- Oxidative double dehalogenation of tetrachlorocatechol by a bio-inspired CuII complex: Formation of chloranilic acid
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Copper(II) complexes of the potentially tripodal N,N,O ligand 3,3-bis(1-methylimidazol-2-yl)propionate (L1) and its conjugate acid HL1 have been synthesised and structurally and spectroscopically characterised. The reaction of equimolar amounts of ligand and CuII resulted in the complexes [Cu(L1)]n(X)n (X = OTf-, PF 6-; n = 1,2), for which a new bridging coordination mode of L1 is inferred. Although these complexes showed moderate catecholase activity in the oxidation of 3,5-di-tert-butylcatechol, surprising reactivity with the pseudo-substrate tetrachlorocatechol was observed. A chloranilato-bridged dinuclear CuII complex was isolated from the reaction of [Cu(L1)]n(PF6)n with tetrachlorocatechol. This stoichiometric oxidative double dehalogenation of tetrachlorocatechol to chloranilic acid by a biomimetic copper(II) complex is unprecedented. The crystal structure of the product, [Cu2(Ca)Cl2(HL1) 2], shows a bridging bis-bidentate chloranilato (ca) ligand and ligand L1 coordinated as its conjugate acid (HL1) in a tridentate fashion. Magnetic susceptibility studies revealed weak antiferromagnetic coupling (J = -35 cm-1) between the two copper centres in the dinuclear complex. Dissolution of the green complex [Cu2(Ca)Cl2(HL1) 2] resulted in the formation of new pink-purple mononuclear compound [Cu(Ca)(HL1)(H2O)], the crystal structure of which was determined. It showed a terminal bidentate chloranilato ligand and N,N-bidentate coordination of ligand HL1, which illustrates the flexible coordination chemistry of ligand L1.
- Bruijnincx, Pieter C.A.,Viciano-Chumillas, Marta,Lutz, Martin,Spek, Anthony L.,Reedijk, Jan,Van Koten, Gerard,Gebbink, Robertus J.M. Klein
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- Kinetic study of the fast step of the alkaline hydrolysis of p-chloranil using stopped flow technique
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The alkaline hydrolysis of p-chloranil or 2,3,5,6-tetrachloro-1,4-benzoquinone (C6Cl4O2,Q) was studied, using stopped flow spectrophotometry and Electron Spin Resonance techniques (E.S.R.). In the present study it was shown for the first time, that a free radical is produced chemically and that it can account for the propagation of the reaction. It was found that in alkaline conditions chloranil in a `Michael' fashion undergoes 1,2 addition being hydrolyzed and in turn produces a chloranil free radical (Q·). The hydrolysis then proceeds via a number of intermediates yielded by this radical and a number of different products is formed. The formation of these products, both quantitatively and qualitatively has a strong dependence on the concentration of the OH- species and chloranil. The various possible routes of the hydrolysis are studied either spectrophotometrically or by E.S.R. Two different intermediates are observed absorbing at 426 nm and at 540 nm, respectively. Each species was formed and destroyed within 10 s to 30 min depending on the exact conditions. The reaction rate constants for the formation and the decay of the intermediates was estimated using the Guggenheim method. At both wavelengths the rate constants seem to have a complex relation to the concentration of the anion.
- Lafis,Konidari,Veltsistas,Tzerpos,Karayannis
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- Catalytic activity of heteroaromatic N-oxides in the hydrolysis of 2,3,5,6-tetrachloro-p-benzoquinone
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Pyridine and quinoline N-oxides catalyze hydrolysis of 2,3,5,6-tetrachloro- p-benzoquinone to 2,5-dichloro-3,6-dihydroxy-p-benzoquinone in dilute aqueous acetonitrile. Their catalytic activity is much higher than that of the corresponding azines. Quinoline N-oxides react with 2,3,5,6-tetrachloro-p- benzoquinone in a concentrated dioxane solution in the presence of water to give 2,5-dichloro-3,6-dihydroxy-p-benzoquinone salts. Pleiades Publishing, Inc. 2006.
