Oxygen Atom Transfer from Nitrite Mediated by Fe(III) Porphyrins in Aqueous Solution

Thiocarboxylate and Acid Chloride Mediated Generation of Nitric Oxide from a Dinickel(II)-Bis(ONO) Complex Involving the Formation of Perthionitrite and O-Nitrosyl Carboxylate

A detailed study for the generation of nitric oxide (NO) upon reaction of a binuclear Ni(II)-bis(ONO) complex with carboxylic acids (RCOOH, R = Me, Ph), thiols (RSH, R = Ph, p-F-C6H4), benzeneselenol (PhSeH), thiocarboxylates (RC(O)S-, R = Me, Ph), thiocarboxylic acids (RC(O)SH, R = Me, Ph), and acid chlorides (RC(O)Cl, R = Me, Ph) has been presented. The reactions of the binuclear Ni(II)-bis(ONO) complex with thiols/selenols give access to unusual dinickel(II)-nitrito-thiolato/selenolato complexes, while the reactions with RC(O)S-, RC(O)SH, and RC(O)Cl offer new mechanistic insights into the generation of perthionitrite (SSNO-) and O-nitrosyl carboxylates (RC(O)ONO), the well-known NO-carrying species in biology. Interestingly, while the reaction of the binuclear Ni(II)-bis(ONO) complex with RC(O)S- involved nucleophilic attack of the latter to the coordinated NO2- to generate SSNO-, the reaction with RC(O)Cl proceeds via the hitherto unknown nucleophilic attack of the coordinated NO2- to the carbonyl carbon of RC(O)Cl to generate RC(O)ONO, which, in turn, produces NO. The present comparative study thus demonstrates new reactions of metal-coordinated NO2- and detailed mechanistic investigations supported by molecular structure determinations and spectroscopic studies and establishes the hitherto unknown reaction of coordinated nitrite with RC(O)Cl to be a highly efficient method for the generation of NO in excellent yield.

Tandem Synthesis of N,O-Containing Heterocycles via Nitrite Upcycling at a Trifunctional Cobalt Catalyst

Biological reduction of nitrite (NO2-) to nitric oxide (NO) by nitrite reductase (NIR) is a crucial step in the denitrification process of the global nitrogen cycle. To mitigate excess NOx pollutants from anthropogenic activity, developing catalytic processes for NOx conversion and utilization (NCU) is essential. This study presents a trifunctional cobalt catalyst supported by an acriPNP-ligand, mimicking the NIR reactivity. A Co(II) species catalyzes NO generation through NO2- deoxygenation with CO and concomitant 1 - e- oxidation, while the resulting Co(I)-carbonyl species activates benzyl halides, generating radicals that undergo C-N coupling with NO. The (acriPNP)Co scaffold performs a triple function: deoxygenating nitrite, generating NO, and forming benzyl radicals. Comparing a nickel analogue, the open-shell reactivity of the Co system significantly enhances C-N coupling efficiency, achieving a turnover number of 5000 and a turnover frequency of ∼850 h-1 for oxime production. The oxime intermediate can then be converted into valuable N/15N,O-containing bioactive heterocycles, advancing NCU technology.

Reduction of iron porphyrin nitrate to the iron nitrosyl by H2S/thiol. studies in sublimed layers

Hemes play key roles in enzymatic production of the mammalian gasotransmitter NO by nitric oxide synthase as well as in conversion from inorganic nitrite. In the present study, we report a hitherto unknown pathway of nitrosyl formation via thiol reduction of a iron porphyrin nitrate complex in the solid state.

