Speciation and NMR relaxation studies of VO(IV) complexes with several O-donor containing ligands: oxalate, malonate, maltolate and kojate

Potentiometric, spectral and 1H NMR relaxation studies are reported on the complex formation of VO(IV) with the bidentate chelating ligands oxalic and malonic acids, of a dicarboxylic nature, and maltol and kojic acid, of the hydroxy-4-pyrone type. Complexes with stochiometries VOA and VOA2 were characterised in aqueous solution. Two main features were established for the complex systems. It was found that five-membered ring chelation favours the cis arrangement in the bis complexes of maltol and kojic acid to a higher extent than in those of oxalic and malonic acids. Moreover, previous investigations of VO(IV) binding to maltol or its analogues did not consider the formation of hydroxo species. Formation of dihydroxo bridged dimeric complexes of stoichiometry (VOAH−1)2 and monomeric hydroxo species VOA2H−1 or VOA2(OH), are now taken into account in order to explain the equilibrium and spectroscopic results. 1H NMR relaxation measurements strongly suggest an equilibrium between the two isomers of the bis complexes.