{Fe(NO)2}9 Dinitrosyl Iron Complex Acting as a Vehicle for the NO Radical

Insight into the photodynamic mechanism and protein binding of a nitrosyl iron-sulfur [Fe2S2(NO)4]2- cluster

Iron-sulfur cluster conversion and nitrosyl modification are involved in regulating their functions and play critical roles in signaling for biological systems. Hereby, the photo-induced dynamic process of (Me4N)2[Fe2S2(NO)4] was monitored using time-resolved electron paramagnetic resonance (EPR) spectra, MS spectra and cellular imaging methods. Photo-irradiation and the solvent affect the reaction rates and products. Spectroscopic and kinetic studies have shown that the process involves at least three intermediates: spin-trapped NO free radical species with a gav at 2.040, and two other iron nitrosyl species, dinitrosyl iron units (DNICs) and mononitrosyl iron units (MNICs) with gav values at 2.031 and 2.024, respectively. Moreover, the [Fe2S2(NO)4]2- cluster could bind with ferritin and decompose gradually, and a binding state of dinitrosyl iron coordinated with Cys102 of the recombinant human heavy chain ferritin (rHuHF) was finally formed. This study provides insight into the photodynamic mechanism of nitrosyl iron - sulfur clusters to improve the understanding of physiological activity.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

One-Pot Photosynthesis of Cubic Fe@Fe3O4 Core-Shell Nanoparticle Well-Dispersed in N-Doping Carbonaceous Polymer Using a Molecular Dinitrosyl Iron Precursor

Nanoscale zerovalent iron (NZVI) features potential application to biomedicine, (electro-/photo)catalysis, and environmental remediation. However, multiple-synthetic steps and limited ZVI content prompt the development of a novel strategy for efficient preparation of NZVI composites. Herein, a dinitrosyl iron complex [(N₃MDA)Fe(NO)₂] (1-N₃MDA) was explored as a molecular precursor for one-pot photosynthesis of a cubic Fe@Fe₃O₄ core-shell nanoparticle (ZVI% = 60%) well-dispersed in an N-doping carbonaceous polymer (NZVI@NC). Upon photolysis of 1-N₃MDA, photosensitizer Eosin Y, and sacrificial reductant TEA, the α-diimine N₃MDA and noninnocent NO ligands (1) enable the slow reduction of 1-N₃MDA into an unstable [(N₃MDA)Fe(NO)₂]^- species, (2) serve as a capping reagent for controlled nucleation of zerovalent Fe atom into Fe nanoparticle, and (3) promote the polymerization of degraded Eosin Y with N₃MDA yielding an N-doping carbonaceous matrix in NZVI@NC. This discovery of a one-pot photosynthetic process for NZVI@NC inspires continued efforts on its application to photolytic water splitting and ferroptotic chemotherapy in the near future.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Insight into the Electronic Structure of Biomimetic Dinitrosyliron Complexes (DNICs): Toward the Syntheses of Amido-Bridging Dinuclear DNICs

The ubiquitous function of nitric oxide (NO) guided the biological discovery of the natural dinitrosyliron unit (DNIU) [Fe(NO)2] as an intermediate/end product after Fe nitrosylation of nonheme cofactors. Because of the natural utilization of this cofactor for the biological storage and delivery of NO, a bioinorganic study of synthetic dinitrosyliron complexes (DNICs) has been extensively explored in the last 2 decades. The bioinorganic study of DNICs involved the development of synthetic methodology, spectroscopic discrimination, biological application of NO-delivery reactivity, and translational application to the (catalytic) transformation of small molecules. In this Forum Article, we aim to provide a systematic review of spectroscopic and computational insights into the bonding nature within the DNIU [Fe(NO)2] and the electronic structure of different types of DNICs, which highlights the synchronized advance in synthetic methodology and spectroscopic tools. With regard to the noninnocent nature of a NO ligand, spectroscopic and computational tools were utilized to provide qualitative/quantitative assignment of oxidation states of Fe and NO in DNICs with different redox levels and ligation modes as well as to probe the Fe-NO bonding interaction modulated by supporting ligands. Besides the strong antiferromagnetic coupling between high-spin Fe and paramagnetic NO ligands within the covalent DNIU [Fe(NO)2], in polynuclear DNICs, the effects of the Fe···Fe distance, nature of the bridging ligands, and type of bridging modes on the regulation of the magnetic coupling among paramagnetic DNIU [Fe(NO)2] are further reviewed. In the last part of this Forum Article, the sequential reaction of {Fe(NO)2}10 DNIC [(NO)2Fe(AMP)] (1-red) with NO(g), HBF4, and KC8 establishes a synthetic cycle, {Fe(NO)2}9-{Fe(NO)2}9 DNIC [(NO)2Fe(μ-dAMP)2Fe(NO)2] (1) → {Fe(NO)2}9 DNIC [(NO2)Fe(AMP)][BF4] (1-H) → {Fe(NO)2}10 DNIC 1-red → DNIC 1, for the transformation of NO into HNO/N2O. Of importance, the NO-induced transformation of {Fe(NO)2}10 DNIC 1-red and [(NO)2Fe(DTA)] (2-red; DTA = diethylenetriamine) unravels a synthetic strategy for preparation of the {Fe(NO)2}9-{Fe(NO)2}9 DNICs [(NO)2Fe(μ-NHR)2Fe(NO)2] containing amido-bridging ligands, which hold the potential to feature distinctive physical properties, chemical reactivities, and biological applications.

