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Albumin, mucin and oxyhemoglobin act as carriers of the newly synthesized nitrosyl iron complex [Fe2(SC6H3Cl2)2(NO)4] and participate in its transformation.

The chemical reaction of a cationic ironnitrosyl complex with thiourea ligands of the composition [Fe(SC(NH2)2)2(NO)2]+ in aerobic aqueous conditions was studied via kinetic and quantum chemical modeling. An implicit expression is found for the kinetic curve which describes this process. The mechanism of interaction of the complex with molecular oxygen in the presence of water molecules was studied with density functional method. An optimized geometry of the initial, final, intermediate and transition states were obtained and the, energy profiles of the reactions were constructed. It was shown that the explicit inclusion of the water molecules in the reaction of the interaction of the complex with oxygen and the further transformation of the resulting products demonstrated not only the mechanism of the NO release, but also the intramolecular transformation of NO into NO2, NO2−, NO3−, ONOO−. A mechanism was proposed for the formation of a OH radical in the system and its release, along with the previously mentioned products, into solution.

High-molecular-weight dinitrosyl iron complexes (DNICs) are formed in living systems and are a stable depot of nitrogen monoxide (NO). In this work, using experimental and theoretical methods, we investigated the interaction of their synthetic analog, a promising cardiotropic complex of the composition [Fe(SC(NH

New amphiphilic terpolymers of N-vinylpyrrolidone, poly(ethylene glycol) methyl ether methacrylate (Mn 500 g/mol) and triethylene glycol dimethacrylate (branching agent) were synthesized via radical polymerization with and without 1-decanethiol as a chain transfer agent. They consisted of ca. 10 mol% (di)methacrylate units and had effective absolute molecular weight of about 26–600 kDa. The terpolymers were studied in polar media with dynamic light scattering, and critical concentration of aggregation and sizes of individual macromolecules and their aggregates were determined. Nanostructures of terpolymers containing 0.25–0.75% of fluorescent zinc tetraphenylporphyrinate (ZnTPP) were obtained that had the hydrodynamic radii less than 100 nm in aqueous solutions. The molar extinction coefficient of the Soret band at 425 nm of the encapsulated ZnTPP and the effective terpolymer binding constant for ZnTPP and the stability of terpolymers were found to be dependent on the presence of hydrophobic –SC10H21 groups in terpolymer chains. Terpolymer 1 did not affect significantly the viability of Vero or HeLa cells and was able to penetrate cells with high efficiency. Thus, new non-toxic amphiphilic terpolymers of N-vinylpyrrolidone can be promising platforms for delivery of hydrophobic biologically active compounds.

In this work a new donor of nitric oxide (NO) with antibacterial properties, namely nitrosyl iron complex of [Fe(C6H5C-SNH2)2(NO)2][Fe(C6H5C-SNH2)(S2O3)(NO)2] composition (complex I), has been synthesized and studied. Complex I was produced by the reduction of the aqueous solution of [Fe2(S2O3)2(NO)2]2− dianion by the thiosulfate, with the further treatment of the mixture by the acidified alcohol solution of thiobenzamide. Based on the structural study of I (X-ray analysis, quantum chemical calculations by NBO and QTAIM methods in the frame of DFT), the data were obtained on the presence of the NO…NO interactions, which stabilize the DNIC dimer in the solid phase. The conformation properties, electronic structure and free energies of complex I hydration were studied using B3LYP functional and the set of 6–31 + G(d,p) basis functions. The effect of an aquatic surrounding was taken into account in the frame of a polarized continuous model (PCM). The NO-donating activity of complex I was studied by the amperometry method using an “amiNO-700” sensor electrode of the “inNO Nitric Oxide Measuring System”. The antibacterial activity of I was studied on gram-negative (Escherichia coli) and gram-positive (Micrococcus luteus) bacteria. Cytotoxicity was studied using Vero cells. Complex I was found to exhibit antibacterial activity comparable to that of antibiotics, and moderate toxicity to Vero cells.

