Not Limited to Iron: A Cobalt Heme–NO Model Facilitates N–N Coupling with External NO in the Presence of a Lewis Acid to Generate N₂O

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N₂O. We previously reported that a heme Fe–NO model engages in this N−N bond-forming reaction with NO. We now demonstrate that (OEP)Co^II(NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX₃ (X=F, C₆F₅) to generate N₂O. DFT calculations support retention of the Co^II oxidation state for the experimentally observed adduct (OEP)Co^II(NO⋅BF₃), the presumed hyponitrite intermediate (P^.+)Co^II(ONNO⋅BF₃), and the porphyrin π-radical cation by-product of this reaction, and that the π-radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous-to-ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO-to-N₂O conversion reaction.