Mechanistic Insights into Nitric Oxide Oxygenation (NOO) Reactions of {crno}5 and {Cono}8.

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 (O-2(center dot-)), generate [(BPMEN) Co-II(NO2-)(2)] (Co-II-NO2-, 3) and [(BPMEN)Cr-III(NO2-)Cl-](+) (Cr-III-NO2-, 4), respectively, with O-2 evolution. Furthermore, analysis of these NOO reactions and tracking of the N-atom using N-15-labeled NO ((NO)-N-15) revealed that the N-atoms of 3 (CoII-15NO2-) and 4 (CrIII-15NO2-) derive from the nitrosyl ((NO)-N-15) moieties of 1 and 2, respectively. This work represents a comparative study of oxidation reactions of {CoNO}(8) vs. {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.