Bond Characterization of a Unique Thiathiophthene Derivative: Combined Charge Density Study and X‑ray Absorption Spectroscopy

Thiathiophthene (TTP), a planar molecule with two fused heterocyclic five-membered rings and an essentially linear S–S–S bond, is a molecule of great interest due to its unique chemical bondings. To elucidate the remarkable bonding nature, a combined experimental and theoretical study on the electron density distribution of 2,5-dimethyl-3,4-trimethylene-6a-TTP (<b>1</b>) is investigated based on a multipole model through high-resolution X-ray diffraction data experimentally and on the density functional calculations (DFT) theoretically. In addition, S K-edge X-ray absorption spectroscopy (XAS) is measured to verify the chemical bonding concerning the sulfur atoms. The molecule can be firmly described as 10π electron with aromatic character among the eight atoms, S₃C₅, of the two fused five-membered rings plus three-center four-electron σ character along the S–S–S bond. Such bonding description is verified with the calculated XAS spectrum, where the pre-edge absorption for transitions from S 1s to π* and σ* are located. The three-center four-electron S–S–S σ bond makes the terminal S atoms richer in electron density than the central one

Bond Characterization of a Unique Thiathiophthene Derivative: Combined Charge Density Study and X‑ray Absorption Spectroscopy

Thiathiophthene (TTP), a planar molecule with two fused heterocyclic five-membered rings and an essentially linear S–S–S bond, is a molecule of great interest due to its unique chemical bondings. To elucidate the remarkable bonding nature, a combined experimental and theoretical study on the electron density distribution of 2,5-dimethyl-3,4-trimethylene-6a-TTP (<b>1</b>) is investigated based on a multipole model through high-resolution X-ray diffraction data experimentally and on the density functional calculations (DFT) theoretically. In addition, S K-edge X-ray absorption spectroscopy (XAS) is measured to verify the chemical bonding concerning the sulfur atoms. The molecule can be firmly described as 10π electron with aromatic character among the eight atoms, S₃C₅, of the two fused five-membered rings plus three-center four-electron σ character along the S–S–S bond. Such bonding description is verified with the calculated XAS spectrum, where the pre-edge absorption for transitions from S 1s to π* and σ* are located. The three-center four-electron S–S–S σ bond makes the terminal S atoms richer in electron density than the central one

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

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) [Fe₂(μ-SPh)₂(NO)₄]^2–(<b>1</b>) with {Fe­(NO)₂}¹⁰-{Fe­(NO)₂}¹⁰ core, the reduced-form RRE [Fe₂(μ-SPh)₂(NO)₄]⁻(<b>3</b>) with {Fe­(NO)₂}⁹-{Fe­(NO)₂}¹⁰ core, and RRE [Fe₂(μ-SPh)₂(NO)₄] (<b>4</b>) with {Fe­(NO)₂}⁹-{Fe­(NO)₂}⁹ 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>_eff) 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_NO stretching frequencies (1761, 1720 cm^–1 (<b>4</b>), 1680, 1665 cm^–1 (<b>3</b>), and 1646, 1611, 1603 cm^–1 (<b>1</b>)) associated with the higher transition energy of N_1s →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>_eff of Fe derived from the transition energy of Fe_1s → Fe_3d (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>

Bond Characterization of a Unique Thiathiophthene Derivative: Combined Charge Density Study and X‑ray Absorption Spectroscopy

Thiathiophthene (TTP), a planar molecule with two fused heterocyclic five-membered rings and an essentially linear S–S–S bond, is a molecule of great interest due to its unique chemical bondings. To elucidate the remarkable bonding nature, a combined experimental and theoretical study on the electron density distribution of 2,5-dimethyl-3,4-trimethylene-6a-TTP (<b>1</b>) is investigated based on a multipole model through high-resolution X-ray diffraction data experimentally and on the density functional calculations (DFT) theoretically. In addition, S K-edge X-ray absorption spectroscopy (XAS) is measured to verify the chemical bonding concerning the sulfur atoms. The molecule can be firmly described as 10π electron with aromatic character among the eight atoms, S₃C₅, of the two fused five-membered rings plus three-center four-electron σ character along the S–S–S bond. Such bonding description is verified with the calculated XAS spectrum, where the pre-edge absorption for transitions from S 1s to π* and σ* are located. The three-center four-electron S–S–S σ bond makes the terminal S atoms richer in electron density than the central one

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 the Reactivity and Electronic Structure of Dinuclear Dinitrosyl Iron Complexes

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) [Fe₂(μ-SPh)₂(NO)₄]^2–(<b>1</b>) with {Fe­(NO)₂}¹⁰-{Fe­(NO)₂}¹⁰ core, the reduced-form RRE [Fe₂(μ-SPh)₂(NO)₄]⁻(<b>3</b>) with {Fe­(NO)₂}⁹-{Fe­(NO)₂}¹⁰ core, and RRE [Fe₂(μ-SPh)₂(NO)₄] (<b>4</b>) with {Fe­(NO)₂}⁹-{Fe­(NO)₂}⁹ 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>_eff) 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_NO stretching frequencies (1761, 1720 cm^–1 (<b>4</b>), 1680, 1665 cm^–1 (<b>3</b>), and 1646, 1611, 1603 cm^–1 (<b>1</b>)) associated with the higher transition energy of N_1s →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>_eff of Fe derived from the transition energy of Fe_1s → Fe_3d (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 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 the Dinuclear {Fe(NO)₂}¹⁰{Fe(NO)₂}¹⁰ and Mononuclear {Fe(NO)₂}¹⁰ Dinitrosyliron Complexes

