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

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 W^IV₃O₁₃ group of α-Keggin framework and approximate C_3v symmetry. The edge-shared W^IV₃ triad contains terminal aqua ligands with a mean bond length of 2.15(4)Å on each W^IV atom which is linked by W^IV–W^IV 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 W^IV–W^IV bond formation in the W^IV₃ triad is concerted, pushing atom B down (≈0.4 Å) toward the corner-shared W^VI₃ triad. As a result, an expansion (≈0.03 Å) of the W^VI⋯ W^VI distances in the corner-shared W^VI₃ triad occurs with a shortening (≈0.4 Å) in B ⋯ W^VI distances, and is accompained by slight elongations (<0.07 Å) in both the W^VI⋯ W^VI distances in the edge-shared W^VI₃ triad and W^VI⋯ W^IV distances between the corner-sharing W^VIO₆ and W^IVO₆ octahedra.