Applications of 51V NMR spectroscopy to studies of the complexation of vanadium(V) by α-hydroxycarboxylic acids

Vanadate complexes of alpha-hydroxycarboxylic acids form readily and reversibly in aqueous solution. 2-Methyl-2-hydroxypropanoic acid (alpha-hydroxyisobutyric acid) provides a convenient system to demonstrate the utilization of V-51-NMR spectroscopy in the study of the formation of various products and to provide examples of procedures used to determine product stoichiometries and equilibrium constants. a-Hydroxyisobutyric acid forms three major products with 1:1, 2:2 and 2:3 ligand to vanadium stoichiometries. The formation of products with this ligand is compared with reactions with similar ligands, 2-hydroxypropanoic acid (lactic acid) and 2-ethyl-2-hydroxybutanoic acid and to ligands of other oxidation states, most notably 1,2 glycols and oxalic acid. The dependence of product coordination geometry on ligand oxidation state is discussed. These studies may have relevance to the possible role of a vanadium(V) oxidation state in vanadium-dependent nitrogen fixation, an alternate to the molybdenum-dependent system. (C) 2002 Published by Elsevier Science B.V.