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

Abstract

A reversible redox reaction ({Fe­(NO)<sub>2</sub>}<sup>9</sup> DNIC [(NO)<sub>2</sub>Fe­(N­(Mes)­(TMS))<sub>2</sub>]<sup>−</sup> (<b>4</b>) ⇄ oxidized-form DNIC [(NO)<sub>2</sub>Fe­(N­(Mes)­(TMS))<sub>2</sub>] (<b>5</b>) (Mes = mesityl, TMS = trimethylsilane)), characterized by IR, UV–vis, <sup>1</sup>H/<sup>15</sup>N 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><sub>total</sub> = 0) may be best described as the delocalized aminyl radical [(N­(Mes)­(TMS))<sub>2</sub>]<sub>2</sub><sup>–•</sup> stabilized by the electron-deficient {Fe<sup>III</sup>(NO<sup>–</sup>)<sub>2</sub>}<sup>9</sup> motif, that is, substantial spin is delocalized onto the [(N­(Mes)­(TMS))<sub>2</sub>]<sub>2</sub><sup>–•</sup> such that the highly covalent dinitrosyl iron core (DNIC) is preserved. In addition to IR, EPR (<i>g</i> ≈ 2.03 for {Fe­(NO)<sub>2</sub>}<sup>9</sup>), 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)<sub>2</sub>}<sup>10</sup> vs 7113.4–7113.9 eV for {Fe­(NO)<sub>2</sub>}<sup>9</sup>), the <sup>15</sup>N NMR spectrum of [Fe­(<sup>15</sup>NO)<sub>2</sub>] was also explored to serve as an efficient tool to characterize and discriminate {Fe­(NO)<sub>2</sub>}<sup>9</sup> (δ 23.1–76.1 ppm) and {Fe­(NO)<sub>2</sub>}<sup>10</sup> (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<sup>III</sup>(NO<sup>–</sup>)<sub>2</sub>}<sup>9</sup>–[N­(Mes)­(TMS)]<sub>2</sub><sup>–•</sup>]. This result may explain why all tetrahedral DNICs containing monodentate-coordinate ligands isolated and characterized nowadays are confined in the {Fe­(NO)<sub>2</sub>}<sup>9</sup> and {Fe­(NO)<sub>2</sub>}<sup>10</sup> DNICs in chemistry and biology

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