Kinetics and mechanism of oxidation of glyoxylic acid by a phenolate-amide-amine coordinated cobalt (III) metal ion

Kinetic and mechanistic studies of a reaction provide valuable insights into the pathways of chemical transformations, enabling better control over reaction conditions (such as pH, temperature, and nature of solvent) to optimize yields and minimize undesired reactions. Due to the biocompatibility of cobalt and the versatile redox chemistry of its coordination complexes, a cobalt(III) ion coordinated with a hexadentate ligand containing two phenolate-O, two carboxamide-N, and two sec-amine-N donor atoms has been selected to react with glyoxylic acid (HGl). The kinetics of oxidation of HGl by the octahedral cobalt(III) complex, [CoIII(HL)].9H2O, H4L = 1,8-bis(2-hydroxybenzamido)-3,6-diazaoctane was investigated in the temperature range 20.0–45.0 °C, pH range 1.93 ≤ pH ≤ 4.98 and at an ionic strength, I = 0.3 mol dm−3 (NaClO4). The overall reaction is believed to be proceed through an equilibrium pre-association of [CoIIIHL] with HGl and its conjugate base Gl− followed by a slow electron transfer. Final products of the reaction were identified as formic acid, CO2 and Co(II). The rate and activation parameters of the reaction are reported. In the studied pH range, no evidence of direct coordination of HGl with the metal centre was found by experiment as well as theoretical calculation. Spin-polarized density functional theory-based studies are performed to investigate the interaction of HGl, and its oxidised products such as the formic acid and CO2 with Co(III)H2L complex, where the active sites and the binding energies of the reactants and products with the host complex is determined. The theoretical studies show that HGl binds through –OH site and CO site of COOH group with phenolate-O and amide-NH of the Co-complex with a binding energy value of −0.98 eV. Binding of the molecule leads to stretching of OH bond by ∼9 % compared to that of the free molecule indicating the fact that the OH-site involve in the non-covalent H-bonding driven electron transfer process.