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The reduced forms of Roussin’s red esters [Fe2(μ-RS)2(NO)4]n− (n = 1, 2; R = Ph, Pr) and their possible decomposition products were studied by quantum chemical calculations. The energy diagram of the processes that occur in a dichloromethane solution was constructed. According to this diagram, the monoanions are much more stable than the corresponding dianions. Possible dissociation paths of the dianions in solution were proposed. The anions with the propyl ligand are more stable than the anions with the phenyl ligand.

Deoxyhemoglobin (Hb) stabilizes the cationic nitrosyl iron complex with cysteamine {Fe2[S(CH2)2NH3]2(NO)4}SO4·2.5H2O (CysAm), by slowing down its hydrolysis. In the absence of Hb, the electrochemical detection of NO release in the course of the hydrolysis using a sensor electrode gave the rate constant of (5.2±0.2)·10−5 s−1. The release of NO is a reversible process, and the amount of released NO is 1.4% of the CysAm concentration. In the presence of Hb, NO is released much more slowly, and the reaction is more intense than that in the absence of Hb. The adsorption of CysAm by an Hb molecule results in NO release from the CysAm-Hb complex with a rate constant of 1·10−8 s−1. The analysis of the Hb surface revealed the possible location of the cation-binding sites, which reversibly bind the cationic CysAm complex. The kinetic parameters of NO release from CysAm in the absence and in the presence of Hb were studied by the kinetic modeling.

A new method is developed for the synthesis of the binuclear iron tetranitrosyl complex with 5-nitropyridine-2-thiolate, [Fe2(SC5H3N2O2)2(NO)4] (I), from 2,2’-dithiobis(5-nitropyridine) and hydrazine hydrate in an aqueous-alcohol solution followed by the replacement of the thiosulfate ligands in an aqueous solution of salt Na2[Fe2(S2O3)2(NO)4] · 4H2O by functional 5-nitropyridine-2-thiolates. It is established by the X-ray diffraction method that the complex has the structure of the μ-SR type and crystallizes as two polymorphs. The Moessbauer spectral parameters at 293 K (quadrupole splitting ΔE Q = 1.243(1) mm/s, isomeric shift δFe = 0.095(1) mm/s, and absorption line width Γ = 0.263(1) mm/s) for studied compound I are similar to the isoelectronic complexes with thiophenoxide, pyridyl-2-thiolate, and pyrimidyl-2-thiolate. According to the electrochemical analysis data, complex I is a more efficient donor of NO in hydrolytic decomposition in protic media with NO formation than its isostructural analog with the unsubstituted pyridylthiolate ligand. The maximum amount of NO generated by complex I in a 1% aqueous solution of dimethyl sulfoxide at T = 25°C and pH 7.0 is 7.4 nmoles and increases twofold with an increase in the acidity of the medium (to pH 6.5) or temperature (37°C).

Reduction of the complex [Fe2(SC6H5)2(NO)4] in an aprotic solvent was studied by cyclic voltammetry in a wide range of potential scan rates. It was established that transfer of the first electron is reversible and the redox potential of this reaction was determined. Further reduction of the complex is irreversible because the product of attachment of the second electron is unstable and partially decomposes during the characteristic time of potential scan. The molecular and electronic structures of mono- and dianion of the complex as well as its theoretical redox potential value were calculated using the density functional theory methods with the local (BP86) and hybrid (B3LYP) functionals. The former functional better describes the geometry of the complex while the latter gives a better insight into its electronic structure. The extra negative charge is delocalized over NO groups, phenyl ligands, and iron atoms. The calculated redox potentials of one-electron reduction of the complexes are close to the experimental values obtained by analyzing cyclic voltammograms. Attachment of the second electron opens the decomposition channel of the complex, which is also consistent with experimental data.

The kinetic characteristics of the reaction of adenosine triphosphoric acid (ATP) with the thiosulfate-nitrosyl iron complex (TNCI, donor of nitrogen monoxide) were obtained by the spectrofluorimetric method. The shape of the kinetic curves indicate a complicated mechanism of interaction between ATP and TNCI. The kinetic model for this process was proposed that satisfactorily describes the experimental data. The compositions and structures of the reaction products were determined. Iron in the reaction products is presented by two forms: FeII and FeIII. The structures for the [ATP-Fe2+S] and [ATP-Fe3+S] complexes were proposed.

Quantum chemical calculations of the structure and solvation energies of mono- and dianions of Roussin’s red salt esters [Fe2(μ-RS)2(NO)4] (R = Me, Et, Pri, But) in solutions in THF and acetonitrile were carried out. In monoanions, an additional electron is localized on one Fe(NO)2 fragment, which leads to significant structural distortion of the anion compared to neutral molecule. The second electron is localized on the other Fe(NO)2 fragment; this causes symmetrization of the dianion geometry. There are good linear correlations between the calculated and experimental redox potentials of these systems. A relationship was proposed for estimation of the redox potentials of related iron—nitrosyl complexes. The standard redox potentials of the complex with R = Ph in water, DMSO, and acetonitrile evaluated using this expression lie between −0.5 and −0.6 V.

