Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC) and Insight into DNIC Electronic Structure via Nuclear Resonance Vibrational Spectroscopy

Nitric oxide inhibits ten-eleven translocation DNA demethylases to regulate 5mC and 5hmC across the genome

DNA methylation at cytosine bases of eukaryotic DNA (5-methylcytosine, 5mC) is a heritable epigenetic mark that can regulate gene expression in health and disease. Enzymes that metabolize 5mC have been well-characterized, yet the discovery of endogenously produced signaling molecules that regulate DNA methyl-modifying machinery have not been described. Herein, we report that the free radical signaling molecule nitric oxide (NO) can directly inhibit the Fe(II)/2-OG-dependent DNA demethylases ten-eleven translocation (TET) and human AlkB homolog 2 (ALKBH2). Physiologic NO concentrations reversibly inhibited TET and ALKBH2 demethylase activity by binding to the mononuclear non-heme iron atom which formed a dinitrosyliron complex (DNIC) preventing cosubstrates (2-OG and O2) from binding. In cancer cells treated with exogenous NO, or cells endogenously synthesizing NO, there was a global increase in 5mC and 5-hydroxymethylcytosine (5hmC) in DNA, the substrates for TET, that could not be attributed to increased DNA methyltransferase activity. 5mC was also elevated in NO-producing cell-line-derived mouse xenograft and patient-derived xenograft tumors. Genome-wide DNA methylome analysis of cells chronically treated with NO (10 days) demonstrated enrichment of 5mC and 5hmC at gene-regulatory loci which correlated to changes in the expression of NO-regulated tumor-associated genes. Regulation of DNA methylation is distinctly different from canonical NO signaling and represents a novel epigenetic role for NO.

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.

Insight into the relevance of dinitrosyl iron complex (DNIC) formation in the absence of thiols in aqueous media

DNIC can be formed in aqueous media in the absence of thiols via mechanisms that depend exclusively on Fe(II) and NO. However, these reactions do not take place at intracellular concentrations of Fe(II) and NO, reinforcing the relevance of thiols to assist Fe(II) to Fe(I) reduction during DNIC formation in biological media.

Phosphine/thiolate-containing dinitrosyl cobalt complexes (DNCCs): synthesis, characterization, interconversion, X-ray diffraction identification and NO release

Cobalt carbonyl/nitrosyl complexes, (PPh3)(CO)2Co(NO) (1) and (PPh3)2(CO)Co(NO) (2), were obtained by reacting (CO)3Co(NO) with one equiv. and two equiv. of PPh3, respectively. The process of isoelectronic replacement of CO with NO+ resulted in the formation of a cationic complex {Co(NO)2}10 [(PPh3)2Co(NO)2][BF4] (3). Complex (PPh3)(SPh)Co(NO)2 (4), which contains a thiophenolate ligand, was synthesized by ligand exchange of complex 3 with [PPh4][SPh] in a 1 : 1 molar ratio in THF solution. The addition of one equiv. of [PPh4][SPh] to complex 4 led to the formation of complex [PPh4][(SPh)2Co(NO)2] (5). The interconversions among complexes 1-5 were substantiated with the application of IR spectroscopy and X-ray single-crystal diffraction techniques. Notably, complex 4 exhibited commendable NOs (nitric oxide species: NO+/˙NO/NO-) transfer capabilities in the presence of [Fe(TPP)Cl] (5,10,15,20-tetraphenyl-21H,23H-porphine iron(III) chloride).

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Bacterial nitric oxide reductase (NorBC) models employing click chemistry

Bacterial NO Reductase (NorBC or cNOR) is a membrane-bound enzyme found in denitrifying bacteria that catalyzes the two-electron reduction of NO to N₂O and water. The mechanism by which NorBC operates is highly debated, due to the fact that this enzyme is difficult to work with, and no intermediates of the NO reduction reaction could have been identified so far. The unique active site of NorBC consists of a heme b₃/non-heme Fe_B diiron center. Synthetic model complexes provide the opportunity to obtain insight into possible mechanistic alternatives for this enzyme. In this paper, we present three new synthetic model systems for NorBC, consisting of a tetraphenylporphyrin-derivative clicked to modified BMPA-based ligands (BMPA = bis(methylpyridyl)amine) that model the non-heme site in the enzyme. These complexes have been characterized by EPR, IR and UV-Vis spectroscopy. The reactivity with NO was then investigated, and it was found that the complex with the BMPA-carboxylate ligand as the non-heme component has a very low affinity for NO at the non-heme iron site. If the carboxylate functional group is replaced with a phenolate or pyridine group, reactivity is restored and formation of a diiron dinitrosyl complex was observed. Upon one-electron reduction of the nitrosylated complexes, following the semireduced pathway for NO reduction, formation of dinitrosyl iron complexes (DNICs) was observed in all three cases, but no N₂O could be detected.

