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A series of branched high-molecular-weight alkoxyamines (HAAs) based on polystyrene of different molecular weight were synthesized using nitroxide radicals generated in situ in the presence of conjugated dinitrones (N,N-dimethylglyoxaldinitrone, N,N-di-tert-butylgly-oxaldinitrone, and N,N-diphenylglyoxaldinitrone). Structural features of the products obtained were studied by MALDI-TOF mass spectrometry. Modification of the synthesized HAAs in the presence of azobisisobutyronitrile, carbon tetrabromide, dodecyl mercaptane, 4,5,5-trimethyl-2,2-diethyl-2,5-dihydroimidazole-1-oxyl, and 3,5-di-tert-butylbenzoquinone, as well as thermolysis of the HAAs in the presence of atmospheric oxygen showed that the nitroxide fragments are located within the polymer chain irrespective of the initial structure of the conjugated dinitrone. The molecular weight characteristics and conformational properties of the nitroxide-containing linear macromolecules and polymers were studied by static and dynamic light scattering and by viscometry. In most cases, the calculated values of the ρ-parameter (Rg/Rh) and the Zimm viscosity factor of the branched polystyrene samples synthesized using conjugated dinitrones are lower than those of linear analogues. [ABSTRACT FROM AUTHOR]

The structures of low-molecular-weight chitosan oligomers and their two nitroxide derivatives were examined using the force field molecular dynamics (MD) and density functional theory calculations with implicit and explicit solvent models. Rotamers of both neutral oligomers and those protonated at amino group were determined and the influence of the chain length and solvent (water) on their stability was studied. Bent conformations of the chitosan chain were found to be the most stable in the gas phase, whereas a linear structure of the polysaccharide is more preferred in water. Conformational transitions between the bent structures can occur only via the linear conformation of chitosan. According to the MD calculations, the nitroxides linked at the amino group do not form intramolecular hydrogen bonds with polar groups of the polysaccharide. The nitroxide substituents enhance the conformational flexibility of chitosan, but the linear conformation of the substituted oligomers remains most populated in water. [ABSTRACT FROM AUTHOR]

A method for the synthesis of solvates of dinuclear heterospin complexes of bis(hexafluoroacetylacetonato)copper(ii) with 2-(4-methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LMe) and 2-(4-ethylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) of the composition [Cu(hfac)2LR]2 · Solv (Solv = benzene, bromobenzene, toluene, o-xylene, p-xylene) was developed. It was found that a decrease in temperature induces structural transformations of the solid phases of the complexes followed by significant changes in the distances between the paramagnetic centers in the intramolecular exchange clusters {Cu2+-O·-} from 2.321 to 1.974 Å. As a result, the temperature dependences of the effective magnetic moment μeff(T) exhibit magnetic features similar to spin transitions. An analysis of the shape of the μeff(T) curves suggested that the magnetic properties of the compounds under study depend primarily on the molecular packing. A comparison of the magneto-structural correlations typical of the [Cu(hfac)2LR]2 · Solv complexes studied in this work with the data obtained earlier for analogous solvates of heterospin polymer chain complexes showed that the spin transition characteristics of the [Cu(hfac)2LR]2 · Solv systems are much less sensitive to the change of the solvent than the corresponding characteristics of the heterospin polymers containing solvate molecules in the interchain space. The magnetic characteristics of the heterospin dimer molecules depend primarily on which cavity between the dimers is filled with solvate molecules. For solvates with monoclinic symmetry, changes in the solvent molecules occupying same-type cavities have almost no effect on the spin transition characteristics in the heterospin exchange cluster. [ABSTRACT FROM AUTHOR]

The cytotoxicity and antioxidant properties of water-soluble (at physiological pH) chitosan- (poly)nitroxides (CPNs) containing pyrroline-based nitroxide and having different molecular weights of the polysaccharide backbone were studied on normal (Vero) and tumor (HepG2) cell models. The compounds were found to exhibit low cytotoxicity and to effectively reduce the level of reactive oxygen species (ROS) generated upon the breakdown of tert-butyl hydroperoxide in normal cells. In tumor cells, the CPNs have little effect on the ROS level under the oxidative stress. The results show that CPNs also represent a promising platform for the development of cell delivery systems for biologically active compounds. [ABSTRACT FROM AUTHOR]

