SOLID-STATE AND SOLUTION STUDIES OF TUNGSTEN(VI) OXOTETRAALKOXIDES

Variable-temperature H-1 NMR studies of the oxoalkoxides, WO(OR)4 (R = Me, Et, Pr(i), 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) angstrom (average); R = cyclo-C6H11, 2.044(4) and 2.250(4) angstrom] and coplanar with the terminal oxo groups, the longer W-OR bonds being trans to the short W = O bonds. Tungsten-183 NMR chemical shifts of the compounds with R = Me, Et, cyclo-C6H11, Bu(t) or C6H3Pr(i)2-2,6 are very sensitive to the nature of the alkoxo ligand, and range from delta -62.9 for [{WO(OMe)4}2] to -493.6 for mononuclear [WO(OC6H3Pr(i)2-2,6)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) angstrom.