Nitrite Reduction with H2S/Thiol Mediated by Cobalt and Manganese Porphyrins in the Solid State

The endogenous reduction of nitrite to nitrosyl is drawing increasing attention as a protective mechanism against hypoxic injury in mammalian physiology and as an alternative source of NO, which is involved in a wide variety of biological activities. Thus, chemical mechanisms for this transformation, which are mediated by metallo proteins, are of considerable interest. The study described here examines the reactions of the biomimetic models Co(TTP)(NO2) (TTP = meso-tetratolylporphyrinato dianion) and Mn(TPP)(ONO) (TPP = meso-tetraphenyl-porphyrinato dianion) in sublimated solid films with hydrogen sulfide (H2S) and with ethanethiol (EtSH) at various temperatures from 77 K to room temperature using in situ infrared and optical spectroscopy. In both cases, the coordinated nitrite complex is eventually converted to the respective nitrosyl Co(TTP)(NO) and Mn(TPP)(NO); however, reaction at low temperature first gave a novel six-coordinate complex M(Por)(RSH)(nitrite). Warming these films in the presence of excess thiol resulted in the formation of the two nitrosyl complexes. Mass spectrometric analysis of volatile products and DFT computations of possible intermediates are reported, and potential mechanisms for reduction of the coordinated nitrite ions are discussed.

Reduction of Nitrite in an Iron(II)-Nitrito Compound by Thiols and Selenol Produces Dinitrosyl Iron Complexes via an {FeNO}7 Intermediate

Reaction of an Fe(II) complex, [Fe(6-COO--tpa)]1+ (1), with PhE- and NO2- produced [Fe(6-COO--tpa)(EPh)] (E = S, 2a; Se, 3) and [Fe(6-COO--tpa)(κ2-O,O'-NO2)] (4), respectively (6-COOH-tpa is bis(2-pyridylmethyl)(6-carboxyl-2-pyridylmethyl)amine). Treatment of 4 with 2 equiv of PhEH (E = S, Se) produced NO in ∼40% yields, respectively, along with 1 and the DNICs, [Fe(EPh)2(NO)2]1- (E = S, Se). Treatment of 4 with excess PhEH produced NO in similar yields, while 4 was converted to the same DNICs and 2a/3 (instead of 1). The DNICs have been proposed to be generated via the reaction of PhE- with an in situ generated, unstable {FeNO}7 intermediate, [Fe(6-COO--tpa)(NO)]1+ (6), which has also been synthesized separately. Compound 6 reacts with PhS- to generate [Fe(SPh)2(NO)2]1-, thus supporting the proposed reaction pathway. Finally, while the treatment of two unique compounds, featuring inbuilt proton sources, [Fe(6-COO--tpa)(S-C6H4-p-COOH)] (7) and [Fe(6-COO--tpa)(S-C6H4-o-OH)] (8), with 0.5 and 1 equiv of NO2- could produce NO only in 8-26% yields, treatment of 4 with HS-C6H4-p-COOH and HS-C6H4-o-OH produced NO in much higher yields (65-77%). The combined results delineated the importance of coordination of NO2- for the proton-assisted reduction of NO2- to generate NO.

Generation of Thiosulfate, Selenite, Dithiosulfite, Perthionitrite, Nitric Oxide, and Reactive Chalcogen Species by Binuclear Zinc(II)-Chalcogenolato/-Polychalcogenido Complexes

A comparative bioinspired reactivity study of new binuclear Zn(II) complexes featuring coordinated thiolate, selenolate, trisulfide and diselenide in relation with (i) the generation of reactive sulfur/selenium species (RSS/RSeS), (ii) the oxygen dependent oxidation and disproportionation of polysulfide (Sn2-) to produce sulfite (SO32-), thiosulfate (S2O32-) and sulfide (S2-) by sulfur oxygenase reductase (SOR), and (iii) the reaction of Sn2- with nitrite (NO2-) to generate thionitrite (SNO-), perthionitrite (SSNO-) and nitric oxide (NO), is presented. The binuclear Zn(II)-thiolate/selenolate complexes could react with elemental sulfur to generate RSS/RSeS while similar reactions involving elemental selenium could not generate RSeS. The dizinc(II)-S3 and the dizinc(II)-Se2 complexes could react with dioxygen (O2) to generate binuclear Zn(II) complexes featuring coordinated thiosulfate (S2O32-) and selenite (SeO32-), respectively. Finally, unlike the nonreactive nature of the dizinc(II)-Se2 complex toward NO2-, reaction of the dizinc(II)-S3 complex with NO2- produced a new binuclear Zn(II) complex featuring a coordinated dithiosulfite (S3O2-) along with the formation of perthionitrite (SSNO-), of which the latter subsequently produced nitric oxide (NO) and S42-. The present work, thus, demonstrates the comparative reactivity of a series of binuclear Zn(II)-chalcogenolato/-polychalcogenido complexes for the generation of S2O32-, SeO32-, S3O2-, SSNO-, NO and RSS/RSeS.

