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

Abstract

The kinetics of reactions of S₂O₃^2– 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 (c_acac) over the range pH 4.2–8.5. The observed dependences of the reaction rates on pH and c_acac arise from hydrolytic equilibria of [Mn(acac)₃]. The reaction is first order in [S₂O₃^2–] but consumes no S₂O₃^2–. Instead, 1 mol of acetylacetone is consumes per mol of Mn^III reduced. EPR and kinetic data suggest a S₂O₃^2–/S₂O₃^– catalytic cycle which provides a low-energy path for the acac → Mn^III 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.