FTIR investigation of the intermediates formed in the reaction of nitroprusside and thiolates

Iron(II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and S-Nitrosothiols through Reversible Fe-S Interaction

Heme and non-heme iron in biology mediate the storage/release of NO^• from S-nitrosothiols as a means to control the biological concentration of NO^•. Despite their importance in many physiological processes, the mechanisms of N-S bond formation/cleavage at Fe centers have been controversial. Herein, we report the interconversion of NO^• and S-nitrosothiols mediated by Fe^II/Fe^III chloride complexes. The reaction of 2 equiv of S-nitrosothiol (Ph₃CSNO) with [Cl₆Fe^II₂]^2- results in facile release of NO^• and formation of iron(III) halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and Mössbauer spectroscopy, support the interaction of the S atom with the Fe^II center. This is in contrast to the proposed mechanism of NO^• release from the well-studied "red product" κ¹-N bound S-nitrosothiol Fe^II complex, [(CN)₅Fe(κ¹-N-RSNO)]^3-. Additionally, Fe^III chloride can mediate NO^• storage through the formation of S-nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO^• regenerates Ph₃CSNO with the Fe^II source trapped as the S = 3/2 {FeNO}⁷ species [Cl₃FeNO]^-, which is inert toward further coordination and activation of S-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO^•/thiolate and S-nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO^•.

Solving the 170-Year-Old Mystery About Red-Violet and Blue Transient Intermediates in the Gmelin Reaction

The Gmelin reaction between nitroprusside and sulfides in aqueous solution is known to produce two transient intermediates with distinct colors: an initial red-violet intermediate that subsequently converts into a blue intermediate. In this work, we use a combination of multinuclear ((17) O, (15) N, (13) C) NMR, UV/Vis, IR spectroscopic techniques and quantum chemical computation to show unequivocally that the red-violet intermediate is [Fe(CN)5 N(O)S](4-) and the blue intermediate is [Fe(CN)5 N(O)SS)](4-) . While the formation of [Fe(CN)5 N(O)S](4-) has long been postulated in the literature, this study provides the most direct proof of its structure. In contrast, [Fe(CN)5 N(O)SS)](4-) represents the first example of any metal coordination complex containing a perthionitro ligand. The new reaction pathways found in this study not only provide clues for the mode of action of nitroprusside for its pharmacological activity, but also have broader implications to the biological role of H2 S, potential reactions between H2 S and nitric oxide donor compounds, and the possible biological function of polysulfides.

Direct NMR detection of the unstable “red product” from the reaction between nitroprusside and 2-mercaptosuccinic acid

First NMR characterization of the unstable “red product” produced from the reaction between nitroprusside and organic thiolates.

Reactivity of iron(II)-bound nitrosyl hydride (HNO, nitroxyl) in aqueous solution

The reactivity of coordinated nitroxyl (HNO) has been explored with the [Fe(II)(CN)(5)HNO](3-) complex in aqueous medium, pH 6. We discuss essential biorelevant issues as the thermal and photochemical decompositions, the reactivity toward HNO dissociation, the electrochemical behavior, and the reactions with oxidizing and reducing agents. The spontaneous decomposition in the absence of light yielded a two-electron oxidized species, the nitroprusside anion, [Fe(II)(CN)(5)NO](2-), and a negligible quantity of N(2)O, with k(obs)≈5×10(-7)s(-1), at 25.0°C. The value of k(obs) represents an upper limit for HNO release, comparable to values reported for other structurally related L ligands in the [Fe(II)(CN)(5)L](n-) series. These results reveal that the FeN bond is strong, suggesting a significant σ-π interaction, as already postulated for other HNO-complexes. The [Fe(II)(CN)(5)HNO](3-) ion showed a quasi-reversible oxidation wave at 0.32 V (vs normal hydrogen electrode), corresponding to the [Fe(II)(CN)(5)HNO](3-)/[Fe(II)(CN)(5)NO](3-),H(+) redox couple. Hexacyanoferrate(III), methylviologen and the nitroprusside ion have been selected as potential oxidants. Only the first reactant achieved a complete oxidation process, initiated by a proton-coupled electron transfer reaction at the HNO ligand, with nitroprusside as a final oxidation product. Dithionite acted as a reductant of [Fe(II)(CN)(5)HNO](3-), in a 4-electron process, giving NH(3). The high stability of bound HNO may resemble the properties in related Fe(II) centers of redox active enzymes. The very minor release of N(2)O shows that the redox conversions may evolve without disruption of the FeN bonds, under competitive conditions with the dissociation of HNO.

