Peroxovanadium(V) complexes of glycolic acid as studied by NMR spectroscopy

Abstract

Multinuclear (1H, 13C, 17O, 51V) 1D and 2D NMR spectroscopy has been used to characterize the peroxovanadium(V) complexes of glycolic acid in aqueous solution. One 2:2:2 (metal:ligand:peroxo) complex, together with a 1:1:1 and a 2:2:1 species, are found in the pH range 1-7. The 2:2:2 complex is a monoperoxo (one peroxide unit per vanadium atom) dinuclear species having a V2O3 4+ seven-coordinated metal centre. In this structure, the two vanadium atoms are triple bridged, two of those bridges being formed by oxygen atoms of the hydroxyl group of the acid. The 1:1:1 species has a seven-coordinated VO3+ metal centre. Glycolic acid bonds to the vanadium atoms in a bidentate way, through both the carboxylic and the hydroxyl groups. The peroxo groups are bound in the equatorial plane relative to the apical V=O and the geometry around each vanadium atom is close to pentagonal bipyramidal. The 2:2:1 complex is similar to the 2:2:2 species, except for one of the vanadium centres, which is now a five-coordinated oxovanadium centre. Three additional complexes are found in very small amounts for some pH and concentration conditions. Further support for the proposal of monoperoxovanadium species is given by UV-visible spectroscopy results.