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A new tungstoboric species, [BW 13 O 46 H 3 ] 8-, which was identified in solution, is an intermediate between [BW 11 O 39 H] 8and the so-called hexagonal acid. This heteropolyacid was obtained in high yield by a new synthetic route. A single-crystal X-ray diffraction study showed that its formula is H 21 [B 3 W 39 O 132 ]‚69H 2 O. The heteropolyanion is formed by three BW 13 O 46 subunits linked together by sharing vertices leading to C 3h symmetry. Each subunit is constituted by a undecatungstic anion derived from the Keggin structure, capped by a µ-oxoditungstic group, W 2 O 11 . The BO 4 tetrahedron is significantly distorted, one of the B-O bonds being longer (1.58 Å) than the three others (1.46 Å). The conversions between the three species [BW 11 O 39 H] 8-, [BW 13 O 46 H 3 ] 8and [B 3 W 39 O 132 ] 21are rapid and reversible. On the contrary, the irreversible formation of the Keggin heteropolyanion, R-[BW 12 O 40 ] 5-, is observed by heating of the solution of [BW 13 O 46 H 3 ] 8at pH 1-5.

The reaction of tungstate ions with the gamma-10-tungstosilicate in mixed water-ethanol (v/v) yields the corresponding isomer of the 12-tungstosilicate isolated as its tetrabutylammonium salt. It was characterized by means of (183)W NMR, infrared, Raman, and UV spectroscopies and was identified with the isomer resulting from the Keggin structure (alpha-isomer) by rotation of two tritungstic groups by pi/3. Cyclic voltammetry shows that the reducibility increases in the sequence alpha-beta-gamma as the number of rotated tritungstic groups. Whereas the gamma-12-tungstosilicate anion is unstable in pure aqueous solution whatever the pH value, it is stable in mixed aqueous-organic or pure organic solvents, even at boiling temperatures. In contrast, the two- and four-electron-reduced blue species are stable in aqueous media but not the one-electron-reduced blue. Study of the (183)W NMR spectrum of the two-electron-reduced species showed that the two spin-paired electrons are delocalized on all the W atoms on the NMR time scale but have a larger residency time on one group of four atoms.