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′O₄^2–(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.