Study of the interaction between oxovanadium(IV) and hydroxamic acids

The interaction between oxovanadium(IV) and primary hydroxamic acids (aceto- (HAha), benzohydroxamic acid (HBha) or N-hydroxy-N'-[3-(N-hydroxy-carbamoyl)-propyl]-lieptane-1,7-dicarboxamide (H(2)Diha)) as well as the corresponding N-methyl substituted hydroxamic acids (HMeAha, HMeBha, H(2)MeDiha) was studied by pH-potentiometric and pH-stat methods in aqueous solution. Stoichiometry and stability constants of the VO(IV) complexes formed in the pH range 2.0-5.0 were determined and the effect of the substituents on the C or N atoms of the hydroxamic group is discussed. The dihydroxamic acids are more effective oxovanadium(IV) binders than the monohydroxamic ones. Above pH = 4.5-5.0 (depending upon ligand excess), slow redox reaction was found in all the systems studied. During this process, beside the oxidation of VO(IV) to the appropriate V(V)-hydroxamate complex, formation of amides as reduction products was detected under anaerobic conditions. The same reaction can also take place when stirring sparingly soluble VOA(2) type complexes (A: Bha or MeBha) in water in the absence of oxygen. The exact stoichiometry of the redox reaction was determined by pH-stat measurements and found to be 2VO(R1CON(R-2)O)(2) + R1CON(R-2)OH + H2O = 2[VO2(R1ON(R-2)O)(2)]- + 2H(+) + R1CON(R-2)H (R-1: CH3, C6H5; R-2: H, CH3) for the monohydroxamates. The N-methyl hydroxamic acids tend to oxidise VO2+ significantly slower than the corresponding primary ones. (c) 2005 Elsevier Ltd. All riehts reserved.