Characterization of decavanadate and decaniobate solutions by Raman spectroscopy

Abstract

The decaniobate ion, (Nb₁₀ = [Nb₁₀O₂₈]⁶⁻) being isoelectronic and isostructural with the decavanadate ion (V₁₀ = [V₁₀O₂₈]⁶⁻), but chemically and electrochemically more inert, has been useful in advancing the understanding of V₁₀ toxicology and pharmacological activities. In the present study, the solution chemistry of Nb₁₀ and V₁₀ between pH 4 and 12 is studied by Raman spectroscopy. The Raman spectra of V₁₀ show that this vanadate species dominates up to pH 6.45 whereas it remains detectable until pH 8.59, which is an important range for biochemistry. Similarly, Nb₁₀ is present between pH 5.49 and 9.90 and this species remains detectable in solution up to pH 10.80. V₁₀ dissociates at most pH values into smaller tetrahedral vanadate oligomers such as V₁ and V₂, whereas Nb₁₀ dissociates into Nb₆ under mildly (10 > pH > 7.6) or highly alkaline conditions. Solutions of V₁₀ and Nb₁₀ are both kinetically stable under basic pH conditions for at least two weeks and at moderate temperature. The Raman method provides a means of establishing speciation in the difficult niobate system and these findings have important consequences for toxicology activities and pharmacological applications of vanadate and niobate polyoxometalates.