Interaction of N-hydroxyacetamide with vanadate in aqueous solution

Abstract

The equilibria between aqueous vanadate and N-hydroxyacetamide (acetohydroxamic acid, HL, CH₃CONHOH) have been studied at 25 °C in 0.15 mol dm^–3 NaCl medium by combined potentiometric, spectrophotometric, ⁵¹V and ¹⁷O NMR methods. Complexes form in the range pH 3 to 11, and many of their formation constants have been determined. The neutral species HV(HL) and HV(HL)₂ predominate below pH 4.5 and with HL∶V > 2∶1, where V is an abbreviation for [H₂VO₄]^–. These notional formulae do not specify the possible co-ordination or loss of water molecules from the complex anions and the charges shown are the overall species charges rather than any algebraic sum. From pH 4.5 to 8.5 the main species are [V(HL)₂]^– and, to a lesser extent, [V(HL)]^–. Oxygen-17 NMR data strongly suggest that [V(HL)₂]^– has octahedral co-ordination, with a cis-VO₂⁺ unit and two bidentate L^– ligands, one of which can lose another proton above pH 8.5 to form [VL(HL)]^2–. The species [V(HL)]^– also deprotonates above pH 9. The minor oligomeric species [V₂(HL)₃]^n– and [V₂(HL)₄]^n– also form, at higher concentrations of V and HL.