An NMR study of aqueous isopolytungstate, molybdotungstate and tungstovanadate solutions

Isopoly tungstates have been studied in aqueous solution between pH« 8-1, using 183W, 170, 7Li and *H NMR spectroscopy. The first isopolyanions to form upon acidification are the paratungstates-A and -B. Their resonances are largely assigned, and paratungstate-B is shown to protonate with pKg = 4.59. Its protonated form has at least two isomers separated by a detectably slow proton-exchange process. On further acidification, paratungstate-B loses one tungsten atom to give an anion with no symmetry, which is identified as the solution form of vp-metatungstate. This in turn protonates with pKa =2.65. Six metatungstate species with Keggin structures are also observed at lower pH values. Two have the P-Keggin structures. In each case direct structural information is provided from the solution state. The rotational correlation times and average interproton distances of the a- and P-[H2Wj204o]6' species are deduced from their *H NMR relaxation data, and similar data are used to assist in identifying the three internally monoprotonated a-and P- Keggin anions. Using 183W and 95Mo NMR spectroscopy, molybdotungstates in aqueous solution have been examined over a wide range of metal-ion ratios in the pH range 3-6. Molybdenum is shown to substitute into heptatungstate forming a range of isomers [MoxW7_x024]6* (x = 1 to 6) with a positional preference for the “ends” and least for the “central” W atoms. Assignments for 19 of the isomers present have been proposed. The ratios of the equilibrium constants for a given mixed-metal species with respect to the separate constants for the formation of heptatungstate from tungstate, and heptamolybdate from molybdate have been determined using the least-squares computer program LAKE. Mo is also shown to substitute only once into paratungstate-B and the a-Keggin species. Structures for these anions have been proposed and the substituted paratungstate-B is shown to protonate with pKa = 4.9. Tungstovanadates in aqueous solution have been investigated using 51V, 183W and 170NMR spectroscopy over a wide range of metal-ion concentration and temperature in the range pH 1.2-7. The structures of the anions _/ac-[W3V3019]5', m-[W4V2019]4', c«-[HW4V2019]3’, [W5V019]3- and a-[H2WnVO40]7- have been confirmed. Sound evidence is presented for species /ner-[W3V3019]5' and mmj-[W4V2019]4'. Sly-Sly correlation spectroscopy has established the unequivocal assignment of the [WV9028]5- species with the tungsten replacing one of the “capping” vanadium atoms. [WV9028]5" is also shown to protonate primarily at the bridging oxygens furthest away from W, with pK,=2.1. Tentative evidence is also presented for the solution species [W8V2032]6*, and more heavily substituted a- and 3- metatungstates. Calculations on quantitative 51V NMR data (25 °C, 0.6 mol dm-3 NaCl) using the least-squares computer program LAKE have established the formation constants for the major species. The chemical speciation is illustrated in distribution diagrams