Oxidation of Tiron by (pyrophosphato)manganese(iii). Kinetics and mechanism

The kinetics of oxidation of 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron) by tris(pyrophosphato)manganese(III) in aqueous solution has been studied by the stopped-flow technique in the pH interval 1–6. Rapid-scan UV-Vis spectrophotometry in the 2–70 ms range revealed two parallel reactions, namely oxidation to the o-quinone and disproportionation to manganese(II) and soluble manganese(IV). The redox reaction exhibits first-order kinetics in both MnIII and Tiron in the whole pH range investigated. The second-order rate constant shows a marked pH dependence with a maximum at pH = 2.4. The proposed reaction mechanism involves formation of two sets of Tiron–pyrophosphato–manganese(III) mixed-ligand intermediates, bonded via a catecholato or via a sulfonato group, respectively. They decompose to the quinone or the Mn(IV)/Mn(II) product in a rate-determining electron transfer or disproportionation. Remarkably, disproportionation is also first-order in both reactants, which implies a novel type of catalysis by Tiron, due to the greater substitutional lability of sulfonato vs. catecholato bonded Tiron.