- Ryzhakov,Rodina
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- IR, 1H-NMR, UV-Vis and thermal studies on the rhodamine 6G chargetransfer complexes
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Charge-transfer (CT) complexes formed between Rhodamine 6G (Rh6G) as donor with iodine, CLA and PA as acceptors have been studied spectrophotometrically. The synthesis and characterization of Rhodamine 6G CT-complexes of iodine, [(Rh6G)2]I+.I3-, chloranilic acid (CLA), [(Rh6G)(CLA)] and picric acid (PA) [(Rh6G)(PA)] were described. These complexes are readily prepared from the reaction of Rh6G with I2, CLA and PA with CHCl3 solvent. Mid and far IR, 1H-NMR, UV-Vis techniques, elemental analyses (CHN) and thermal studies, characterize the three new Rhodamine 6G charge-transfer complexes. Modified Benesi-Hildebrand method was applied to the determination of association constant and molar extinction coefficient.
- Refat, Moamen S.,Killa, Hamada M.A.,El-Sayed
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experimental part
p. 137 - 146
(2012/05/31)
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- The reaction of the OH radical with pentafluoro-, pentachloro-, pentabromo- and 2,4,6-triiodophenol in water: Electron transfer vs. addition to the ring
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The OH-radical-induced dehalogenation of pentafluorophenol (F5C6OH), pentachlorophenol (Cl5C6OH), pentabromophenol (Br5C6OH) and 2,4,6-triiodophenol (I3H2C6OH) in water has been studied by pulse radiolysis in basic solution where these compounds are deprotonated and hence slightly water soluble. Hydroxyl radicals react with these phenolates both by electron transfer and by addition. Electron transfer yields hydroxide ions and the corresponding phenoxyl radicals (X5C6O and I3H2C6O); these were also generated independently, to the exclusion of OH-adduct radicals, by reacting the phenolates with N3 radicals [k(N3 + F5C6O-) = 4.9 × 109 dm3 mol-1 s-1, λmax(F5C6O) = 395 nm; k(N3 + Cl5C6O-) = 5.7 × 109 dm3 mol-1 s-1, λmax(Cl5C6O) = 452 nm; k(N3 + Br5C6O-) = 6.5 × 109 dm3 mol-1 s-1, λmax(Br5C6O) = 476 nm; k(N3 + I3H2C6O-) = 5.6 × 109 dm3 mol-1 s-1, λmax(I3H2C6O) = 540 nm]. Hydroxyl radical addition to the pentahalophenolates is followed by rapid halide elimination, giving rise to hydroxytetrahalophenoxyl radical anions (X4O-C6O). The latter exhibit absorption maxima near those of the pentahalophenoxyl radicals. This prevents a proper determination of the relative importance of the two processes by optical detection. However, these two processes distinguish themselves by their behaviour with respect to the stoichiometry and kinetics of the production of ionic conducting species. In basic solution, electron transfer causes a conductivity increase due to the formation of OH- whereas addition followed by HX elimination and deprotonation of the X4OHC6O radical results in a conductivity drop. The evaluation of the conductivity change at 8 μs after the radiolytic pulse has ended, reveals that about 27%, 53%, 73%, and 97% of the OH radicals react by electron transfer with F5C6O-, Cl5C6O-, Br5C6O- and I3H2C6O-, respectively. Further conductivity change occurs during the bimolecular termination of the halophenol-derived radicals (t1/2 9 and 4 × 109 dm3 mol-1 s-1) and continues into progressively longer times, owing to the hydrolysis of unstable HX-releasing products, on account of the replacement of OH- by halide/halophenolate ions. Additionally, further halide is released on a time scale of minutes and hours. The rates of the conductivity change in the time range from a few ms to several tens of seconds are proportional to the OH- concentration.
- Fang, Xingwang,Schuchmann, Heinz-Peter,Von Sonntag, Clemens
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p. 1391 - 1398
(2007/10/03)
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