Inorganic Polysulfides in Solution: Structural Properties and Conformational Isomerism.

We present a comprehensive theoretical examination of the structural properties of dianionic polysulfides [Sn]2- (n = 2-6), their conjugated monoacids [HSn]- (n = 2-6), and a selection of 1e--oxidized radical anions [Sn]•- (n = 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. The interconversions between the isomeric structures proceed on the order of hundreds of picoseconds and are energetically similar to the isomerization processes in substituted cyclohexanes. We also conducted a detailed analysis of the stability of different isomers of the radical anion [S4]•- in solution. Our findings highlight the significant influence of the solvent on the isomerizations, a result that could be particularly relevant for enhancing the performance of metal-sulfur batteries.

NO/H2S "Crosstalk" Reactions. The Role of Thionitrites (SNO-) and Perthionitrites (SSNO-).

The redox chemistry of H2S 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 H2S 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 N2O, NO2-, NH2OH/NH3, and S8. Thionitrous acid, generated either in the direct reaction of NO + H2S or through the transnitrosation of RSNO's (nitrosothiols) with H2S at pH 7.4, is best described as a mixture of rapidly interconverting isomers, {(H)SNO}. It is reactive in different competitive modes, with a half-life of a few seconds at pH 7.4 for homolytic cleavage of the N-S bond, and could be deprotonated at pH values of up to ca. 10, giving SNO-, a less reactive species than {(H)SNO}. The latter mixture can also react with HS-, giving HNO and HS2- (hydrogen disulfide), a S0(sulfane)-transfer reagent toward {(H)SNO}, leading to SSNO-, a moderately stable species that slowly decomposes in aqueous sulfide-containing solutions in the minute-hour time scale, depending on [O2]. The previous characterization of HSNO/SNO- and SSNO- is critically discussed based on the available chemical and spectroscopic evidence (mass spectrometry, UV-vis, 15N NMR, Fourier transform infrared), together with computational studies including quantum mechanics/molecular mechanics molecular dynamics simulations that provide a structural and UV-vis description of the solvatochromic properties of cis-SSNO- acting as an electron donor in water, alcohols, and aprotic acceptor solvents. In this way, SSNO- is confirmed as the elusive "yellow intermediate" (I412) emerging in the aqueous crosstalk reactions, in contrast with its assignment to polysulfides, HSn-. The analysis extends to the coordination abilities of {(H)SNO}, SNO-, and SSNO- into heme and nonheme iron centers, providing a basis for best unraveling their putative specific signaling roles.

Remarkable Changes of the Acidity of Bound Nitroxyl (HNO) in the [Ru(Me3[9]aneN3)(L2)(NO)] n+ 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. A series of three closely related {RuNO}6,7 complexes have been prepared and structurally characterized, namely [Ru(Me3[9]aneN3)(L2)(NO)]3+/2+, with L2 = 2,2'-bipyridine, 4,4'-dimethoxy-2,2'-bipyridine, and 2,2'-bipyrimidine. These species have also been thoroughly studied in solution, allowing for a systematic exploration of their electrochemical properties in a wide pH range, thus granting access and characterization of the elusive {RuNO}8 systems. Modulation of the electronic density in the {RuNO} fragment introduced by changing the bidentate coligand L2 produced only subtle structural modifications but affected dramatically other properties, most noticeably the redox potentials of the {RuNO}6,7 couples and the acidity of bound HNO, which spans over a range of almost three pH units. Controlling the acidity of coordinated HNO by the rational design of coordination compounds is of fundamental relevancy in the field of inorganic chemistry and also fuels the growing interest of the community in understanding the role that different HNO-derived species can play in biological systems.

Nitrosodisulfide [S2NO]- (perthionitrite) is a true intermediate during the "cross-talk" of nitrosyl and sulfide.

