NMR spectroscopy study of the peroxovanadium(V) complexes of l-malic acid

Continuing the solution speciation studies of peroxovanadium(V) complexes with α-hydroxycarboxylic acids, we now report on the complexes formed with l-malic acid, studied by multinuclear (1H, 13C, 51V) 1D and 2D NMR spectroscopy. The system V(V)–l-malic acid–H2O2 in aqueous solution was found to involve a large number of species: nine major peroxovanadium(V) complexes in the pH range 2–7, the structures of which have been deduced, and eight weaker complexes in very low concentrations. The former are monoperoxo species with the stoichiometries (metal:acid:peroxide) 2:2:2 (two complexes), 2:2:1 (two complexes), 2:1:1 (two complexes) and 1:1:1 (three complexes). The 2:2:2 complexes have seven-coordinated metal centres which are doubly bridged by the hydroxyl oxygen atom of malic acid. In one case, all three functional groups of malic acid are bound to each of the two VO3+ metal centres, whereas in the other the C4 carboxylic group of the ligand is dangling, two possibilities being advanced for the vanadium centres: either one V2O34+ centre or two VO3+ centres. The two complexes of stoichiometry 2:2:1 are the result of degradation (loss of one peroxide unit) of the 2:2:2 species in which l-malic acid is a bidentate ligand; they have seven- and five-coordinated metal centres, respectively, the peroxo and the oxo centres, and the malate groups are equally bidentate. The two complexes of stoichiometry 2:1:1 have presumably V2O42+ metal centres and all three functional groups of l-malic acid are involved in chelation; isomerism is interpreted as due to which of the two non-equivalent V(V) atoms is the peroxo centre. The three 1:1:1 complexes have either seven- or six-coordinated VO3+ metal centres; in one of the complexes, l-malic acid acts as a tridentate ligand, whereas in the other two, the ligand is bidentate.