19 research outputs found

    Formation Pathway of Roussin’s Red Ester (RRE) via the Reaction of a {Fe(NO)<sub>2</sub>}<sup>10</sup> Dinitrosyliron Complex (DNIC) and Thiol: Facile Synthetic Route for Synthesizing Cysteine-Containing DNIC

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    Transformation of {Fe­(NO)<sub>2</sub>}<sup>10</sup> dinitrosyliron complex (DNIC) Fe­(CO)<sub>2</sub>(NO)<sub>2</sub> into [{Fe­(NO)<sub>2</sub>}<sup>9</sup>]<sub>2</sub> Roussin’s red ester (RRE) [(μ-S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>2</sub>)­Fe­(NO)<sub>2</sub>]<sub>2</sub> (<b>3</b>) triggered by cysteamine via the reaction pathway (intermediates) [{Fe­(NO)<sub>2</sub>}<sup>10</sup>]<sub>2</sub>­[(NO)<sub>2</sub>­Fe­(μ-CO)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>1</b>) → {Fe­(NO)<sub>2</sub>}<sup>9</sup>­{Fe­(NO)<sub>2</sub>}<sup>10</sup>­[(NO)<sub>2</sub>Fe­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>2</sub>)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>2</b>) → RRE <b>3</b> is demonstrated. The <b>1</b>-to-<b>2</b>-to-<b>3</b> conversion is promoted by proton transfer followed by O<sub>2</sub> oxidation and deprotonation. Additionally, a study on facile conversion of complex <b>3</b> to complexes [(SR)­(S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] [SR = 2-aminoethanethiolate (<b>4</b>), benzenethiolate (<b>5</b>)] and [(CysS))­(S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>6</b>) via reaction with thiols and the further utility of complex <b>5</b> as a template for synthesizing mixed-thiolate-containing reduced RRE (rRRE) [(μ-SC<sub>6</sub>H<sub>5</sub>)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe<sub>2</sub>(NO)<sub>4</sub>] (<b>7</b>) provide the methodology for the synthesis and isolation of neutral, pure cysteine-/mixed-thiolate-containing DNIC/RRE. Compared to the conversion of complex <b>2</b> to complex <b>3</b> via reaction with O<sub>2</sub>, diphenyl disulfide triggers oxidation of complex <b>2</b> to lead to formation of the neutral {Fe­(NO)<sub>2</sub>}<sup>9</sup> DNIC <b>5</b> and RRE <b>3</b>. S–S bond activation of diphenyl disulfide by rRRE <b>2</b> may support the decay (oxidation) of rRRE species in ToMOC via the reduction of adjacent protein residues such as cystins, proposed by Lippard

    Formation Pathway of Roussin’s Red Ester (RRE) via the Reaction of a {Fe(NO)<sub>2</sub>}<sup>10</sup> Dinitrosyliron Complex (DNIC) and Thiol: Facile Synthetic Route for Synthesizing Cysteine-Containing DNIC

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    Transformation of {Fe­(NO)<sub>2</sub>}<sup>10</sup> dinitrosyliron complex (DNIC) Fe­(CO)<sub>2</sub>(NO)<sub>2</sub> into [{Fe­(NO)<sub>2</sub>}<sup>9</sup>]<sub>2</sub> Roussin’s red ester (RRE) [(μ-S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>2</sub>)­Fe­(NO)<sub>2</sub>]<sub>2</sub> (<b>3</b>) triggered by cysteamine via the reaction pathway (intermediates) [{Fe­(NO)<sub>2</sub>}<sup>10</sup>]<sub>2</sub>­[(NO)<sub>2</sub>­Fe­(μ-CO)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>1</b>) → {Fe­(NO)<sub>2</sub>}<sup>9</sup>­{Fe­(NO)<sub>2</sub>}<sup>10</sup>­[(NO)<sub>2</sub>Fe­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>2</sub>)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>­NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>2</b>) → RRE <b>3</b> is demonstrated. The <b>1</b>-to-<b>2</b>-to-<b>3</b> conversion is promoted by proton transfer followed by O<sub>2</sub> oxidation and deprotonation. Additionally, a study on facile conversion of complex <b>3</b> to complexes [(SR)­(S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] [SR = 2-aminoethanethiolate (<b>4</b>), benzenethiolate (<b>5</b>)] and [(CysS))­(S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe­(NO)<sub>2</sub>] (<b>6</b>) via reaction with thiols and the further utility of complex <b>5</b> as a template for synthesizing mixed-thiolate-containing reduced RRE (rRRE) [(μ-SC<sub>6</sub>H<sub>5</sub>)­(μ-S­(CH<sub>2</sub>)<sub>2</sub>NH<sub>3</sub>)­Fe<sub>2</sub>(NO)<sub>4</sub>] (<b>7</b>) provide the methodology for the synthesis and isolation of neutral, pure cysteine-/mixed-thiolate-containing DNIC/RRE. Compared to the conversion of complex <b>2</b> to complex <b>3</b> via reaction with O<sub>2</sub>, diphenyl disulfide triggers oxidation of complex <b>2</b> to lead to formation of the neutral {Fe­(NO)<sub>2</sub>}<sup>9</sup> DNIC <b>5</b> and RRE <b>3</b>. S–S bond activation of diphenyl disulfide by rRRE <b>2</b> may support the decay (oxidation) of rRRE species in ToMOC via the reduction of adjacent protein residues such as cystins, proposed by Lippard

