Oxidation of glyoxylic acid by a mononuclear manganese(IV) complex of 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane: a kinetics and mechanistic study

Abstract

The oxidation of glyoxylic acid (HGl) by Mn^IVL {L^4- = tetra deprotonated 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane} was investigated in the pH range 1.67-10.18, at 25-45 °C and 0.5 M ionic strength. The reaction exhibited biphasic kinetics with Mn^IIIL^- as the reactive intermediate. Mn^IV was reduced to Mn^II. The products of oxidation of HGl were identified as formic acid and CO₂ in acidic medium, and oxalate in basic medium, consistent with the stoichiometry: -Δ[Mn^IV]/-Δ[HGl] = 1. In acidic medium, both Mn^IVL and Mn^IIIL^- formed outer-sphere adducts with the neutral HGl {HC(OH)₂COOH} molecule, with an association constant Q_av of 28 and 70 M^-1, respectively. A similar adduct formation was not observed for the glyoxylate mono anion {Gl^-, CH(OH)₂(CO₂^-)} and glyoxylate dianion {Gl^2-, CH(OH)(O^-)CO₂^-}. The rate and activation parameters for the various paths are reported and an outer-sphere electron transfer mechanism is suggested.