Reduced Molybdovanadophosphates

The equilibrium speciation in the aqueous H+/MoO42−/HVO42−/HPO42−/e− system, 0.600 M Na(Cl) medium at 25 °C, was characterised by potentiometric [H+] (glass electrode) and {e−} (platinum electrode) titration techniques as fundamental tools, complemented by 51VV NMR (131.6 MHz) and VIV ESR spectroscopy. The study has been focused on solutions with the Keggin ratio, (Mo + V)/P = 12:1 and Mo/V > 5, in the pH range 1.4−4.5, where anions having the formula [HzMo12−x−yVVxVIVyPO40](3+x+2y−z)− are predominant species. Added numbers of electrons per vanadium(V) (e−/VV) have been kept less than one to exclude reduction of molybdenum(VI). Six reduced molybdovanadophosphate species have been identified. The [Mo11VIVPO40]5− species [logβ = 154.15 ± 0.03 (3σ)] and the mixed-valence species [Mo10VVVIVPO40]6− (logβ = 155.84 ± 0.02) can both be monoprotonated. These species have pKa values of 0.93 and 3.68, respectively. A “fully” reduced Keggin species, [H2Mo10VIV2PO40]5− (logβ = 171.27 ± 0.06), and a lacunary [H4Mo10VIVPO39]5− species (logβ = 142.0 ± 0.2) were found as well. Except for the molybdovanadophosphate species, the only reduced species formed, in the present investigation, is the vanadyl ion, VO2+. logKred (2 H+ + VO2+ + e ⇄ VO2+ + H2O) was determined to 17.15 ± 0.01 (E0 = 1014.5 ± 0.6 mV).