NO Reduction to N2O Triggered by a Dinuclear Dinitrosyl Iron Complex via the Associated Pathways of Hyponitrite Formation and NO Disproportionation

In spite of the comprehensive study of the metal-mediated conversion of NO to N2O disclosing the conceivable processes/mechanism in biological and biomimetic studies, in this study, the synthesis cycles and mechanism of NO reduction to N2O triggered by the electronically localized dinuclear {Fe(NO)2}10-{Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(NO)2(μ-bdmap)Fe(NO)2(THF)] (1) (bdmap = 1,3- bis(dimethylamino)-2-propanolate) were investigated in detail. Reductive conversion of NO to N2O triggered by complex 1 in the presence of exogenous ·NO occurs via the simultaneous formation of hyponitrite-bound {[Fe2(NO)4(μ-bdmap)]2(κ4-N2O2)} (2) and [NO2]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO2)] (3) (NO disproportionation yielding N2O and complex 3). EPR/IR spectra, single-crystal X-ray diffraction, and the electrochemical study uncover the reversible redox transformation of {Fe(NO)2}9-{Fe(NO)2}9 [Fe2(NO)4(μ-bdmap)(μ-OC4H8)]+ (7) ↔ {Fe(NO)2}10-{Fe(NO)2}9 1 ↔ {Fe(NO)2}10-{Fe(NO)2}10 [Fe(NO)2(μ-bdmap)Fe(NO)2]- (6) and characterize the formation of complex 1. Also, the synthesis study and DFT computation feature the detailed mechanism of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 DNIC 1 reducing NO to N2O via the associated hyponitrite-formation and NO-disproportionation pathways. Presumably, the THF-bound {Fe(NO)2}9 unit of electronically localized {Fe(NO)2}10-{Fe(NO)2}9 complex 1 served as an electron buffering reservoir for accommodating electron redistribution, and the {Fe(NO)2}10 unit of complex 1 acted as an electron-transfer channel to drive exogeneous ·NO coordination to yield proposed relay intermediate κ2-N,O-[NO]--bridged [Fe2(NO)4(μ-bdmap)(μ-NO)] (A) for NO reduction to N2O.

Study of Translocation of Stabilized NO Forms through Rat Skin using Electron Paramagnetic Resonance Method

We studied the effect of dinitrosyl-iron complexes with N-acetyl-L-cysteine as a thiol-containing ligand (DNIC-Acc) after transdermal administration to rats. Electron paramagnetic resonance spectroscopy with a lipophilic NO spin trap (a complex of iron and diethyldithiocarbamate ions) showed that DNIC-Acc administration significantly increased the total level of NO in the lung and liver tissues of the animal, which was accompanied by a slight decrease in the mean BP (<10%).

Synthesis and Structure of Iron (II) Complexes of Functionalized 1,5-Diaza-3,7-Diphosphacyclooctanes

In order to synthesize new iron (II) complexes of 1,5-diaza-3,7-diphosphacyclooctanes with a wider variety of the substituents on ligands heteroatoms (including functionalized ones, namely, pyridyl groups) and co-ligands, it was found that these ligands with relatively small phenyl, benzyl, and pyridin-2-yl substituents on phosphorus atoms in acetonitrile formed bis-P,P-chelate cis-complexes [L₂Fe(CH₃CN)₂]²⁺ (BF₄)₂⁻, whereas P-mesityl-substituted ligand formed only monoligand P,P-complex [LFe(CH₃CN)₄]²⁺ (BF₄)₂⁻. 3,7-dibenzyl-1,5-di(1′-(R)-phenylethyl)-1,5-diaza-3,7-diphosphacyclooctane reacted with hexahydrate of iron (II) tetrafluoroborate in acetone to give an unusual bis-ligand cationic complex of the composition [L₂Fe(BF₄)]⁺ BF₄⁻, where two fluorine atoms of the tetrafluoroborate unit occupied two pseudo-equatorial positions at roughly octahedral iron ion, according to X-ray diffraction data. 1,5-diaza-3,7-diphosphacyclooctanes replaced tetrahydrofurane and one of the carbonyl ligands of cyclopentadienyldicarbonyl(tetrahydrofuran)iron (II) tetrafluoroborate to form heteroligand complexes [CpFeL(CO)]⁺BF₄⁻. The structural studies in the solid phase and in solutions showed that diazadiphosphacyclooctane ligands of all complexes adopted chair-boat conformations so that their nitrogen atoms were in close proximity to the central iron ion. The redox properties of the obtained complexes were performed by the cyclic voltammetry method.

