Radical Dianion Research Articles

Two new dibenzobicyclic penta- and hexacoordinated tin compounds

Two new dibenzobicyclic penta- andhexacoordinated tin compounds (1–2) were prepared, derived from bis(2-hydroxy-3,5-di-tert-butyl-phenol)imine and dichloro-dimethylstannane and trichloro-n-butylstannane, respectively. Each compound is an example of two different coordination forms of the ligand. The crystalline complexes have been characterized by magnetic moment, infrared and mass spectrometry, elemental analysis, and X-ray diffraction studies. Compound 1 crystallized in the monoclinic system, space group P21/n; it contains a five-coordinated trigonal bipyramidal tin atom with methyl groups in equatorial positions and a planar dibenzobicyclic ring. It is a paramagnetic compound (μeff = 1.59 μB) due to the radical dianion nature of the ligand. Compound 2 crystallized also in the monoclinic system, space group P21/c; it contains a six-coordinated tin atom with an octahedral geometry; the dibenzobicyclic ring is planar, and the chlorine atoms are in trans positions. It is diamagnetic with a delocalized monoanion structure of the ligand. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:321–326, 1998

Two new dibenzobicyclic penta‐ and hexacoordinated tin compounds

Two new dibenzobicyclic penta- andhexacoordinated tin compounds (1–2) were prepared, derived from bis(2-hydroxy-3,5-di-tert-butyl-phenol)imine and dichloro-dimethylstannane and trichloro-n-butylstannane, respectively. Each compound is an example of two different coordination forms of the ligand. The crystalline complexes have been characterized by magnetic moment, infrared and mass spectrometry, elemental analysis, and X-ray diffraction studies. Compound 1 crystallized in the monoclinic system, space group P21/n; it contains a five-coordinated trigonal bipyramidal tin atom with methyl groups in equatorial positions and a planar dibenzobicyclic ring. It is a paramagnetic compound (μeff = 1.59 μB) due to the radical dianion nature of the ligand. Compound 2 crystallized also in the monoclinic system, space group P21/c; it contains a six-coordinated tin atom with an octahedral geometry; the dibenzobicyclic ring is planar, and the chlorine atoms are in trans positions. It is diamagnetic with a delocalized monoanion structure of the ligand. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:321–326, 1998

Electrochemical and Spectroelectrochemical Characterization of Some Polypyrrole/Anthraquinone Sulfonate Films.

We have fabricated composite films of polypyrrole/quinone sulfonate via electropolymerization. The FTIR investigation indicated the incorporation of quinone sulfonate in the polypyrrole (PPy) film to compensate for a positive charge in the polymer chain. Further, the elemental analysis of PPy/9,10-anthraquinone-2-sulfonate (AQS−) showed that 0.29 of the sulfonate was incorporated per pyrrole unit. Cyclic voltammetric results in dimethylsulfoxide and dimethylformamide solutions showed that the two 1-electron transfer steps could be assigned to the formation of radical dianion (AQS2−.) and trianion (AQS3−). The positive shift of the first cathodic peak for the incorporated AQSP− indicated a strong interaction between the dianion and the cationic sites of the polymer. The PPy/AQS- film worked as a cation-exchanger at electrode potentials below about −0.3 V vs. Ag/AgCl (saturated KCl) because of the creation of the anionic sites of the reduced quinone.

Oxidation intermediates of α-glucosyl rutin by pulse radiolysis

Transient intermediates generated in the reaction of α-glucosyl rutin (G-rutin) with the OH radical or the azide radical were investigated by pulse radiolysis. The OH radical reacted with G-rutin to produce the OH-adduct radical, followed by deprotonation in neutral and acidic solutions. In alkaline solutions, the OH-adduct di-anion radical formed from the di-anions of G-rutin were dehydrated to the phenoxyl radical. On the other hand, G-rutin was oxidized to one-electron oxidized cation radical by the azide radical. The pK a values of the OH radical adduct were found to be 6.83 ± 0.10 and 8.87 ± 0.13. While, the pK a values of the phenoxyl radical of G-rutin were found to be 7.52 ± 0.11 and 9.95 ± 0.13.

