Mechanistic insights into nitric oxide oxygenation (NOO) reactions of {CrNO}5 and {CoNO}8

Abstract

Here, we report the nitric oxide oxygenation (NOO) reactions of two distinct metal nitrosyls {Co–nitrosyl (S = 0) vs. Cr–nitrosyl (S = 1/2)}. In this regard, we synthesized and characterized [(BPMEN)Co(NO)]²⁺ ({CoNO}⁸, 1) to compare its NOO reaction with that of [(BPMEN)Cr(NO)(Cl⁻)]⁺ ({CrNO}⁵, 2), having a similar ligand framework. Kinetic measurements showed that {CrNO}⁵ is thermally more stable than {CoNO}⁸. Complexes 1 and 2, upon reaction with the superoxide anion (O₂˙⁻), generate [(BPMEN)Co^II(NO₂⁻)₂] (Co^II–NO₂⁻, 3) and [(BPMEN)Cr^III(NO₂⁻)Cl⁻]⁺ (Cr^III–NO₂⁻, 4), respectively, with O₂ evolution. Furthermore, analysis of these NOO reactions and tracking of the N-atom using ¹⁵N-labeled NO (¹⁵NO) revealed that the N-atoms of 3 (Co^II–¹⁵NO₂⁻) and 4 (Cr^III–¹⁵NO₂⁻) derive from the nitrosyl (¹⁵NO) moieties of 1 and 2, respectively. This work represents a comparative study of oxidation reactions of {CoNO}⁸vs. {CrNO}⁵, showing different rates of the NOO reactions due to different thermal stability. To complete the NOM cycle, we reacted 3 and 4 with NO, and surprisingly, only 3 generated {CoNO}⁸ species, while 4 was unreactive towards NO. Furthermore, the phenol ring nitration test, performed using 2,4-di-tert-butylphenol (2,4-DTBP), suggested the presence of a proposed peroxynitrite (PN) intermediate in the NOO reactions of 1 and 2.