Remote Positioning of Cations Tunes Catalytic Fe-Mediated Nitrogen Fixation Selectivity for Hydrazine Instead of...

Understanding the basis of product selectivity is a central issue in catalyst design. Catalytic nitrogen reduction (N₂R) provides a salient example; whereas ammonia (NH₃) is the common product of N₂R, hydrazine (N₂H₄) is produced under certain conditions. Using mechanism-guided design, we report a strategy for tuning redox potential that enables selective reduction of dinitrogen to hydrazine by iron complexes in polar protic media. Incorporation of cationic trimethylammonium (NMe₃ ⁺) or proton-responsive dimethylamino (NMe₂) groups into a tris(phosphino)borane (P₃ ^B) ligand framework affords redox-tunable iron precatalysts that operate efficiently in methanol. Computational analyses reveal that these ligand modifications anodically shift the reduction potential of an iron hydrazido (Fe═NNH₂) intermediate by >400 mV, thereby influencing the key branch point for hydrazine versus ammonia. Critical to success is positioning the cationic charges remote from the Fe-N₂ binding site to preserve the high degree of N₂ activation required for functionalization. Newly prepared tricationic iron complexes, soluble and stable in polar protic media, catalyze N₂R with N-fixed yields of up to 73% per reducing equivalent consumed, and with hydrazine selectivity exceeding 20:1 over ammonia. This work highlights the use of remote electrostatic effects to tune multi-electron catalytic product profiles from a 6e^- to a 4e^- product.