NO and N2O Release from the Trityl Diazeniumdiolate Complexes [M(O2N2CPh3)3]- (M = Fe, Co)

Reaction of MBr₂ with 3 equiv of [K(18-crown-6)][O₂N₂CPh₃] generates the trityl diazeniumdiolate complexes [K(18-crown-6)][M(O₂N₂CPh₃)₃] (M = Co, 2; Fe, 3) in good yields. Irradiation of 2 and 3 using 371 nm light led to NO formation in 10 and 1% yields (calculated assuming a maximum of 6 equiv of NO produced per complex), respectively. N₂O was also formed during the photolysis of 2, in 63% yield, whereas photolysis of 3 led to the formation of N₂O, as well as Ph₃CN(H)OCPh₃, in 37 and 5% yields, respectively. These products are indicative of diazeniumdiolate fragmentation via both C-N and N-N bond cleavage pathways. In contrast, oxidation of complexes 2 and 3 with 1.2 equiv of [Ag(MeCN)₄][PF₆] led to N₂O formation but no NO formation, suggesting that diazeniumdiolate fragmentation occurs exclusively via C-N bond cleavage under these conditions. While the photolytic yields of NO are modest, they represent a 10- to 100-fold increase compared to the previously reported Zn congener, suggesting that the presence of a redox-active metal center favors NO formation upon trityl diazeniumdiolate fragmentation.

Substrate-Gated Transformation of a Pre-Catalyst into an Iron-Hydride Intermediate [(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2]- for Catalytic Dehydrogenation of Dimethylamine Borane

Continued efforts are made on the development of earth-abundant metal catalysts for dehydrogenation/hydrolysis of amine boranes. In this study, complex [K-18-crown-6-ether][(NO)₂Fe(μ-^MePyr)(μ-CO)Fe(NO)₂] (3-K-crown, ^MePyr = 3-methylpyrazolate) was explored as a pre-catalyst for the dehydrogenation of dimethylamine borane (DMAB). Upon evolution of H_2(g) from DMAB triggered by 3-K-crown, parallel conversion of 3-K-crown into [(NO)₂Fe(N,N'-^MePyrBH₂NMe₂)]^- (5) and an iron-hydride intermediate [(NO)₂(CO)Fe(μ-H)Fe(CO)(NO)₂]^- (A) was evidenced by X-ray diffraction/nuclear magnetic resonance/infrared/nuclear resonance vibrational spectroscopy experiments and supported by density functional theory calculations. Subsequent transformation of A into complex [(NO)₂Fe(μ-CO)₂Fe(NO)₂]^- (6) is synchronized with the deactivated generation of H_2(g). Through reaction of complex [Na-18-crown-6-ether][(NO)₂Fe(η²-BH₄)] (4-Na-crown) with CO_(g) as an alternative synthetic route, isolated intermediate [Na-18-crown-6-ether][(NO)₂(CO)Fe(μ-H)Fe(CO)(NO)₂] (A-Na-crown) featuring catalytic reactivity toward dehydrogenation of DMAB supports a substrate-gated transformation of a pre-catalyst [(NO)₂Fe(μ-^MePyr)(μ-CO)Fe(NO)₂]^- (3) into the iron-hydride species A as an intermediate during the generation of H_2(g).

