Speciation 98: Abstracts

Peroxovanadates have recently received wide attention due to their biochemical significance, most notably insulin-mimetic behavior.¹ Although there have been several speciation studies performed on vandate-peroxo systems,^2-4 no pH independent formation constants have been reported. A detailed and thorough potentiometric (glass electrode) and ⁵¹V NMR (Bruker AMX-500 MHz) spectroscopic investigation of the H⁺-H₂VO₄^--H₂O₂ system has therefore been performed (25 C, 0.15 M Na(Cl)). This medium was chosen to represent physiological conditions and an extensive pH (0.5-10.5) and vanadate concentration range (1-80 mM) was covered. The computer program LAKE,⁵ designed to simultaneously treat multimethod data, has been used to establish the entire speciation in the system.

Peroxide interacts with vanadate in the whole pH range studied and equilibration is fast, except when initially formed decavanadates slowly decompose. Six major complexes were found, VX^2-, VX₂^-, VX₂^2-, VX₃^2-, V₂X₄^3- and VX⁺ (X = peroxo ligand) and three minor complexes, V₂X^3- and two isomeric V₂X₂^3- species, which are only observed in the pH range 7-10 at high V_tot concentrations. The species are strongly dependent on the H₂O₂/V ratio as shown in the figure (V_tot = 20 mM, pH = 8.1). The VX^2- species is only present at low ratios (cf. lower spectrum). However, under such conditions a substantial amount of vanadium has not reacted with peroxide. The four larger peaks to the left represent mono-, di-, tetra- and pentameric vanadate species and the small peaks originate from the minor vanadoperoxide species. The VX₂^n- species are "strong" and at H₂O₂/V = 2 (middle spectrum) all vanadium is bound in peroxo species. The VX₃^2- species becomes predominant at excess of peroxide (upper spectrum). The VX⁺ species occurs as a predominant species but only at low pH.

Formation constants will be given for all species and the equilibrium conditions illustrated in distribution diagrams. Structural aspects will be discussed. Some preliminary results from the vanadoperoxoimidazole system will also be presented.

References

1. Orvig, C. et al. in Metal Ions in Biological Systems, Vol 31, Sigel, H., Sigel, A. (Ed), Marcel Dekker, 1995.
2. Harrison, A. T., Howarth, O. W., J. Chem. Soc. Dalton Trans. 1985, 1173-1177.
3. Jaswal, J., Tracey, A. S., Inorg. Chem. 1991, 30, 3718-3722.
4. Conte, V., Di Furia, F., Moro, S., J. Mol. Cat. A: Chemical, 1997, 120, 93-99.
5. Ingri, N., Andersson, I., Pettersson, L., Yagasaki, A., Andersson, L., Holmström, K., Acta Chem. Scand., 1996, 50, 717-734.

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