Nitrosodisulfide S2NO- 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 S2NO- 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 141.6(2)°, respectively. [1](BF4)2 can be stored indefinitely under argon. Solutions of [1](2+) show no signs of decomposition when protected from air and light. The electron paramagnetic resonance X-band spectrum at 85 K in vitrified acetonitrile (MeCN) shows signals consistent with an S = (1)/2 spin state, better described as Ru(II)NO(•) (g = [2.030, 1.993, 1.880] and A = [11.0, 30.4, 3.9]/10(-4) cm(-1)). In water, the {RuNO}(7) species reacts with O2 in a 1:4 stoichiometry. The reaction is first-order in both reactants with k = (1.9 ± 0.2) M(-1) s(-1) at 25 °C (ΔH(⧧) = 11.5 ± 0.3 kJ mol(-1); ΔS(⧧) = -189 ± 1 J K(-1) mol(-1)). Solutions of [1](2+) evolve NO when irradiated a 365 nm with ϕNO = 0.024 and 0.090 mol einstein(-1) in H2O and MeCN, respectively.

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). The reaction kinetics between the {RuNO}(6) complex and the nucleophile OH(-) is also presented. The incorporation of tridentate and bidentate ligands in the coordination sphere prevents labilization issues associated with the trans effect when attaining the reduced states of the nitrosyl group. This allows for a consistent interpretation of the changes in the main geometrical parameters: Ru-N and N-O distances, Ru-N-O angle, and the νNO frequency and electronic transitions. We explore the redox properties in acetonitrile and aqueous solutions, and provide a potential (E1/2) - pH (Pourbaix) diagram for the three diatomic nitrosyl-bound species, as well as for HNO and NO2(-), including the report of the pKa of the [Ru(Me3[9]aneN3)(bpy)(HNO)](2+) ion, 9.78 ± 0.15 at 25.0 °C.

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.

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). The progress of the addition process of HS(-) into the bound NO(+) ligand was monitored by stopped-flow UV/Vis/EPR and FTIR spectroscopy, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R=[HS(-)]/[NP]. The initial adduct [Fe(CN)(5)N(O)SH](3-) (AH, λ(max) ≈570 nm) forms in the course of a reversible process, with k(ad)=190±20 M(-1)s(-1) , k(-ad)=0.3±0.05 s(-1) . Deprotonation of AH (pK(a)=10.5±0.1, at 25.0 °C, I=1 M), leads to [Fe(CN)(5)N(O)S](4-) (A, λ(max)=535 nm, ε=6000±300 M(-1) cm(-1) ). [Fe(CN)(5)NO](.)(3-) and HS(2)(.)(2-) radicals form through the spontaneous decomposition of AH and A. The minor formation of the [Fe(CN)(5)NO](3-) ion equilibrates with [Fe(CN)(4)NO](2-) through cyanide labilization, and generates the "g=2.03" iron-dinitrosyl, [Fe(NO)(2)(SH)(2)](-) , which is labile toward NO release. Alternative nucleophilic attack of HS(-) on AH and A generates the reactive intermediates [Fe(CN)(5)N(OH)(SH)(2)](3-) and [Fe(CN)(5)N(OH)(S)(SH)](4-) , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH(3) and hydrogen disulfide, HS(2)(-) . N(2)O is also produced at pH≥11. Biological relevance relates to the role of NO, NO(-) , and other bound intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.

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. Different N(2)/N(2)O ratios obtained with these three porphyrinates strongly suggest distinctive mechanistic scenarios: while [Fe(III)(TEPyP)](5+) and [Fe(III)(MP11)] formed unknown steady-state porphyrinic intermediates in the presence of HA, [Fe(III)(TPPS)](3-) led to the well characterized soluble intermediate, [Fe(II)(TPPS)NO](4-). Free-radical formation was only evidenced for [Fe(III)(TEPyP)](5+), as a consequence of a metal centered reduction. We discuss the catalytic pathways of HA disproportionation on the basis of the distribution of gaseous products, free radicals formation, the nature of porphyrinic intermediates, the Fe(II)/Fe(III) redox potential, the coordinating capabilities of each complex, and the kinetic analysis. The absence of NO(2)(-) revealed either that no HAO-like activity was operative under our reaction conditions, or that NO(2)(-), if formed, was consumed in the reaction milieu.

