Thiosulphate-catalysed reduction of tris(acetylacetonato)-manganese(III) and its hydrolytic derivatives in aqueous perchlorate media

Abstract

The kinetics of reactions of S2O32– with [Mn(acac)3](Hacac = acetylacetone) and its derivatives [Mn(acac)2(H2O)2]+ and [Mn(acac)2(H2O)(OH)] have been investigated in aqueous perchlorate media containing different total quantities of acetylacetone (cacac) over the range pH 4.2–8.5. The observed dependences of the reaction rates on pH and cacac arise from hydrolytic equilibria of [Mn(acac)3]. The reaction is first order in [S2O32–] but consumes no S2O32–. Instead, 1 mol of acetylacetone is consumes per mol of MnIII reduced. EPR and kinetic data suggest a S2O32–/S2O3– catalytic cycle which provides a low-energy path for the acac → MnIII electron transfer. Rate constants for reduction of [Mn(acac)3](k1), [Mn(acac)2(H2O)2]+(k2) and [Mn(acac)2(H2O)(OH)](k3) follow the sequence k1 < k2≈k3. An inner-sphere mechanism seems plausible for the k2 and the k3 paths; k1 probably involves an outer-sphere process.