Synthesis, structure, and molecular modeling of a titanoniobate isopolyanion

Polyoxoniobate chemistry, both in the solid state and in solution is dominated by [Nb6O19](8-), the Lindquist ion. Recently, we have expanded this chemistry through use of hydrothermal synthesis. The current publication illustrates how use of heteroatoms is another means of diversifying polyoxoniobate chemistry. Here we report the synthesis of Na-8[Nb8Ti2O28](.)34H(2)O [(1) under bar] and its structural characterization from single-crystal X-ray data. This salt crystallizes in the P-1 space group (a = 11.829(4) Angstrom, b = 12.205(4) Angstrom, c = 12.532(4) Angstrom, alpha = 97.666(5)degrees, beta = 113.840(4)degrees, gamma = 110.809(4)degrees), and the decameric anionic cluster [Nb8Ti2O28](8-) has the same cluster geometry as the previously reported [Nb10O28](6-) and [V10O28](6-). Molecular modeling studies of [Nb10O28](6-) and all possible isomers of [Nb8Ti2O28](8-) suggest that this cluster geometry is stabilized by incorporating the Ti4+ into cluster positions in which edge-sharing is maximized. In this manner, the overall repulsion between edge-sharing octahedra within the cluster is minimized, as Ti4+ is both slightly smaller and of lower charge than Nb5+. Synthetic studies also show that while the [Nb10O28](6-) cluster is difficult to obtain, the [Nb8Ti2O28](8-) cluster can be synthesized reproducibly and is stable in neutral to basic solutions, as well. Published by Elsevier Inc.