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

Multinuclear (H-1,C-13, O-17, V-51) 1D and 2D NMR spectroscopy has been use d to characterize the peroxovanadium(V) complexes of glycolic acid in aqueo us 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 V2O34+ seven-coordinated metal centre. In this structure, the two vanadiu m atoms are triple bridged, two of those bridges being formed by oxygen ato ms of the hydroxyl group of the acid. The 1:1:1 species has a seven-coordin ated VO3+ metal centre. Glycolic acid bonds to the vanadium atoms in a bide ntate 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 vanad ium centres, which is now a fve-coordinated oxovanadium centre. Three addit ional complexes are found in very small amounts for some pH and concentrati on conditions. Further support for the proposal of monoperoxovanadium speci es is given by UV-visible spectroscopy results. (C) 2000 Elsevier Science B .V. All rights reserved.