Quantitative EPR Spin Trapping. 1. Nitrogen Dioxide Radicals and Nitrite Ions from Energetic Materials in Alkaline Aqueous Solution

A new application of electron paramagnetic resonance (EPR) spin trapping is described using the nitromethane aci anion (CH2NO2-) spin trapping technique for quantitative measurement of nitrogen dioxide radicals (•NO2) and nitrite ions (NO2-) in strong alkaline aqueous solution. The relative concentration of a spin adduct radical, dinitromethyl dianion radical [-O2N−CHNO2•-] (I), produced by ultraviolet photolysis, is shown to be proportional to the concentration of NO2- dissolved in alkaline solution. This method is applied to estimate the •NO2 concentrations produced by decomposition of two energetic nitramine compounds: cyclotrimethylenetrinitramine, C6H6N6O6 (RDX), and ammonium dinitramide, NH4+N(NO2)2- (ADN), by using calibration plots of the spin concentration of I versus time of photolysis. Calibrations were prepared using sodium nitrite (NaNO2) or ammonium nitrate (NH4NO3) which behave similarly to the energetic compounds with respect to formation of I.

Gem-Nitronitroso Dianion Radicals R-C (NO2)(NO)2- Formed in the Reaction between Sodium Nitrite/Ascorbate and the Sulfur Compounds Methionine, (Methylthio)acetic Acid and S-Methylcysteine Studied by EPR Spectroscopy.

gem-Nitronitroso Dianion Radicals R-C (NO2)(NO)2- Formed in the Reaction between Sodium Nitrite/Ascorbate and the Sulfur Compounds Methionine, (Methylthio)acetic Acid and S-Methylcysteine Studied by EPR Spectroscopy.

ChemInform Abstract: Acidities of Radical Anions (pKHA‐×) Derived from Nitro‐ Substituted Aromatic Weak Acids and the Formation of Radical Dianions.

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Acidities of Radical Anions (pKHA−•) Derived from Nitro-Substituted Aromatic Weak Acids and the Formation of Radical Dianions

A simple method has been devised for measuring the equilibrium acidities in DMSO of radical anions by using the equation pKHA−• = pKHA + 16.8[Erd(HA) − Erd(A-) ], which is based on a thermodynamic cycle. The acidities of 24 nitro-substituted aromatic radical anions have been measured in this way. The reversible reduction potentials to form the radical anions and those of their conjugate bases to form the radical dianions were obtained by cyclic voltammetry. The points for the irreversible Erd(HA) values for 6 nitroaromatic compounds were found to fit on or close to the line of a plot of Erd(HA) vs σp- for the reversible Erd(HA) values for 11 nitroaromatic compounds. The σp- value for p-NH2 was estimated to be about −1.0 from this plot. Relative radical dianion stabilization energies have been derived from the equilibrium acidities of the radical anions and the reduction potentials of the parent acids to provide an estimate of the stabilities of the radical dianions, relative to that of the p--OC6H4NO2-• r...

EPR spectroscopic study of the radical oxidation of hydroxypurines in aqueous solution: acid–base properties of the derived radicals

EPR spectroscopy has been employed to study the reactions of SO4˙– and OH˙ radicals with xanthine, 8-methylxanthine, hypoxanthine and 2-hydroxypurine.Radicals obtained on the reaction of SO4˙– with these hydroxypurines resulted, depending on the pH, from one-electron oxidation of either the parent compound or its anion, followed by deprotonation. For xanthine and 8-methylxanthine the respective radical anion and radical dianion were observed, for hypoxanthine the species detected correspond to the neutral radical and radical anion, whereas for 2-hydroxypurine only its radical anion was observed.The pKa values of the radical anion of xanthine and 8-methylxanthine were estimated as 12.0 ± 0.1 and 12.1 ± 0.5, respectively, whereas the pKa value of the hypoxanthine neutral radical was 7.7 ± 0.1.In the reaction with OH˙, only the radical anion of xanthine was detected.

Bond Dissociation Free Energies (BDFEs) of the Acidic H-A Bonds in HA(*)(-) Radical Anions by Three Different Pathways.

Cleavage of radical anions, HA(*)(-), have been considered to give either H(*) + A(-) (path a) or H(-) + A(*) (path b), and factors determining the preferred mode of cleavage have been discussed. It is conceivable that cleavage to give a proton and a radical dianion, HA(*)(-) right harpoon over left harpoon H(+) + A(*)(2)(-) (path c), might also be feasible. A method, based on a thermodynamic cycle, to estimate the bond dissociation free energy (BDFE) by path c has been devised. Comparison of the BDFEs for cleavage of the radical anions derived from 24 nitroaromatic OH, SH, NH, and CH acids by paths a, b, c has shown that path c is favored thermodynamically.

