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 (1D) 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.