Solid state and solution studies of tungsten(VI) oxotetraalkoxides

Abstract

Variable-temperature ¹H NMR studies of the oxoalkoxides, WO(OR)₄(R = Me, Et, Prⁱ, or cyclo-C₆H₁₁), show these compounds to be dynamic in solution. For R = Me or Et, limiting spectra have been obtained which are consistent with alkoxide-bridged binuclear structures, and for R = Me or cyclo-C₆H₁₁, X-ray diffraction studies confirm that these structures are adopted in the solid state. The alkoxide bridges are asymmetric [R = Me, 2.032(7) and 2.242(12)Å(average); R =cyclo-C₆H₁₁, 2.044(4) and 2.250(4)Å] and coplanar with the terminal oxo groups, the longer W–OR bonds being trans to the short WO bonds. Tungsten-183 NMR chemical shifts of the compounds with R = Me, Et, cyclo-C₆H₁₁, Bu^t or C₆H₃Prⁱ₂-2,6 are very sensitive to the nature of the alkoxo ligand, and range from δ–62.9 for [{WO(OMe)₄}₂] to –493.6 for mononuclear [WO(OC₆H₃Prⁱ₂-2,6)₄] Possible solution equilibria are discussed in the light of these NMR results. An attempt to prepare [WO(OC₆H₄Me-4)₄] yielded [W(OC₆H₄Me-4)₆], and an X-ray crystal structure determination showed this compound to be octahedral with an average W–O bond length of 1.895(9)Å.