Nitrosyl compounds based on the iron-sulfur clusters are a promising class of inorganic nitric oxide donors. We studied cytotoxic properties of one of such NO donors, viz., the nitrosyl [2Fe-2S] complex with phenyl ligands (Ph-complex). The Ph-complex induces the HeLa and H1299 tumor cell death. The Ph-complex and cisplatin used in combination exhibit synergistic cytotoxicity. During studies of the poly(ADP-ribose) polymerase degradation, it was found that the HeLa cell death induced by the Ph-complex proceeds presumably by the mechanism of apoptosis. In the MCF7 cells, the Ph-complex causes induction of p53 protein expression and changes in its apparent molecular weight.

The photochromic ring closure in diarylethylene was studied by femtosecond laser spectroscopy. The absorption spectrum of the initial excited state under pulse excitation at 305 nm was observed. The kinetic scheme of transitions from the initial excited state to the closed-ring isomer as the final product is proposed.

The exchange coupling constants (J) were calculated and the spin density distributions were analyzed in the B3LYP/TZV approximation for the complex anions [L2M(1)IIILM(2)IIL2]n−, where L is ligand (L is oxalate, oxamide, dithiooxamide, hydroxamate) and M(1) and M(2) are atoms of the tri- and divalent 3d-transition metals, respectively, and n- is the charge of the anion. The largest J values were found for the complexes formed by the CrIII-NiII and CrIII-CoII pairs with the dithiooxamide ligands. Differences between the calculated and experimental J values are at most a few cm−1.

The reaction of the nitric oxide donor, viz., iron tetranitrosyl complex bearing the thiosulfate ligand (TNIC), with adenosine triphosphoric acid (ATP) was studied. By spectrofluorometry and electronic microscopy, the formation of the reaction products of ATP both with TNIC and with Na2S2O3 and FeSO4 was shown and the complexation constants of ATP with these compounds were calculated.

Single crystals of an iron complex with 1-methyltetrazole-5-thiol of the formula [Fe2(SC2H3N4)2(NO)4] (I) were obtained and examined by X-ray diffraction. According to electrochemical data, tetranitrosyl binuclear complex I rapidly decomposes in protic solvents with elimination of NO. The maximum amount of NO generated by complex I in 1% aqueous DMSO is ∼900 nmol. This amount is reduced by half 15 min after the beginning of the decomposition under anaerobic conditions. The dinitrosyl mononuclear intermediates [Fe(SC2H3N4)2(NO)]− and [Fe(SC2H3N4)2(NO)2]− were detected in solutions and identified by EPR spectroscopy and mass spectrometry. The low number of spins per complex in solutions indicates that the mononuclear complexes undergo further decomposition into NO and the species [Fe(SC2H3N4)3]−, [SC2H3N4]−, and [Fe4S3(NO)7]−. Complex I was found to be substantially more stable in DMSO than in methanol and 1% aqueous DMSO.

The kinetics of erythrocyte hemolysis and intra-erythrocyte hemoglobin oxidation under the action of synthetic sulfur-nitrosyl iron complexes was studied. The complexes capable of releasing nitric oxide due to spontaneous hydrolytic decomposition was studied. The addition of these complexes to a 0.2% suspension of mouse erythrocytes results in hemolysis. The kinetic curves of hemolysis exhibit an induction period, whose duration is different for each complex. The hemolysis is preceded by hemoglobin oxidation with nitric oxide penetrating into the cell. The oxidation of hemoglobin follows the first-order rate equation. The apparent first-order rate constants characterizing the NO-donating ability of each complex were determined. The hemolytic effect of the studied complexes is suggested to be related to the formation of peroxynitrite inside erythrocytes. Peroxynitrite is the cytotoxic product of interaction of nitric oxide and the superoxide radical anion.

The hydrolysis of the iron nitrosyl complex [Fe2(μ2-SC4H3N2)2(NO)4](C4H3N2S− is pyrimidine-2-thiolate) in the presence of hemoglobin (Hb) is accompanied by the NO release into a solution. In the absence of Hb, the starting complex is oxidized by nitric oxide that is released into a solution, which leads to further transformations of NO, nitric oxide being present in the solution only partially. The effective rate constant for the decomposition of the complex is high and depends on its concentration. On the one hand, in the presence of Hb, NO molecules rapidly and irreversibly bind to Hb to form HbNO, which is the intermediate in the nitric oxide metabolism. On the other hand, the reversible binding of the iron nitrosyl complex to the surface functional groups of Hb leads to a decrease in its concentration in a solution and deceleration of the formation of NO. Therefore, Hb can ensure the complete and more prolonged assimilation of NO.

