The First Niobasilsesquioxanes^§

Synthetic routes leading to the first polyhedral oligosilsesquioxane derivatives incorporating niobium are reported. Tetrasilanol (c-C6H11)6Si6O7(OH)4 (2), for which an improved laboratory-scale synthesis is reported, cleanly reacts with Nb(OEt)5 to afford the dinuclear niobasilsesquioxane [(c-C6H11)6Si6O11NbOEt]2 (3), which slowly converts to the tetranuclear μ-oxo species 4 in the presence of moisture. A structurally different μ-ethoxy-bridged dinuclear niobasilsesquioxane, [(c-C6H11)7Si7O12NbOEt(μ-OEt)]2 (6), is readily accessible from (c-C6H11)7Si7O9(OH)3 (5) and Nb(OEt)5. The molecular structures of 2, 3, 4, and 6 have been determined by X-ray diffraction.