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]⁻ (<b>4</b>) ⇄ oxidized-form DNIC [(NO)₂Fe­(N­(Mes)­(TMS))₂] (<b>5</b>) (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 <b>5</b> (<i>S</i>_total = 0) may be best described as the delocalized aminyl radical [(N­(Mes)­(TMS))₂]₂^–• stabilized by the electron-deficient {Fe^III(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 (<i>g</i> ≈ 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 <b>5</b> is the first structurally characterized tetrahedral DNIC formulated as covalent–delocalized [{Fe^III(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