By an example of the iron cysteamine nitrosyl complex {Fe2[S(CH2)2NH3]2(NO)4}SO4··2.5H2O (CAC), it was shown for the first time that the hydrolysis of this NO donor in the presence of ferrocytochrome c (cyt c2+) affords the iron nitrosyl complex NO-cyt c2+, which serves as the NO depot. The rate constant of NO release from CAC was determined from the kinetics of the formation of NO-cyt c2+. At pH 3.0 the rate constant is (2.7±0.1)·10−3 s−1. Ferrocytochrome c produces a less stabilizing effect on CAC than deoxyhemoglobin (Hb). Thus in the presence of cyt c2+, the reaction is completed in 1 h, whereas NO is released from a solution of CAC (2·10−4 mol L−1) in the presence of Hb during 40 h. The previously unknown stabilization of iron nitrosyl complexes by hemoglobin was found.

By an example of cysteamine iron nitrosyl complex {Fe2[S(CH2)2NH3]2(NO)4}SO4 • 2.5H2O (CAC) it was shown for the first time that the NO donor hydrolysis in the presence of ferricytochrome c (cyt c3+) affords the iron nitrosyl complex NO—cyt c3+. It was found that cyt c3+ can serve as a depot for NO evolved during the hydrolysis of CAC. In the presence of CAC, the rate of NO—cyt c3+ complex decomposition to NO and cyt c3+ depends on the molar ratio [cyt c3+]: [CAC] and at [cyt c3+]: [CAC] = 0.3 it was found to be lower than that in decomposition of CAC in the absence of cyt c3+. As a result, the total NO evolving process becomes 5.6 times more prolonged. The number of NO groups evolved from CAC can be determined by the reaction of CAC with cyt c3+ in the presence of ferricyanide: at most one NO group is evolved to a solution in the spontaneous hydrolysis of CAC (pH 7.0), and no less than three of them are evolved from oxidized CAC.

An increase in the effective magnetic moment and in the widths of the EPR lines of crystals composed of complexes of chromium(III) oxalates with cations of spiropyrans under the action of crystallization water was revealed. It was established that the dehydration-rehydration of the samples is a reversible process. Upon dehydration of the crystals at a temperature above 35°C, the triplet magnetism of the molecules of the spiropyrans weakens, strengthening upon rehydration. The contributions from the chromium oxalate and from the thermally induced states of the spiropyran molecules to the magnetization were isolated. The energies of thermal activation of the triplet states of the spiropyran molecules were determined.

Dinuclear iron tetranitrosyl complex with the composition [Fe2(SPh)2(NO)4] (1) was synthesized and its single crystals and polycrystals were studied by X-ray diffraction, IR spectroscopy, and elemental analysis. The decomposition products of complex 1 were investigated by electrochemical method and mass spectrometry. The mass spectrum of a solution of complex 1 shows two groups of ions: the primary decomposition products of 1 in solution (the complex ions [Fe(SPh)(NO)2(NO2)]−, [Fe(SPh)2(NO)]−, and [Fe(SPh)2(NO)2]−) and a series of the ions [FeO2 + n(NO)]− and [FeO3 + n(NO)]− (n = 0–4), which are formed in secondary reactions. The structures of the complexes, which were formed through the Fe-NO bond dissociation and the replacement of the NO ligand by aqua and oxygen ligands in complex 1, and the structure of the complex [FeO3]− were studied by quantum chemical modeling.

Single crystals of the new cationic chromene, 7-methyl-3,3-diphenyl-3H-pyrano[3,2-f-quinolinium iodide (C25H20NO)I (1), were synthesized. The crystal structure of the new compound was studied, and quantum chemical calculation for the open and closed forms were carried out. The bifunctional compounds containing mono- and bimetallic 3d metal (tris)oxalates with the chromenium cation, (C25H20NO)3[Cr(C2O4)3] · 4H2O (2) and (C25H20NO)[CrMn(C2O4)3] · H2O (3), were prepared. Compound 1 is paramagnetic due to low-lying thermally excited states of the chromene molecules. At low temperatures (∼2 K), the paramagnetic states are frozen, and the compound becomes diamagnetic. Compound 2 is paramagnetic and its magnetic properties are determined mainly by the Cr3+ ions and the thermally induced paramagnetic states of the chromene molecules. At high temperatures, the magnetic moment of compound 3 consists of the contributions of the paramagnetic Cr3+ and Mn2+ ions and the thermally induced paramagnetic states of chromenes. At low temperatures (2–3 K), the thermally induced magnetism of organic molecules is frozen, and the magnetically ordered (and, probably, spin-glass) state is observed in the two-dimensional network of metal oxalates (Tc = 3 K in the zero magnetic field). The UV irradiation leads to an increase in the magnetic moment of the compound in the paramagnetic region due to the generation of radiation defects.

