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(afaCy)3Fe]OTf2 (N(afaCy)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(afaCy)3Fe]OTf2 complex. An alternative pathway showcases the reduction of the bound NO complex, [N(afaCy)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.