Oxidation of sulfur(IV) by tris(acetylacetonato)manganese(III) and its hydrolytic derivatives: possible examples of substitution-controlled redox processes
Abstract
The manganese(III) complex [Mn(acac)3](Hacac = acetylacetone) hydrolyses partially to [Mn(acac)2(H2O)2]+ and [Mn(acac)2(H2O)(OH)] in aqueous perchlorate media containing excess acetylacetone and equilibriated between pH 4.0 and 8.6 Both [Mn(acac)2(H2O)2]+ and [Mn(acac)2(H2O)(OH)] oxidise SIV to SVI quantitatively. In the presence of excess SIV, the observed kinetics between 20 and 40 °C are best explained by the reaction sequence (i)–(iii) where Ki1 and Ki2 are the formation constants for the [Mn(acac)2(H2O)2]++ HSO3– [graphic omitted] [Int1] [graphic omitted] Products (i), [Mn(acac)2(H2O)2]++ SO32– [graphic omitted] [Int2] [graphic omitted] Products (ii), [Mn(acac)2(H2O)(OH)]+ SO32– [graphic omitted] Products (iii), hydrogen-bonded ion-pair intermediates [Int1] and [Int2] respectively. At 30 °C and I= 1.0 mol dm–3(NaClO4), Ki1= 30 dm3 mol–1, Ki2= 100 dm3 mol–1, k1= 7.0 × 10–4 s–1, k2= 3.55 × 10–4 s–1 and k3= 0.41 dm3 mol–1 s–1. The rate-determining steps appear to be the transformation of [Int1], [Int2] and [Mn(acac)2(H2O)(OH)] to bona-fide inner-sphere adducts (not shown in the above sequence). Subsequent rapid redox processes via sulfur(V) intermediates led to the products. The complex [Mn(acac)3] was found to be kinetically inactive despite its abundance in the reaction media and only acts as a buffer for [Mn(acac)2(H2O)2]+ and [Mn(acac)2(H2O)(OH)].