Kinetics and mechanism of the oxidation of Mn(ii)aq by bromate and peroxomonosulfate in the presence of molybdate to form [MnIVMo9O32]6−

Abstract

The oxidation of Mn(II) by both BrO₃⁻ and HSO₅⁻ in the presence of MoO₄²⁻, in weakly acidic solution over the pH ranges 3.9–5.5 and 4.4–5.5, respectively, results in the formation of the heteropolyoxomolybdate [MnMo₉O₃₂]⁶⁻ in each case. The kinetics of oxidation were studied at 40.0 °C for BrO₃⁻ and 30.0 °C for HSO₅⁻, along with temperature dependence studies, and for each oxidant were found to exhibit solution autocatalytic behaviour. For BrO₃⁻ the oxidation kinetics followed the expanded rate expression

+d[MnMo₉O₃₂⁶⁻]/dt = k_AC(1)[Mn²⁺][MnMo₉O₃₂⁶⁻][HMoO₄⁻]²[BrO₃⁻]

based on an examination of the individual [BrO₃⁻], pH and actual [MoO₄²⁻] dependences, with a value for k_AC(1) of 9.09(34) × 10⁶ dm¹² mol⁻⁴ s⁻¹. For HSO₅⁻ the oxidation kinetics followed the expanded two-term rate expression

+d[MnMo₉O₃₂⁶⁻]/dt = k_AC(2a)[Mn²⁺][MnMo₉O₃₂⁶⁻][HMoO₄⁻]⁻⁵[SO₅⁻] + k_AC(2b)[Mn²⁺][MnMo₉O₃₂⁶⁻][HMoO₄⁻]⁻⁵[HSO₅⁻]

based on [HSO₅⁻], pH and [MoO₄²⁻] dependences. The values of k_AC(2a) and k_AC(2b) are 4.6(4) × 10⁻¹¹ dm⁻⁹ mol³ s⁻¹ and 2.6(4) × 10⁻¹⁵ dm⁻⁹ mol³ s⁻¹. For BrO₃⁻ oxidation, from the composition of the transition state, it is proposed that the product [MnMo₉O₃₂]⁶⁻ species combines with Mn(II) and two HMoO₄⁻ ions to generate a Mn(II)-substituted lacunary [Mn^IIMn^IVMo₁₁O₃₉]⁶⁻ anion based on a Keggin structure, with the Mn(IV) located at the centre of the polyoxomolybdate framework. Extended-Hückel molecular orbital calculations have been used to investigate the stability of the proposed Mn(II)-substituted lacunary species, based on an α-Keggin structure, relative to the unassembled components. For HSO₅⁻ oxidation, the two parallel pathways indicate oxidation by both HSO₅⁻ and its deprotonated form SO₅⁻. The two mechanisms reflect the differences in how BrO₃⁻ and HSO₅⁻ operate oxidatively and have been highlighted by the facile nature of polyoxomolybdate polymerization. In each case following oxidation of Mn(II) to Mn(IV) fast separation of the two Mn(IV) centres must subsequently occur, along with rapid assembly of the polyoxomolybdate frameworks around each centre to yield the product species.