Ordered nanowires of photochromic antiferromagnet SpFeMn(C2O4)3 (Sp is 1-{1′,3′,3′-trime-thyl-6-nitro-5′-chlorspiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) are synthesized in anodized aluminum oxide (AAO) pores 20 nm and 200 nm in diameter. A change in the magnetic properties of the material due to its nanostructuring is observed, resulting in the suppression of canted spin ordering (spincanted phase) in favor of the uniaxial antiferromagnet phase.

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp3Cr(C2O4)3 molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr3+ and Cr4+ ions and spiropyran molecules Sp0 and Sp+. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr3+ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm−1 and E = 0.024 cm−1. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr3+ and Sp0. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp+ and the SpI salt is observed.

The analysis of temperature dependences of magnetic moment of several new iron nitrosyl complexes in solid phase allowed the value of exchange interaction (J) between two iron ions to be estimated in each type complexes and a systematic decrease of J together with the increase in the ESR linewidth with a distance between iron ions to be observed. It was found that exchange interaction in the complexes involves two components, namely, direct exchange interaction and indirect one transferred through organic ligands.

New binuclear tetranitrosyl iron complex with pyrimidin-2-yl of the μ2-S type [Fe2(SC4H3N2)2(NO)4] (1) was synthesized by the exchange reaction of thiosulfate ligands in the [Fe(S2O3)2(NO)2]3s- anion for pyrimidin-2-yl ligands. The crystal structure of complex 1 was studied by single-crystal X-ray diffraction analysis. According to the X-ray diffraction data, pyrimidin-2-yl is coordinated to the iron atom in the thiol form. According to the quantum chemical calculations, the low stability of complex 1 is related to a possibility of formation of the coordination bond of the iron atom with the atom of the pyrimidine cycle of the ligand after NO group detachment. The ability of complex 1 to donate NO and the kinetics of its hydrolysis in aqueous solutions were studied by electrochemical analysis using sensor electrodes ami NO-700, by spectrophotometry in the pH interval from 6.0 to 7.76, and in the reaction with hemoglobin. Complex 1 is most stable in a neutral medium and more vigorously evolves NO in acidic and alkaline media.

The NO-donor ability of new binuclear tetranitrosyl complexes of the μ-N—C—S type, namely, bis(5-methylbenzimidazol-2-ylthio)- (1), bis(benzimidazol-2-ylthio)- (2), and bis(benzothiazol-2-ylthio)(tetranitrosyl)diiron (3), was studied in aqueous solutions by spectrophotometry. All kinetic regularities obtained for complexes 1–3 are well described in terms of formalism of pseudo-first-order reactions. The apparent first-order reaction rate constants for NO evolution by the complexes to solution were determined. Complexes 1–3 are good donors of NO. The structures of the complexes and the effect of their stabilization by hemoglobin were compared. The stabilization effect is explained by different basicities of the sulfur-containing ligands in the complexes studied.

One of the effects of endogenous NO is the formation of dinitrosyl iron complexes (DNICs), which, along with nitrosothiols, have been identified in all life forms, i.e., in the cells of bacteria, plants, and mammals [1]. Nitrosyl nonheme iron complexes have been identified by EPR as products of interaction of NO with some iron‐sulfur proteins and other iron-containing proteins [2‐5]. DNICs are intermediates in the iron-catalyzed decomposition and formation of S- nitrosothiols [6] and are considered as NO reservoirs and transporters in vivo. These results initiated the development of methods of synthesis and study of the physical and chemical properties of synthetic DNIC analogues [7] with the aim of their practical use as NO donors in medicine and biology [8, 9]. In the present paper, we report the results of the mass spectral and electrochemical studies of the products of hydrolysis of the tetranitrosyl iron thiosulfate complex Na 2 [Fe 2 ( µ -S 2 O 3 ) 2 (NO) 4 )] · 4 H 2 O (TNIC) in aqueous solutions. Study of the antimetastatic effect of the TNIC on three experimental models—melanoma B16, LL carcinoma, and AKATOL—showed that the complex inhibits metastatic growth. The combined administration of a low ineffective dose of the TNIC with cisplatin (or adriablastin) in leukemia P 388‐bearing mice leads to 100% survival among the laboratory animals [10]. Studying the TNIC decomposition products in solutions and determining the structure and properties of intermediates formed during hydrolysis are important for an understanding of molecular genetic mechanisms of action of exogenous NO donors of this class.

Single crystals of 1,3,3,7′-tetramethylspiro[indoline-2,2′-2H-pyrano[3,2-f]quinolinium] tris(oxalato)chromate(III), in which the cation exists in the open merocyanine form, (Mc)3Cr(ox)3·11.56H2O (ox is C2O42−), were obtained for the first time. The molecular and crystal structure of the salt was established by X-ray diffraction. The crystal structure consists of two types of layers (cationic and anionic). In the cationic layer, the Mc+ cations are parallel to each other and form a close-packed arrangement. The anionic layer consists of oxalatochromate complexes and water molecules linked together by hydrogen bonds. The salt does not display photochromic properties typical of salts of cationic spiropyrans. However, UV irradiation (355 nm) causes a change in the average spin of the sample from 3/2 to 1 and a change in the Weiss constant from −7 to 7 K.

