Sodium decavanadate

Sodium decavanadate describes any member of the family of inorganic compounds with the formula Na₆[V₁₀O₂₈](H₂O)_n. These are sodium salts of the orange-colored decavanadate anion [V₁₀O₂₈]⁶⁻.^[1] Numerous other decavanadate salts have been isolated and studied since 1956 when it was first characterized.^[2]

Sodium decavanadate

Identifiers
CAS Number	(anhydrous): 12315-57-0 N
3D model (JSmol)	(anhydrous): Interactive image
ChemSpider	(anhydrous): 34997594
EC Number	(anhydrous): 235-375-1
PubChem CID	(anhydrous): 118856830
InChI InChI=1S/6Na.28O.10V/q6*+1;28*-2;10*+5 Key: WSNCYQDYQWKFLZ-UHFFFAOYSA-N
SMILES (anhydrous): [O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[O-2].[Na+].[Na+].[Na+].[Na+].[Na+].[Na+].[V].[V].[V].[V].[V].[V].[V].[V].[V].[V]
Properties
Chemical formula	Na6[V10O28]
Molar mass	1419.6 g/mol
Appearance	orange solid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N (what is YN ?) Infobox references

Preparation

The preparation of decavanadate is achieved by acidifying an aqueous solution of ortho-vanadate:^[1]

10 Na₃[VO₄] + 24 HOAc → Na₆[V₁₀O₂₈] + 12 H₂O + 24 NaOAc

The formation of decavanadate is optimized by maintaining a pH range of 4–7. Typical side products include metavanadate, [VO₃]⁻, and hexavanadate, [V₆O₁₆]²⁻, ions.^[1]

Structure

The decavanadate ion consists of 10 fused VO₆ octahedra and has D_2h symmetry.^[3][4][5] The structure of Na₆[V₁₀O₂₈]·18H₂O has been confirmed with X-ray crystallography.^[6]

Figure 1: structure of decavanadate ion with equivalent V and O atoms indicated

The decavanadate anions contains three sets of equivalent V atoms (see fig. 1).^[3] These include two central VO₆ octahedra (V_c) and four each peripheral tetragonal-pyramidal VO₅ groups (V_a and V_b). There are seven unique groups of oxygen atoms (labeled A through G). Two of these (A) bridge to six V centers, four (B) bridge three V centers, fourteen of these (C, D and E) span edges between pairs of V centers, and eight (F and G) are peripheral.

The oxidation state of vanadium in decavanadate is +5.

Acid-base properties

Aqueous vanadate (V) compounds undergo various self-condensation reactions.^[7] Depending on pH, major vanadate anions in solution include VO₂(H₂O)₄²⁺, VO₄³⁻, V₂O₇³⁻, V₃O₉³⁻, V₄O₁₂⁴⁻, and V₁₀O₂₈⁶⁻. The anions often reversibly protonate.^[5] Decavanadate forms according to this equilibrium:^[2][7]

H₃V₁₀O₂₈³⁻ ⇌ H₂V₁₀O₂₈⁴⁻ + H⁺

H₂V₁₀O₂₈⁴⁻ ⇌ HV₁₀O₂₈⁵⁻ + H⁺

HV₁₀O₂₈⁵⁻_(aq) ⇌ V₁₀O₂₈⁶⁻ + H⁺

The structure of the various protonation states of the decavanadate ion has been examined by ⁵¹V NMR spectroscopy.^[5][7] Each species gives three signals; with slightly varying chemical shifts around −425, −506, and −523 ppm relative to vanadium oxytrichloride; suggesting that rapid proton exchange occurs resulting in equally symmetric species.^[8] The three protonations of decavanadate have been shown to occur at the bridging oxygen centers, indicated as B and C in figure 1.^[8]

Decavanadate is most stable in pH 4–7 region.^[1][4][7] Solutions of vanadate turn bright orange at pH 6.5, indicating the presence of decavanadate. Other vanadates are colorless. Below pH 2.0, brown V₂O₅ precipitates as the hydrate.^[3][7]

V₁₀O₂₈⁶⁻ + 6H⁺ + 12H₂ ⇌ 5V₂O₅

Potential uses

Decavanadate has been found to inhibit phosphoglycerate mutase, an enzyme which catalyzes step 8 of glycolysis. In addition, decavandate was found to have modest inhibition of Leishmania tarentolae viability, suggesting that decavandate may have a potential use as a topical inhibitor of protozoan parasites.^[9]

Related decavanadates

Many decavanadate salts have been characterized. NH₄⁺, Ca²⁺, Ba²⁺, Sr²⁺, and group I decavanadate salts are prepared by the acid-base reaction between V₂O₅ and the oxide, hydroxide, carbonate, or hydrogen carbonate of the desired positive ion.^[1]

6 NH₃ + 5 V₂O₅ + 3 H₂O ⇌ (NH₄)₆[V₁₀O₂₈]

Other decavanadates:

(NH₄)₆[V₁₀O₂₈]·6H₂O^[2]

K₆[V₁₀O₂₈]·9H₂O^[2]

K₆[V₁₀O₂₈]·10H₂O^[1][2][3]

Ca₃[V₁₀O₂₈]·16H₂O^[2][3]

K₂Mg₂[V₁₀O₂₈]·16H₂O^[2][3]

K₂Zn₂[V₁₀O₂₈]·16H₂O^[1][2][3]

Cs₂Mg₂[V₁₀O₂₈]·16H₂O^[3]

Cs₄Na₂[V₁₀O₂₈]·10H₂O^[10]

K₄Na₂[V₁₀O₂₈]·16H₂O^[11]

Sr₃[V₁₀O₂₈]·22H₂O^[10]

Ba₃[V₁₀O₂₈]·19H₂O^[10]

[(C₆H₅)₄P]H₃V₁₀O₂₈·4CH₃CN^[8]

Ag₆[V₁₀O₂₈]·4H₂O^[12][13]

Naturally occurring decavanadates include:

Ca₃V₁₀O₂₈·17 H₂O (Pascoite)

Ca₂Mg(V₁₀O₂₈)·16H₂O (Magnesiopascoite)

Na₄Mg(V₁₀O₂₈)·24H₂O (Huemulite)