Inorganic Polysulfides in Solution: Structural Properties and Conformational Isomerism.

We present a comprehensive theoretical examination of the structural properties of dianionic polysulfides [S] ( = 2-6), their conjugated monoacids [HS] ( = 2-6), and a selection of 1e-oxidized radical anions [S] ( = 2-4), in aqueous and dimethyl sulfoxide (DMSO) solutions. We investigated the structures and stabilities of various conformational isomers within these families of compounds by employing Quantum Mechanics-Molecular Mechanics (QM-MM) Molecular Dynamics (MD) simulations. The explicit inclusion of solvent molecules in the calculations revealed stable conformational structures that were previously unreported and might have appreciable concentrations in real systems.

NO/HS "Crosstalk" Reactions. The Role of Thionitrites (SNO) and Perthionitrites (SSNO).

The redox chemistry of HS with NO and other oxidants containing the NO group is discussed on a mechanistic basis because of the expanding interest in their biological relevance, with an eye open to the chemical differences of HS and thiols RSH. We focus on the properties of two "crosstalk" intermediates, SNO (thionitrite) and SSNO (perthionitrite, nitrosodisulfide) based in the largely controversial status on their identity and chemistry in aqueous/nonaqueous media, en route to the final products NO, NO, NHOH/NH, and S. Thionitrous acid, generated either in the direct reaction of NO + HS or through the transnitrosation of RSNO's (nitrosothiols) with HS at pH 7.

Remarkable Changes of the Acidity of Bound Nitroxyl (HNO) in the [Ru(Me[9]aneN)(L)(NO)] Family ( n = 1-3). Systematic Structural and Chemical Exploration and Bioinorganic Chemistry Implications.

This work demonstrates that the acidity of nitroxyl (HNO) coordinated to a metal core is significantly influenced by its coordination environment. The possibility that NO complexes may be the predominant species in physiological environments has implications in bioinorganic chemistry and biochemistry. This (apparently simple) result pushed us to delve into the basic aspects of HNO coordination chemistry.

Nitrosodisulfide [SNO] (perthionitrite) is a true intermediate during the "cross-talk" of nitrosyl and sulfide.

Nitrosodisulfide SNO is a controversial intermediate in the reactions of S-nitrosothiols with HS that produce NO and HNO. QM-MM molecular dynamics simulations combined with TD-DFT analysis contribute to a clear identification of SNO in water, acetone and acetonitrile, accounting for the UV-Vis signatures and broadening the mechanistic picture of N/S signaling in biochemistry.

Structural, Spectroscopic, and Photochemical Investigation of an Octahedral NO-Releasing {RuNO}(7) Species.

[Ru(Me3[9]aneN3)(bpy)(NO)](BF4)2 ([1](BF4)2) was explored by single-crystal X-ray diffractometry, leading to the first crystal structure of an octahedral {RuNO}(7) complex. The metal resides on the center of a distorted octahedron, with dN-O and ∠Ru-N-O at 1.177(3) Å and 141.

Nitrosyl-centered redox and acid-base interconversions in [Ru(Me3[9]aneN3)(bpy)(NO)](3,2,1+). The pKa of HNO for its nitroxyl derivative in aqueous solution.

This work reports the preparation of a new 6-coordinated nitrosyl compound and its use as a model to explore the redox and acid-base properties of the three redox states of bound nitrosyl (formally NO(+), NO(•), NO(-)/HNO) in {RuNO}(6,7,8) species. We prepared the octahedral {RuNO}(6) complex [Ru(Me3[9]aneN3)(bpy)(NO)](3+) (Me3[9]aneN3: 1,4,7-trimethyl-1,4,7-triazacyclononane; bpy = 2,2'-bipyridine), and the related [Ru(Me3[9]aneN3)(bpy)(NO2)](+) nitro derivative. The compounds were characterized by chemical analysis, X-ray diffraction, NMR, IR, and UV-vis spectroscopies, cyclic voltammetry (CV), UV-vis/IR spectroelectrochemistry, and theoretical calculations (DFT, (TD)DFT).

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.

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.

Addition and redox reactivity of hydrogen sulfides (H2S/HS⁻) with nitroprusside: new chemistry of nitrososulfide ligands.

The nitroprusside ion [Fe(CN)(5)NO](2-) (NP) reacts with excess HS(-) in the pH range 8.5-12.5, in anaerobic medium ("Gmelin" reaction).

Disproportionation of hydroxylamine by water-soluble iron(III) porphyrinate compounds.

The reactions of hydroxylamine (HA) with several water-soluble iron(III) porphyrinate compounds, namely iron(III) meso-tetrakis-(N-ethylpyridinium-2yl)-porphyrinate ([Fe(III)(TEPyP)](5+)), iron(III) meso-tetrakis-(4-sulphonatophenyl)-porphyrinate ([Fe(III)(TPPS)](3-)), and microperoxidase 11 ([Fe(III)(MP11)]) were studied for different [Fe(III)(Porph)]/[HA] ratios, under anaerobic conditions at neutral pH. Efficient catalytic processes leading to the disproportionation of HA by these iron(III) porphyrinates were evidenced for the first time. As a common feature, only N(2) and N(2)O were found as gaseous, nitrogen-containing oxidation products, while NH(3) was the unique reduced species detected.