Paramagnetic (tris)oxalates with the composition (SP)3M(C2O4)3·4H2O (M = Cr (2), Mn (3), Fe (4), and Co (5)) were synthesized for the first time based on the spiropyran salt SP+Br− (1) with the SP+ cation containing the pyridine ring in the aliphatic chain of the benzopyran moiety, which was also prepared in the present study. According to X-ray diffraction data, the SP+ cations in the starting salt 1 are close-packed thus hindering photochromic transformations. The spin for paramagnet 4 is 5/2, which corresponds to the FeIII atom in the high-spin state, the spin of the system remaining unchanged under UV irradiation. In complex 3, a crossover from ferromagnetic to antiferromagnetic correlations was observed after the replacement of the inorganic cation with the organic spiropyran cation.

X-ray analysis of mononuclear dinitrosyl iron complex [Fe(SC 2 H 3 N 3 )(SC 2 H 2 N 3 )(NO) 2 ]·0.5H 2 O ( 1 ) has been performed at 100 K (the crystallographic data are: a = 18.5006(3) A, b = 9.5207(1) A, c = 13.5719(2) A, β = 99.8860(7)°, V = 2355.04(6) A 3 , space group C 2/ c , Z = 8). From precision study of 1 , distribution maps of deformation electron density (DED) have been obtained. Both DED and topological parameters clearly indicate that Fe–S bonds in 1 are similar, though originally one of the molecules of ligand 1,2,3-triazole-3-thione coordinates the Fe atom in a thiol form (A), while the other in a thione form (B). These bonds can be described as interatomic interactions of a peak–peak type. The geometry and bond lengths in 5-membered cycles for A and B forms are rather similar. Fe–NO bonds can be described as interactions of a peak–hole type. The thermal motion of NO groups (even at 100 K) is highly anisotropic (this being an intrinsic feature of NO groups), thus suggesting mobility of the Fe–NO bond.

The magnetic properties of the binuclear nitrosyl-iron complexes Fe2(SC3H5N2)2(NO)4 are investigated. It is demonstrated that several types of particles, such as dimers with a pair of spins 1/2, dimers with a pair of spins 5/2, and paramagnetic particles with spin 3/2, make a contribution to the magnetic properties of the complexes. A decrease in the temperature below 25 K leads to a change in the shape of the EPR spectra corresponding to these dimers, so that Lorentzian lines (homogeneous broadening) transform into Gaussian lines (inhomogeneous broadening). This is accompanied by a stepwise change in the EPR line width and g factors. The change in the line shape indicates that complexes become asymmetric at low temperatures, possibly, due to the decrease in the spin exchange frequency below the frequency of the microwave field of the spectrometer.

The molecular and electronic structures of different isomers of a mononuclear dinitrosyl iron complex [Fe(SC2H3N3)(SC2H2N3)(NO)2] were calculated by the B3LYP and PBE density functional methods. Both theoretical approaches provide good agreement between the calculated and experimental geometry of the lowest-lying isomer (bond lengths differ by 0.02–0.04 A and bond angles differ by 2–3° in terms of root-mean square values). A feature of the complex is an intramolecular hydrogen bond N-H...N between the thiolate and thione ligands, which causes equalization of the Fe-S and S-C bond lengths. The ground state of the system has a spin of 1/2 and exists at antiparallel orientation of the spin (S = 3/2) of the Fe atom with formal electron configuration d7 and two local spins (S = 1/2) of the NO ligands. Although each NO group has a small negative charge, which is mainly localized on the O atom, the Fe-NO bond can be treated as similar to homeopolar one. This corresponds to the effective trivalent state of Fe with an oxidation state of 1+. Both theoretical methods correctly reproduce the experimental structure of the IR spectrum, but the PBE functional provides a better description of absolute positions of spectral lines, whereas the B3LYP functional gives a somewhat better description of the relative intensities of spectral components. In spite of similar geometric parameters of coordination of two NO groups, the splitting between the NO stretching bands is rather large (58 cm−1); this value is satisfactorily reproduced in theoretical calculations. A strong intramolecular hydrogen bond causes a large frequency shift of the N-H stretching vibrations corresponding to a broad absorption band in the region 2300–2600 cm−1.