We synthesized 1-ethylimidazolyl-substituted nitronyl nitroxides, i.e., 2-(1-ethylimidazol-4-yl)- (L) and 2-(1-ethylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide-1-oxyl (L). The stable radical L is an ethyl analog of 2-(1-methylimidazol-5-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide-1-oxyl (L) described earlier, the reaction of which with Cu(hfac) (hfac is 1,1,1,5,5,5-hexafluoropentane-2,4-dionate) leads to the formation of the [Cu(hfac)(L)] jumping crystals. The reaction of Cu(hfac) with L with reagent ratios 1: 2 and 1: 1 yields heterospin complexes [Cu(hfac)(L)] and [Cu(hfac)L], respectively. X-ray diffraction study of the mononuclear complex [Cu(hfac)(L)] determined that the compound has a packing similar to that of jumping crystals studied earlier, with the only difference being that the O...O contacts between neigh- boring nitroxide groups were found to be 0.3-0.5 Å longer than in [Cu(hfac)(L)]. As a result of the lengthening of these contacts, [Cu(hfac)(L)] crystals lack chemomechanical activi- ty. We found that when cooling crystals of binuclear complex [Cu(hfac)L] below 50 K, the antiferromagnetic exchange between unpaired electrons of the >N-•O groups of neighboring molecules leads to the full spin-pairing of the nitroxides, with only the Cu ions contributing to the residual paramagnetism of the compound. [ABSTRACT FROM AUTHOR]

5-Formylpyrrolyl-substituted nitronyl and imino nitroxide radicals HL and HL were synthesized. Their solid phases are formed by packing pairs of the molecules. In the {HL...HL} pairs, the dominant interaction is the ferromagnetic exchange with J/kB = 8.8 K (Hamiltonian $$H = 2J\left( {\overrightarrow {{s_1}} \overrightarrow {{s_2}} } \right)$$ ). The ferromagnetic exchange occurs also in the heterospin molecules [Ni(L)], [Cu(L)], and [Ni(L)(MeOH)]. In the complexes [Ni(L)] and [Cu(L)], a small change in the mutual orientation of the coordinated ligands has a considerable effect on the value and the sign of the energy of exchange interactions between the unpaired electrons of the metal ion and paramagnetic ligands. [ABSTRACT FROM AUTHOR]

The synthesis, structures, and magnetochemical data for the heterospin chain polymer complexes Cu(hfac)L and Cu(hfac)L·0.5Solv, where hfac is the hexafluoroacetylacetonate anion, L and L are 2-(1-allyl-1 H-pyrazol-4-yl)- and 2-(1-butyl-1 H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyls, respectively, Solv is isopropylbenzene or tert-butylbenzene, were described. These polycrystalline solids were found to exhibit similar magnetic anomalies. Thus, the effective magnetic moment first decreases and then increases with temperature, resulting in the specific minimum in the curve μ( T). The study of the magneto-structural correlations showed that the appearance of the minimum is attributed to different factors. For Cu(hfac)L, this is the phase transition accompanied by the structural rearrangement of the exchange cluster >N-·O-Cu-O·-N<, which leads to a change in the energy of the exchange interaction between the unpaired electrons of the paramagnetic centers. By contrast, for Cu(hfac)L·0.5PrPh and Cu(hfac)L·0.5BuPh, the appearance of the minimum is a consequence of the coexistence of exchange interaction energies with opposite signs, while the structure of the solid phase remains unchanged. [ABSTRACT FROM AUTHOR]

A method was developed for the synthesis of a nitronyl nitroxide containing cyclopentane substituents in positions 4 and 5 of the imidazoline ring, viz., 2-(3-pyridyl)-4,5-bis(spiropentyl)-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyl (L). The reaction of Cu hexafluoroacetylacetonate with L affords different products depending on the reaction conditions: mononuclear [Cu(hfac)(L)], binuclear [Cu(hfac)L], tetranuclear {[Cu(hfac)](L)}, or chain polymer {[Cu(hfac)](L)}. Temperature changes induce structural transformations accompanied by a change in the spin state in exchange clusters in the solid [Cu(hfac)L] and {[Cu(hfac)](L)}. [ABSTRACT FROM AUTHOR]

A procedure was developed for the synthesis of the kinetically stable nitroxide 2-( N- tert-butyl- N-hydroxyamino)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole-3-oxide-1-oxyl containing the sterically hindered hydroxylamino group. The crystal and molecular structure of this compound was determined. The isolation of this nitroxide hydroxylamine in the individual state enabled a new procedure for the preparation of its oxidation product, viz., the corresponding nitroxide biradical. The results of quantum chemical calculations of the electron exchange in the biradical molecule are in good agreement with the experimental data on the triplet ground state of the molecule in the crystal obtained earlier. [ABSTRACT FROM AUTHOR]