Distortion of the [FeNO]2 Core in Flavodiiron Nitric Oxide Reductase Models Inhibits N-N Bond Formation and Promotes Formation of Unusual Dinitrosyl Iron Complexes: Implications for Catalysis and Reactivity

Flavodiiron nitric oxide reductases (FNORs) carry out the reduction of nitric oxide (NO) to nitrous oxide (N₂O), allowing infectious pathogens to mitigate toxic levels of NO generated in the human immune response. We previously reported the model complex [Fe₂(BPMP)(OPr)(NO)₂](OTf)₂ (1, OPr^- = propionate) that contains two coplanar NO ligands and that is capable of quantitative NO reduction to N₂O [White et al. J. Am. Chem. Soc. 2018, 140, 2562-2574]. Here we investigate, for the first time, how a distortion of the active site affects the ability of the diiron core to mediate N₂O formation. For this purpose, we prepared several analogues of 1 that contain two monodentate ligands in place of the bridging carboxylate, [Fe₂(BPMP)(X)₂(NO)₂]^3+/1+ (2-X; X = triflate, 1-methylimidazole, or methanol). Structural data of 2-X show that without the bridging carboxylate, the diiron core expands, leading to elongated (O)N-N(O) distances (from 2.80 Å in 1 to 3.00-3.96 Å in 2-X) and distorted (O)N-Fe-Fe-N(O) dihedral angles (from coplanarity (5.9°) in 1 to 52.9-85.1° in 2-X). Whereas 1 produces quantitative amounts of N₂O upon one-electron reduction, N₂O production is substantially impeded in 2-X, to an initial 5-10% N₂O yield. The main products after reduction are unprecedented hs-Fe^II/{Fe(NO)₂}^9/10 dinitrosyl iron complexes (DNICs). Even though mononuclear DNICs are stable and do not show N-N coupling (since it is a spin-forbidden process), the hs-Fe^II/{Fe(NO)₂}^9/10 DNICs obtained from 2-X show unexpected reactivity and produce up to quantitative N₂O yields after 2 h. The implications of these results for the active site structure of FNORs are discussed.

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.

NO Coupling by Nonclassical Dinuclear Dinitrosyliron Complexes to Form N2O Dictated by Hemilability

Selective coupling of NO by a nonclassical dinuclear dinitrosyliron complex (D-DNIC) to form N₂O is reported. The coupling is facilitated by the pyridinediimine (PDI) ligand scaffold, which enables the necessary denticity changes to produce mixed-valent, electron-deficient tethered DNICs. One-electron oxidation of the [{Fe(NO)₂}]₂^10/10 complex Fe₂(^PyrrPDI)(NO)₄ (4) results in NO coupling to form N₂O via the mixed-valent {[Fe(NO)₂]₂}^9/10 species, which possesses an electron-deficient four-coordinate {Fe(NO)₂}¹⁰ site, crucial in N-N bond formation. The hemilability of the PDI scaffold dictates the selectivity in N-N bond formation because stabilization of the five-coordinate {Fe(NO)₂}⁹ site in the mixed-valent [{Fe(NO)₂}]₂^9/10 species, [Fe₂(^Pyr2PDI)(NO)₄][PF₆] (6), does not result in an electron-deficient, four-coordinate {Fe(NO)₂}¹⁰ site, and hence no N-N coupling is observed.

Dinitrosyl Iron Complexes (DNICs). From Spontaneous Assembly to Biological Roles

Dinitrosyl iron complexes (DNICs) are spontaneously and rapidly generated in cells. Their assembly requires nitric oxide (NO), biothiols, and nonheme iron, either labile iron or iron-sulfur clusters. Despite ubiquitous detection by electron paramagnetic resonance in NO-producing cells, the DNIC's chemical biology remains only partially understood. In this Forum Article, we address the reaction mechanisms for endogenous DNIC formation, with a focus on a labile iron pool as the iron source. The capability of DNICs to promote S-nitrosation is discussed in terms of S-nitrosothiol generation associated with the formation and chemical reactivity of DNICs. We also highlight how elucidation of the chemical reactivity and the dynamics of DNICs combined with the development of detection/quantification methods can provide further information regarding their participation in physiological and pathological processes.

