Ammonia offers a potential solution for efficiently storing and distributing molecular hydrogen in chemical bonds of a gas that liquefies at modest pressures.1 Our research groups designed a RuII precatalyst [(tpy ({NMe2}2bpy)Ru(NH3)][PF6]2 (tpy = terpyridine, {NMe2}2bpy = 4,4- bis(dimethylamino)-2,2-bipyridine) that oxidizes ammonia to nitrogen, protons and electrons electrocatalytically under mild conditions. Several Ru intermediates have been proposed in the catalytic cycle,2 and experiments are consistent with the first N–N bond on the way to N2 arising from nucleophilic attack of NH3 at nitrogen of an elusive RuIV=NH intermediate to form a RuII hydrazine complex that has been synthesized independently. However, several questions about the mechanism remain.2,3 We are studying the details of this catalytic system by several approaches. One involves chemical syntheses of proposed intermediates in the electrocatalytic cycle and studying their stoichiometric reactions to probe their catalytic relevance. In this regard, we have developed chemical syntheses of the RuIII complexes [(tpy)({NMe2}2bpy)RuIII(NH3)][Z]3, where Z = PF6, BArF24. In the proposed catalytic cycle, a RuIII(NH3) intermediate is thought to undergo base-catalyzed, redox disproportionation to generate RuII(NH3) and RuIV(NH) in a 1:1 ratio. When our RuIII(NH3) complexes are treated with hindered bases, two RuII complexes are formed in a 2:1 ratio. We shall present details of our synthetic approach and results from isotopic labeling and spectroscopic measurements designed to both shed light on the mechanism of this stoichiometric reaction, and determine its relevance to electrocatalytic NH3 oxidation.

Synthesis of ruthenium complexes relevant to the mechanism of catalytic oxidation of ammonia to molecular nitrogen, protons, and electrons

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.