Electrocatalytic Ammonia Oxidation with a Highly Preorganized Metal–Metal Cooperative Diruthenium Complex

Abstract

The catalytic oxidation of ammonia is relevant in the context of zero-carbon energy scenarios, for instance, in fuels cells or for hydrogen storage. Here, we report the diruthenium complex [LRu2 (py)4 (μ1,2 -N2 H4 )](PF6 )3 (1(PF6 )3 ; HL = 3,5-bis(bipyridyl)pyrazole) and demonstrate its ability to catalyze the oxidation of ammonia at a low overpotential of 0.76 V. Notably, complex 1(PF6 )3 is a molecular catalyst with a highly preorganized dinuclear substrate binding pocket to enforce the close proximity of two ammonia molecules and induce metal–metal cooperativity. The formation of N2 and H2 from ammonia during constant potential electrolysis was analyzed using gas chromatography as well as mass spectrometry in combination with15 N-labeling. Detailed electrochemical and spectroscopic studies, including voltammetry-coupled ESI-MS and gas phase photodissociation spectroscopy, provided mechanistic insights and identified [LRu2 (py)4 (N2 H2 )]3+ (33+ ) as a key intermediate, and DFT computations were used to evaluate possible reaction pathways as well as the electronic structures of relevant species. The combined experimental and computational findings allowed to propose a catalytic cycle where N–N coupling occurs at an early stage of the multi-PCET sequence in species [LRu2 (py)4 (NH2 )2 ]3+ , giving 13+ and then 33+ en route to N2 . Furthermore, [LRu2 (py)4 (MeCN)2 ]3+ (23+ ) was identified as an off-cycle product leading to gradual catalyst deactivation.