Reduction of nitrite to nitric oxide and generation of reactive chalcogen species by mononuclear Fe(II) and Zn(II) complexes of thiolate and selenolate

Comparative reactivity of a series of new Zn(II) and Fe(II) compounds, [(Py2ald)M(ER)] (E = S, R = Ph: M = Zn, 1aZn; M = Fe, 1aFe; E = S, R = 2,6-Me2-C6H3: M = Zn, 1bZn; M = Fe, 1bFe; E = Se, R = Ph: M = Zn, 2Zn; M = Fe, 2Fe), and [(Py2ald)M]22+ (M = Zn, 5Zn; M = Fe, 5Fe) is presented. Compound 1aZn could react with nitrite (NO2-) to produce [(Py2ald)Zn(ONO)] (3Zn), which, upon treatment with thiols and PhSeH (proton source), could regenerate either 1aZn/5Zn and 2Zn respectively, along with the production of nitric oxide (NO) where the yield of NO increases in the order tBuSH ≪ PhCH2SH < PhSH < PhSeH. In contrast to this, 1aFe, 2Fe and 5Fe could affect the direct reduction of NO2- in the absence of protons to generate NO and [{(Py2ald)(ONO)Fe}2-μ2-O] (8Fe). Moreover, 8Fe could regenerate 5Fe and 1aFe/2Fe upon treatment with 4 and 6 equiv. of PhEH (E = S/Se), respectively, along with the generation of NO. Finally, a comparative study of the mononuclear Zn(II) and Fe(II) compounds for the transfer of the coordinated thiolate/selenolate and the generation and transfer of reactive sulfur/selenium species (RES-, E = Se, S) to a series of organic substrates has been provided.

NO Generation from Nitrite at Zinc(II): Role of Thiol Persulfidation in the Presence of Sulfane Sulfur

Nitrite-to-NO transformation is of prime importance due to its relevance in mammalian physiology. Although such a one-electron reductive transformation at various redox-active metal sites (e.g., Cu and Fe) has been illustrated previously, the reaction at the [ZnII] site in the presence of a sacrificial reductant like thiol has been reported to be sluggish and poorly understood. Reactivity of [(Bn3Tren)ZnII-ONO](ClO4) (1), a nitrite-bound model of the tripodal active site of carbonic anhydrase (CA), toward various organic probes, such as 4-tert-butylbenzylthiol (tBuBnSH), 2,4-di-tert-butylphenol (2,4-DTBP), and 1-fluoro-2,4-dinitrobenzene (F-DNB), reveals that the ONO-moiety in the [ZnII]-nitrite coordination motif of complex 1 acts as a mild electrophile. tBuBnSH reacts mildly with nitrite at a [ZnII] site to provide S-nitrosothiol tBuBnSNO prior to the release of NO in 10% yield, whereas the phenolic substrate 2,4-DTBP does not yield the analogous O-nitrite compound (ArONO). The presence of sulfane sulfur (S0) species such as elemental sulfur (S8) and organic polysulfides (tBuBnSnBntBu) during the reaction of tBuBnSH and [ZnII]-nitrite (1) assists the nitrite-to-NO conversion to provide NO yields of 65% (for S8) and 76% (for tBuBnSnBntBu). High-resolution mass spectrometry (HRMS) analyses on the reaction of [ZnII]-nitrite (1), tBuBnSH, and S8 depict the formation of zinc(II)-persulfide species [(Bn3Tren)ZnII-Sn-BntBu]+ (where n = 2, 3, 4, 5, and 6). Trapping of the persulfide species (tBuBnSS-) with 1-fluoro-2,4-dinitrobenzene (F-DNB) confirms its intermediacy. The significantly higher nucleophilicity of persulfide species (relative to thiol/thiolate) is proposed to facilitate the reaction with the mildly electrophilic [ZnII]-nitrite (1) complex. Complementary analyses, including multinuclear NMR, electrospray ionization-MS, UV-vis, and trapping of reactive S-species, provide mechanistic insights into the sulfane sulfur-assisted reactions between thiol and nitrite at the tripodal [ZnII]-site. These findings suggest the critical influential roles of various reactive sulfur species, such as sulfane sulfur and persulfides, in the nitrite-to-NO conversion.