Water-soluble polymer compositions of the lipophilic complex of cis-bis[(nitroxyethyl)-isonicotinamide-N]-tetrachloroplatinum(iv) with antitumor activity were prepared. The complex was solubilized by amphiphilic copolymers of N-vinylpyrrolidone (VP) with (di)methacrylates synthesized by radical copolymerization in toluene in the absence of any inhibitors of polymer chain growth. Aqueous buffer solutions of the nanostructures were studied by dynamic light scattering, and the influence of the concentrations of the complex and copolymer and temperature on the nanoparticle size (hydrodynamic radius) was estimated. The nanostructures based on the VP copolymer with triethylene glycol dimethacrylate are shown to be thermosensitive and decompose when the temperature rises to physiologically significant values. According to the TEM data, the polymer particles are spherical and contain inclusions of the PtIV complex ∼4 nm in size. The polymer compositions were studied using CV, TGA, and DSC methods. The results of IR spectroscopic analysis of the polymer compositions and quantum chemical modeling indicate the formation of the hydrogen bond between the NH groups of the complex and VP copolymer.

The structures of H-complexes of the anthracycline antibiotic doxorubicin with poly-N-vinylpyrrolidone segments in an aqueous medium were examined using the density functional theory and in terms of the quantum theory “Atoms in molecule” (QTAIM analysis). Quantum chemical calculations of key geometric and energy parameters of the complexes were carried out and the thermodynamic stabilities of the complexes relative to one another were compared. Based on the results obtained, possible structures of the system under study in aqueous solutions are described and the assumption is made that N-vinylpyrrolidone copolymers can be used as polymer carriers for the targeted delivery of doxorubicin.

Using metformin, a broad spectrum drug, and the branched amphiphilic copolymer of N-vinylpyrrolidone and triethylene glycol dimethacrylate with a molar ratio of comonomers of 80 : 20 and a weight average molecular weight of 1.96 × 105 hybrid materials are synthesized and characterized in aqueous solutions and in the solid state. It is shown that in aqueous buffer solutions they occur as nanostructures, whose behavior and dynamic parameters depend on the concentration of metformin and copolymer and temperature. According to quantum-chemical modeling, the NH2 groups of metformin form hydrogen bonds via the carbonyl group of N-vinylpyrrolidone and the ester and ether groups of dimethacrylate. The effective constant of binding metformin with the copolymer is determined using electron adsorption spectroscopy data. It is found that the polymer composite containing 20 wt % metformin exerts an inhibiting effect on the key enzyme of the sorbitol pathway of glycose metabolism—aldose reductase.

The kinetic and thermodynamic parameters of the concerted decomposition of cyclohexenes of various structures were calculated by two methods: the method of three intersecting parabolas and quantum chemical modeling (hybrid density functional theory B3LYP method and the 6–311++G** basic set). The activation energies, rate constants, and enthalpies for nine decomposition reactions were determined for the first time. The parameters of the decomposition of cyclohexenes calculated by two independent methods are in good agreement with each other.

The structures of a number of the products for the reaction of the NO donor (cation of the dinitrosyl iron complex [Fe(SC(NH2)2)2(NO)2] • Cl•H2O) with poly-N-vinylpyrrolidone in water were studied using the density functional method and QTAIM analysis. The quantum chemical calculations of the basic geometric and energy parameters of the formed compounds were performed. The expected forms of existence of this system in aqueous solutions were described on the basis of the obtained results. The retention of the NO-donor activity in the formed polymeric structures is related to the preservation of the Fe(NO)2 fragment in them.

Quantum-chemical modeling of the retro-Diels–Alder reaction is performed for the cycles of different structure as examples: with (a) alkyl substituents and (b) heteroatoms in a cycle. A topological analysis of the electron density distribution function (QTAIM) is used to analyze transient states and intermediate compounds close to them. Using the obtained data on the structure of transient states and the reaction mechanism, an algorithm for calculating the activation energy of concerted ring cleavage reaction is created within intersecting parabolas (M3IP) approach. The effect the structure of the initial cycle has on the course of the reaction is determined.

