Résumé
The electrocatalytic reduction of dinitrogen to ammonia by molecular complexes is fundamentally limited by poorly understood proton–electron transfer sequences and catalyst deactivation pathways. Here we report a detailed mechanistic investigation of nitrogen reduction mediated by a Mo(VI) nitride complex involving a bis-aryloxy-carbene ligand. Combined electrochemical, spectroscopic, and computational studies reveal stepwise electron transfer and proton transfer processes proceeding through Mo(V) imide and Mo(IV) amide intermediates. Notably, we demonstrate that a Mo(V) imide intermediate undergoes a key disproportionation reaction producing the corresponding amide species, ultimately enabling NH3 formation. We also establish that the catalytic activity is impeded by competitive chloride coordination and the formation of a stable Mo(III) dimer that prevents N2 binding. These findings identify critical mechanistic bottlenecks in molecular N2 electroreduction and establish clear ligand-design criteria for suppressing deactivation pathways and enabling efficient ammonia synthesis under mild conditions.