    Nitrite Activation to Nitric Oxide via One-fold Protonation of Iron(II)‑<i>O</i>,<i>O</i>‑nitrito Complex: Relevance to the Nitrite Reductase Activity of Deoxyhemoglobin and Deoxyhemerythrin

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    The reversible transformations [(Bim)<sub>3</sub>Fe­(κ<sup>2</sup>-O<sub>2</sub>N)]­[BF<sub>4</sub>] (<b>3</b>) ⇌ [(Bim)<sub>3</sub>Fe­(NO)­(κ<sup>1</sup>-ONO)]­[BF<sub>4</sub>]<sub>2</sub> (<b>4</b>) were demonstrated and characterized. Transformation of <i>O</i>,<i>O</i>-nitrito-containing complex <b>3</b> into [(Bim)<sub>3</sub>Fe­(μ-O)­(μ-OAc)­Fe­(Bim)<sub>3</sub>]<sup>3+</sup> (<b>5</b>) along with the release of NO and H<sub>2</sub>O triggered by 1 equiv of AcOH implicates that nitrite-to-nitric oxide conversion occurs, in contrast to two protons needed to trigger nitrite reduction producing NO observed in the protonation of [Fe<sup>II</sup>-nitro] complexes

    Nitrite Activation to Nitric Oxide via One-fold Protonation of Iron(II)‑<i>O</i>,<i>O</i>‑nitrito Complex: Relevance to the Nitrite Reductase Activity of Deoxyhemoglobin and Deoxyhemerythrin

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    The reversible transformations [(Bim)<sub>3</sub>Fe­(κ<sup>2</sup>-O<sub>2</sub>N)]­[BF<sub>4</sub>] (<b>3</b>) ⇌ [(Bim)<sub>3</sub>Fe­(NO)­(κ<sup>1</sup>-ONO)]­[BF<sub>4</sub>]<sub>2</sub> (<b>4</b>) were demonstrated and characterized. Transformation of <i>O</i>,<i>O</i>-nitrito-containing complex <b>3</b> into [(Bim)<sub>3</sub>Fe­(μ-O)­(μ-OAc)­Fe­(Bim)<sub>3</sub>]<sup>3+</sup> (<b>5</b>) along with the release of NO and H<sub>2</sub>O triggered by 1 equiv of AcOH implicates that nitrite-to-nitric oxide conversion occurs, in contrast to two protons needed to trigger nitrite reduction producing NO observed in the protonation of [Fe<sup>II</sup>-nitro] complexes

    Noticiero de Vigo : diario independiente de la mañana: Ano XXVIII Número 11586 - 1913 novembro 26

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    Insertion of CS<sub>2</sub> into the thermally unstable nickel­(III) hydride [PPN]­[Ni­(H)­(P­(<i>o</i>-C<sub>6</sub>H<sub>3</sub>-3-SiMe<sub>3</sub>-2-S)<sub>3</sub>)] (<b>1</b>), freshly prepared from the reaction of [PPN]­[Ni­(OC<sub>6</sub>H<sub>5</sub>)­P­(C<sub>6</sub>H<sub>3</sub>-3-SiMe<sub>3</sub>-2-S)<sub>3</sub>] and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin; pin = OCMe<sub>2</sub>CMe<sub>2</sub>O) in tetrahydrofuran at −80 °C via a metathesis reaction, readily affords [PPN]­[Ni<sup>III</sup>(κ<sup>1</sup>-S<sub>2</sub>CH)­(P­(<i>o</i>-C<sub>6</sub>H<sub>3</sub>-3-SiMe<sub>3</sub>-2-S)<sub>3</sub>)] (<b>2</b>) featuring a κ<sup>1</sup>-S<sub>2</sub>CH moiety

    Insight into the Reactivity and Electronic Structure of Dinuclear Dinitrosyl Iron Complexes

