Selective Stepwise Reduction of Nitrate and Nitrite to Dinitrogen or Ammonia

This study reports a method for the selective reduction of NO₃^– and NO₂^– to N₂ or NH₃, extending prior work in our lab where NO₃^– was reduced to NO by [N(afa^Cy)₃Fe]OTf₂ (N(afa^Cy)₃ = tris(5-cyclohexyl-amineazafulvene-2-methyl)amine, OTf = triflate). The first pathway involves the reduction of NO₂^– to N₂, where the NO generated in the initial step is transformed to N₂O by PPh₃ and further reduced to N₂ by the [N(afa^Cy)₃Fe]OTf₂ complex. An alternative pathway showcases the reduction of the bound NO complex, [N(afa^Cy)₃Fe(NO)]²⁺, to NH₃ using chemical reductants, albeit with a modest yield of 29%. Confirmation of the nitrogen source as NO is established through ¹⁵N labeling studies. Hydroxylamine (NH₂OH) is proposed as a plausible intermediate in the reduction of bound NO, supported by independent NH₂OH reduction experiments and computational studies. Nature employs a well-orchestrated, stepwise process involving several enzymes to reduce N-containing oxyanions, and this approach provides valuable insights into the stepwise reduction mechanisms of nitrate and nitrite, yielding NH₃ or N₂ as the product.