The structure of metformin (dimethylbiguanide) complexes with the copolymer of N-vinylpyrrolidone and triethylene glycol dimethacrylate in aqueous solutions is studied by means of DFT (Gaussian 09 software; TPSSh functional; 6-31G*//6-311++G** basis set) and the QTAIM model (AIMALL software). It is shown that both metformin and water molecules coordinate via the oxygen atom of the С=O bond of lactam rings, and that this process is exothermic. One oxygen atom can coordinate one or two water molecules, and water can form bridges connecting two adjacent C=O groups in addition to common terminal coordination. The formation of the second hydration shell after the formation of the first is possible due to the chain addition of water molecules; its structure depends on the mutual orientation of the С=О groups. It was shown that metformin is oriented with respect to electron donor groups in a strictly determined way; it can displace water upon coordination with the copolymer.

Dinitrosyl iron complexes (DNICs) are a depot and potential source of free NO in organisms. Their synthetic analog, N-ethylthiourea DNIC [Fe(SC(NH

Mesoporous polymer networks were prepared via the cross-linking radical copolymerization of non-toxic hydrophilic N-vinylpyrrolidone (VP) with triethylene glycol dimethacrylate (TEGDM) and poly(ethylene glycol) methyl ester methacrylate (PEGMMA) in bulk, using appropriate soluble and thermodynamically compatible macromolecular additives with a branched structure as porogens. The branched copolymers of various monomer compositions were obtained by radical copolymerization in toluene, controlled by 1-decanethiol, and these materials were characterized by a wide set of physical chemical methods. The specific surface areas and surface morphology of the polymer networks were determined by nitrogen low-temperature adsorption or Rose Bengal (RB) sorption, depending on the copolymer compositions and scanning electron microscopy. The electrochemical properties of RB before and after its encapsulation into a branched VP copolymer were studied on a glassy carbon electrode and the interaction between these substances was observed. Quantum chemical modeling of RB-VP or RB-copolymer complexes has been carried out and sufficiently strong hydrogen bonds were found in these systems. The experimental and modeling data demonstrate the high potency of such mesoporous polymer networks as precursors of molecularly imprinted polymers for the recognition of fluorescent dyes as nanomarkers for biomedical practice.

An original approach is proposed for creating the molecularly imprinted polymer and the carrier of protein molecules. Specifically, a thermally and chemically stable branched N-vinylpyrrolidone copolymer was used as the model of an imprint biomolecule and the porogen in the three-dimensional radical copolymerization of N-vinylpyrrolidone and triethylene glycol dimethacrylate. Removal of the branched copolymer left cavities available for binding biomolecules to the copolymer matrix due to its mesoporous structure.

New cis-bis[N-(2-nitroxyethyl)isonicotinamide-N]platinum(iv) tetrachloride hybrid materials were obtained via its solubilization by nanoparticles of N-vinylpyrrolidone copolymer with dimethacrylates and characterized. The materials containing 2% of PtIV complex cause a cytotoxic effect on the tumor A-172 cells; IC50 values were ca. 106 μm after incubation for 48 h.

Reactions of concerted decomposition of cyclic molecules (Diels—Alder retro-reactions) were studied using quantum chemical modeling and the method of intersecting parabolas (M3IP model). According to results of quantum chemical calculations and topological analysis of transition states (TSs) of five concerted decomposition reactions of cyclic molecules, all TSs represent six-membered rings in which the bonds being cleaved are weak covalent ones. The presence of ring heteroatoms (N, O, or S) has a strong impact on the TS geometry and on the energy characteristics of the reactions. The M3IP model was used to process the experimental results to develop an algorithm for calculating the activation energies (E) of the reactions in question and the classical potential barriers to thermally neutral reaction (Ee0). Factors influencing the energies E were determined and assessed. These include the enthalpy of reaction, substituents, ring heteroatoms, the force constants of bonds, and the bond dissociation energies. A comparison of the activation energies E and the enthalpies of reaction ΔH obtained from the M3IP and density functional B3LYP/6-311++G** calculations revealed good agreement between them.

Using EPR spectroscopy, it has been established that monomers affect the decomposition rate of their polymerization initiators by forming complexes with them. Several structures of the complexes have been optimized, and their decomposition mechanisms, the activation energies of the chain initiation and propagation reactions, and the heat of polymerization have been determined by means of quantum chemistry methods. In the absence of radical inhibitors (evacuation), the polymerization proceeds with high efficiency (98%) at room temperature without stimulation of the system with external energy.

