Study on the reaction of 2,3-dihydroxybenzoic acid with molybdenum in aqueous solution. I. Synthesis and characterization of the oligomeric complexes formed

Dimeric or oligomeric oxo-complexes of Mo(VI) with 2,3-dihydroxybenzoic acid were prepared in aqueous solutions in the presence or not of K2S2O5 (acting as a reducing agent) in various conditions. The complexes were found to contain the cis-(Mo2O5)2+cis-(Mo2O5)2+ core and the ligands in the catecholate, semiquinonate or mixed valence oxidation form, depending on the reaction conditions and especially on the presence or not of the reductant. The isolated complexes in the presence or absence of reductant and the oxidation products in solution in the presence of air were studied via elemental, thermogravimetric and electrochemical analysis, Infrared, Raman, NMR and ESR spectroscopies and Electrospray Mass Spectra. The general molecular formula for the complexes is {[(PPh4)2(Mo2O5L2X2] · xH2O)}n, where the coordinated ligand’s L oxidation form varies and X involves coordinated water or hydroxyl group depending on the ligand oxidation state.