Disproportionation of O-methylhydroxylamine catalyzed by aquapentacyanoferrate(II)

The aquapentacyanoferrate(II) ion, [Fe(II)(CN)(5)H(2)O](3-), catalyzes the disproportionation reaction of O-methylhydroxylamine, NH(2)OCH(3), with stoichiometry 3NH(2)OCH(3) → NH(3) + N(2) + 3CH(3)OH. Kinetic and spectroscopic evidence support an initial N coordination of NH(2)OCH(3) to [Fe(II)(CN)(5)H(2)O](3-) followed by a homolytic scission leading to radicals [Fe(II)(CN)(5)(•)NH(2)](3-) (a precursor of Fe(III) centers and bound NH(3)) and free methoxyl, CH(3)O(•), thus establishing a radical path leading to N-methoxyamino ((•)NHOCH(3)) and 1,2-dimethoxyhydrazine, (NHOCH(3))(2). The latter species is moderately stable and proposed to be the precursor of N(2) and most of the generated CH(3)OH. Intermediate [Fe(III)(CN)(5)L](2-) complexes (L = NH(3), H(2)O) form dinuclear cyano-bridged mixed-valent species, affording a catalytic substitution of the L ligands promoted by [Fe(II)(CN)(5)L](3-). Free or bound NH(2)OCH(3) may act as reductants of [Fe(III)(CN)(5)L](2-), thus regenerating active sites. At increasing concentrations of NH(2)OCH(3) a coordinated diazene species emerges, [Fe(II)(CN)(5)N(2)H(2)](3-), which is consumed by the oxidizing CH(3)O(•), giving N(2) and CH(3)OH. Another side reaction forms [Fe(II)(CN)(5)N(O)CH(3)](3-), an intermediate containing the nitrosomethane ligand, which is further oxidized to the nitroprusside ion, [Fe(II)(CN)(5)NO](2-). The latter is a final oxidation product with a significant conversion of the initial [Fe(II)(CN)(5)H(2)O](3-) complex. The side reaction partially blocks the Fe(II)-aqua active site, though complete inhibition is not achieved because the radical path evolves faster than the formation rates of the Fe(II)-NO(+) bonds.

Competitive adsorption study of CO2 and SO2 on CoII3[CoIII(CN)6]2 using DRIFTS

Diffuse reflectance infrared Fourier transform spectroscopy was used to study the competitive adsorption of CO(2) and SO(2) on the cobalt Prussian blue analogue Co(II)(3)[Co(III)(CN)(6)](2) at 298 K. Characteristic peaks for adsorbed CO(2) and SO(2) species were identified and their relative areas, measured simultaneously as a function of pressure at 298 K, varied in accordance with a Langmuir-Freundlich isotherm fitted to both gases in the low-coverage Henry's Law limit. Evidence for co-adsorption of trace water was also obtained, as well as the apparent formation of an analogous cobalt nitroprusside compound as a reaction product under certain conditions. The several aspects of the adsorption of CO(2) and SO(2) determined in this work point to an important role for real-time diffuse reflectance infrared measurements in adsorption studies, particularly in the case of competitive adsorption where the occurrence and fate of molecular-level markers arising from more than one adsorbed species can be monitored simultaneously. Depending on the application, this may more than offset certain quantitative limitations of the technique that confine measurements to a relatively narrow set of experimental conditions and demand careful consideration of the effects of sample preparation and treatment.

Catalytic disproportionation of N-alkylhydroxylamines bound to pentacyanoferrates

The substituted hydroxylamines, CH(3)N(H)OH (N-methylhydroxylamine) and (CH(3))(2)NOH (N,N-dimethylhydroxylamine), disproportionate catalytically to the corresponding alkylamines and oxidation products, only in the presence of [Fe(CN)(5)H(2)O](3-). Substitution kinetic measurements suggest an initial coordination step to Fe(ii). Two parallel N- and O-coordination modes are considered with the subsequent formation of Fe(iii), free aminyl (RNCH(3)) and nitroxide (RN(CH(3))O) radicals (R = H, CH(3)). With CH(3)N(H)OH, bound nitrosomethane, CH(3)NO, has been characterized by UV-visible and IR spectroscopies. The mechanism is discussed on the basis of common and differential features with respect to the disproportionation of hydroxylamine catalyzed by the same Fe-fragment.