To find the factors favorable for the appearance of chemomechanical activity of heterospin crystals, a series of new heterospin complexes were synthesized and characterized. It includes [Cu(tfac)L]·2CHCl, [Cu(tfac)L]·2EtOH, [[Cu(pfu)]L]·1/2CHCl, [Cu(pfh)L]·1/2CHCl, [Cu(piv)L]·2MeOH, [Co(piv)L], [ $$ Cu(hfac)_2 L^{CD_3 } _2 $$], [Cu(hfac)L]·CHCl, and [Cu(hfac)L] (L, $$ L^{CD_3 } $$, and L are N-methylimidazolyl-, N-trideuteriomethylimidazolyl-, and N-methyltriazolyl-substituted nitronyl nitroxides, respectively; tfac, hfac, pfu, pfh, and piv are the charged coordinated diamagnetic ligands 1,1,5,5-tetrafluoropentane-2,4-dionate, 1,1,1,5,5,5-hexafluoropentane-2,4-dionate, 1,1,1,2,2,3,3,4,4,8,8,9,9,10,10,11,11,11-octadecafluoroundecane-5,7-dionate, 1,1,1,5,5,6,6,6-octafluorohexane-2,4-dionate, and 2,2-dimethylpropionate, respectively). The crystal and molecular structures of all compounds were determined. The results of the X-ray diffraction study of the complex [Ni(hfac)L] synthesized earlier are reported. In the solid state of the complexes [Cu(pfh)L] and [Co(piv)L], the paramagnetic ligands are cis-coordinated to the central atom in a monodentate fashion via the donor N atom of the imidazole ring. In the dinuclear complexes [[Cu(pfu)]L] and [Cu(hfac)L], the paramagnetic ligands are also in cis positions but act as bridges through coordination of the donor N atoms of the azole ring and the O atom of the nitronyl nitroxide moiety to different Cu ions. In the solid complexes $$ Cu(hfac)_2 L^{CD_3 } _2 $$ , [Cu(hfac)L], [Cu(tfac)L]·2CHCl, [Cu(tfac)L]·2EtOH, and [Cu(piv)L]·2MeOH, the nitronyl nitroxide radicals in the mononuclear heterospin molecules are in trans positions. The packing motif in the crystal structures of the complexes $$ Cu(hfac)_2 L^{CD_3 } _2 $$ , [Cu(tfac)L]·2CHCl, and [Cu(tfac)L]·2EtOH is the same as that in the previously studied complexes [M(hfac)L] exhibiting chemomechanical activity. Among the complexes under consideration, only crystals of $$ Cu(hfac)_2 L^{CD_3 } _2 $$ can exhibit chemomechanical activity, that is to make jumps upon heating or irradiation. The results of the present study suggest that the packing of the solid-state structure plays a key role in the generation of mechanical activity of the crystals. [ABSTRACT FROM AUTHOR]

The cation of 2,2,6,6-tetramethyl-1,4-dioxopiperidinium perchlorate disproportionates in acidic aqueous medium to give piperidinoxyl and/or 1-hydroxypiperidine, nitrone, and acetone. A mechanism of this reaction was proposed and the rate constants for its particular steps were calculated. [ABSTRACT FROM AUTHOR]

Parameters of exchange interactions in heterospin chain polymeric complexes of Cu(hfac)2 (hfac is hexafluoroacetylacetonate anion) with pyrazolyl-substituted nitronyl nitroxides LR (R = Me, Et, Pr, Bu) were estimated using quantum chemical computational methods. The magnetic properties of the considered chain polymeric complexes can be described within the framework of the model of isolated exchange clusters. Experimental data on the structural dynamics of chains polymeric with “head-to-tail” (R = Me) and “head-to-head” (R = Et, Pr, Bu) motifs are discussed in the context of the concept of gradual phase transitions. Based on the analysis performed, a hypothesis of microscopic mechanisms of magnetic transitions in crystals of this type of compounds was proposed. [ABSTRACT FROM AUTHOR]

Addition products of 4-hydroximino-2,2,6,6-tetramethylpiperidine-1-oxyl with 5-aryl-2,3-dicyano-1-ethylpyrazinium salts were isolated for the first time. Crystallographic data on the three-dimensional structures of σH-adducts bearing a stable radical moiety were obtained. [ABSTRACT FROM AUTHOR]