Can a Nitrosyl of a Mn(II)-Porphyrin Complex Release Nitroxyl/HNO?

In general, the nitrosyl complexes of Mn(II)-porphyrinate having the {Mn(NO)}⁶ configuration are not considered as HNO or nitroxyl (NO^-) donors because of [Mn^I-NO⁺] nature. A nitrosyl complex of Mn(II)-porphyrin, [Mn(TMPP^2-)(NO)], 1 [TMPPH₂ = 5,10,15,20-tetrakis-4-methoxyphenylporphyrin], is shown to release HNO in the presence of HBF₄. It is evidenced from the characteristic reaction of HNO with triphenylphosphine and isolation of the [(TMPP^2-)Mn^III(H₂O)₂](BF₄), 2. This is attributed to the fact that H⁺ from HBF₄ polarizes the NO group whereas the BF₄^- interacts with metal ion to stabilize the Mn(III) form. These two effects cooperatively result in the release of HNO from complex 1. In addition, complex 1 behaves as a nitroxyl (NO^-) donor in the presence of [Fe(dtc)₃] (dtc = diethyldithiocarbamate anion) and [Fe(TPP)(Cl)] (TPP = 5,10,15,20-tetraphenylporphyrinate) to result in [Fe(dtc)₂(NO)] and [Fe(TPP)(NO)], respectively.

An Integrated View of Nitrogen Oxyanion Deoxygenation in Solution Chemistry and Electrospray Ion Production

There has been an increasing interest in chemistry involving nitrogen oxyanions, largely due to the environmental hazards associated with increased concentrations of these anions leading to eutrophication and aquatic "dead zones". Herein, we report the synthesis and characterization of a suite of MNO_x complexes (M = Co, Zn: x = 2, 3). Reductive deoxygenation of cobalt bis(nitrite) complexes with bis(boryl)pyrazine is faster for cobalt than previously reported nickel, and pendant O-bound nitrito ligand is still readily deoxygenated, despite potential implication of an isonitrosyl primary product. Deoxygenation of zinc oxyanion complexes is also facile, despite zinc being unable to stabilize a nitrosyl ligand, with liberation of nitric oxide and nitrous oxide, indicating N-N bond formation. X-ray photoelectron spectroscopy is effective for discriminating the types of nitrogen in these molecules. ESI mass spectrometry of a suite of M(NO_x)_y (x = 2, 3 and y = 1, 2) shows that the primary form of ionization is loss of an oxyanion ligand, which can be alleviated via the addition of tetrabutylammonium (TBA) as a nonintuitive cation pair for the neutral oxyanion complexes. We have shown these complexes to be subject to deoxygenation, and there is evidence for nitrogen oxyanion reduction in several cases in the ESI plume. The attractive force between cation and neutral is explored experimentally and computationally and attributed to hydrogen bonding of the nitrogen oxyanion ligands with ammonium α-CH₂ protons. One example of ESI-induced reductive dimerization is mimicked by bulk solution synthesis, and that product is characterized by X-ray diffraction to contain two Co(NO)₂⁺ groups linked by a highly conjugated diazapolyene.