Performing organic chemistry with inorganic compounds: electrophilic reactivity of selected nitrosyl complexes

The inorganic nitrosyl (NO(+)) complexes [Fe(CN) 5NO](2-), [Ru(bpy)2(NO)Cl](2+), and [IrCl 5(NO)](-) are useful reagents for the nitrosation of a variety of organic compounds, ranging from amines to the relatively inert alkenes. Regarding [IrCl 5(NO)](-), its high electrophilicity and inertness define it as a unique reagent and provide a powerful synthetic route for the isolation and stabilization of coordinated nitroso compounds that are unstable in free form, such as S-nitrosothiols and primary nitrosamines. Related to the high electrophilicity of [IrCl 5(NO)](-), an unusual behavior is described for its PPh 4(+) salt in the solid state, showing an electronic distribution represented by Ir(IV)-NO(*) instead of Ir (III)-NO(+) (as for the K(+) and Na(+) salts).

A Surprisingly Stable S-Nitrosothiol Complex

In this work we present for the first time an X-ray structure of a coordinated S-nitrosothiol obtained by reaction of the extremely reactive K[IrCl5NO] with benzylmercaptan in acetonitrile. This surprisingly stable compound, trans-K[IrCl4(CH3CN)N(O)SCH2Ph], was isolated in high yield (80%) and fully characterized by FTIR, 1H NMR, and ESI-MS. To our knowledge this is the first example of a coordinated S-nitrosothiol that has been isolated.

The Reactions of Nitrosyl Complexes with Cysteine

The reaction kinetics of a set of ruthenium nitrosyl complexes, {(X)5MNO}n, containing different coligands X (polypyridines, NH3, EDTA, pz, and py) with cysteine (excess conditions), were studied by UV-vis spectrophotometry, using stopped-flow techniques, at an appropriate pH, in the range 3-10, and T = 25 degrees C. The selection of coligands afforded a redox-potential range from -0.3 to +0.5 V (vs Ag/AgCl) for the NO+/NO bound couples. Two intermediates were detected. The first one, I1, appears in the range 410-470 nm for the different complexes and is proposed to be a 1:1 adduct, with the S atom of the cysteinate nucleophile bound to the N atom of nitrosyl. The adduct formation step of I1 is an equilibrium, and the kinetic rate constants for the formation and dissociation of the corresponding adducts were determined by studying the cysteine-concentration dependence of the formation rates. The second intermediate, I2, was detected through the decay of I1, with a maximum absorbance at ca. 380 nm. From similar kinetic results and analyses, we propose that a second cysteinate adds to I1 to form I2. By plotting ln k1(RS-) and ln k2(RS-) for the first and second adduct formation steps, respectively, against the redox potentials of the NO+/NO couples, linear free energy plots are obtained, as previously observed with OH- as a nucleophile. The addition rates for both processes increase with the nitrosyl redox potentials, and this reflects a more positive charge at the electrophilic N atom. In a third step, the I2 adducts decay to form the corresponding Ru-aqua complexes, with the release of N2O and formation of cystine, implying a two-electron process for the overall nitrosyl reduction. This is in contrast with the behavior of nitroprusside ([Fe(CN)5NO]2-; NP), which always yields the one-electron reduction product, [Fe(CN)5NO]3-, either under substoichiometric or in excess-cysteine conditions.

Sodium nitroprusside: mechanism of NO release mediated by sulfhydryl-containing molecules

Sodium nitroprusside (SNP) is among the most widely studied nitric oxide donors, and its capability of producing NO seems to depend on its interaction with sulfhydryl-containing molecules present in vivo. The aim of this research has been the study of the mechanism of interaction between SNP and sulfhydryl-containing compounds, such as cysteine and glutathione, through detection by EPR, UV-vis, and IR spectroscopy of both the radical and nonradical species involved. An electron-transfer process can be invoked as the key step, which leads to the formation of the reduced SNP radical, the main detectable radical intermediate, and the corresponding S-nitrosothiol, the ending product of NO that can be considered the real storage and transporters of NO. When cysteine was used, a second radical species (A) is detectable: it can be accounted for by the interaction of a byproduct with unreacted cysteine.

Release of NO from Reduced Nitroprusside Ion. Iron-Dinitrosyl Formation and NO-Disproportionation Reactions

