A solid-state NMR, X-ray diffraction, and Ab initio investigation into the structures of novel tantalum oxyfluoride clusters

A series of tantalum oxyfluoride materials containing the [Ta ₄F₁₆O₄]^4- and [Ta₈F ₂₄O₁₂]^8- anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta₄F₁₆O₄] ^4- cluster is an eight-membered ring, whereas the [Ta ₈F₂₄O₁₂]^8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (ID) ¹⁹F magic angle spinning (MAS) NMR, ¹³C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) ¹⁹F MAS NMR spectra. The DQ ¹⁹F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio ¹⁹F NMR chemical shift calculations were used to assist in the interpretation of the ¹⁹F NMR spectra. The influence of scalar relativistic and Ta-F spin-orbit coupling on the ¹⁹F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.