CV and NMR study on the reaction of Mo(VI) with 3,4-dihydroxybenzoic acid and ascorbic acid in aqueous solution

The gradual release of the ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) from its molybdenum complex in the presence of ascorbic acid (AscA) in a weakly acidic aqueous solution (pH ∼ 3.5) is described. We observed that the formation of the 3,4-DHBA-semiquinone oxidation state and the semidehydroascorbate is a pre-requisite for the release of the 3,4-DHBA ligand. The interaction of these radicals leads at the same time to the further degradation of AscA resulting in, among other compounds, threonic acid which participates in the reaction with molybdenum. The comparison of the complexing ability indicated that threonic acid competes with protocatechuate, while ascorbic acid is a less good ligand for the Mo(VI). Solution studies on the reaction mechanism were performed by cyclic voltammetry, NMR spectroscopy and UV–Vis spectroscopy. Isolated precipitates were investigated by NMR spectroscopy. The antioxidant properties of 3,4-DHBA and AscA were also compared using the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH).