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 [Ta4F16O4](4-) and [Ta8F24O12](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 [Ta8F24O12](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) F-19 magic angle spinning (MAS) NMR, C-13 cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) F-19 MAS NMR spectra. The DQ F-19 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 F-19 NMR chemical shift calculations were used to assist in the interpretation of the F-19 NMR spectra. The influence of scalar relativistic and Ta-F spin-orbit coupling on the F-19 NMR shielding calculation arising from bonding to tantalum atoms, is also addressed.