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

The Mn<SUP>IV</SUP> complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane (Mn<SUP>IV</SUP>L) 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<SUP>III</SUP>L<SUP>-</SUP>is the reactive intermediate. The product of oxidation of ascorbic acid (H<SUB>2</SUB>Asc) is dehydroascorbic acid and that of oxalic acid (H<SUB>2</SUB>OX) is CO<SUB>2</SUB>, while Mn<SUP>II</SUP> is the end product from Mn<SUP>IV</SUP>. Both Mn<SUP>IV</SUP>L and Mn<SUP>III</SUP>L- form outer sphere adducts with H<SUB>2</SUB>Asc and H<SUB>2</SUB>OX with high values of equilibrium constants of formation (Q&lt;10<SUP>2</SUP> dm<SUP>3</SUP> mol<SUP>-1</SUP>, I = 0.5 mol dm<SUP>-3</SUP>, 25.8 &#176;C, 1.5% v/v MeOH+H<SUB>2</SUB>O). 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<SUP>IV</SUP>/<SUP>III</SUP>L, H<SUB>2</SUB>A), (Mn<SUP>IV</SUP>/<SUP>III</SUP>L, HA<SUP>-</SUP>) (H<SUB>2</SUB>A = H<SUB>2</SUB>Asc, H<SUB>2</SUB>OX) and for (Mn<SUP>IV</SUP>L, H<SUB>2</SUB>Asc)+H<SUB>2</SUB>Asc, (Mn<SUP>III</SUP>L<SUP>-</SUP>, HAsc<SUP>-</SUP>)+HAsc<SUP>-</SUP> are reported. There was no evidence of direct coordination of the reductants to the Mn<SUP>IV/III</SUP> center indicating an outer sphere (ET) mechanism