In this work, transformation of DNIC with thiourea ligands, [Fe(SС(NH2)2)2(NO)2]Сl∙H2O (I), in an aqueous buffer solution at neutral pH and in the presence of reduced glutathione (GSH) has been studied. Decomposition of the complex was shown to occur in the buffer solution, with removal of the first thiourea ligand having a higher rate (k=(3.8 ± 1.3)∙10-4 s-1) than the NO group release (k=(0.7 ± 0.2)∙10-4 s-1). This corroborates a quantum-chemical model of I decomposition suggested earlier. Using UV–Vis spectrophotometry and EPR spectrometry, the interaction of I with GSH in solution was shown to yield a new binuclear tetranitrosyl iron complex (TNIC) with two GS−- ligands (II). Using quantum-chemical modeling, the reaction scheme was suggested, which involves replacement of one thiourea ligand by GS−, and then dimerization of the forming complexes accompanied by removal of the second thiourea ligand. Theoretical UV–Vis spectra were calculated and interpreted for the initial I and for the product of the substitution reaction, II.

Redox reactions of cationic nitrosyl iron complexes with thiourea and its aliphatic derivatives have been studied using cyclic voltammetry technique and DFT investigation. Experimental values of the redox potentials have been determined and compared with results of DFT calculations. Using quantum-chemical modeling, structures of the forming reduced and oxidized forms have been obtained and their electronic structures have been studied. The redox processes in these systems were shown to be complicated and accompanied by the complexes decomposition. It is very important for understanding of their reactivity.

Hydrated copolymers of N-vinylpyrrolidone (VP) with triethylene glycol dimethacrylate as a promising platform for biologically active compounds (BAC) were investigated by different physical chemical methods (dynamic light scattering, infrared spectroscopy, thermal gravimetric analysis , and differential scanning calorimetry) and the quantum chemical modeling of water coordination by the copolymers in a solution. According to the quantum chemical simulation, one to two water molecules can coordinate on one O-atom of the lactam ring of VP units in the copolymer. Besides the usual terminal coordination, the water molecule can form bridges to bind two adjacent C=O groups of the lactam rings of VP units. In addition to the first hydration shell, the formation of a second one is also possible due to the chain addition of water molecules, and its structure depends on a mutual orientation of C=O groups. We showed that N,N-dimethylbiguanidine hydrochloride (metformin) as a frontline drug for the treatment of type 2 diabetes mellitus can be associated in aqueous solutions with free and hydrated C=O groups of the lactam rings of VP units in studied copolymers. Based on the characteristics of the H-bonds, we believe that the level of the copolymer hydration does not affect the behavior and biological activity of this drug, but the binding of metformin with the amphiphilic copolymer will delight in the penetration of a hydrophilic drug across a cell membrane to increase its bioavailability.

By means of quantum-chemical calculations using Density Functional Theory, Quantum Theory of Atoms in Molecules, and Natural Bond Orbitals, theoretical modeling of intermolecular interactions has been performed for eight nitrosyl iron complexes with aliphatic thiourea ligands, which was aimed at discovering the presence of the NO…NO intermolecular interactions and at studying the possibility of the NO…NO supramolecular synthon formation in their crystalline structure for explaining their unusual magnetic properties. Such interactions were shown to be either stacking or T-like interactions, depending on the relative position of nitrosyl ligands and energetically corresponding to Van der Waals bonds. Mainly LP(O), π (NO), and π*(NO) orbitals in various combinations participate in their formation, with π (FeN), π(FeО), and LP(N) orbitals hardly being participants. The involvement of the NO bond orbitals results in quenching the orbital moment of the NO groups. If NO groups are isolated from intermolecular interactions, they can preserve the unquenched orbital moment.

