Vanadium(V) Complexes of α-Hydroxycarboxylic Acids in Aqueous Solution

51V NMR and IR spectroscopic studies of the complexes formed between vanadate and the α-hydroxylic acid ligands, (S)-2-hydroxypropanoic acid (l-(+)-lactic acid), 2-hydroxy-2-methylpropanoic acid, and 2-ethyl-2-hydroxybutanoic acid were carried out for aqueous 1 M ionic strength (NaCl) solutions. Three major products in V to L stoichiometries of 1:1, 2:2, and 3:2 were identified from vanadate and ligand concentration studies, while a pH variation study allowed charge states to be determined. At pH 7.06, the formation constants for the predominant reactions were (26 ± 1) M -1, (V + L ⇄ VL); (6.8 ± 0.4) × 103 M-1, (2VL ⇄ V2L2); and (3.5 ± 0.3) × 103 M-1, (V2L2 + V ⇄ V3L2). Dissolution studies of various crystalline products were carried out for aqueous, nonaqueous, and mixed solvent systems. These studies combined with information available from X-ray structural studies provided a basis for the assignment of solution state structures. Pentacoordinate vanadium in a trigonal-bipyramidal geometry was proposed for the both the 1:1 and 2:2 complexes when in aqueous solution. Observed changes in 51V chemical shift patterns were consistent with a cis fusion in octahedral coordination for the central vanadium of the 3:2 complex, while the remaining vanadiums retained a pentacoordinate geometry.