A reagent for heteroatom borylation, including iron mediated reductive deoxygenation of nitrate yielding a dinitrosyl complex

4,4'-Bipyridyl is shown to be a catalyst for transfer of pinacolboryl groups from (Bpin)₂ to nitrogen heterocycles and to Me₃SiN₃. Using stoichiometric (Bpin)₂(pyrazine) or (Bpin)₂(bipyridine) in an analogous manner, an aromatic nitro group is deoxygenated and subsequently borylated, and four-fold deoxygenation of (DIM)Fe(NO₃)₂(MeCN) to yield the dinitrosyl complex (DIM)Fe(NO)₂ is facile. The co-product O(Bpin)₂ is the quantitative fate of the removed oxo groups. With borylation of both nitrogen heterocycles and doubly deoxygenating two nitrates coordinated to a single metal center, broad spectrum methodology is demonstrated.

Syntheses and characterizations of iron complexes of bulky o-phenylenediamide ligand

We report the reactivity of the iron complexes of a bulky phenylenediamide ligand.

Incorporation of Nitroprusside on Silica Nanoparticles-A Strategy for Safer Use of This NO Donor in Therapy

Silica-based nanoparticles have been developed as powerful platforms for drug delivery and might also prevent undesired side effects of drugs. Here, a fast method to synthesize positively charged mesoporous silica nanoparticles (ζ = 20 ± 0.5 mV, surface area = 678 m² g^-1, and 2.3 nm of porous size) was reported. This nanomaterial was employed to anchor sodium nitroprusside (SNP), a vasodilator drug with undesired cyanide release. A remarkable incorporation of 323.9 ± 7.55 μmol of SNP per gram of nanoparticle was achieved, and a series of studies of NO release were conducted, showing efficient release of NO along with major cyanide retention (ca. 64% bound to nanoparticle). Biological assays with mammalian cells showed only a slight drop in cell viability (13%) at the highest concentration (1000 μM), while SNP exhibited an LC₅₀ of 228 μM. Moreover, pharmacological studies demonstrated similar efficacy for vasodilation and sGC-PKG-VASP pathway activation when compared to SNP alone. Altogether, this new SNP silica nanoparticle has great potential as an alternative for wider and safer use of SNP in medicine with lower cyanide toxicity.

Transformation of the hydride-containing dinitrosyl iron complex [(NO)2Fe(η2-BH4)]− into [(NO)2Fe(η3-HCS2)]−via reaction with CS2

Hydride-insertion reactivity of DNIC [(NO)₂Fe(η²-BH₄)]⁻ promotes the reductive transformation of CS₂ into DNIC [(NO)₂Fe(η³-HCS₂)]⁻ featuring Fe 3d_z2-to-HCS₂ π* backbonding interaction.

Electrocatalytic Water Reduction Beginning with a {Fe(NO)2}10-Reduced Dinitrosyliron Complex: Identification of Nitrogen-Doped FeO x(OH) y as a Real Heterogeneous Catalyst

Electron paramagnetic resonance, IR, single-crystal X-ray diffraction, and density functional theory computation reveal that the electronic structure of α-diimine-coordinated {Fe(NO)₂}¹⁰-reduced dinitrosyliron complexes (DNICs) may best be described as [{Fe(NO)₂}¹⁰-L^•], with the added electron residing mainly on the α-diimine ligand framework. The combination of electrochemistry, gas chromatography, Fourier transform infrared, X-ray photoelectron spectroscopy, and scanning electron microscopy-energy-dispersive X-ray studies demonstrates that the cathodic potential promotes/triggers the transformation of an α-diimine-coordinated {Fe(NO)₂}¹⁰-reduced DNIC into a particulate deposit on the electrode, and electrodeposited-film electrodes, CFeO and CFeNO, are kinetically dominant electrocatalysts responsible for hydrogen evolution reaction (HER) from water with quantitative Faradaic efficiency. In comparison with the CFeO electrode reaching a current density of 10 mA/cm² with an overpotential of 333 mV for HER, the nitrogen-doped iron oxide electrode, CFeNO, requires 147 mV of overpotential to achieve a current density of 10 mA/cm² in a 1 M NaOH aqueous solution. The CFeNO electrode exhibits higher kinetic efficiency (Tafel slope of 59 mV/dec) than the CFeO electrode (Tafel slope of 122 mV/dec) in alkaline conditions. As opposed to high R_ct (74.3 Ω) displayed by the CFeO electrode, the smaller charge-transfer resistance ( R_ct) of the CFeNO electrode (34.0 Ω) demonstrated that the better HER catalytic activity may be ascribed to the incorporation of nitrogen into iron oxide architecture, which increases the surface roughness and electroconductivity of the CFeNO electrode (56.9% iron content and nitrogen electron-donating effect) and improves HER catalysis by polarizing the incoming water molecule (acting as a proton tray). This result implicates that a (NH₄)₂SO₄-assisted nitrogen-doping strategy is a direct and effective method to realize synergistic regulation of the reaction dynamics, catalytically active sites and electronic conductivity, endowing this nitrogen-doped material CFeNO electrode as a promising HER electrocatalyst under alkaline conditions.