Insight into One-Electron Oxidation of the {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC): Aminyl Radical Stabilized by [Fe(NO)2] Motif

A reversible redox reaction ({Fe­(NO)2}9 DNIC [(NO)2Fe­(N­(Mes)­(TMS))2]− (4) ⇄ oxidized-form DNIC [(NO)2Fe­(N­(Mes)­(TMS))2] (5) (Mes = mesityl, TMS = trimethylsilane)), characterized by IR, UV–vis, 1H/15N NMR, SQUID, XAS, single-crystal X-ray structure, and DFT calculation, was demonstrated. The electronic structure of the oxidized-form DNIC 5 (Stotal = 0) may be best described as the delocalized aminyl radical [(N­(Mes)­(TMS))2]2–• stabilized by the electron-deficient {FeIII(NO–)2}9 motif, that is, substantial spin is delocalized onto the [(N­(Mes)­(TMS))2]2–• such that the highly covalent dinitrosyl iron core (DNIC) is preserved. In addition to IR, EPR (g ≈ 2.03 for {Fe­(NO)2}9), single-crystal X-ray structure (Fe–N­(O) and N–O bond distances), and Fe K-edge pre-edge energy (7113.1–7113.3 eV for {Fe­(NO)2}10 vs 7113.4–7113.9 eV for {Fe­(NO)2}9), the 15N NMR spectrum of [Fe­(15NO)2] was also explored to serve as an efficient tool to characterize and discriminate {Fe­(NO)2}9 (δ 23.1–76.1 ppm) and {Fe­(NO)2}10 (δ −7.8–25.0 ppm) DNICs. To the best of our knowledge, DNIC 5 is the first structurally characterized tetrahedral DNIC formulated as covalent–delocalized [{FeIII(NO–)2}9–[N­(Mes)­(TMS)]2–•]. This result may explain why all tetrahedral DNICs containing monodentate-coordinate ligands isolated and characterized nowadays are confined in the {Fe­(NO)2}9 and {Fe­(NO)2}10 DNICs in chemistry and biology.