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-). [Fe(II)(CN)(5)(HNO)](3-) is the first non-heme iron-nitroxyl complex prepared in aqueous solution that is reversibly redox-active under biologically relevant conditions.

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.

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 compound [(Por)Ru(NO)(H 2O)]BF 4 undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm (-1) for OEP (*-) and 1290 cm (-1) for TPP (*-)), from the small shift of approximately 20 cm (-1) for nu NO and from the EPR signal at g iso approximately 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm (-1) shift in nu NO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF 4, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm (-1) shift in nu NO. The EPR response of the NO (*) complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)] (+), where X = H 2O or pyridines, was calculated to be centered at the porphyrin pi system, whereas the LUMO of [(TPP)Ru(NO)(X)] (+) has about 50% pi*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H 2O)] (+) occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)] (+) is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)] (+) is correlated with the stability of the reduced form as opposed to that of the aqua complex.

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. The nitrosonium complex, [Fe(CN)(5)(NO)](2-), is formed through a reductive nitrosylation reaction of [Fe(III)(CN)(5)(H(2)O)](2-) with NO, or, alternatively, through the coordination of NO(2)(-) to [Fe(II)(CN)(5)(H(2)O)](3-) and further proton-assisted dehydration. It is extremely inert toward NO(+)-dissociation, and behaves as an electrophile toward different bases: OH(-), amines, thiolates, etc. Also, SNP releases NO upon UV-vis photo-activation, with formation of [Fe(III)(CN)(5)(H(2)O)](2-). The more electron rich [Fe(CN)(5)(NO)](3-) may be prepared from [Fe(II)(CN)(5)(H(2)O)](3-) and NO, and is also highly inert toward the dissociation of NO (k = 1.6 x 10(-5) s(-1), 25.0 degrees C, pH 10.2). It reacts with O(2) leading to SNP, with the intermediacy of a peroxynitrite adduct. The [Fe(CN)(5)(NO)](3-) ion is labile toward the release of trans-cyanide, forming the [Fe(CN)(4)(NO)](2-) ion. Both complexes exist in a pH-dependent equilibrium, and decompose thermally in the hours time scale, releasing cyanides and NO. The latter may further bind to [Fe(CN)(4)(NO)](2-) with formation of a singlet dinitrosyl species, [Fe(CN)(4)(NO)(2)](2-), which in turn is unstable toward disproportionation into SNP and N(2)O, and toward the parallel formation of a tetrahedral paramagnetic dinitrosyl compound, [Fe(CN)(2)(NO)(2)]. Emerging studies with the putative nitroxyl complex, [Fe(CN)(5)(HNO)](3-), should allow for a complete picture of the three nitrosyl ligands in the same pentacyano fragment. The present Perspective, based on an adequate characterization of structural and spectroscopic properties, will focus on the kinetic and mechanistic description of the above mentioned reactions, which display a versatile scenario, fundamentally related to the biologically relevant processes associated with NO reactivity.

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 179.1(12) degrees , typical for a {RuNO}6 description. The [Ru(bpy)(tpm)NO]3+ ion (I) has been characterized by 1H NMR and IR spectroscopies (nu(NO) = 1959 cm(-1)) and through density functional theory calculations. Intense electronic transitions in the 300-350-nm region are assigned through time-dependent (TD)DFT as intraligand pi --> pi for bpy and tpm. The dpi --> pi(bpy) metal-to-ligand charge-transfer transitions appear at higher energies. Aqueous cyclic voltammetric studies show a reversible wave at 0.31 V (vs Ag/AgCl, 3 M Cl-), which shifts to 0.60 V in MeCN, along with the onset of a wave of an irreversible process at -0.2 V. The waves are assigned to the one- and two-electron reductions centered at the NO ligand, leading to species with {RuNO}(7) and {RuNO}(8) configurations, respectively. Controlled potential reduction of I in MeCN led to the [Ru(bpy)(tpm)NO]2+ ion (II), revealing a significant downward shift of nu(NO) to 1660 cm(-1) as well as changes in the electronic absorption bands. II was also characterized by electron paramagnetic resonance, showing an anisotropic signal at 110 K that arises from an S = 1/2 electronic ground state; the g-matrix components and hyperfine coupling tensor resemble the behavior of related {RuNO}7 complexes. Both I and II were characterized through their main reactivity modes, electrophilic and nucleophilic, respectively. The addition of OH- into I generated the nitro complex, with k(OH) = 3.05 x 10(6) M(-1) s(-1) (25 degrees C). This value is among the highest obtained for related nitrosyl complexes and correlates with ENO+/NO, the one-electron redox potential. Complex II is a robust species toward NO release, although a conversion to I was observed in the presence of O2. This reaction afforded a second-order rate law with k = 3.5 M(-1) s(-1) (25 degrees C). The stabilization of the NO radical complex is attributed to the high positive charge of the precursor and to the geometrical and electronic structure as determined by the neutral tpm ligand.