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    A combination of N/S/Fe K-edge X-ray absorption spectroscopy (XAS), X-ray diffraction data, and density functional theory (DFT) calculations provides an efficient way to unambiguously delineate the electronic structures and bonding characters of Fe–S, N–O, and Fe–N bonds among the direduced-form Roussin’s red ester (RRE) [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>]<sup>2–</sup>(<b>1</b>) with {Fe­(NO)<sub>2</sub>}<sup>10</sup>-{Fe­(NO)<sub>2</sub>}<sup>10</sup> core, the reduced-form RRE [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>]<sup>−</sup>(<b>3</b>) with {Fe­(NO)<sub>2</sub>}<sup>9</sup>-{Fe­(NO)<sub>2</sub>}<sup>10</sup> core, and RRE [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>] (<b>4</b>) with {Fe­(NO)<sub>2</sub>}<sup>9</sup>-{Fe­(NO)<sub>2</sub>}<sup>9</sup> core. The major contributions of highest occupied molecular orbital (HOMO) 113α/β in complex <b>1</b> is related to the antibonding character between Fe­(d) and Fe­(d), Fe­(d), and S atoms, and bonding character between Fe­(d) and NO­(π*). The effective nuclear charge (<i><i>Z</i></i><sub>eff</sub>) of Fe site can be increased by removing electrons from HOMO to shorten the distances of Fe···Fe and Fe–S from <b>1</b> to <b>3</b> to <b>4</b> or, in contrast, to increase the Fe–N bond lengths from <b>1</b> to <b>3</b> to <b>4</b>. The higher IR ν<sub>NO</sub> stretching frequencies (1761, 1720 cm<sup>–1</sup> (<b>4</b>), 1680, 1665 cm<sup>–1</sup> (<b>3</b>), and 1646, 1611, 1603 cm<sup>–1</sup> (<b>1</b>)) associated with the higher transition energy of N<sub>1s</sub> →σ*­(NO) (412.6 eV (<b>4</b>), 412.3 eV (<b>3</b>), and 412.2 eV (<b>1</b>)) and the higher <i><i>Z</i></i><sub>eff</sub> of Fe derived from the transition energy of Fe<sub>1s</sub> → Fe<sub>3d</sub> (7113.8 eV (<b>4</b>), 7113.5 eV (<b>3</b>), and 7113.3 eV (<b>1</b>)) indicate that the N–O bond distances of these complexes are in the order of <b>1 > 3 > 4</b>. The N/S/Fe K-edge XAS spectra as well as DFT computations reveal the reduction of complex <b>4</b> yielding complex <b>3</b> occurs at Fe, S, and NO; in contrast, reduction mainly occurs at Fe site from complex <b>3</b> to complex <b>1</b>

    Insight into the Reactivity and Electronic Structure of Dinuclear Dinitrosyl Iron Complexes

    No full text
    A combination of N/S/Fe K-edge X-ray absorption spectroscopy (XAS), X-ray diffraction data, and density functional theory (DFT) calculations provides an efficient way to unambiguously delineate the electronic structures and bonding characters of Fe–S, N–O, and Fe–N bonds among the direduced-form Roussin’s red ester (RRE) [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>]<sup>2–</sup>(<b>1</b>) with {Fe­(NO)<sub>2</sub>}<sup>10</sup>-{Fe­(NO)<sub>2</sub>}<sup>10</sup> core, the reduced-form RRE [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>]<sup>−</sup>(<b>3</b>) with {Fe­(NO)<sub>2</sub>}<sup>9</sup>-{Fe­(NO)<sub>2</sub>}<sup>10</sup> core, and RRE [Fe<sub>2</sub>(μ-SPh)<sub>2</sub>(NO)<sub>4</sub>] (<b>4</b>) with {Fe­(NO)<sub>2</sub>}<sup>9</sup>-{Fe­(NO)<sub>2</sub>}<sup>9</sup> core. The major contributions of highest occupied molecular orbital (HOMO) 113α/β in complex <b>1</b> is related to the antibonding character between Fe­(d) and Fe­(d), Fe­(d), and S atoms, and bonding character between Fe­(d) and NO­(π*). The effective nuclear charge (<i><i>Z</i></i><sub>eff</sub>) of Fe site can be increased by removing electrons from HOMO to shorten the distances of Fe···Fe and Fe–S from <b>1</b> to <b>3</b> to <b>4</b> or, in contrast, to increase the Fe–N bond lengths from <b>1</b> to <b>3</b> to <b>4</b>. The higher IR ν<sub>NO</sub> stretching frequencies (1761, 1720 cm<sup>–1</sup> (<b>4</b>), 1680, 1665 cm<sup>–1</sup> (<b>3</b>), and 1646, 1611, 1603 cm<sup>–1</sup> (<b>1</b>)) associated with the higher transition energy of N<sub>1s</sub> →σ*­(NO) (412.6 eV (<b>4</b>), 412.3 eV (<b>3</b>), and 412.2 eV (<b>1</b>)) and the higher <i><i>Z</i></i><sub>eff</sub> of Fe derived from the transition energy of Fe<sub>1s</sub> → Fe<sub>3d</sub> (7113.8 eV (<b>4</b>), 7113.5 eV (<b>3</b>), and 7113.3 eV (<b>1</b>)) indicate that the N–O bond distances of these complexes are in the order of <b>1 > 3 > 4</b>. The N/S/Fe K-edge XAS spectra as well as DFT computations reveal the reduction of complex <b>4</b> yielding complex <b>3</b> occurs at Fe, S, and NO; in contrast, reduction mainly occurs at Fe site from complex <b>3</b> to complex <b>1</b>

