Syntheses and Structures of the Tungstoborate Anions

A new tungstoboric species, [BW₁₃O₄₆H₃]^8-, which was identified in solution, is an intermediate between [BW₁₁O₃₉H]^8- and 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 H₂₁[B₃W₃₉O₁₃₂]·69H₂O. The heteropolyanion is formed by three BW₁₃O₄₆ 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, W₂O₁₁. The BO₄ 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 [BW₁₁O₃₉H]^8-, [BW₁₃O₄₆H₃]^8- and [B₃W₃₉O₁₃₂]^21- are rapid and reversible. On the contrary, the irreversible formation of the Keggin heteropolyanion, α-[BW₁₂O₄₀]^5-, is observed by heating of the solution of [BW₁₃O₄₆H₃]^8- at pH 1−5.