Antiferromagnetic Exchange and Metal-Metal Bonding in Roussin's Black Sulfur and Selenium Salts

Atom-efficient syntheses of the tetraethylammonium Roussin black sulfur and selenium salts ((Et₄N)[Fe₄E₃(NO)₇], E = S, Se) as well as their ¹⁵N-labeled counterparts are described herein. Broken-symmetry DFT calculations were conducted on both complexes to model an antiferromagnetic interaction between the apical {FeNO}⁷ unit, S_ap = 3/2, and the three basal {Fe(NO)₂}⁹ units, S_bas = 1/2. The calculated J values are -1813 and -1467 cm^-1 for the sulfur and selenium compounds, respectively. The mechanism for antiferromagnetic exchange in both compounds was deduced to be direct exchange on the basis of the partially overlapping magnetic orbitals with orbital density only residing on the Fe-centers. The obtained Mössbauer parameters are most consistent with the calculated M_S = 0 broken-symmetry state for both complexes. The values for J have been determined with variable-temperature ¹⁵N NMR experiments. Values of -1660 and -1430 cm^-1 for the sulfur and selenium compounds, respectively, were obtained by fits to the variable-temperature NMR data, further validating the broken-symmetry M_S = 0 model of the electronic structure.

Cell-Penetrating Delivery of Nitric Oxide by Biocompatible Dinitrosyl Iron Complex and Its Dermato-Physiological Implications

After the discovery of endogenous dinitrosyl iron complexes (DNICs) as a potential biological equivalent of nitric oxide (NO), bioinorganic engineering of [Fe(NO)₂] unit has emerged to develop biomimetic DNICs [(NO)₂Fe(L)₂] as a chemical biology tool for controlled delivery of NO. For example, water-soluble DNIC [Fe₂(μ-SCH₂CH₂OH)₂(NO)₄] (DNIC-1) was explored for oral delivery of NO to the brain and for the activation of hippocampal neurogenesis. However, the kinetics and mechanism for cellular uptake and intracellular release of NO, as well as the biocompatibility of synthetic DNICs, remain elusive. Prompted by the potential application of NO to dermato-physiological regulations, in this study, cellular uptake and intracellular delivery of DNIC [Fe₂(μ-SCH₂CH₂COOH)₂(NO)₄] (DNIC-2) and its regulatory effect/biocompatibility toward epidermal cells were investigated. Upon the treatment of DNIC-2 to human fibroblast cells, cellular uptake of DNIC-2 followed by transformation into protein-bound DNICs occur to trigger the intracellular release of NO with a half-life of 1.8 ± 0.2 h. As opposed to the burst release of extracellular NO from diethylamine NONOate (DEANO), the cell-penetrating nature of DNIC-2 rationalizes its overwhelming efficacy for intracellular delivery of NO. Moreover, NO-delivery DNIC-2 can regulate cell proliferation, accelerate wound healing, and enhance the deposition of collagen in human fibroblast cells. Based on the in vitro and in vivo biocompatibility evaluation, biocompatible DNIC-2 holds the potential to be a novel active ingredient for skincare products.

Stepwise nitrosylation of the nonheme iron site in an engineered azurin and a molecular basis for nitric oxide signaling mediated by nonheme iron proteins

Mononitrosyl and dinitrosyl iron species, such as {FeNO}⁷, {FeNO}⁸ and {Fe(NO)₂}⁹, have been proposed to play pivotal roles in the nitrosylation processes of nonheme iron centers in biological systems. Despite their importance, it has been difficult to capture and characterize them in the same scaffold of either native enzymes or their synthetic analogs due to the distinct structural requirements of the three species, using redox reagents compatible with biomolecules under physiological conditions. Here, we report the realization of stepwise nitrosylation of a mononuclear nonheme iron site in an engineered azurin under such conditions. Through tuning the number of nitric oxide equivalents and reaction time, controlled formation of {FeNO}⁷ and {Fe(NO)₂}⁹ species was achieved, and the elusive {FeNO}⁸ species was inferred by EPR spectroscopy and observed by Mössbauer spectroscopy, with complemental evidence for the conversion of {FeNO}⁷ to {Fe(NO)₂}⁹ species by UV-Vis, resonance Raman and FT-IR spectroscopies. The entire pathway of the nitrosylation process, Fe(ii) → {FeNO}⁷ → {FeNO}⁸ → {Fe(NO)₂}⁹, has been elucidated within the same protein scaffold based on spectroscopic characterization and DFT calculations. These results not only enhance the understanding of the dinitrosyl iron complex formation process, but also shed light on the physiological roles of nitric oxide signaling mediated by nonheme iron proteins.