The reversible redox transformations [(NO)₂Fe­(S^tBu)₂]⁻ ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂^2– ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂^– ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂ and [cation]­[(NO)₂Fe­(SEt)₂] ⇌ [cation]₂[(NO)₂Fe­(SEt)₂] (cation = K⁺-18-crown-6 ether) are demonstrated. The countercation of the {Fe­(NO)₂}⁹ dinitrosyliron complexes (DNICs) functions to control the formation of the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ dianionic reduced Roussin’s red ester (RRE) [PPN]₂[Fe­(μ-SR)­(NO)₂]₂ or the {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNIC [K⁺-18-crown-6 ether]₂[(NO)₂Fe­(SR)₂] upon reduction of the {Fe­(NO)₂}⁹ DNICs [cation]­[(NO)₂Fe­(SR)₂] (cation = PPN⁺, K⁺-18-crown-6 ether; R = alkyl). The binding preference of ligands [OPh]⁻/[SR]⁻ toward the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ motif of dianionic reduced RRE follows the ligand-displacement series [SR]⁻ > [OPh]⁻. Compared to the Fe K-edge preedge energy falling within the range of 7113.6–7113.8 eV for the dinuclear {Fe­(NO)₂}⁹{Fe­(NO)₂}⁹ DNICs and 7113.4–7113.8 eV for the mononuclear {Fe­(NO)₂}⁹ DNICs, the {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNICs and the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ dianionic reduced RREs containing S/O/N-ligation modes display the characteristic preedge energy 7113.1–7113.3 eV, which may be adopted to probe the formation of the EPR-silent {Fe­(NO)₂}¹⁰-{Fe­(NO)₂}¹⁰ dianionic reduced RREs and {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNICs in biology. In addition to the characteristic Fe/S K-edge preedge energy, the IR ν_NO spectra may also be adopted to characterize and discriminate [(NO)₂Fe­(μ-S^tBu)]₂ [IR ν_NO 1809 vw, 1778 s, 1753 s cm^–1 (KBr)], [Fe­(μ-S^tBu)­(NO)₂]₂^– [IR ν_NO 1674 s, 1651 s cm^–1 (KBr)], [Fe­(μ-S^tBu)­(NO)₂]₂^2– [IR ν_NO 1637 m, 1613 s, 1578 s, 1567 s cm^–1 (KBr)], and [K-18-crown-6 ether]₂[(NO)₂Fe­(SEt)₂] [IR ν_NO 1604 s, 1560 s cm^–1 (KBr)]

Insight into the Dinuclear {Fe(NO)₂}¹⁰{Fe(NO)₂}¹⁰ and Mononuclear {Fe(NO)₂}¹⁰ Dinitrosyliron Complexes

The reversible redox transformations [(NO)₂Fe­(S^tBu)₂]⁻ ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂^2– ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂^– ⇌ [Fe­(μ-S^tBu)­(NO)₂]₂ and [cation]­[(NO)₂Fe­(SEt)₂] ⇌ [cation]₂[(NO)₂Fe­(SEt)₂] (cation = K⁺-18-crown-6 ether) are demonstrated. The countercation of the {Fe­(NO)₂}⁹ dinitrosyliron complexes (DNICs) functions to control the formation of the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ dianionic reduced Roussin’s red ester (RRE) [PPN]₂[Fe­(μ-SR)­(NO)₂]₂ or the {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNIC [K⁺-18-crown-6 ether]₂[(NO)₂Fe­(SR)₂] upon reduction of the {Fe­(NO)₂}⁹ DNICs [cation]­[(NO)₂Fe­(SR)₂] (cation = PPN⁺, K⁺-18-crown-6 ether; R = alkyl). The binding preference of ligands [OPh]⁻/[SR]⁻ toward the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ motif of dianionic reduced RRE follows the ligand-displacement series [SR]⁻ > [OPh]⁻. Compared to the Fe K-edge preedge energy falling within the range of 7113.6–7113.8 eV for the dinuclear {Fe­(NO)₂}⁹{Fe­(NO)₂}⁹ DNICs and 7113.4–7113.8 eV for the mononuclear {Fe­(NO)₂}⁹ DNICs, the {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNICs and the {Fe­(NO)₂}¹⁰{Fe­(NO)₂}¹⁰ dianionic reduced RREs containing S/O/N-ligation modes display the characteristic preedge energy 7113.1–7113.3 eV, which may be adopted to probe the formation of the EPR-silent {Fe­(NO)₂}¹⁰-{Fe­(NO)₂}¹⁰ dianionic reduced RREs and {Fe­(NO)₂}¹⁰ dianionic reduced monomeric DNICs in biology. In addition to the characteristic Fe/S K-edge preedge energy, the IR ν_NO spectra may also be adopted to characterize and discriminate [(NO)₂Fe­(μ-S^tBu)]₂ [IR ν_NO 1809 vw, 1778 s, 1753 s cm^–1 (KBr)], [Fe­(μ-S^tBu)­(NO)₂]₂^– [IR ν_NO 1674 s, 1651 s cm^–1 (KBr)], [Fe­(μ-S^tBu)­(NO)₂]₂^2– [IR ν_NO 1637 m, 1613 s, 1578 s, 1567 s cm^–1 (KBr)], and [K-18-crown-6 ether]₂[(NO)₂Fe­(SEt)₂] [IR ν_NO 1604 s, 1560 s cm^–1 (KBr)]