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

The reversible transformations [(Bim)₃Fe­(κ²-O₂N)]­[BF₄] (<b>3</b>) ⇌ [(Bim)₃Fe­(NO)­(κ¹-ONO)]­[BF₄]₂ (<b>4</b>) were demonstrated and characterized. Transformation of <i>O</i>,<i>O</i>-nitrito-containing complex <b>3</b> into [(Bim)₃Fe­(μ-O)­(μ-OAc)­Fe­(Bim)₃]³⁺ (<b>5</b>) along with the release of NO and H₂O 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^II-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

The reversible transformations [(Bim)₃Fe­(κ²-O₂N)]­[BF₄] (<b>3</b>) ⇌ [(Bim)₃Fe­(NO)­(κ¹-ONO)]­[BF₄]₂ (<b>4</b>) were demonstrated and characterized. Transformation of <i>O</i>,<i>O</i>-nitrito-containing complex <b>3</b> into [(Bim)₃Fe­(μ-O)­(μ-OAc)­Fe­(Bim)₃]³⁺ (<b>5</b>) along with the release of NO and H₂O 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^II-nitro] complexes

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

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

Iron(III) Bound by Hydrosulfide Anion Ligands: NO-Promoted Stabilization of the [Fe^III–SH] Motif

Spontaneous transformation of the thermally stable [HS]⁻-bound {Fe­(NO)₂}⁹ dinitrosyl iron complex (DNIC) [(HS)₂Fe­(NO)₂]⁻ (<b>1</b>) into [(NO)₂Fe­(μ-S)]₂^2– (Roussin’s red salt (RRS)) along with release of H₂S, probed by NBD-SCN (NBD = nitrobenzofurazan), was observed when DNIC <b>1</b> was dissolved in water at ambient temperature. The reversible transformation of RRS into DNIC <b>1</b> (RRS → DNIC <b>1</b>) in the presence of H₂S was demonstrated. In contrast, the thermally unstable hydrosulfide-containing mononitrosyl iron complex (MNIC) [(HS)₃Fe^III(NO)]⁻ (<b>3</b>) and [Fe^III(SH)₄]⁻ (<b>5</b>) in THF/DMF spontaneously dimerized into the first structurally characterized Fe^III–hydrosulfide complexes [(NO)­(SH)­Fe­(μ-S)]₂^2– (<b>4</b>) with two {Fe­(NO)}⁷ motifs antiferromagnetically coupled and [(SH)₂Fe­(μ-S)]₂^2– (<b>6</b>) resulting from two Fe^III (<i>S</i> = 5/2) centers antiferromagnetically coupled to yield an <i>S</i> = 0 ground state with thermal occupancy of higher spin states, respectively. That is, the greater the number of NO ligands bound to [2Fe2S], the larger the antiferromagnetic coupling constant. On the basis of DFT computation and the experimental (and calculated) reduction potential (<i>E</i>_1/2) of complexes <b>1</b>, <b>3</b>, and <b>5</b>, the NO-coordinate ligand(s) of complexes <b>1</b> and <b>3</b> serves as the stronger electron-donating ligand, compared to thiolate, to reduce the effective nuclear charge (<i>Z</i>_eff) of the iron center and prevent DNIC <b>1</b> from dimerization in an organic solvent (MeCN)