Dinitrosyliron Complex [(PMDTA)Fe(NO)₂]: Intermediate for Nitric Oxide Monooxygenation Activity in Nonheme Iron Complex

Despite a comprehensive study on the biosynthesis and function of nitric oxide, biological metabolism of nitric oxide, especially when its concentration exceeds the cytotoxic level, remains elusive. Oxidation of nitric oxide by O2 in aqueous solution has been known to yield NO2-. On the other hand, a biomimetic study on the metal-mediated conversion of NO to NO2-/NO3- via O2 reactivity disclosed a conceivable pathway for aerobic metabolism of NO. During the NO-to-NO3- conversion, transient formation of metal-bound peroxynitrite and subsequent release of •NO2 via O-O bond cleavage were evidenced by nitration of tyrosine residue or 2,4-di-tert-butylphenol (DTBP). However, the synthetic/catalytic/enzymatic cycle for conversion of nitric oxide into a nitrite pool is not reported. In this study, sequential reaction of the ferrous complex [(PMDTA)Fe(κ2-O,O′-NO2)(κ1-O-NO2)] (3; PMDTA = pentamethyldiethylenetriamine) with NO(g), KC8, and O2 established a synthetic cycle, complex 3 → {Fe(NO)2}9 DNIC [(PMDTA)Fe(NO)2][NO2] (4) → {Fe(NO)2}10 DNIC [(PMDTA)Fe(NO)2] (1) → [(PMDTA)(NO)Fe(κ2-O,N-ONOO)] (2) → complex 3, for the transformation of nitric oxide into nitrite. In contrast to the reported reactivity of metal-bound peroxynitrite toward nitration of DTBP, peroxynitrite-bound MNIC 2 lacks phenol nitration reactivity toward DTBP. Presumably, the [(PMDTA)Fe] core in {Fe(NO)}8 MNIC 2 provides a mononuclear template for intramolecular interaction between Fe-bound peroxynitrite and Fe-bound NO-, yielding Fe-bound nitrite stabilized in the form of complex 3. This [(PMDTA)Fe]-core-mediated concerted peroxynitrite homolytic O-O bond cleavage and combination of the O atom with Fe-bound NO- reveals a novel and effective pathway for NO-to-NO2- transformation. Regarding the reported assembly of the dinitrosyliron unit (DNIU) [Fe(NO)2] in the biological system, this synthetic cycle highlights DNIU as a potential intermediate for nitric oxide monooxygenation activity in a nonheme iron system.