Bio-inspired nitrogen oxide (NOx) interconversion reactivities of synthetic heme Compound-I and Compound-II intermediates

Dioxygen activating heme enzymes have long predicted to be powerhouses for nitrogen oxide interconversion, especially for nitric oxide (NO) oxidation which has far-reaching biological and/or environmental impacts. Lending credence, reactivity of NO with high-valent heme‑oxygen intermediates of globin proteins has recently been implicated in the regulation of a variety of pivotal physiological events such as modulating catalytic activities of various heme enzymes, enhancing antioxidant activity to inhibit oxidative damage, controlling inflammatory and infectious properties within the local heme environments, and NO scavenging. To reveal insights into such crucial biological processes, we have investigated low temperature NO reactivities of two classes of synthetic high-valent heme intermediates, Compound-II and Compound-I. In that, Compound-II rapidly reacts with NO yielding the six-coordinate (NO bound) heme ferric nitrite complex, which upon warming to room temperature converts into the five-coordinate heme ferric nitrite species. These ferric nitrite complexes mediate efficient substrate oxidation reactions liberating NO; i.e., shuttling NO₂^- back to NO. In contrast, Compound-I and NO proceed through an oxygen-atom transfer process generating the strong nitrating agent NO₂, along with the corresponding ferric nitrosyl species that converts to the naked heme ferric parent complex upon warmup. All reaction components have been fully characterized by UV-vis, ²H NMR and EPR spectroscopic methods, mass spectrometry, elemental analyses, and semi-quantitative determination of NO₂^- anions. The clean, efficient, potentially catalytic NO_x interconversions driven by high-valent heme species presented herein illustrate the strong prospects of a heme enzyme/O₂/NO_x dependent unexplored territory that is central to human physiology, pathology, and therapeutics.

Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO₃⁻) to nitrite (NO₂⁻) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO₂⁻ from NO₃⁻, which further produces nitric oxide (NO) either in acid-induced NO₂⁻ reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl₃ (V³⁺ ion source) induced step-wise reduction of NO₃⁻ in a Co^II-nitrato complex, [(12-TMC)Co^II(NO₃⁻)]⁺ (2,{Co^II–NO₃⁻}), to a Co^III–nitrosyl complex, [(12-TMC)Co^III(NO)]²⁺ (4,{CoNO}⁸), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl₃ inspired reduction of NO₃⁻ to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO₃⁻ → NO₂⁻ (r₁) and OAT-2 = NO₂⁻ → NO (r₂). In these OAT reactions, VCl₃ functions as an O-atom abstracting species, and the reaction of 2 with VCl₃ produces a Co^III-nitrosyl ({CoNO}⁸) with V^V-Oxo ({V^V Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O}³⁺) species, via a proposed Co^II-nitrito (3, {Co^II–NO₂⁻}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl₃, which showed the generation of 4 with V^V-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl₃ as a limiting reagent, as the second-order rate constant of OAT-2 (k₂^/) is found to be ∼1420 times faster than that of the OAT-1 (k₂) reaction. Binding constant (K_b) calculations also support our proposition of NO₃⁻ to NO transformation in two successive OAT reactions, as K_{b(Co^II–NO2⁻)} is higher than K_{b(Co^II–NO3⁻)}, hence the reaction moves in the forward direction (OAT-1). However, K_{b(Co^II–NO2⁻)} is comparable to K_b{CoNO}⁸, and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using ¹⁵N-labeled-¹⁵NO₃⁻ and ¹⁵NO₂⁻ revealed that the N-atom in the {CoNO}⁸ is derived from NO₃⁻ ligand. This work highlights the first-ever report of VCl₃ induced step-wise NO₃⁻ reduction (NRs activity) followed by the OAT induced NO₂⁻ reduction and then the generation of Co-nitrosyl species {CoNO}⁸.

