Synthesis and characterization of Zr(IV) polyoxotungstates as molecular analogues of zirconia-supported tungsten catalysts

A new zirconium-containing polyoxotungstate [{W5O18Zr(mu-OH)}(2)](6-) 2 was prepared by alkalinization of a solution of the monomeric form [W5O18Zr(H2O)](2-) 1. The crystal structure of ((Bu4N)-Bu-n)(6)[{W5O18Zr(eta-OH)}(2)](.)2H(2)O (monoclinic; space group C2/c; a = 26.086, b = 17.018, c = 32.891 Angstrom; beta = 112.23degrees) reveals that two Lindqvist-type units {ZrW5O18}(2-) (shortly ZrW5) 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 W-183 NMR spectrum of 2 in acetonitrile is consistent with the solid-state structure assuming a dynamic process implying likely the mu-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.