Electrocatalytic Oxidation of Ammonia by (Salen)ruthenium(III) Ammine Complexes: Direct Evidence for a Ruthenium(VI) Nitrido Active Intermediate

The electrocatalytic oxidation of ammonia using molecular catalysts has attracted much attention recently due to its potential for fuel cell applications. In this study, we report the electrocatalytic ammonia oxidation (AO) by [RuIII(salchda)(NH3)(CH3CN)]+ (RuNH3, salchda = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) and its bromo derivative. Controlled potential electrolysis at 0.65 V versus Fc+/0 for 3.2 h of a solution of RuNH3 and NH3 in CH3CN produced N2 with a TON of 26 and Faradaic efficiency (FE) close to 100%. The TON was increased to 79 and 147 when electrolysis was carried out at 0.7 and 0.80 V vs Fc+/0, respectively, with FE maintained at >99%, which are the highest among molecular ruthenium catalysts. An active intermediate was detected and shown to be the corresponding ruthenium(VI) nitrido complex [RuVI(salchda)(N)]+ (RuN) by direct comparison with an authentic sample of RuN, which we have previously synthesized and fully characterized. Direct kinetic studies on the oxidation of NH3 to N2 have also been carried out and the results are consistent with parallel electrophilic attack of NH3 by RuN and bimolecular N···N coupling of RuN to produce N2. DFT calculations have also been performed to support the proposed mechanism.