Mapping the Catalytic Cycle of Ru-Catalyzed Ammonia Oxidation

Abstract

This study presents a mechanism of ammonia oxidation catalyzed by [Ru^II(NH₃)][PF₆]₂ (Ru=Ru(tpy)(dmabpy)) complexes. All intermediates, including the elusive [Ru^III(NH₂)]²⁺ and [Ru^IV(NH)]²⁺, were detected, with several intermediates characterized spectroscopically. The catalytic cycle follows multiple reaction pathways branching at the key intermediates [Ru^III(NH₂)]²⁺ and [Ru^II(N₂H₄)]²⁺. The competition between the pathways is influenced by ammonia concentration and the local proton concentration generated during electrooxidation at the anode. In the N−N bond-forming steps, dimerization of [Ru^III(NH₂)]²⁺ and nucleophilic attack of [Ru^IV(NH)]²⁺ by ammonia compete, while in the subsequent [Ru^II(N₂H₄)]²⁺ oxidation, direct oxidation competes with proton-catalyzed disproportionation. These findings provide molecular-level insights into the catalytic cycle and offer guidance for developing more efficient ruthenium-based ammonia oxidation catalysts.