Isocyanide Ligation\nEnables Electrochemical Ammonia\nFormation in a Synthetic Cycle for N<sub>2</sub> Fixation

Transition-metal-mediated splitting of N<sub>2</sub> to\nform metal\nnitride complexes could constitute a key step in electrocatalytic\nnitrogen fixation, if these nitrides can be electrochemically reduced\nto ammonia under mild conditions. The envisioned nitrogen fixation\ncycle involves several steps: N<sub>2</sub> binding to form a dinuclear\nend-on bridging complex with appropriate electronic structure to cleave\nthe N<sub>2</sub> bridge followed by proton/electron transfer to release\nammonia and bind another molecule of N<sub>2</sub>. The nitride reduction\nand N<sub>2</sub> splitting steps in this cycle have differing electronic\ndemands that a catalyst must satisfy. Rhenium systems have had limited\nsuccess in meeting these demands, and studying them offers an opportunity\nto learn strategies for modulating reactivity. Here, we report a rhenium\nsystem in which the pincer supporting ligand is supplemented by an\nisocyanide ligand that can accept electron density, facilitating reduction\nand enabling the protonation/reduction of the nitride to ammonia under\nmild electrochemical conditions. The incorporation of isocyanide raises\nthe N–H bond dissociation free energy of the first N–H\nbond by 10 kcal/mol, breaking the usual compensation between p<i>K</i><sub>a</sub> and redox potential; this is attributed to\nthe separation of the protonation site (nitride) and the reduction\nsite (delocalized between Re and isocyanide). Ammonia evolution is\naccompanied by formation of a terminal N<sub>2</sub> complex, which\ncan be oxidized to yield bridging N<sub>2</sub> complexes including\na rare mixed-valent complex. These rhenium species define the steps\nin a synthetic cycle that converts N<sub>2</sub> to NH<sub>3</sub> through an electrochemical N<sub>2</sub> splitting pathway, and\nshow the utility of a second, tunable supporting ligand for enhancing\nnitride reactivity.