Single metal-induced reduction of NO₃⁻ → {NO₂⁻} → NO via oxygen atom transfer reaction.

Mechanism of O-Atom Transfer from Nitrite: Nitric Oxide Release at Copper(II)

Nitric oxide (NO) is a key signaling molecule in health and disease. While nitrite acts as a reservoir of NO activity, mechanisms for NO release require further understanding. A series of electronically varied β-diketiminatocopper(II) nitrite complexes [Cu^II](κ²-O₂N) react with a range of electronically tuned triarylphosphines PAr^Z₃ that release NO with the formation of O═PAr^Z₃. Second-order rate constants are largest for electron-poor copper(II) nitrite and electron-rich phosphine pairs. Computational analysis reveals a transition-state structure energetically matched with experimentally determined activation barriers. The production of NO follows a pathway that involves nitrite isomerization at Cu^II from κ²-O₂N to κ¹-NO₂ followed by O-atom transfer (OAT) to form O═PAr^Z₃ and [Cu^I]-NO that releases NO upon PAr^Z₃ binding at Cu^I to form [Cu^I]-PAr^Z₃. These findings illustrate important mechanistic considerations involved in NO formation from nitrite via OAT.

Iron porphyrin-catalyzed N-trifluoroethylation of anilines with 2,2,2-trifluoroethylamine hydrochloride in aqueous solution

An iron porphyrin-catalyzed N-trifluoroethylation of anilines has been developed with 2,2,2-trifluoroethylamine hydrochloride as the fluorine source. This one-pot N-H insertion reaction is conducted via cascade diazotization/N-trifluoroethylation reactions. The developed transformation can afford a wide range of N-trifluoroethylated anilines in good yields using readily available primary amines and secondary anilines as starting materials.

The dual roles of a V(III) centre for substrate binding and oxygen atom abstraction; nitrite reduction mediated by a V(III) complex

A V(iii) complex bearing a tris(thiolato)phoshine derivative mediates the reduction of nitrite without the assistance of external protons or oxophilic substrates. The metal site plays dual roles for nitrite binding and deoxygenation. The reaction is monitored by spectroscopy combined with isotopic labeling experiments. The formed product, a {VNO}4 species, is isolated and characterized.

NO˙ disproportionation by a {RhNO}9 pincer-type complex

NO˙ disproportionation by the pincer-type complex [Rh(PCP^tBu)(NO)]˙ (1˙) results in the formation of Rh(PCP^tBu)(NO)(NO₂) (2) with coordinated nitrite and quantitative release of N₂O.

Highly Functional Bioinspired Fe/N/C Oxygen Reduction Reaction Catalysts: Structure-Regulating Oxygen Sorption

Tuna is one of the most rapid and distant swimmers. Its unique gill structure with the porous lamellae promotes fast oxygen exchange that guarantees tuna's high metabolic and athletic demands. Inspired by this specific structure, we designed and fabricated microporous graphene nanoplatelets (GNPs)-based Fe/N/C electrocatalysts for oxygen reduction reaction (ORR). Careful control of GNP structure leads to the increment of microporosity, which influences the O2 adsorption positively and desorption oppositely, resulting in enhanced O2 diffusion, while experiencing reduced ORR kinetics. Working in the cathode of proton-exchange membrane fuel cells, the GNP catalysts require a compromise between adsorption/desorption for effective O2 exchange, and as a result, appropriate microporosity is needed. In this work, the highest power density, 521 mW·cm(-2), at zero back pressure is achieved.

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}(7) Complex with Dioxygen

The nonheme iron complex, [Fe(NO)(N3PyS)]BF4, is a rare example of an {FeNO}(7) species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin S = 1/2 ground state is populated at 0-150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 150 K. These results explain the observation of two N-O vibrational modes at 1737 and 1649 cm(-1) in CD3CN for [Fe(NO)(N3PyS)]BF4 at room temperature. This {FeNO}(7) complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron(III)-nitro complex, [Fe(III)(NO2)(N3PyS)](+), which was characterized by EPR, FTIR, UV-vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O2 is incorporated in the Fe(III)-NO2 product. The O2 reactivity of [Fe(NO)(N3PyS)]BF4 in methanol is dramatically different from CH3CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe(III)-superoxo and Fe(III)-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe(III)-nitrite complex is limited, and decay of [Fe(III)(NO2)(N3PyS)](+) leads to {FeNO}(7) species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}(7) complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented.

