Electrochemical Conversion of 5-Hydroxymethylfurfural to 2,5-Furandicarboxaldehyde Using Mn(III)-Schiff Base Catalysts

2,5-furandicarboxaldehyde (DFF) is one of the most promising biomass-based building blocks for the synthesis of biobased polymers. DFF can be obtained from 5-hydroxymethylfurfural (HMF), a fructose derivate, and it is a key molecule in the sequence of reactions of furan chemistry to develop biobased plastics. In this frame, four manganese(III)-Schiff base complexes 1-4 have been obtained. The general formula for the complexes, MnLn(OCN)(H2O/CH3OH)m (Ln being the Schiff base ligands L1-L4, formed as the result of the condensation of different substituted hydroxybenzaldehydes with diverse diamines, and m = 1-3), has been confirmed by characterization through different analytical and spectroscopic techniques. X-ray crystallographic studies for 1 and 2 showed tetragonally distorted octahedral structures, where the Schiff base was placed in the equatorial coordination positions of the Mn(III) ion. Complexes 1 and 2 behaved as efficient catalysts in the oxidation of HMF to DFF in an electrolytic reaction at pH 8.5, with phosphate buffer at room temperature, with conversion rates of 70-80%. On the other hand, complexes 3 and 4, where the axial position was sterically less accessible, yielded only an 11% conversion of HMF to DFF. The results indicate that a correct selection of metal complexes allows the development of a new efficient way to obtain DFF.