Stepwise nitrosylation from Fe(ii) to {FeNO}⁷, {FeNO}⁸ and then to {Fe(NO)₂}⁹ is reported for the first time in the same protein scaffold, providing deeper understanding of the detailed mechanism of dinitrosyl iron complex formation.

Toward Frameworks with Multiple Aligned and Interactive Fe(CO)3 Rotators: Syntheses and Structures of Diiron Complexes Linked by Two trans-Diaxial α,ω-Diphosphine Ligands Ar2P(CH2)nPAr2

Reactions of (η⁴-benzylideneacetone)Fe(CO)₃ and the α,ω-diphosphines Ar₂P(CH₂)_nPAr₂ afford the trigonal bipyramidal diiron tetraphosphorus complexes trans,trans-(CO)₃Fe[Ar₂P(CH₂)_nPAr₂]₂Fe(CO)₃ (n/Ar = 3/Ph 3, 4/Ph 4a, 4/p-tol 4b; 56-19%). Crystal structures establish essentially parallel P-Fe-P axes, iron-iron distances of 5.894(9)-5.782(1) Å (3) and 6.403(1)-6.466(1) Å (4a,b), and van der Waals radii of 4.45 Å for the Fe(CO)₃ rotators, the planes of which are offset by 0.029-1.665 Å. Analogous reactions of Ph₂P(CH₂)₆PPh₂ yield the square pyramidal monoiron complex trans-(CO)₃Fe[Ph₂P(CH₂)₆PPh₂] (6', 31%), a rare case where a diphosphine spans trans basal positions (∠P-Fe-P 147.4(2)°). Both 3 and 6' exhibit two CO ¹³C NMR signals at room temperature, indicating slow exchange on the NMR time scale, which in the former could entail Fe(CO)₃/Fe(CO)₃ gearing. Under analogous conditions, 4a,b exhibit one signal. Previously reported adducts of Fe(CO)₃ and Ph₂P(CH₂)_nPPh₂ are surveyed (1:1, n = 1-5; 2:2, n = 5), and the IR ν_C≡O band patterns and energies of all complexes analyzed with the aid of DFT calculations. The diiron complexes are preferred thermodynamically. Attention is given to limiting types of Fe(CO)₃/Fe(CO)₃ interactions in the diiron complexes.

Molecular understanding of heteronuclear active sites in heme-copper oxidases, nitric oxide reductases, and sulfite reductases through biomimetic modelling

Heme-copper oxidases (HCO), nitric oxide reductases (NOR), and sulfite reductases (SiR) catalyze the multi-electron and multi-proton reductions of O2, NO, and SO32-, respectively. Each of these reactions is important to drive cellular energy production through respiratory metabolism and HCO, NOR, and SiR evolved to contain heteronuclear active sites containing heme/copper, heme/nonheme iron, and heme-[4Fe-4S] centers, respectively. The complexity of the structures and reactions of these native enzymes, along with their large sizes and/or membrane associations, make it challenging to fully understand the crucial structural features responsible for the catalytic properties of these active sites. In this review, we summarize progress that has been made to better understand these heteronuclear metalloenzymes at the molecular level though study of the native enzymes along with insights gained from biomimetic models comprising either small molecules or proteins. Further understanding the reaction selectivity of these enzymes is discussed through comparisons of their similar heteronuclear active sites, and we offer outlook for further investigations.

Biological and Bioinspired Inorganic N-N Bond-Forming Reactions

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N2O), dinitrogen (N2), and hydrazine (N2H4) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes as well as synthetic model complexes.

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.

Structures and Properties of Dinitrosyl Iron and Cobalt Complexes Ligated by Bis(3,5-diisopropyl-1-pyrazolyl)methane

Two dinitrosyl iron and cobalt complexes [Fe(NO)2(L1”)](BF4) and [Co(NO)2(L1”)](BF4) are synthesized and characterized, supported by a less hindered bidentate nitrogen ligand bis(3,5-diisopropyl-1-pyrazolyl)methane (denoted as L1”), are surprisingly stable under argon atmosphere. X-ray structural analysis shows a distorted tetrahedral geometry. Spectroscopic and structural parameters of the dinitrosyl iron and cobalt complexes are consistent with the previous reported {Fe(NO)2}9 and {Co(NO)2}10. Two N–O and M–N(O) stretching frequencies and their magnetic properties are also consistent with the above electronic structural assignments. We explored the dioxygen reactivities of the obtained dinitrosyl complexes. Moreover, the related [FeCl2(L1”)], [Co(NO3)2(L1”)], and [Co(NO2)2(L1”)] complexes are also characterized in detail.