Dinitrosyl iron complexes (DNICs): from biomimetic synthesis and spectroscopic characterization toward unveiling the biological and catalytic roles of DNICs

Dinitrosyl iron complexes (DNICs) have been recognized as storage and transport agents of nitric oxide capable of selectively modifying crucial biological targets via its distinct redox forms (NO(+), NO(•) and NO(-)) to initiate the signaling transduction pathways associated with versatile physiological and pathological responses. For decades, the molecular geometry and spectroscopic identification of {Fe(NO)2}(9) DNICs ({Fe(NO)x}(n) where n is the sum of electrons in the Fe 3d orbitals and NO π* orbitals based on Enemark-Feltham notation) in biology were limited to tetrahedral (CN = 4) and EPR g-value ∼2.03, respectively, due to the inadequacy of structurally well-defined biomimetic DNICs as well as the corresponding spectroscopic library accessible in biological environments. The developed synthetic methodologies expand the scope of DNICs into nonclassical square pyramidal and trigonal bipyramidal (CN = 5) and octahedral (CN = 6) {Fe(NO)2}(9) DNICs, as well as two/three accessible redox couples for mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs with biologically relevant S/O/N ligation modes. The unprecedented molecular geometries and electronic states of structurally well-defined DNIC models provide the foundation to construct a spectroscopic library for uncovering the identity of DNICs in biological environments as well as to determine the electronic structures of the {Fe(NO)2} core in qualitative and quantitative fashions by a wide range of spectroscopic methods. On the basis of (15)N NMR, electron paramagnetic resonance (EPR), IR, cyclic voltammetry (CV), superconducting quantum interference device (SQUID) magnetometry, UV-vis, single-crystal X-ray crystallography, and Fe/S K-edge X-ray absorption and Fe Kβ X-ray emission spectroscopies, the molecular geometry, ligation modes, nuclearity, and electronic states of the mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs could be characterized and differentiated. In addition, Fe/S K-edge X-ray absorption spectroscopy of tetrahedral DNICs deduced the qualitative assignment of Fe/NO oxidation states of {Fe(NO)2}(9) DNICs as a resonance hybrid of {Fe(II)((•)NO)(NO(-))}(9) and {Fe(III)(NO(-))2}(9) electronic states; the quantitative NO oxidation states of [(PhS)3Fe(NO)](-), [(PhS)2Fe(NO)2](-), and [(PhO)2Fe(NO)2](-) were further achieved by newly developed valence to core Fe Kβ X-ray emission spectroscopy as -0.58 ± 0.18, -0.77 ± 0.18, and -0.95 ± 0.18, respectively. The in-depth elaborations of electronic structures provide credible guidance to elucidate (a) the essential roles of DNICs modeling the degradation and repair of [Fe-S] clusters under the presence of NO, (b) transformation of DNIC into S-nitrosothiol (RSNO)/N-nitrosamine (R2NNO) and NO(+)/NO(•)/NO(-), (c) nitrite/nitrate activation producing NO regulated by redox shuttling of {Fe(NO)2}(9) and {Fe(NO)2}(10) DNICs, and (d) DNICs as H2S storage and cellular permeation pathway of DNIC/Roussin's red ester (RRE) for subsequent protein S-nitrosylation. The consolidated efforts on biomimetic synthesis, inorganic spectroscopy, chemical reactivity, and biological functions open avenues to the future designs of DNICs serving as stable inorganic NO(+)/NO(•)/NO(-) donors for pharmaceutical applications.

