Electrocatalytic Nitrite Reduction in Neutral Water with Ni(II) and Co(II) Macrocycle Complexes: Catalytic Evaluation and Mechanistic Elucidation

Abstract

AbstractNitrite anions, of anthropogenic origin, in the environment disrupt the nitrogen cycle making the electrocatalytic reduction of nitrite a significant objective. Homogeneous Ni(II) and Co(II) complexes bearing a macrocyclic supporting ligand consisting of a tridentate redox active bis(imino)pyridine moiety coupled with a tertiary amine donor site are effective and selective for the electrocatalytic reduction of nitrite to ammonium ion and hydroxylamine in buffered (pH 7) aqueous solutions. Controlled potential coulometry at potentials from −0.98 to −1.05 V vs Ag/AgCl in 4‐morpholinepropanesulfonic acid (MOPS) buffer yielded ammonium as the major product with Faradaic efficiencies ranging from 76–90 %. A foot‐of‐the‐wave analysis yielded calculated turn‐over frequencies of 61 s−1and 28 s−1 for the Ni and Co complexes, respectively. The Ni complex displayed a higher selectivity for NH4+/NH2OH than the Co analog. A computational examination of the catalytic mechanism of the Ni complex was used to support and elucidate the proposed chemical steps, provide some energetic details of the electron and proton transfers, and present a rationale for the selectivity of this reduction.