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

The catalytic oxidation of ammonia is relevant in the context of sustainable energy scenarios, for instance in fuels cells or for hydrogen storage. Here we report the diruthenium complex [LRu2(py)4(μ1,2-N2H4)](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 the first example for 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 spec-trometry in connection with 15N-labeling. Detailed electrochemical and spectroscopic studies, including voltammetry-coupled ESI-MS and gas phase photodissociation spectroscopy, provided mechanistic insights and identified [LRu2(N2H2)]3+ (33+) as a key intermediate, and DFT was 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(NH2)2]3+, giving 13+ and then 33+ en route to N2. Furthermore, [LRu2(MeCN)2]3+ (23+) was identified as an off-cycle product leading to gradual catalyst deactivation.