The kinetics and mechanism of the thermal decomposition of the one-electron reduction product of [Fe(CN)(5)NO](2-) (nitroprusside ion, NP) have been studied by using UV-vis, IR, and EPR spectroscopy and mass-spectrometric and electrochemical techniques in the pH range of 4-10. The reduction product contains an equilibrium mixture of [Fe(CN)(4)NO](2-) and [Fe(CN)(5)NO](3-) ions. The first predominates at pH <8 and is formed by the rapid release of trans-cyanide from [Fe(CN)(5)NO](3-), which, in turn, is the main component at pH >9-10. Both nitrosyl complexes decay by first-order processes with rate constants around 10(-5) s(-1) (pH 6-10) related to the dissociation of NO. The decomposition is enhanced at pH 4 by 2 orders of magnitude with protons (and also metal ions) favoring the release of cyanides from the [Fe(CN)(4)NO](2-) ions and the ensuing rapid delivery of NO. At pH 7, an EPR-silent intermediate I(1) is detected (nu(NO), 1695 and 1740 cm(-1)) and assigned to the trans-[Fe(II)(CN)(4)(NO)(2)](2-) ion, an {Fe(NO)(2)}(8) species. At pH 6-8, I(1) induces a disproportionation process with formation of N(2)O and the regeneration of nitroprusside in a 1:2 molar ratio. At lower pHs, I(1) leads, competitively, to a second paramagnetic (S = 1/2) dinitrosyl intermediate I(2), [Fe(CN)(2)(NO)(2)](1-), a new member of a series of four-coordinate {Fe(L)(2)(NO)(2)} complexes (L = thiolates, imidazole, etc.), described as {Fe(NO)(2)}(9). Other decomposition products are hexacyanoferrate(II) or free cyanide, depending on the pH, and precipitates of the Prussian-Blue type. This study throws light on the conditions favoring rapid release of NO, to promote vasodilatory effects upon NP injection, and describes new processes related to dinitrosyl formation and NO disproportionation, which are also relevant to the diverse biological processes associated with NO and N(2)O processing.

Theoretical characterization of stable eta1-N2O-, eta2-N2O-, eta1-N2-, and eta2-N2-bound species: intermediates in the addition reactions of nitrogen hydrides with the pentacyanonitrosylferrate(II) ion

The addition of nitrogen hydrides (hydrazine, hydroxylamine, ammonia, azide) to the pentacyanonitrosylferrate(II) ion has been analyzed by means of density functional calculations, focusing on the identification of stable intermediates along the reaction paths. Initial reversible adduct formation and further decomposition lead to the eta(1)- and eta(2)-linkage isomers of N(2)O and N(2), depending on the nucleophile. The intermediates (adducts and gas-releasing precursors) have been characterized at the B3LYP/6-31G level of theory through the calculation of their structural and spectroscopic properties, modeling the solvent by means of a continuous approach. The eta(2)-N(2)O isomer is formed at an initial stage of adduct decompositions with the hydrazine and azide adducts. Further conversion to the eta(1)-N(2)O isomer is followed by Fe-N(2)O dissociation. Only the eta(1)-N(2)O isomer is predicted for the reaction with hydroxylamine, revealing a kinetically controlled N(2)O formation. eta(1)-N(2) and eta(2)-N(2) isomers are also predicted as stable species.

Mechanistic studies on the interaction of reduced cobalamin (vitamin B12r) with nitroprusside

The electron-transfer reaction between reduced cobalamin (Cbl(II)) and sodium pentacyanonitrosylferrate(II) (sodium nitroprusside, NP), as well as the subsequent processes following the electron-transfer step, were investigated by spectroscopic (UV-vis, (1)H NMR, EPR), electrochemical (CV, DPV) and kinetic (stopped-flow) techniques. In an effort to clarify the complex reaction pattern observed at physiological pH, systematic spectroscopic and kinetic studies were undertaken as a function of pH (1.8-9) and NP concentration (0.0001 - 0.09 M). The kinetics of the electron-transfer reaction was studied under pseudo-first-order conditions with respect to NP. The reaction occurs in two parallel paths of different order, viz. pseudo-first and pseudo-second order with respect to the NP concentration, respectively. The contribution of each path depends on pH and the [NP]/[Cbl(II)] ratio. At low pH and total NP concentration (pH < 3, [NP]/[Cbl(II)] approximately 1), the cyano-bridged successor complex [Cbl(III)-(mu-NC)-Fe(I)(CN)(3)(NO(+))](-) (1(s)()) is the final reaction product formed in an inner-sphere electron transfer reaction that is coupled to the release of cyanide from coordinated nitroprusside. At higher pH, subsequent reactions were observed which involve the attack of cyanide released in the electron transfer step on the initially formed cyano-bridged species, and lead to the formation of Cbl(III)CN and [Fe(I)(CN)(4)(NO(+))](2)(-). The strong dependence of the rate and mechanism of the subsequent reactions on pH is attributed to the large variation in the effective nucleophilicity of the cyanide ligand in the studied pH range. An alternative electron-transfer pathway observed in the presence of excess NP involves the reaction of the precursor complex [Cbl(II)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](2)(-) (1(p)()) with NP to give [Cbl(III)-(mu-NC)-Fe(II)(CN)(4)(NO(+))](-) (2) and reduced nitroprusside, [Fe(CN)(5)NO](3)(-), as the initial reaction products. Analysis of the kinetic data allowed elucidation of the rate constants for the inner- and outer-sphere electron-transfer pathways. The main factors which influence the kinetics and thermodynamics of the observed electron-transfer steps are discussed on the basis of the spectroscopic, kinetic and electrochemical results. A general picture of the reaction pathways that occur on a short (s) and long (min to h) time scale as a function of pH and relative reactant concentrations is derived from the experimental data. In addition, the release of NO resulting from the one-electron reduction of NP by Cbl(II) was monitored with the use of a sensitive NO electrode. The results obtained in the present study are discussed in reference to the possible influence of cobalamin on the pharmacological action of nitroprusside.

