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
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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
