Improving Molecular Iron Ammonia Oxidation Electrocatalysts via Substituent Effects that Modulate Standard Potential and Stability

Molecular ammonia oxidation (AO) catalysis is a rapidly evolving research area. Among the catalysts studied, featuring metals including ruthenium, iron, manganese, nickel, and copper, polypyridyl iron complexes are attractive owing to their fast catalytic rates and significant turnover numbers (TON). Building upon our previous work on AO using the polypyridyl systems [(TPA)Fe(MeCN)₂]²⁺ and [(BPM)Fe(MeCN)₂]²⁺, this study investigates factors that impact rate and TON within and across catalyst series based on these polypyridyl ligand frameworks. The synthesis and analysis of derivatives functionalized in the 4-pyridyl position with electron-donating and electron-withdrawing groups (NMe₂, OMe, CF₃) are described; a combination of electroanalytical, UV–vis, and NMR analyses provide insights into the relative importance of catalyst standard potential (E°) and 4-pyridyl substituent to rate and stability. These findings constrain hypotheses rationalizing the nature of improved catalysis by comparing two classes of polypyridyl ligands for [(Lₐᵤₓ)Fe(MeCN)₂]²⁺ species and help define a roadmap for future catalyst development. For the most active catalyst studied herein, [(BPM^(OMe))Fe(MeCN)₂]²⁺, a TON of 381 is demonstrated after 48 h of sustained catalysis.