Structural characterization of the brown six-electron-reduced form of dodecatungstoborate, K5[BW12O37(H2O)3]·13.5H2O

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Abstract
Prolonged photolysis of aqueous solutions containing α-K5[BW12O40]·9H2O 1 and MeOH at pH 0.1 led to the formation of the diamagnetic six-electron-reduced species K5[BW12O37(H2O)3]·13.5H2O 2, via successive disproportionations between the protonated two-electron reduced species [H2BW12O40]5–. Single-crystal X-ray diffraction analyses of colourless crystals of 1 and dark brown crystals of 2 showed that six electrons contribute to the formation of a trigonal edge-shared WIV 3O13 group of α-Keggin framework and approximate C3v symmetry. The edge-shared WIV 3 triad contains terminal aqua ligands with a mean bond length of 2.15(4)Å on each WIV atom which is linked by WIV–WIV bonds [mean 2.543(3)Å]. The aqua ligand is also bound to lattice water and neighbouring anion oxygen atoms by hydrogen bonds. The magnitude of the O(terminal)–W–O(µ) bond angles for each WO6 octahedron can be associated with the asymmetry of the axial W–O bond distances, due to the electrostatic repulsive interaction of µ-O atoms with terminal-O and µ4-O atoms. The contraction (≈0.77 Å) of the W ⋯ W distances due to the WIV–WIV bond formation in the WIV 3 triad is concerted, pushing atom B down (≈0.4 Å) toward the corner-shared WVI 3 triad. As a result, an expansion (≈0.03 Å) of the WVI⋯ WVI distances in the corner-shared WVI 3 triad occurs with a shortening (≈0.4 Å) in B ⋯ WVI distances, and is accompained by slight elongations (VI⋯ WVI distances in the edge-shared WVI 3 triad and WVI⋯ WIV distances between the corner-sharing WVIO6 and WIVO6 octahedra.

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