    Nitrate-to-Nitrite-to-Nitric Oxide Conversion Modulated by Nitrate-Containing {Fe(NO)<sub>2</sub>}<sup>9</sup> Dinitrosyl Iron Complex (DNIC)

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    Nitrosylation of high-spin [Fe­(κ<sup>2</sup>-O<sub>2</sub>NO)<sub>4</sub>]<sup>2<b>–</b></sup> (<b>1</b>) yields {Fe­(NO)}<sup>7</sup> mononitrosyl iron complex (MNIC) [(κ<sup>2</sup>-O<sub>2</sub>NO)­(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>3</sub>Fe­(NO)]<sup>2<b>–</b></sup> (<b>2</b>) displaying an <i>S</i> = 3/2 axial electron paramagnetic resonance (EPR) spectrum (<i>g</i><sub>⊥</sub> = 3.988 and <i>g</i><sub>∥</sub> = 2.000). The thermally unstable nitrate-containing {Fe­(NO)<sub>2</sub>}<sup>9</sup> dinitrosyl iron complex (DNIC) [(κ<sup>1</sup>-ONO<sub>2</sub>)<sub>2</sub>Fe­(NO)<sub>2</sub>]<sup><b>–</b></sup> (<b>3</b>) was exclusively obtained from reaction of HNO<sub>3</sub> and [(OAc)<sub>2</sub>Fe­(NO)<sub>2</sub>]<sup><b>–</b></sup> and was characterized by IR, UV–vis, EPR, superconducting quantum interference device (SQUID), X-ray absorption spectroscopy (XAS), and single-crystal X-ray diffraction (XRD). In contrast to {Fe­(NO)<sub>2</sub>}<sup>9</sup> DNIC [(ONO)<sub>2</sub>Fe­(NO)<sub>2</sub>]<sup><b>–</b></sup> constructed by two monodentate O-bound nitrito ligands, the weak interaction between Fe(1) and the distal oxygens O(5)/O(7) of nitrato-coordinated ligands (Fe(1)···O(5) and Fe(1)···O(7) distances of 2.582(2) and 2.583(2) Å, respectively) may play important roles in stabilizing DNIC <b>3</b>. Transformation of nitrate-containing DNIC <b>3</b> into N-bound nitro {Fe­(NO)}<sup>6</sup> [(NO)­(κ<sup>1</sup>-NO<sub>2</sub>)­Fe­(S<sub>2</sub>CNEt<sub>2</sub>)<sub>2</sub>] (<b>7</b>) triggered by bis­(diethylthiocarbamoyl) disulfide ((S<sub>2</sub>CNEt<sub>2</sub>)<sub>2</sub>) implicates that nitrate-to-nitrite conversion may occur via the intramolecular association of the coordinated nitrate and the adjacent polarized NO-coordinate ligand <b>(</b>nitrosonium<b>)</b> of the proposed {Fe­(NO)<sub>2</sub>}<sup>7</sup> intermediate [(NO)<sub>2</sub>(κ<sup>1</sup>-ONO<sub>2</sub>)­Fe­(S<sub>2</sub>CNEt<sub>2</sub>)<sub>2</sub>] (<b>A</b>) yielding {Fe­(NO)}<sup>7</sup> [(NO)­Fe­(S<sub>2</sub>CNEt<sub>2</sub>)<sub>2</sub>] (<b>6</b>) along with the release of N<sub>2</sub>O<sub>4</sub> (·NO<sub>2</sub>) and the subsequent binding of ·NO<sub>2</sub> to complex <b>6</b>. The N-bound nitro {Fe­(NO)}<sup>6</sup> complex <b>7</b> undergoes Me<sub>2</sub>S-promoted O-atom transfer facilitated by imidazole to give {Fe­(NO)}<sup>7</sup> complex <b>6</b> accompanied by release of nitric oxide. This result demonstrates that nitrate-containing DNIC <b>3</b> acts as an active center to modulate nitrate-to-nitrite-to-nitric oxide conversion

    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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    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

    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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    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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