Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: Synthesis and crystal structures of heteronuclear hexametalate derivatives

Oxygen-17 NMR studies indicated that the hexametalates [MW(5)O(19)](n-) or their derivatives [(MeO)MW(5)O(18)]((n-1)-) (M = Ti, Zr, V, Nb, Ta, Mo or W) can be obtained by hydrolysis of the appropriate mixture of metal alkoxides in the presence of M'O-4(2-) (M' = W or Mo) with remarkable selectivity in some cases, giving a very efficient method of preparing O-17-enriched polyoxometalates. The crystal structure of [NBu(4)(n)](3)[(MeO)TiW5O18] . 0.5MeCN shows the terminal Ti-OMe bond in the anion to have Ti-O 1.760(10) Angstrom and Tl-O-C 150.1(12)degrees and also reveals W-O bridging bond length alternations due to the substitution of Ti(OMe)(3+) for WO4+ in the [W6O19](2-) structure. Hydrolysis of [NBu(4)(n)](2)[(MeO)NbW5O18] gives [NBu(4)(n)](4)[(NbW5O18)(2)O] and the crystal structure revealed two eclipsed NbW5O18 oxide fragments joined by a strictly linear Nb-O-Nb linkage with Nb-O 2.264(8) Angstrom.