Polyoxometalates from heteropoly “brown” precursors. A new structural class of mixed valence heteropolytungstates, [(XO4)WIV3WVI17O62Hx]n−

Abstract

Reduction of the α-Keggin anion [Xn+O4W12O36][8 − n]− (X = H22+, B3+, Si4+) by six electrons results in the known tungsten “brown” species [Xn+O4(H2O)3WIV3WVI9O33][8 − n]− in which three W atoms have been reduced from WVI to WIV, forming a metal–metal bonded triad. The WIV atoms have terminal water coordinated in place of terminal oxo groups. Additional tungstate can condense onto these water molecules in aqueous solution between pH = 4 and 6.5 to form the species reported here, [(XO4)WIV3WVI17O62Hx]y−. The boron derivative (X = B3+) is more stable than the metatungstate (X = H22+), both of which have been characterized by elemental analysis, 183W NMR and X-ray crystal structure analysis. Eight additional tungstate groups condense in the form of a partial Keggin structure containing two triads and one dyad which is rotated 60° relative to a hypothetical α-isomer.