Three redox states of nitrosyl: NO+, NO*, and NO-/HNO interconvert reversibly on the same pentacyanoferrate(II) platform.

Not so elusive: [Fe(II)(CN)(5)(HNO)](3-) has been characterized spectroscopically after the two-electron reduction of nitroprusside (see scheme). The complex is stable at pH 6, slowly decomposing to [Fe(CN)(6)](4-) and N(2)O. It is deprotonated at increasing pH value with oxidation of bound NO(-) to [Fe(II)(CN)(5)(NO)](3-).

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)).

Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study.

The kinetics of the reaction between aqueous solutions of Na2[Fe(CN)5NO].2H2O (sodium pentacyanonitrosylferrate(II), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling (15NO), in the pH range 7.1-9.

Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: structural, spectroelectrochemical (IR, UV-visible, and EPR), and theoretical studies.

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)] ( n+ ) (where Por (2-) = tetraphenylporphyrin dianion (TPP (2 (-) )) or octaethylporphyrin dianion (OEP (2-)) and X = H 2O ( n = 1, 2, 3) or pyridine, 4-cyanopyridine, or 4- N,N-dimethylaminopyridine ( n = 1, 0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H 2O)]BF 4 is established as an {MNO} species with an almost-linear RuNO arrangement at 178.1(3) degrees .

The coordination chemistry of nitrosyl in cyanoferrates. An exhibit of bioinorganic relevant reactions.

Sodium nitroprusside (SNP, Na(2)[Fe(CN)(5)(NO)].2H(2)O) is a widely used NO-donor hypotensive agent, containing the formally described nitrosonium (NO(+)) ligand, which may be redox-interconverted to the corresponding one-electron (NO) and two-electron (NO(-)/HNO) reduced bound species. Thus, the chemistry of the three nitrosyl ligands may be explored with adequate, biologically relevant substrates.

New ruthenium nitrosyl complexes with tris(1-pyrazolyl)methane (tpm) and 2,2'-bipyridine (bpy) coligands. Structure, spectroscopy, and electrophilic and nucleophilic reactivities of bound nitrosyl.

The new compound [Ru(bpy)(tpm)NO](ClO4)3 [tpm = tris(1-pyrazolyl)methane; bpy = 2,2'-bipyridine] has been prepared in a stepwise procedure that involves the conversion of [Ru(bpy)(tpm)Cl]+ into the aqua and nitro intermediates, followed by acidification. The diamagnetic complex crystallizes to exhibit distorted octahedral geometry around the metal, with the Ru-N(O) bond length 1.774(12) A and the RuNO angle 179.

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.

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.

The photorelease of nitrogen monoxide (NO) from pentacyanonitrosyl coordination compounds of group 8 metals.

The photodetachment of NO from [M(II)(CN)5NO]2- with M = Fe, Ru, and Os, upon laser excitation at various wavelengths (355, 420, and 480 nm) was followed by various techniques. The three complexes showed a wavelength-dependent quantum yield of NO production Phi(NO), as measured with an NO-sensitive electrode, the highest values corresponding to the larger photon energies. For the same excitation wavelength the decrease of Phi(NO) at 20 degrees C in the order Fe > Ru >> Os, is explained by the increasing M-N bond strength and inertness of the heavier metals.

The [Ru(Hedta)NO](0.1-) system: structure, chemical reactivity and biological assays.

The [Ru(II)(Hedta)NO(+)] complex is a diamagnetic species crystallizing in a distorted octahedral geometry, with the Ru-N(O) length 1.756(4) A and the RuNO angle 172.3(4) degrees .

Metal-catalyzed anaerobic disproportionation of hydroxylamine. Role of diazene and nitroxyl intermediates in the formation of N2, N2O, NO+, and NH3.

The catalytic disproportionation of NH(2)OH has been studied in anaerobic aqueous solution, pH 6-9.3, at 25.0 degrees C, with Na(3)[Fe(CN)(5)NH(3)].

Establishing the NO oxidation state in complexes [Cl(5)(NO)M](n-), M = Ru or Ir, through experiments and DFT calculations.

Predominantly NO-centered reduction was observed by EPR and IR spectroelectrochemistry to occur reversibly at low temperatures for [Cl(5)Ir(NO)](-). In contrast, the [Cl(5)Ru(NO)](2-) ion was found to undergo only irreversible reduction but reversible oxidation to a ruthenium(III) species at -40 degrees C. DFT calculations were used to establish the electronic structures and to rationalise the different stabilities.

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.

Kinetics and mechanism of the interaction of nitric oxide with pentacyanoferrate(II). Formation and dissociation of [Fe(CN)5NO]3 -.

The interaction of NO with [Fe(CN)(5)H(2)O](3)(-) (generated by aquation of the corresponding ammine complex) to produce [Fe(CN)(5)NO](3)(-) was studied by UV-vis spectrophotometry. The reaction product is the well characterized nitrosyl complex, described as a low-spin Fe(II) bound to the NO radical. The experiments were performed in the pH range 4-10, at different concentrations of NO, temperatures and pressures.