An electron spin resonance study of complexes of oxovanadium(IV) with simple dicarboxylic acids

Abstract

The pH-dependent formation and stability of a range of oxovanadium(IV) dicarboxylic acid complexes have been studied by e.s.r. spectroscopy. The results from oxalic acid [(CO₂H)₂] and malonic acid [CH₂(CO₂H)₂] complexes show that there is significant stabilization with respect to oxovanadium(IV) oxidation/hydrolysis, in agreement with published work using electrochemical techniques. This behaviour can be explained by chelate formation, the five-membered chelate ring with oxalate being a slightly more favoured arrangement than the six-membered ring with malonate, where evidence of distortion is seen in the spectral parameters. Addition of further CH₂ groups in succinic acid [(CH₂)₂(CO₂H)₂] and glutaric acid [(CH₂)₃(CO₂H)₂] results in corresponding decreases in complex stability, with the latter giving results similar to those obtained with fumaric acid [trans-(CH)₂(CO₂H)₂], where no chelation is possible. With maleic acid [cis-(CH)₂(CO₂H)₂] a range of complexes is seen, and these have a significantly greater stability range than was observed with succinic acid.