Synthesis and Characterization of Zr(IV) Polyoxotungstates as Molecular Analogues of Zirconia-Supported Tungsten Catalysts

A new zirconium-containing polyoxotungstate [{W 5 O 18 Zr(μ-OH)} 2 ] 6- 2 was prepared by alkalinization of a solution of the monomeric form [W 5 O 18 Zr(H 2 O)] 2- 1 . The crystal structure of ( n Bu 4 N) 6 [{W 5 O 18 Zr(μ-OH)} 2 ]·2H 2 O (monoclinic; space group C 2/ c; a = 26.086, b = 17.018, c = 32.891 Å; β = 112.23°) reveals that two Lindqvist-type units {ZrW 5 O 18 } 2- (shortly ZrW 5 ) are linked through two hydroxo bridges (Zr−OH−Zr) with Zr in sevenfold coordination. Both compounds 1 and 2 were characterized by IR and Raman spectroscopy. EXAFS analysis of 1 at the W L III -edge agrees with the hypothesis of a monosubstituted Lindqvist structure. The 183 W NMR spectrum of 2 in acetonitrile is consistent with the solid-state structure assuming a dynamic process implying likely the μ-hydroxo bridge. A brief discussion is given on the relevance of molecular zirconium-containing polyoxotungstates to heterogeneous catalysis since these compounds can be envisioned as molecular analogues of zirconia-supported tungsten catalysts.