Mechanistic study of the reactions of L-ascorbic acid and oxalic acid with an octahedral manganese(IV) complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane

Abstract

The Mn^IV complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane (Mn^IVL) with phenolate-amido-amine coordination is reduced by L-ascorbic acid and oxalic acid obeying overall 1:1 stoichiometry. The reactions are biphasic and Mn^IIIL^-is the reactive intermediate. The product of oxidation of ascorbic acid (H₂Asc) is dehydroascorbic acid and that of oxalic acid (H₂OX) is CO₂, while Mn^II is the end product from Mn^IV. Both Mn^IVL and Mn^IIIL- form outer sphere adducts with H₂Asc and H₂OX with high values of equilibrium constants of formation (Q<10² dm³ mol^-1, I = 0.5 mol dm^-3, 25.8 °C, 1.5% v/v MeOH+H₂O). The adduct formation is diffusion controlled and is attributed to hydrogen bonding interactions between the reactants. The rate constants for the electron transfer in (Mn^IV/^IIIL, H₂A), (Mn^IV/^IIIL, HA^-) (H₂A = H₂Asc, H₂OX) and for (Mn^IVL, H₂Asc)+H₂Asc, (Mn^IIIL^-, HAsc^-)+HAsc^- are reported. There was no evidence of direct coordination of the reductants to the Mn^IV/III center indicating an outer sphere (ET) mechanism.