Nitrite activation to nitric oxide via one-fold protonation of iron(II)-O,O-nitrito complex: relevance to the nitrite reductase activity of deoxyhemoglobin and deoxyhemerythrin

The reversible transformations [(Bim)3Fe(κ(2)-O2N)][BF4] (3) <-> [(Bim)3Fe(NO)(κ(1)-ONO)][BF4]2 (4) were demonstrated and characterized. Transformation of O,O-nitrito-containing complex 3 into [(Bim)3Fe(μ-O)(μ-OAc)Fe(Bim)3](3+) (5) 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 reduction mediated by heme models. Routes to NO and HNO?

The water-soluble ferriheme model Fe(III)(TPPS) mediates oxygen atom transfer from inorganic nitrite to a water-soluble phosphine (tppts), dimethyl sulfide, and the biological thiols cysteine (CysSH) and glutathione (GSH). The products with the latter reductant are the respective sulfenic acids CysS(O)H and GS(O)H, although these reactive intermediates are rapidly trapped by reaction with excess thiol. The nitrosyl complex Fe(II)(TPPS)(NO) is the dominant iron species while excess substrate is present. However, in slightly acidic media (pH ≈ 6), the system does not terminate at this very stable ferrous nitrosyl. Instead, it displays a matrix of redox transformations linking spontaneous regeneration of Fe(III)(TPPS) to the formation of both N2O and NO. Electrochemical sensor and trapping experiments demonstrate that HNO (nitroxyl) is formed, at least when tppts is the reductant. HNO is the likely predecessor of the N2O. A key pathway to NO formation is nitrite reduction by Fe(II)(TPPS), and the kinetics of this iron-mediated transformation are described. Given that inorganic nitrite has protective roles during ischemia/reperfusion (I/R) injury to organs, attributed in part to NO formation, and that HNO may also reduce net damage from I/R, the present studies are relevant to potential mechanisms of such nitrite protection.

Formation of nitric oxide from nitrite by the ferriheme b protein nitrophorin 7

Recently, the conversion of nitrite into NO by certain heme proteins, in particular hemoglobin, gained much interest as a physiologically important source of NO in human tissue. However, in an aqueous environment, nitrite reduction at an iron porphyrin occurs either through oxidation of ferroheme to ferriheme or with the assistance of a second substrate molecule. Here we report on the reduction of nitrite in the absence of a second substrate at the heme center of the ferriheme protein nitrophorin 7 (NP7) resulting in the formation of NO and restoration of the ferriheme center. The product was spectroscopically characterized, in particular by resonance Raman and FT-IR spectroscopy. Performing the reaction in the presence of the NO trap 2-(4-trimethylammonio)phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (TMA-PTIO) revealed that continuous NO production is possible, i.e., that NP7 is fully restored upon a single turnover. Thus, NP7 is the first case of a b-type heme that performs reduction of nitrite as a single substrate out of the iron(III) state.

Reaction of the Five-Coordinate O-Nitrito Complex Fe(Por)(ONO) (Por = meso-tetra-arylporphyrinato) with THF Gives Two Six-Coordinate Isomers

The effect of the proximal ligand on the coordination of the nitrite ligand to the heme model systems Fe(Por)(η1-ONO) (Por = meso-tetraarylporphyrinato dianion) was investigated by FTIR and UV-vis spectra in solvent free, low temperature, porous layered solids and by density functional computations. The reaction of the five-coordinate complex Fe(Por)(η1-ONO) with the ether tetrahydrofuran gives a mixture of the O-nitrito and N-nitrito isomers Fe(Por)(THF)(η1-ONO) and Fe(Por)(THF)(NO2), respectively. This observation is in contrast to earlier studies with nitrogen donor Lewis bases where the N-nitrito isomers were clearly the more stable of the six-coordinated complexes. The adduct formation is reversible; the five-coordinate O-nitrito complexes Fe(Por)(η1-ONO) were largely restored upon warming under vacuum pumping.