Interaction and transformation of the mononuclear cationic dinitrosyl iron complex [Fe(SC(NH2)2)2(NO)2]+ (complex 1) upon binding with bovine serum albumin (BSA) have been explored using kinetic measurements, UV-Vis and fluorescence spectroscopy, and computational molecular modeling. BSA was found to bind up to five molecules of complex 1 per one protein molecule; as a result, the rate of NO release by complex 1 into solution decreases by a factor of 10. The binding constant of complex 1 with BSA measured by the quenching of intrinsic fluorescence of BSA is 5 × 105 М-1. Molecular docking calculations at pH = 7 have determined five-six low-energy binding sites for complex 1 at subunits I and II of BSA. The most stable protein-ligand complexes are located at the protein pockets near Cys34. The spectroscopic measurements and docking calculations have shown that the decomposition product of complex 1, the Fe(NO)2+ fragment, can form an adduct Fe(Cys34)(His39)(NO)2 (complex 2) with the coordination bonds of Fe with atoms S of Cys34 and ND of His39. The structure of complex 2 was supported by the density functional calculations of the absorption spectrum. Decomposition of complex 2 leads to nitrosylation of BSA at atom S of Cys34. Complexes 1 (bound with BSA), 2 and the nitrosylated BSA can serve as NO depot in plasma.

Homogeneous composites based on poly(methyl methacrylate) and its copolymers with methacrylic acid and on the nitrosyl iron complexes with 2-mercaptobenzimidazole and 2-mercaptobenzthiazole were synthesized. An analysis of the experimental data obtained by the methods of small-angle X-ray diffraction and SQUID magnetometry in combination with the results of quantum chemical calculations showed that the nitrosyl complexes did not retain their initial binuclear structure under the matrix isolation conditions: some complexes decompose to mononuclear ones. The percentage ratio of the dimers and monomers depends on the fraction of the methacrylic acid copolymer in the polymeric matrix and is almost independent of the type of thiol ligand in the iron complex.

Concerted cycloaddition reactions were studied by the method of intersecting parabolas (M3IP) and quantum chemical calculations. Experimental data were processed within the framework of the M3IP method and an algorithm for calculating the activation energies (E) and rate constants (k) for reactions from the enthalpies of reactions was developed. The parameters E and k for twelve cycloaddition reactions not studied previously were calculated. Factors affecting the activation energies were established and evaluated; these include the enthalpy of reaction, substituents, and the molecular structure of reactants. Quantum chemical modeling and topological analysis of transition states (TS) of six concerted cycloaddition reactions were performed. Depending on structure of the starting olefins, the TS of reactions can have either a symmetric or asymmetric geometry. This influences their electronic structures, the energies of chemical bonds, and the activation energies of reactions. A comparison of the activation energy values obtained from the M3IP and DFT(B3lyp/6-311++G** ) calculations revealed good agreement between them.

The amphiphilic copolymer of N-vinylpyrrolidone forms in isopropyl alcohol the copolymer particles with a core composed of low polar fragments of triethylene glycol dimethacrylate. In this work, quantum chemical modelling of various variants of the structural products of this copolymer and the μ-S–C–N type tetranitrosyl iron complex with a benzothiazole ligand was carried out. The theoretical IR spectra of these possible structural products are calculated and compared with experiments. Based on the results, it is assumed that the C=O bond of the copolymer’s methacrylate units is coordinated by the mononuclear iron complex, which is formed upon dissociation of the initial binuclear nitrosyl complex via the Fe–N bond of the heterocyclic ligand.

Single crystals of new binuclear tetranitrosyl iron complexes ([Fe2(SR)2(NO)4]) have been synthesized, with R being m-aminophenyl (1) and m-hydroxyphenyl (2). The structure, physical–chemical properties, and NO release of new complexes in comparison with the formerly studied isostructural analog, the complex with R being m-nitrophenyl ligand, were studied by experimental (X-ray, IR, Mossbauer spectroscopy and amperometry) and theoretical (quantum chemical modeling (DFT)) methods.

Possible reactions of Roussin’s red esters [Fe2(μ-SC6H4R)2(NO)4], where R = H, o-NH2, m-NO2, m-OH, or m-NH2, in DMSO solution were investigated by quantum chemical modeling. It was shown that in these systems, numerous reactions can occur, including NO donation, ligand substitution, and decomposition into mononuclear iron nitrosyl complexes. The resulting compounds are also NO donors.