Effects of Steric Congestion on Stilbene Radical Anions and Dianions: DFT Calculations in the Interpretation of Stilbene Radical Anion ESR Spectra.

Effects of Steric Congestion on Stilbene Radical Anions and Dianions: DFT Calculations in the Interpretation of Stilbene Radical Anion ESR Spectra.

Gem-Dinitro and gem-nitronitroso dianion radicals formed in the reaction between sodium nitrite–ascorbate and carbonyl compounds such as aldehydes, alicyclic ketones, lactams and peptides. An EPR study

gem-Dinitro and gem-nitronitroso radical dianion R–C˙(NO2)22– or R–C˙(NO2)(NO)2–(R = alkyl, aryl) are formed in the reaction between sodium nitrite and carbonyl compounds such as acetaldehyde, benzaldehyde, furfural, cyclopentanone, cyclohexanone, lactams, some dipeptides or insulin dissolved in aqueous acid containing a small amount of ascorbic acid. The radicals appeared in the dark at room temperature when the reaction mixtures had been made alkaline. The EPR spectra indicated an interaction of the unpaired electron with two nitrogen nuclei, and also with either a hydrogen or a further nitrogen nucleus in the parent substances. The two nitrogens were non-equivalent in the radicals derived from substances such as acetaldehyde or glycylglycine (nine-line system) owing to the formation of R–C˙(NO2)(NO)2–, but equivalent in the radicals from substances such as cyclopentanone (five-line system) owing to the formation of R–C˙(NO2)22–. The reaction involves nitration/nitrosation of the carbonyl carbon in acid solution with cleavage of C–C bonds or dissociation of a hydrogen ion leading to intermediate nitronitroso or dinitro compounds which are reduced by ascorbic acid to give the observed radicals when the reaction mixtures are made alkaline.

Characterization of titanocene(III) complexes of β-diketonates by electrochemical, spectroscopic and crystallographic methods: stabilization of oxidized and reduced β-diketonate radicals by acetyl and titanocene derivatization, respectively

The preparation and characterization of the titanocene(III)-β-diketonate complexes Ti( η 5-C 5H 5) 2(acetylacetonate), Ti( η 5-C 5H 5) 2(benzoylacetonate), Ti( η 5-C 5H 5) 2( ortho-hydroxy-acetophenonate) and Ti( η 5-C 5H 5) 2( O, O-3-acetyl-1-benzyl-2-hydroxyl-5-methoxyindolate) are described. The molecular structure of Ti( η 5-C 5H 5)(acetylacetonate) was determined by X-ray crystallography (crystal data: monoclinic, space group P2 1/ c, a = 8.338(6), b = 21.71(2), c = 7.86(1) A ̊ , β = 106.9(1)°, U = 1362(3) A ̊ 3 , Z = 4, T = 293 K R = 0.087, R w = 0.093 based on 1015 independent reflections for I>2 σ( I)). The structure was found to be slightly distorted and exhibited normal average TiO and TiC bond lengths of 2.07 and 2.37 Å (average), respectively. Electrochemical data obtained from voltammetric studies on 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole are compared to that from derivatives in which the hydroxy hydrogen has been replaced by an acetyl or a titanocene group. Appropriate derivatization is shown to stabilize the organic oxidation (acetyl) and reduction (titanocene) processes. In the case of the titanocene derivative, both metal and ligand based processes are observed. The titanocene(III) fragment can be reversibly oxidized in a one-electron metal based process which occurs at a potential which is essentially independent of the β-diketonate ligand. The product of this process is the expected Ti(IV) moiety. The ligand coordinated to the titanocene is oxidized at a considerably more positive potential. In contrast, the acetyl derivative, 2-acetoxy-3-acetyl-1-benzyl-5-methoxyindole, undergoes a reversible one-electron oxidation to produce a moderately stable cation radical which is characterized spectroscopically in an in situ electrochemical EPR experiment. The reduction of 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindole leads to reductive deprotonation with the resulting anion not being reducible. However, voltammetric reduction after coordination to a titanocene(III) fragment can be achieved. In the case of Ti( η 5-C 5H 5) 2(benzolacetone) the reduction process becomes reversible at low temperature to yield a moderately stable coordinated dianion radical.