Kinetic, mechanistic, and DFT study of the electrophilic reactions of nitrosyl complexes with hydroxide

We present a kinetic study of OH(-) additions to several nitrosyl complexes containing mainly ruthenium and different coligands (polypyridines, amines, pyridines, cyanides). According to a first-order rate law in each reactant, we propose a fast ion pair formation equilibrium, followed by addition of OH(-) to the [MX(5)NO](n) moieties, with formation of the [MX(5)NO(2)H](n(-1)) intermediates. Additional attack by a second OH(-) gives the final products, [MX(5)NO(2)]((n-2)). A linear plot was found for ln k(4) (the addition rate constant) against the redox potential for nitrosyl reduction, E(NO+/-NO), showing a free-energy relationship with a slope close to 20 V(-1), consistent with an associative mechanism. Theoretical DFT calculated descriptors, as the charge density in the [MNO] moieties and the LUMO energies, qualitatively correlate with the rate constants. A linear to bent transformation was calculated for the nitrosyl complexes, as they evolve to the angular MNO(2)H and MNO(2) complexes. The geometries were optimized for the different complexes and adduct-intermediates, showing significant changes in the relevant distances and angles upon OH(-) addition. IR vibrations and electronic transitions were also calculated. The complete reaction profile was studied for the nitroprusside ion, including the description of the transition state structure. Experimental activation parameters revealed that both the activation enthalpies and entropies increase when going from the negatively charged to the positively charged complexes. As the rate constants increase in the same direction, we conclude that the reactions are entropically driven, compensating, this function, the increase in the activation enthalpies. The latter trend can be explained by the energies involved in angular reorganization after OH(-) coordination, which are larger as the positive charge in the nitrosyl moiety becomes larger. The use of E(NO+/-NO) as a predictive tool for electrophilic reactivity could be extended to similar reactions implying other nucleophiles, such as amines and thiolates.

The electrophilic reactions of pentacyanonitrosylferrate(II) with hydrazine and substituted derivatives. Catalytic reduction of nitrite and theoretical prediction of eta(1)-, eta(2)-N(2)O bound intermediates

The electrophilic reactivity of the pentacyanonitrosylferrate(II) ion, [Fe(CN)(5)NO](2)(-), toward hydrazine (Hz) and substituted hydrazines (MeHz, 1,1-Me(2)Hz, and 1,2-Me(2)Hz) has been studied by means of stoichiometric and kinetic experiments (pH 6-10). The reaction of Hz led to N(2)O and NH(3), with similar paths for MeHz and 1,1-Me(2)Hz, which form the corresponding amines. A parallel path has been found for MeHz, leading to N(2)O, N(2), and MeOH. The reaction of 1,2-Me(2)Hz follows a different route, characterized by azomethane formation (MeNNMe), full reduction of nitrosyl to NH(3), and intermediate detection of [Fe(CN)(5)NO](3)(-). In the above reactions, [Fe(CN)(5)H(2)O](3)(-) was always a product, allowing the system to proceed catalytically for nitrite reduction, an issue relevant in relation to the behavior of the nitrite and nitric oxide reductase enzymes. The mechanism comprises initial reversible adduct formation through the binding of the nucleophile to the N-atom of nitrosyl. The adducts decompose through OH(-) attack giving the final products, without intermediate detection. Rate constants for the adduct-formation steps (k = 0.43 M(-)(1) s(-)(1), 25 degrees C for Hz) decrease with methylation by about an order of magnitude. Among the different systems studied, one-, two-, and multielectron reductions of bound NO(+) are analyzed comparatively, with consideration of the role of NO, HNO (nitroxyl), and hydroxylamine as bound intermediates. A DFT study (B3LYP) of the reaction profile allows one to characterize intermediates in the potential hypersurface. These are the initial adducts, as well as their decomposition products, the eta(1)- and eta(2)-linkage isomers of N(2)O.