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

Potentiometric, spectral and H-1 NMR relaxation studies are reported on the complex formation of VO(IV) with the bidentate chelating ligands oxalic an d malonic acids, of a dicarboxylic nature, and maltol and kojic acid, of th e hydroxy-4-pyrone type. Complexes with stochiometries VOA and VOA(2) were characterised in aqueous solution. Two main features were established for t he 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 high er extent than in those of oxalic and malonic acids. Moreover, previous inv estigations of VO(IV) binding to maltol or its analogues did not consider t he formation of hydroxo species. Formation of dihydroxo bridged dimeric com plexes of stoichiometry (VOAH(-1))(2) and monomeric hydroxo species VOA(2)H (-1) or VOA(2)(OH), are now taken into account in order to explain the equi librium and spectroscopic results. H-1 NMR relaxation measurements strongly suggest an equilibrium between the two isomers of the bis complexes. (C) 2 000 Elsevier Science S.A. All rights reserved.