Insight into chalcogenolate-bound {Fe(NO)2}9 dinitrosyl iron complexes (DNICs): covalent character versus ionic character

The synthesis, characterization and transformation of the thermally unstable {Fe(NO)₂}⁹ dinitrosyl iron complex (DNIC) [(OMe)₂Fe(NO)₂]⁻ (2) were investigated.

Hemilabile Proton Relays and Redox Activity Lead to {FeNO} x and Significant Rate Enhancements in NO2- Reduction

Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(PyrrPDI)(CO)2 (3) and Fe(MorPDI)(CO)2 (4) display 40-fold enhancements in the initial rate of NO2- reduction, with respect to Fe(MeOPDI)(CO)2 (7). Utilizing the proper sterics and p Ka of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} x mononitrosyl iron complexes (MNICs) as intermediates in the NO2- reduction reaction. The {FeNO} x species behave spectroscopically and computationally similar to {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand. These {FeNO} x MNICs facilitate enhancements in the initial rate.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate-alkoxide cluster

Cooperative multimetallic electron transfer to accommodate substrate binding.

A series of NO-bound, iron-functionalized polyoxovanadate–alkoxide (FePOV–alkoxide) clusters have been synthesized, providing insight into the role of multimetallic constructs in the coordination and activation of a substrate. Upon exposure of the heterometallic cluster to NO, the vanadium-oxide metalloligand is oxidized by a single electron, shuttling the reducing equivalent to the {FeNO} subunit to form a {FeNO}⁷ species. Four NO-bound clusters with electronic distributions ranging from [VV3VIV2]{FeNO}⁷ to [VIV5]{FeNO}⁷ have been synthesized, and characterized via¹H NMR, infrared, and electronic absorption spectroscopies. The ability of the FePOV–alkoxide cluster to store reducing equivalents in the metalloligand for substrate coordination and activation highlights the ultility of the metal-oxide scaffold as a redox reservoir.

Electrodeposited-film electrodes derived from a precursor dinitrosyl iron complex for electrocatalytic water splitting

In artificial photosynthesis, water splitting plays an important role for the conversion and storage of renewable energy sources.

N2O Formation via Reductive Disproportionation of NO by Mononuclear Copper Complexes: A Mechanistic DFT Study

The mechanism of the copper(I)-mediated reductive disproportionation reaction of NO to form N₂O was investigated for five different 3,5-substituted tris(pyrazolyl)borate copper complexes (CuTp^R₁,R₂) by means of DFT calculations. A thorough search of the potential surface was performed, using the B3LYP functional with the def2-SVP basis set for optimization purposes and def2-TZVP single-point calculations for constructing the potential energy surface for two of these complexes. The results can be condensed into six competing reaction mechanisms, two of which were more closely investigated using full def2-TZVP optimized potential and free energies. The results consistently predict the same mechanism to have the lowest overall barrier. For all five different complexes, this is found to be in good agreement with the experimental reaction barriers. The key intermediate for the transition from the N-bound reactant to the O-bound product contains a stable (NO)₃ unit with one N-Cu and one O-Cu bond, which was not included in the mechanistic considerations reported in the literature. Further analysis of the charge distribution and the spin density demonstrates the formation of a Cu(II)-(N₂O₂^-) intermediate and the electronic influence of the different ligands.

Ligand-based reduction of nitrate to nitric oxide utilizing a proton-responsive secondary coordination sphere

Utilizing the proton-responsive pyridinediimine ligand [(2,6-ⁱPrC₆H₃)(NCMe)(N(ⁱPr)₂C₂H₄)(NCMe)C₅H₃N] (didpa), the ligand-based reduction of nitrate (NO₃⁻) to nitric oxide (NO) was achieved.

A cobalt–nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen

The cobalt–nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3⁻), and shows a linear Co–N–O unit.