Kinetic studies on the oxidation of dihydroxybenzenes by monomeric manganese (III)- and bis(μ-oxo) manganese (III, IV)–cyclam complexes—phosphate inhibition

Kinetic studies on the oxidation of hydroquinone and catechol by dinuclear, [Mn2 (III/IV)(μ-O)2(cyclam)2](ClO4)3] (1) and mononuclear, trans- [Mn(cyclam)Cl2]Cl (2) have been carried out in aqueous buffer in the range of pH 5.8–7.6 at 30 °C by UV–Vis spectrophotometry (cyclam=1,4,8,11-tetraazacyclotetradecane). In the reaction of 1 and benzenediols, an outer-sphere mechanism is likely to be operative on the basis of available kinetic data. The deprotonated reductants (HA−) are more reactive than the protonated ones (H2A). An inner-sphere mechanism is apparent in the reaction between 2 and hydroquinone as indicated by rate saturation kinetics leading to high values for association constants. The inhibition of rate by phosphate has also been noted for this reaction owing to the binding of phosphate ion with the metal center. This has been rationalized by cyclic voltammetric experiment of complex 1.