Search

A kinetic and mechanistic study is reported of the oxidation of a number of azonaphthol dyes with hydrogen peroxide in aqueous solution, catalysed by some mono and dinuclear manganese(IV) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN). The results of UV-Vis investigations, augmented by EPR and ESI-MS studies, are described for a series of experiments in which concentrations, pH and ionic strength have been varied. The reactions are characterised by an induction period followed by a relatively rapid oxidation. For the dinuclear manganese complex 2, these are consistent with an initial perhydrolysis of the manganese complex involving both the dye anion and HO2-, to give mononuclear manganese species and the operation of a catalytic cycle incorporating MnIIIL(OH)3, O = MnVL(OH)2 and MnIVL(OH)3 (L = Me3TACN) (cf. the reactions of peroxidase enzymes). ESI-MS results provide evidence for the formation and reaction (with the dye) of MnIVL(OH)3. With the mononuclear manganese complex MnIVL(OMe)3, there is a short lag-phase attributed to perhydrolysis by HO2- followed by the same catalytic cycle.

The spin-trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) is shown to provide a simple method to distinguish free-radical reactions which lead to the production of ˙OH from those which proceed via one-electron transfer followed by hydration or nucleophilic substitution of another adduct. In the first case, only an ˙OH adduct is detected; in the latter, signals from the hydroxyl adduct (labelled with 17O when H217O is employed) are accompanied by a second signal believed to result via internal trapping of a radical formed from an intermediate radical cation. This provides a potentially useful mechanistic probe, for example for use in biochemical systems.

The new tripodal thiol ligands 1,4,7-tris(4-mercaptobenzoyl)-1,4,7-triazacyclononane (H3L1) and 1,5,9-tris(4-mercaptobenzoyl)-1,5,9-triazacyclododecane (H3L2) have been synthesised and characterised. On reaction with iron–sulfur clusters [Fe4S4(SR)4]2–(R = Et or But), subsite-differentiated clusters, for example [Fe4S4(L1)(SEt)]2–, are formed. Site-specific reaction at the differentiated iron is demonstrated. The novel clusters have been prepared both in solution and as isolated solids, and characterised mainly by 1H NMR and Mossbauer spectroscopy.

The Mossbauer parameters for a series of tetraalkylammonium salts of [Fe 4 S 4 (Cys) 4 ] 2− (HCys= L -cysteine ethyl ester hydrochloride) have been measured. The quadrupole splitting is found to be independent of cation size, the cluster core is ‘insulated’, and the expected lattice effect is not observed.