Chelate‐Thiolate‐Coordinate Ligands Modulating the Configuration and Electrochemical Property of Dinitrosyliron Complexes (DNICs)

Stable Bimetallic FeII/{Fe(NO)2}9 Moiety Derived from Reductive Transformations of a Diferrous-dinitrosyl Species

A dimeric dithiolate-bridged species, [Fe(NO)(PS2)]₂ (1) containing two {FeNO}⁷ units, can be isolated by treating [Fe(CO)₂(NO)₂] with PS2H₂ (PS2H₂ = bis(2-dimercaptophenyl)phenylphosphine). Crystallographic studies reveal the syn-configuration of NO units and the bridging thiolates in the butterfly shape of the 2Fe2S core. Addition of PPh₃ to the solution of dinuclear 1 leads to the formation of mononuclear {FeNO}⁷ [Fe(NO)(PS2)(PPh₃)] (2) that shows electrochemical responses similar to those of 1. One-electron reduction of 1 with Cp*₂Co or KC₈ results in the isolation of thiolate-bridged bimetallic DNIC, [(PS2)Fe(μ-PS2)Fe(NO)₂]^- ([3]^-), confirmed by several spectroscopies including single-crystal X-ray diffraction studies. The bimetallic DNIC [3]^- is a rare example obtained from the one-electron reduction of a dinuclear Fe-NO {FeNO}⁷ model complex. With the assistance of redox behaviors of 2, electrochemical studies imply that the reduction of 1 leads to the formation of a mononuclear {FeNO}⁸ [Fe(NO)(PS2)(THF)]^- intermediate, which involves disproportionation or NO^- transfer to yield [3]^-. Based on IR data and magnetic properties, the electronic structure of [3]^- can be described as a Fe^II/{Fe(NO)₂}⁹ state. Isolation of the {Fe(NO)₂}⁹ moiety coordinated by the Fe ancillary complex lends strong support to the NO scrambling behavior in the effectiveness of the activity of flavodiiron nitric oxide reductases (FNORs).

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.

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.

Fe in biosynthesis, translocation, and signal transduction of NO: toward bioinorganic engineering of dinitrosyl iron complexes into NO-delivery scaffolds for tissue engineering

The ubiquitous physiology of nitric oxide enables the bioinorganic engineering of [Fe(NO)₂]-containing and NO-delivery scaffolds for tissue engineering.

Nitric Oxide Modulates Endonuclease III Redox Activity by a 800 mV Negative Shift upon [Fe4S4] Cluster Nitrosylation

Here we characterize the [Fe₄S₄] cluster nitrosylation of a DNA repair enzyme, endonuclease III (EndoIII), using DNA-modified gold electrochemistry and protein film voltammetry, electrophoretic mobility shift assays, mass spectrometry of whole and trypsin-digested protein, and a variety of spectroscopies. Exposure of EndoIII to nitric oxide under anaerobic conditions transforms the [Fe₄S₄] cluster into a dinitrosyl iron complex, [(Cys)₂Fe(NO)₂]^-, and Roussin's red ester, [(μ-Cys)₂Fe₂(NO)₄], in a 1:1 ratio with an average retention of 3.05 ± 0.01 Fe per nitrosylated cluster. The formation of the dinitrosyl iron complex is consistent with previous reports, but the Roussin's red ester is an unreported product of EndoIII nitrosylation. Hyperfine sublevel correlation (HYSCORE) pulse EPR spectroscopy detects two distinct classes of NO with ¹⁴N hyperfine couplings consistent with the dinitrosyl iron complex and reduced Roussin's red ester. Whole-protein mass spectrometry of EndoIII nitrosylated with ¹⁴NO and ¹⁵NO support the assignment of a protein-bound [(μ-Cys)₂Fe₂(NO)₄] Roussin's red ester. The [Fe₄S₄]^2+/3+ redox couple of DNA-bound EndoIII is observable using DNA-modified gold electrochemistry, but nitrosylated EndoIII does not display observable redox activity using DNA electrochemistry on gold despite having a similar DNA-binding affinity as the native protein. However, direct electrochemistry of protein films on graphite reveals the reduction potential of native and nitrosylated EndoIII to be 127 ± 6 and -674 ± 8 mV vs NHE, respectively, corresponding to a shift of approximately -800 mV with cluster nitrosylation. Collectively, these data demonstrate that DNA-bound redox activity, and by extension DNA-mediated charge transport, is modulated by [Fe₄S₄] cluster nitrosylation.