Remote Positioning of Cations Tunes Catalytic Fe‐Mediated Nitrogen Fixation Selectivity for Hydrazine Instead of Ammonia in Protic Media

Abstract

ABSTRACT Understanding the basis of product selectivity is a central issue in catalyst design. Catalytic nitrogen reduction (N 2 R) provides a salient example; whereas ammonia (NH 3 ) is the common product of N 2 R, hydrazine (N 2 H 4 ) 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 (NMe 3 + ) or proton‐responsive dimethylamino (NMe 2 ) groups into a tris(phosphino)borane (P 3 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═NNH 2 ) 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–N 2 binding site to preserve the high degree of N 2 activation required for functionalization. Newly prepared tricationic iron complexes, soluble and stable in polar protic media, catalyze N 2 R 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.