied by formation of a terminal N₂ complex, which can be oxidized to yield bridging N₂ complexes including a rare mixed-valent complex. These rhenium species define the steps in a synthetic cycle that converts N₂ to NH₃ through an electrochemical N₂ splitting pathway, and show the utility of a second, tunable supporting ligand for enhancing nitride reactivity.