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 N2 O

Abstract

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N₂ O. 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, C₆ F₅ ) to generate N₂ O. 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-N₂ O conversion reaction.

Keywords: X-ray diffraction; cobalt; density functional calculations; nitrogen oxides; porphyrinoids.