Kinetics and mechanism of the reaction of sulphur(IV) with N,N'-ethylene-bis(sali-cylidiniminato)manganese-(III) in aqueous solution

Abstract

The reaction of (diaqua)(N,N'-ethylene-bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, [Mn(Salen)(SO₃)]^- (S-bonded isomer) via three distinct paths: (i) Mn(Salen)(OH₂)₂⁺+ HSO₃^-→ (k₁); (ii) Mn(Salen)(OH₂)₂⁺+SO₃^2- →(k₂) ); (III) Mn(Salen)(OH₂)(OH) + SO₃^2-→(k₂) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO₃^2-/HSO₃^- may be attributed to the strong trans-activating influence of the S-bonded sulphite. The values of the rate constants (10^-2k/dm²mol^-1s^-1 at 25°C, 1 = 0.3 mol dm^-3), ΔH_i^#/kJ mol^-1 and ΔH_i^#/J K^-1mol^-1respectively are: 2.97±0.27, 42.4±0.2, -55.3±0.6 (i = 1); 11.0 ± 0.8, 33±3, -75 ±10 (i = 2); 20.6 ± 1.9, 32.4 ± 0.2, -72.9 ± 0.6 (i = 3). The trend in reactivity (k₂ > k₁), a small labilizing effect of the coordinated hydroxo group (k₃/k₂ < 2), and substantially low values of ΔS^# suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua-hydroxo complexes is most likely associative interchange (l_a). No evidence for the formation of the O-bonded sulphito complex and the ligand isomerization in the sulphito complex, (Mn^III-OSO₂→ Mn^III-SO₃), ensures the selectivity of the Mn^III centre toward the S-end of the S^IV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce Mn^II, S₂O₆^2- and SO₄^2-. The formation of dithionate is a consequence of the fast dimerization of the SO3-. generated in the rate determining step and also SO₄^2- formation is attributed to the fast scavenging of the SO_3-. by the MnIII species via a redox path. The internal reduction of the Mn^III centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO_3- and the other SO₃^2-. The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (ΔS^# = -(1.3 ± 0.2) × 10² and -(1.6 ± 0.2) × 10² J K^-1 mol^-1 for the paths involving HSO_3- and SO₃^2- respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer.