Alkoxide hydrolysis as a route to early transition-metal polyoxometalates: synthesis and crystal structures of heteronuclear hexametalate derivatives
Abstract
Oxygen-17 NMR studies indicated that the hexametalates [MW5O19]n– or their derivatives [(MeO)MW5O18](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′O42–(M′= W or Mo) with remarkable selectivity in some cases, giving a very efficient method of preparing 17O-enriched polyoxometalates. The crystal structure of [NBun4]3[(MeO)TiW5O18]·0.5MeCN shows the terminal Ti–OMe bond in the anion to have Ti–O 1.760(10)Å and Ti–O–C 150.1(12)° 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 [NBun4]2[(MeO)NbW5O18] gives [NBun4]4[(NbW5O18)2O] 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)Å.