A new inhibitor of phosphodiesterase (PDE), i.e., nitric oxide (NO) donor, a water soluble DNIC Na[Fe(C7H5N4S)2(NO)2]⋅2·.5H2O (1), has been synthesized. The structure of the new complex, its physical–chemical properties in the solid state and in solutions have been studied experimentally (X-ray analysis, IR, UV–Vis, Mossbauer spectroscopy, mass-spectrometry, and amperometry) and theoretically (quantum-chemical modeling by DFT method).

The interaction of a nitrosyl iron complex with molecular oxygen was studied with Density Functional Theory. On the basis our analysis of the geometric and electronic structure of the complex, three variants of coordination of the oxygen molecule on it were investigated: the attachment of molecular oxygen to the iron center, an insertion of oxygen in between the Fe and NO and the bridging of two nitrosyl ligands. All of the coordination complexes require approximately the same amount of energy input to form. In all of the oxygen-enriched nitrosyl complexes we calculated, the doublet state was energetically below the quartet state. Next, we considered decomposition pathways since the products of all of these oxygen-attaching pathways may then decompose further. The result of this decomposition is either the release of nitrogen monoxide or the release of nitrogen dioxide. For all the three processes, transition and intermediate states were found, as well as the final products. The potential energy surfaces of the most important reactions were plotted. Finally, UV spectra of all the possible products of the oxidation of complex by molecular oxygen were calculated.

Density functional quantum chemical calculations of Roussin’s red esters with phenyl [Fe2(SPh)2(NO)4] and ortho-aminophenyl [Fe2(SC6H4NH2)2(NO)4] ligands, as well as products of partial and complete replacement of functional ligands by glutathione were carried out. Main characteristics of the Fe—S bonds in these compounds were estimated within the framework of the NBO approach and the AIM theory. The possibility for ligand substitution reactions to occur was demonstrated. A possible mechanism of these processes is proposed.

Reduction of neutral binuclear tetranitrosyl iron complexes of “μ-N-C-S” structural type of [Fe2(SR)2(NO)4] composition with R being functional azaheterocyclic thiolyls has been studied by cyclic voltammetry in aprotic solvents in a wide range of potential scan rates. Redox potentials of these reactions have been determined. The consecutive transfer of two electrons was shown to be typical for all the complexes. For some complexes the products of the first electron addition are unstable, while for the others the products of reduction by both the first and the second electrons are stable. The calculations of geometric and electronic structures of monoanions, dianions and intermediates forming upon the reduction of the complexes were performed by DFT methods. Using quantum-chemical modeling, electrochemical reduction of the complexes has been studied; their energy and theoretical redox potentials have been calculated.

The enthalpy and activation energy of reactions involving attack by MeO2• and MeO2• on CH2 groups of 2-butyl nitrite and 2-nitrosobutane have been calculated by quantum chemical methods. The abstraction of a hydrogen atom is accompanied, in the former case, by concerted N–O bond breaking and, in the latter case, by concerted C–N bond breaking, resulting in NO• formation. On the basis of the results obtained, an algorithm has been developed within the intersecting parabolas model for calculating the enthalpies, activation energies, and rate constants of these types of reactions involving alkyl, alkoxyl, aminyl, peroxyl, phenoxyl, thiyl, and hydroxyl radicals.

In this work, transformations of the anionic binuclear tetranitrosyl iron complex with thiosulfato ligands (Na2[Fe2(S2O3)2(NO)4]·4H2O) (1) have been studied in aqueous anaerobic and aerobic solutions. Under anaerobic conditions, complex 1 was shown to undergo intramolecular isomerization, in addition to NO generation, due to embedding of the oxygen atom of the thiosulfato ligand in the Fe-S bonds. The complex becomes more stable, so its further transformation, including NO release, is unlikely. The model for the complex decomposition has been suggested, and reaction rate constants have been calculated using the kinetic modeling method. As follows from the experimental data, under aerobic conditions, complex 1 is oxidized and becomes a more effective NO donor. Due to the incorporation of the oxygen atom into the Fe-S bonds, mononuclear nitrosyl intermediates are formed, which have a typical EPR signal (g = 2.03).