Free radicals of electrochemical reduction of derivatives of 3-nitro-1,4-dihydropyridines

By means of ESR, under conditions of eletrochemical generation of particles, the formation of four types of secondary free radicals has been confirmed in the process of electrochemical reduction of molecules of N-unsubstituted derivatives of 3-nitro-1,4-dihydropyridine in dimthylformamide — specifically, a dianion radical of the molecule of the original compound; an anion radical of the corresponding isomeric 4,5-dihydropyridine; a radical of the nitroalkane type; and in addition, for the compound substituted with a nitrobenzene group; a free radical with the nitrobenzene structure. Methods for synthesis of the individual compounds are described, and a scheme is presented for the mechanism of their electrochemical conversion.

Chemistry of iron oxophlorins. 2. Oxidation of the iron(III) octaethyloxophlorin dimer and observation of stepwise, two-electron oxidation of the oxophlorin macrocycle

The oxidation of iron oxophlorin (meso-hydroxyporphyrin) complexes has been studied in order to understand the basic chemical reactivity which may contribute to the heme destruction that is catalyzed by heme oxygenase. Reaction of the iron(III) octaethyloxopblorin dimer [Fe III (OEPO)] 2 with molecular bromine results in the formation of two new, highly oxidized complexes, BrFe III (OEPO . ) and Br 2 Fe III (OEPOx) (where OEPO . and OEPOx are the octaethyloxophlorin dianion radical and the oxidized octaethyloxophlorin monoanion, respectively), in which the experimental data suggest that it is the ligand rather than the iron that has undergone oxidation

Solvent dependence of linewidth studies in the electron spin resonance spectra of the dianion radicals p- and m-nitrobenzoates

Solvent dependence of linewidth studies in the electron spin resonance spectra of the dianion radicals p- and m-nitrobenzoates

TYPE I and TYPE II PHOTOSENSITIZATION BY THE ANTIBACTERIAL DRUG NALIDIXIC ACID. A LASER FLASH PHOTOLYSIS STUDY*

The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.

Elektrochemische und spektroelektrochemische Untersuchungen neuer S-heterochinoider Elektronenakzeptoren, N-Cyanimin-substituierte Thieno[3,2-b]thiophen-2,5-dione / Electrochemical and Spectroelectrochemical Investigations of New S-Heteroquinoid Electron Acceptors, N-Cyanimine Substituted Thieno[3,2-b]thiophene-2,5-diones

Redoxproperties and UV/VIS/NIR-spectra of radical anions and dianions of 3,6-dimethylthieno[3,2-b]thiophen-2,5-dion and 2,5-bis(cyanimino)-2,5-dihydro-thieno[2,3-b]thiophene derivatives are compared.

Decay of electrogenerated dianions of acidic indenes: an ‘indirect’ self-protonation mechanism

The electrode reduction of 1,2,3-triphenylindene, giving rise to the corresponding radical anion and dianion in two successive one-electron steps, was investigated in dimethylformamide with the aim of clarifying the role of the acidic substrate in the decay of the electrogenerated bases. At –50 °C the direct protonation of the radical anion by indene (self-protonation) was too slow to be effective, and the proton transfer from indene to the dianion did appear kinetically unfavourable with respect to solution electron transfer between the same species (comproportionation). However, dianion protonation by an excess of water present in the solution can induce, through the action of OH– ions, the substrate to transform into its conjugate base. The effect resembles that predicted for a self-protonation mechanism and can be termed an ‘indirect’ self-protonation process. The proposed reaction scheme was confirmed by comparison of cyclic voltammetric responses with those derived by mathematical computation. The rate-constant values for the relevant steps, obtained by the fitting of experimental and computed data, are consistent with the corresponding values determined for similar compounds.

Nature of the transient species in the one-electron oxidation of diethyl dithiocarbamate as studied by pulse radiolysis

One-electron oxidation of the diethyl dithiocarbamate (DDC) anion by oxidising radicals such as ˙OH, N3˙, Br2˙–etc. has been studied employing the pulse radiolysis technique. The transient species absorbing in the visible region (λmax 570 nm) has been identified as the dimer anion radical formed by the interaction of the initial neutral radical (Et2NCS2˙) with the parent anion. This species was found to be rather unreactive towards other solutes and decayed, with second-order kinetics, to give disulfiram. The reaction of disulfiram with hydrated electrons (e–aq) gave rise to the same transient spectrum as obtained in the case of one-electron oxidation of DDC. The reaction of e–aq with DDC at pH 9 gave a transient spectrum with λmax 340 nm which has been attributed to a carbon-centred radical dianion (Et2NCS2˙2–).