A molecular magnetic (C25H23N3O3Cl)CrMn(C2O4)3·H2O whose spiropyran cation contains a quaternized pyridine fragment in the side aliphatic chain was synthesized for the first time. The compound possesses the properties of a ferromagnetic with T c = 5.2 K and photochromic properties in the crystalline state. The photochemical properties of the hybrid compound were studied by electronic and IR spectroscopies. Photochromic transformations of the spiropyran cation are accompanied by the appearance of a broad absorption band in the region 400–600 nm in the electronic spectra and by reduction of intensity of the ν(Cspiro-O) IR band at 942 cm−1.

A new photochromic spiropyran of the indoline series, containing a quinoline fragment, has been synthesized. The photochromic properties of the spiropyran and its iodomethylate have been studied. Features of the molecular structure of the compounds were determined by X-ray structural investigations. It was established that the pyridopyran fragments of the cations in the crystal of the iodomethylate salt, unlike those in the crystal of the neutral spiropyran, emerge from 21-stacks, disposed jointly with I− ions, which causes their higher mobility and aids the course of reversible photoconversions on UV irradiation in solution.

The tetranitrosyl iron complex with pyridine-2-yl [Fe2(SC5H4N)2(NO)4] (1) has higher NO-donating activity in 3% aqueous solutions of DMSO (pH 7.0, 25 °C) than the organic NO donor, viz., adduct of NO with diethylenetriamine (NO-adduct). The NO concentration was determined by the spectrophotometric method based on the formation of an NO complex with hemoglobin (Hb). The apparent first-order rate constants of the studied reactions are (6.15±0.6)·10−1 s−1 and (0.8±0.08)·10−1 s−1 for complex 1 and the NO-adduct, respectively, at an Hb concentration of 2·10−1 mol L−1 and the ratio [NO donor]/[Hb] = 10. The effect of Hb and [NO donor]/[Hb] ratio on the rate of NO generation from a solution of complex 1 was studied. For a fourfold decrease in the concentration of complex 1 the reaction rate constant decreases to 0.5·10−4 s−1, whereas the fourfold increase in the Hb concentration results in the stabilization of complex 1.

The formation of protein-bound dinitrosyl-iron complexes (DNIC) in blood plasma and packed red cell fraction has been demonstrated by the EPR method in the experiments on rabbits which were i/v injected with the low-molecular DNIC with thiosulphate. This formation was ensured by transfer of Fe(+)(NO(+))(2) moieties from low-molecular DNIC onto serum albumin or hemoglobin molecules. Protein-bound DNICs appeared immediately after low-molecular DNIC injection followed with gradually decreasing their amounts. The complexes could be detected by EPR technique during more than two days. The addition of water-soluble NO scavenger, the iron complex with N-methyl-d-glucamine dithiocarbamate (MGD) resulted in decomposition of a part of protein-bound DNICs and in effective excretion of secondary products (mainly mononitrosyl-iron complexes with MGD) from the blood flow.

Basing on 1-[(1′,3′,3′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline]-8-yl)methyl] pyridinium chloride, a new bifunctional compound C31H26N3O16CrMn has been synthesized, which exhibits ferromagnetic (Tk = 5.1 K) and photochromic properties in the solid state.

The neutral dinuclear iron nitrosyl complex [Fe2(SC3H5N2)2(NO)4] (1) of the “g = 2.03” family with a ligand analogous to natural mercaptohistidine was synthesized by the metathesis reaction of the thiosulfate ligands in the [Fe2(S2O3)2(NO)4]2− anion with imidazolidine-2-thiolate ligands. The electrochemical determination of nitrogen oxide in solution showed that compound 1 has a lower NO-donor ability compared to the iron complexes with 1-methylimidazole-2-thiol and imidazole-2-thiol synthesized earlier. Study of the magnetic properties of polycrystals of 1 demonstrated that the effective magnetic moment at room temperature is ca. 2.45 µB and corresponds to a molecule containing a pair of the noninteracting spins S = 1/2. This is evidence that each iron coordination unit in complex 1 contains one unpaired electron, and the iron atom is in the low-spin state.

Geometric and electronic structures of the iron thiolate tetranitrosyl complexes Fe2(µ-SC6−n H5−n Nn)2(NO)4, n = 0, 1, 2) were calculated using the B3LYP and PBE density functional methods. The geometric structures of the complexes found by the both theoretical approaches well agree with experiment, and a divergence of bond lengths is at most several hundredths of angstrom. According to the experimental data, the ground state of the system is diamagnetic, with the antiparallel orientation of the local spins 1/2 of the Fe(NO)2 fragments. The NO group bears a small negative charge mostly concentrated on the O atom, and the Fe-NO bond should be considered as homopolar. The formal electronic configuration of Fe is d7, which corresponds to the oxidation state 1+. However, in view of the presence of two homopolar bonds, the Fe atoms in the complexes should be trivalent. The both methods used well reproduce the experimental structure of the IR spectrum, but the PBE functional gives somewhat better description of the absolute position of the lines and the B3LYP functional better describes their relative intensities. The shifts of the vibrational bands of the thiolate ligand in the spectra of the complexes to both long-and short-wavelength regions are theoretically reproduced both qualitatively and semiquantitatively. Comparison of the calculated and measured splittings of the doublet of the stretching NO vibrations made it possible to attribute the appreciable disorder of splitting (15–55 cm−1) to the structural nonequivalence of the NO groups in the complexes. Due to peculiarities of NO binding, even a small difference in the N-O and Fe-N distances (∼0.01 A) affects the frequency of the NO vibrations.