The electrochemical method and quantum chemical modeling were used to study the NO-donor activity of the mononuclear nitrosyl complex with the thiourea ligand and the influence of the pH of the medium on the reaction. It is shown experimentally that this complex evolves nitrogen monoxide and the character of this process depends on the pH of the medium. For a neutral medium, the curve of nitrogen monoxide evolution reaches a plateau, whereas the curve increases linearly with time for an alkaline medium. The quantum chemical modeling (using DFT methods with the B3LYP and BP86 functionals) of possible processes in an aqueous solution of the complex in neutral and alkaline media allowed us to propose the schemes of the NO-donation reaction, which explain the experimental data on the pH effect.

Quantum-chemical study of the Fe(NO) 2 fragment in the Fe(SC(NH 2 ) 2 ) 2 (NO) 2 + cation of mononuclear nitrosyl iron complex has been performed using various approaches in the frame of density functional method. Both local (Bp86, PW91PW91, BLYP, TPSSTPSS), and hybrid (B3LYP, B3PW91, TPSSh) functionals were used in order to understand which of them are the most suitable for description of the electronic and molecular structures of cations of this type. Using NBO and AIM methods, main characteristics of the Fe(NO) 2 fragment bonds were determined. In the Fe(NO) 2 fragment of the complex π-interactions are strong, this being the reason of re-distribution of the electron density along Fe–N–O. However, the Fe–N bond in this complex differs from a common covalent bond because, according to some topological characteristics, it corresponds to the interaction with closed shells, while the ratio of contributions of kinetic and potential energy densities corresponds to covalent bonds. Local functionals overstate energy of the Fe–NO bond, this being due to the overestimation of the energy of orbital mixing of antibonding orbitals of the Fe–N and N–O bonds.

The quantum chemical study of the redox potentials of ten iron nitrosyl complexes with functional bridging μ-SCN-type ligands was performed by the DFT method combined with the polarizable continuum model (PCM) in order to clarify the influence of the ligand on the redox properties of the complex. The geometry optimization in terms of the PCM approach gave redox potentials, which are in the best agreement with the experimental values. The redox potential depends on the nature of the heteroatom in the five-membered azole ring, the number of N atoms in the ring, and the presence of the fused benzene ring in the aromatic system of the ligand.

The substitution of the glutathione ligand for penicillamine and cysteamine ligands in the cationic nitrosyl iron complexes with the penicillamine thiyl ligand, [Fe2(SC5H11NO2)2(NO)4]SO4·2H2O, and the cysteamine thiyl ligand, [Fe2(S(CH)2NH3)2(NO)4]SO4·2H2O, was studied by quantum chemical methods. Quantum chemical calculations were performed with the full geometry optimization of the starting and final complexes by the DFT method using the local BP86 and OPBE functionals. The structures of the intermediate and final complexes were predicted. The S-C bond in the complexes with the penicillamine ligand is longer than in other complexes of this type. The calculated energy of the detachment of the ligand from the complex with penicillamine is lower than that for the complex with the cysteamine ligand. The ligand substitution in the complex with the penicillamine ligand does not require considerable energy and can easily proceed in aqueous solution, whereas this reaction in the complex with the cysteamine ligand is thermodynamically unfavorable.

The geometries and electronic structures of iron-sulfur nitrosyl complexes with azaheterocyclic thiols with μ-SCN bridging ligands, [Fe2(SCN4Ph)2(NO)4] (1) and [Fe2(SCN4Me)2(NO)4] (2), were calculated within the framework of the density functional theory (DFT) with the BP86, TPSS, B3LYP, and OPBE functionals in order to explain differences in their NO-donor activities. Chemical bonding in the complexes was analyzed using the NBO approach, according to Mulliken, and using the Voronoi scheme. NBO analysis of complex 1 revealed a strong interaction between orbitals of the lone electron pairs of the Fe atom and the antibonding orbitals of the Fe-N bond which leads to strengthening of the Fe-NO bond. In complex 2, the Fe-NO bond is more polar than in complex 1.

