Synthesis, Structural Characterization, and Molecular Modeling of Dodecaniobate Keggin Chain Materials

Four new isostructural one-dimensional dodecaniobate Keggin materials, Na12[Ti2O2][TNb12O40]·xH2O and Na10[Nb2O2][TNb12O40]·xH2O with T = (Si or Ge), have been synthesized hydrothermally using a Lindqvist-ion salt, Na7[Nb6O19H]·15H2O, as the precursor. Their structure, consisting of chains of Keggin ions [TNb12O40]16- linked by [Ti2O2]4+ or [Nb2O2]6+ bridges, was solved ab initio from powder diffraction data. The location of the charge-balancing sodium atoms and the water molecules was further investigated by molecular simulations. These compounds were also characterized by IR and solid-state 1H, 29Si, and 23Na MAS NMR spectroscopies. The structural relationships between these and related phases based on similar Keggin ion building units are discussed.