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.

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.

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. Transient absorption data at 420 nm indicate the formation of the [M(III)(CN)5H2O]2- species in less than ca. 1 micros for M = Fe and Ru. The enthalpy content of [Fe(III)(CN)5H2O]2- with respect to the parent [Fe(II)(CN)5NO]2- state is (190 +/- 20) kJ mol(-1), as measured by laser-induced optoacoustic spectroscopy (LIOAS) upon excitation at 480 nm. The production of [Fe(III)(CN)5H2O]2- is concomitant with an expansion of (8 +/- 3) ml mol(-1) consistent with an expansion of the water bound through hydrogen bonds to the CN ligands plus the difference between NO release into the bulk and water entrance into the first coordination sphere. The activated process, as indicated by the relatively strong temperature dependence of the Phi(NO) values and by the temperature dependence of the appearance of the [Fe(III)(CN)5H2O]2- species, as determined by LIOAS, is attributed to NO detachment in less than ca. 100 ns from the isonitrosyl (ON) ligand (MS1 state).

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 . The complex contains one protonated carboxylate (pK(a)=2.7+/-0.1). The [Ru(II)(Hedta)NO(+)] complex undergoes a nitrosyl-centered one-electron reduction (chemical or electrochemical), with E(NO+/NO)=-0.31 V vs SCE (I=0.2 M, pH 1), yielding [Ru(II)(Hedta)NO](-), which aquates slowly: k(-NO)=2.1+/-0.4x10(-3) s(-1) (pH 1.0, I=0.2 M, CF(3)COOH/NaCF(3)COO, 25 degrees C). At pHs>12, the predominant species, [Ru(II)(edta)NO](-), reacts according to [Ru(II)(edta)NO](-)+2OH(-)-->[Ru(II)(edta)NO(2)](3-), with K(eq)=1.0+/-0.4 x 10(3) M(-2) (I=1.0 M, NaCl; T=25.0+/-0.1 degrees C). The rate-law is first order in each of the reactants for most reaction conditions, with k(OH(-))=4.35+/-0.02 M(-1)s(-1) (25.0 degrees C), assignable mechanistically to the elementary step comprising the attack of one OH(-) on [Ru(II)(edta)NO](-), with subsequent fast deprotonation of the [Ru(II)(edta)NO(2)H](2-) intermediate. The activation parameters were DeltaH(#)=60+/-1 kJ/mol, DeltaS(#)=-31+/-3 J/Kmol, consistent with a nucleophilic addition process between likely charged ions. In the toxicity up-and-down tests performed with Swiss mice, no death was observed in all the doses administered (3-9.08 x 10(-5) mol/kg). The biodistribution tests performed with Wistar male rats showed metal in the liver, kidney, urine and plasma. Eight hours after the injection no metal was detected in the samples. The vasodilator effect of [Ru(II)(edta)NO](-) was studied in aortic rings without endothelium, and was compared with sodium nitroprusside (SNP). The times of maximal effects of [Ru(II)(edta)NO](-) and SNP were 2 h and 12 min, respectively, suggesting that [Ru(II)(edta)NO](-) releases NO slowly to the medium in comparison with SNP.