Bi-nuclear neutral sulfur–nitrosyl iron complex [Fe2(SR)2(NO)4] (I) has been obtained by replacement of thiosulfate ligands in dianion [Fe2(S2O3)2(NO)4]2− by 1-methyl-imidazole-2-yl. From X-ray analysis data, the complex has centrosymmetrical dimeric structure, with the iron atoms being linked via μ-N–C–S bridge. From Mossbauer spectroscopy, isomeric shift δFe is 0.180(1) mm/s and quadrupole splitting ΔEQ is 0.928(2) mm/s at T = 290 K. By comparative studying the mass-spectra in the gaseous phase of solid samples decomposition and kinetics of NO release in 1% aqueous solutions of dimethylsulfoxide, using of the ligand with CH3 substituent in position 1 of imidazole-2-thiol was shown to yield a more stable donor of nitrogen monoxide than earlier obtained analog with imidazole-2-thiol, [Fe2(C3H3N2S)2(NO)4].

Bi-nuclear neutral paramagnetic nitrosyl iron complex [Fe2(C3H3N2S)2(NO)4] (I) has been synthesized by the exchange reaction of Na2Fe2(S2O3)2(NO)4·4H2O with imidazole-2-thiol. As it follows from X-ray analysis, complex I (space group P2(1)/n, Z=2) has centrosymmetric dimeric structure, with the iron atoms linked via structural fragment -μ-N–C–S. Isomeric shift of I δFe is 0.196(1) mm/s and quadrupole resolution ΔEQ is 1.109(2) mm/s at T=293 K. As follows from magnetic measurements, there are two paramagnetic centers with spin 1/2 in molecules [Fe2(C3H3N2S)2(NO)4], which are connected by anti-ferromagnetic exchange (−34.6 cm−1).

Binuclear iron nitrosyl complex Na2[Fe2(S2O3)2(NO)4] · 4H2O (I) was synthesized by the reaction of iron(II) sulfate with sodium thiosulfate in the flow of NO gas. According to X-ray diffraction data, the [Fe2(S2O3)2(NO)4]2− anion has binuclear centrosymmetric structure with Fe atoms bonded by the µ-S atoms of thiosulfate groups. The isomeric shift for complex I δ=0.168(1) mm/s and quadrupole splitting ΔE Q =1.288 mm/s at T=80 K. When heated, complex I transforms to Na2[Fe2(S2O3)2(NO)4] (II), whose unit cell parameters found by X-ray diffraction method differ from those of complex I. The process of transformation of I to II was studied by calorimetric method. Complex I transforms to complex II without chemical decomposition, which was confirmed by IR and mass spectroscopy data.

The review surveys methods for the synthesis, as well as structures and properties of sulfur-containing iron nitrosyl complexes serving as models of active sites of [Fe—S] nitrosyl proteins, which are potential donors of nitrogen monoxide.

Neutral paramagnetic binuclear iron nitrosyl complexes, whose structures and properties differ from those of the known Roussin"s red salt esters, were synthesized for the first time. The iron nitrosyl complexes [Fe2(μ2-SR)2(NO)4]·nH2O (η1-S, η1-N; n = 1 or 2; R is 5-amino-1,2,4-triazol-3-yl (1), 1,2,4-triazol-3-yl (2), 1-methyltetrazol-5-yl (3), or benzothiazol-2-yl (4)) were prepared by the exchange reactions of Na2Fe2(S2O3)2(NO)4 with heterocyclic thiols. According to the results of X-ray diffraction analysis, complex 1 has a centrosymmetrical dimeric structure in which the iron atoms are linked through the μ-N—C—S structural fragment. Each Fe atom is bound to the N atom of one ligand and the S atom of another ligand. The isomer shifts of complexes 1—4 have virtually equal values (δFe = 0.291(1)—0.304(1) mm s–1 at T = 85 K), which are twice as large as δFe for Roussin"s red salt esters. The iron atoms in complexes 1—4 have the low-spin configuration d7 (Fe+). The ESR spectra of polycrystalline powders of complexes 1—4 consist of a single Lorentzian line with g = 2.032 and a width of 6—10 mT. The temperature dependence of the magnetic susceptibility of complex 1 in the temperature range of 80—300 K is adequately described by the Curie—Weiss law with θ ≈ 8 K; the effective magnetic moment per iron atom is 1.85 μB.