Reduction of neutral binuclear nitrosyl iron complexes of “μ-S” structural type [Fe2(SR)2(NO)4] with R = 3-nitro-phenol-2-yl, 4-nitro-phenol-2-yl, 5-nitropyridine-2-yl and pyridine-2-yl in aprotic solution has been studied by a cyclic voltammetry (CVA) method at a wide range of potential scan rates. A complex with R = 3-nitro-phenol-2-yl was synthesized for the first time; therefore it was studied by X-ray and Mossbauer spectroscopy. The parameters of the Mossbauer spectrum are: isomer shift δFe = 0.115(1) mm s−1, quadrupole splitting ΔEQ = 1.171(1) mm s−1, and line width = 0.241(1) mm s−1 at 85 K. From the current–voltage curve, the transfer of the first electron was found to be reversible, and the redox-potentials of these reactions were determined. The further reduction of the complexes was determined to be irreversible because the product of the second electron addition is instable and decomposes partially during the potential scan. Calculations of geometric and electronic structures of monoanions and dianions of the complexes under study and their theoretical redox-potentials were performed by DFT methods. Introduction of the electron-acceptor NO2 group into the phenyl and pyridine rings of sulfur-containing ligands of the nitrosyl iron complexes was found to affect the geometry of the anions and the distribution of the additional negative charge, as well as to increase the redox-potential and to facilitate reduction of these complexes.

A new tetranitrosyl binuclear iron complex, [Fe2(SC6H6N)2(NO)4] (1), has been synthesized by two methods. Molecular and crystalline structure of 1 were determined by X-ray analysis; the complex is binuclear of “μ-S” type with ~2.7052(4) A between the irons. The compound crystallizes in monoclinic, space group P21/n, Z = 2; parameters of the unit cell: a = 6.6257(2) A, b = 7.9337(2) A, c = 16.7858(4) A, β = 96.742(2)°, V = 876.26(4) A3. Parameters of Mossbauer spectrum for 1 are: isomer shift δFe = 0.096(1) mm/s, quadrupole splitting ΔEQ = 1.122(1) mm/s, line width 0.264(1) mm/s at 293 K. As follows from the electrochemical analysis of aqueous solutions of 1, it generates NO in protonic media without additional activation. NO amount and the rate of its activation are much higher in acidic solutions than in neutral and alkali ones. The constants of hydrolytic decomposition of 1 were calculated. The geometry and electronic structure of isolated 1 were studied using the density functional theory. Differential...

New tetranitrosyl binuclear iron complex [Fe 2 (SС 7 H 5 N 4 ) 2 (NO) 4 ] ( I ) has been synthesized by interaction of aqueous solutions of anionic salts [Fе(S 2 O 3 ) 2 (NO) 2 ] 3− and [SС 7 H 5 N 4 ] − . The latter one was synthesized by reduction of bis -(1-phenyl-1H-tetrazole-5-yl) disulfide with hydrazine hydrate in ethanol at T = 25 °C. Molecular and crystalline structure of I was determined by X-ray analysis; the complex has binuclear structure of “μ-SCN” type with ∼4.02 A between the iron atoms. Shortened О⋯О contacts (2.81 A) between the NO groups of similar type are observed. Parameters of Mossbauer spectrum for I are: isomer shift δ Fe = 0.311(1) mm/s, quadrupole splitting Δ E Q = 1.044(1) mm/s, line width Γ = 0.267(1) mm/s at 85 K. From SQUID magnetometry data, the temperature and field dependences of the magnetic moment of I are well described in the frame of a simple model of binuclear iron complex with magnetic centers S 1 = S 2 = ½. In solution, binuclear structure of the complex remains, though the NO groups are non-equivalent. For solutions of I five-line hyperfine structure of spectrum (HFS) is observed, g-factor = 2.03. For polycrystals of I , no HFS was observed due to averaged exchange interaction between the electron spins of adjacent complexes. In polycrystals of I , the number of spins per one binuclear complex is I in 1% dimethylsulfoxide (DMSO) aqueous solution is ∼13.8 nM, it halves in 8 min after decomposition starts, and reaches ∼3.8 nM in anaerobic conditions at Т = 25 °С, pH 7.0. This is due, according to quantum-chemical calculations, to the presence of a more stable Fe NO bond in I than in its isostructural analog – nitrosyl iron complex with 1-methyltetrazole-5-yl ( II ).

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.

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.