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 S^IV to S^VI quantitatively. In the presence of excess S^IV, the observed kinetics between 20 and 40 °C are best explained by the reaction sequence (i)–(iii) where K_i1 and K_i2 are the formation constants for the [Mn(acac)₂(H₂O)₂]⁺+ HSO₃^– [graphic omitted] [Int₁] [graphic omitted] Products (i), [Mn(acac)₂(H₂O)₂]⁺+ SO₃^2– [graphic omitted] [Int₂] [graphic omitted] Products (ii), [Mn(acac)₂(H₂O)(OH)]+ SO₃^2– [graphic omitted] Products (iii), hydrogen-bonded ion-pair intermediates [Int₁] and [Int₂] respectively. At 30 °C and I= 1.0 mol dm^–3(NaClO₄), K_i1= 30 dm³ mol^–1, K_i2= 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)].