A new potential donor of nitrogen monoxide, a binuclear iron sulfur nitroso complex, was prepared by exchange reaction of Na2Fe2(S2O3)2(NO)4 with pyridine-2-thiol in the presence of sodium thiosulfate at pH 12. The molecular and crystal structures of [Fe2(μ-SC5H4N)2(NO)4] were studied by X-ray diffraction analysis. The type of iron coordination by pyridine-2-thiol in the presence of a coordinated NO molecule was determined. In vacuum, the structure of the complex is destroyed, which is accompanied by NO evolution, while exposure to UV radiation results in decomposition of the complex and in a release of N2O.

This paper describes a comparative study of the decomposition of two nitrosyl iron complexes (NICs) with penicillamine thiolic ligands [Fe2(SC5H11NO2)2(NO)4]SO4·5H2O (I) and glutathione- (GSH-) ligands [Fe2(SC10H17N3O6)2(NO)4]SO4·2H2O (II), which spontaneously evolve to NO in aqueous medium. NO formation was measured by a sensor electrode and by spectrophotometric methods by measuring the formation of a hemoglobin- (Hb-) NO complex. The NO evolution reaction rate from (I) k1= (4.6 ± 0.1)·10−3 s−1and the elimination rate constant of the penicillamine ligandk2= (1.8 ± 0.2)·10−3 s−1at 25°C in 0.05 M phosphate buffer, pH 7.0, was calculated using kinetic modeling based on the experimental data. Both reactions are reversible. Spectrophotometry and mass-spectrometry methods have firmly shown that the penicillamine ligand is exchanged for GS−during decomposition of 1.5·10−4 M (I) in the presence of 10−3 M GSH, with 76% yield in 24 h. As has been established, such behaviour is caused by the resistance of (II) to decomposition due to the higher affinity of iron to GSH in the complex. The discovered reaction may impede S-glutathionylation of the essential enzyme systems in the presence of (I) and is important for metabolism of NIC, connected with its antitumor activity.

A structure of the [P(C6H5)4][MnFe(C2O4)3] single crystal is studied by X-ray diffraction within the 100–370 K temperature interval. The phase transition observed in the 150–170 K region is probably due to the cooperative ordering of the axial phenyl rings of the organic cations in the special positions.

A crystal of [N(C5H11)4][MnIIFeIII(C2O4)3] was studied by X-ray diffraction analysis: space group C2221, a= 9.653(2) A, b= 16.201(2) A, c= 20.193(4) A. The arrangement of the cations predetermines the formation of the crystal structure from anionic layers of the same chirality. The presence of two types of organic cation does not contradict the formation of crystals with left and right chirality and accounts for the data of Mossbauer spectroscopy, indicating two states of iron.

Iron nitrosyl complexes with general formula [Q4N]2[Fe2(S2O3)2(NO)4] (Q = Me, Et, n-Pr, n-Bu) were synthesized by the exchange reaction of K2[Fe2(S2O3)2(NO)4] with tetraalkylammonium bromides. The molecular and crystal structure of [(CH3)4N]2[Fe2(S2O3)2(NO)4] were studied by X-ray diffraction analysis. The iron atom in the four-membered cycle of the [2Fe–2S] anion is bound to another Fe atom and to two sulfur atoms and is coordinated by two nonequivalent NO groups, each bridging sulfur atom being bound to the SO3group. The structurally equivalent iron atoms are in the state Fe1–(S= 1/2). The Mossbauer spectroscopy method shows that the complexes are diamagnetic due to the strong Fe–Fe bond. It is found that the SO3group provides higher stability of the thiosulfate anion than the anion in Roussin's red salt [Fe2S2(NO)4]2–.

The [n-Bu4N]2[Fe2(μ-S2O3)2(NO)4] complex was studied using X-ray diffraction analysis, cyclic voltammetry, and EPR spectroscopy, and its crystal structure was determined. The redox properties of the [Fe2(μ-S2O3)2(NO)4]2–anion in CH3CN and CH2Cl2solutions were studied. An addition of excess reducer (sodium thiosulfate) to the thiosulfate complex was shown to produce an EPR signal with g= 2.03 typical of the mononuclear iron dinitrosyl complexes. The mechanism for [Fe2(μ-S2O3)2(NO)4]2–reduction was suggested.

The binuclear nitrosyl complexes Q2[Fe2S2(NO)4], where Q=Me, Et, Prn, and Bun, were synthesized. The crystals of [Pr4nN]2Fe2S2(NO)4 were studied by X-ray analysis. The influence of the cation size on the electronic structure and symmetry of its local environment in the synthesized complexes was examined. The decrease in the isomeric shifts in the57Fe Mossbauer spectra is related to the increase in the length of the alkyl substituent chain of the quaternary tetraammonium cation and is consistent with changing the structural parameters: a decrease in the bond angle of the Fe−NO bond and stretching vibrations of the NO groups due to changes in intermolecular contacts.