Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds

This work determines the potential of solid-state NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na₇HNb₆O₁₉·15H₂O (<b>1</b>), K₈Nb₆O₁₉·16H₂O (<b>2</b>), and Na₈Ta₆O₁₉·24.5H₂O (<b>3</b>) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). ¹H MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated clusters A_<i>x</i>H_8–<i>x</i>M₆O₁₉·<i>n</i>H₂O (A = alkali ion; M = Nb, Ta). ⁹³Nb MAS NMR studies at different fields and MAS rotation frequencies have been performed on <b>1</b>. For the first time, the contributions of NbO₅O_μ2H sites were clearly distinguished from those assigned to NbO₆ sites in the hexaniobate cluster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), ⁹³Nb WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample <b>1</b>. The ⁹³Nb NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO₅O_μ2H sites). 1D ²³Na MAS and 2D ²³Na 3QMAS NMR studies of <b>1</b> revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The ²³Na MAS NMR study of <b>3</b> confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in <b>1</b> was also probed by multinuclear MAS NMR (¹H, ²³Na, and ⁹³Nb)