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Hydrogen peroxide induced oxidation of azo dyes catalysed by manganese 1,4,7-triazacyclononane complexes
212
Journal of Inorganic Biochemistry
Abstracts
G19
HYDROGEN PEROXIDE INDUCED OXIDATION OF AZO DYES C A T A L Y S E D BY M A N G A N E S E 1 , 4 , 7 - T R I A Z A C Y C L O N O N A N E COMPLEXES
M.S. N e w t o n a, B.C. Gilbert a, J.R. Lindsay Smith a and J.Oakes b
~Department of Chemistry, University of York, York, YO1 5DD, U.K. bUnilever Research, Port Sunlight Laboratory, Quarry Road East, Wirral L63 3JW, U.K. Dinuclear manganese complexes with a ligand based on 1,4,7-triazacyclononane (TACN) are good structural models for the oxygen evolving centre (OEC) and catalase as these complexes have a dinuclear la-OXOmanganese centre that can shuttle between different oxidation states [ 1]. It has also been shown that the dinuclear Mn complexes can rearrange into mono- and tetra nuclear species under certain conditions [ 1]. Mn-TACN complexes have also been found to be effective catalysts for the bleaching of stains by H202 at low temperatures in detergents [2]. Little is known about the reactions of dyes with H202 under the same conditions. We report here our results on the Mn-TACN catalysed oxidative degradation of a selection of substituted 13-naphthol azo dyes with the general structure (1) as models for textiles colourants. Kinetics of the bleaching of azo dyes with H202 catalysed by three different manganese complexes (2,3 and 4) in aqueous solution (pH = 10, T = 30°C) have J been studied by UV-Vis spectroscopy and by monitoring the H202 concentration; reactive intermediates have been studied in parallel ESR experiments. These y,i N~ resuks reveal that the oxidations follow two kinetic profiles: with catalysts (2) and ~ o , (3) the reactions show an initial lag phase whereas with (4) they do not. The lag phase is the result of the build-up of catalytic active species. It has been observed that the active species that bleach the dyes also catalyse the decomposition of H202 (cf. catalase activity). None of the Mn-TACN complexes, except for an 1 asymmetric mixed valence Mn-TACN complex [3], has been reported previously to bring about the catalytic disproportionation of H202 in the absence of dyes. The stoichiometry of these oxidations and their dependence on dye structure and pH has been investigated. Based on these studies, mechanisms have been proposed and tested by computer simulations.
/ (PF6) 2
[(L)2Mn2(~-O)31
[(L)Mn(OMe)3]
\
1<'">
(PF6) 2
Y [(L')Mn2(~-O)2(P-CH3COO)I
2
3
References 1. K.Wieghardt, et al., J. Am. Chem. Soc., U0, 7398-7411 (1988) 2. R.Hage, et al. Nature 369, 637-639 (1993) 3. K.Wieghardt, et al., J. Chem. Soc., Chem. Commun., 1780-1782 (1992)
4
Journal of Inorganic Biochemistry
Abstracts
G19
HYDROGEN PEROXIDE INDUCED OXIDATION OF AZO DYES C A T A L Y S E D BY M A N G A N E S E 1 , 4 , 7 - T R I A Z A C Y C L O N O N A N E COMPLEXES
M.S. N e w t o n a, B.C. Gilbert a, J.R. Lindsay Smith a and J.Oakes b
~Department of Chemistry, University of York, York, YO1 5DD, U.K. bUnilever Research, Port Sunlight Laboratory, Quarry Road East, Wirral L63 3JW, U.K. Dinuclear manganese complexes with a ligand based on 1,4,7-triazacyclononane (TACN) are good structural models for the oxygen evolving centre (OEC) and catalase as these complexes have a dinuclear la-OXOmanganese centre that can shuttle between different oxidation states [ 1]. It has also been shown that the dinuclear Mn complexes can rearrange into mono- and tetra nuclear species under certain conditions [ 1]. Mn-TACN complexes have also been found to be effective catalysts for the bleaching of stains by H202 at low temperatures in detergents [2]. Little is known about the reactions of dyes with H202 under the same conditions. We report here our results on the Mn-TACN catalysed oxidative degradation of a selection of substituted 13-naphthol azo dyes with the general structure (1) as models for textiles colourants. Kinetics of the bleaching of azo dyes with H202 catalysed by three different manganese complexes (2,3 and 4) in aqueous solution (pH = 10, T = 30°C) have J been studied by UV-Vis spectroscopy and by monitoring the H202 concentration; reactive intermediates have been studied in parallel ESR experiments. These y,i N~ resuks reveal that the oxidations follow two kinetic profiles: with catalysts (2) and ~ o , (3) the reactions show an initial lag phase whereas with (4) they do not. The lag phase is the result of the build-up of catalytic active species. It has been observed that the active species that bleach the dyes also catalyse the decomposition of H202 (cf. catalase activity). None of the Mn-TACN complexes, except for an 1 asymmetric mixed valence Mn-TACN complex [3], has been reported previously to bring about the catalytic disproportionation of H202 in the absence of dyes. The stoichiometry of these oxidations and their dependence on dye structure and pH has been investigated. Based on these studies, mechanisms have been proposed and tested by computer simulations.
/ (PF6) 2
[(L)2Mn2(~-O)31
[(L)Mn(OMe)3]
\
1<'">
(PF6) 2
Y [(L')Mn2(~-O)2(P-CH3COO)I
2
3
References 1. K.Wieghardt, et al., J. Am. Chem. Soc., U0, 7398-7411 (1988) 2. R.Hage, et al. Nature 369, 637-639 (1993) 3. K.Wieghardt, et al., J. Chem. Soc., Chem. Commun., 1780-1782 (1992)
4