Electronic and Geometric Symmetry in Chalcogen-Bonded Telluronium Cations Revealed by ¹²⁵ Te NMR and DFT

Chalcogen bonding (ChB) interactions involving group 16 elements are of growing interest due to their utility in supramolecular chemistry and catalysis. We present a detailed spectroscopic and theoretical investigation of ChB in a series of telluronium cation and Lewis base cocrystals, using 125Te and 31P solid-state NMR spectroscopy (SSNMR), single-crystal and powder X-ray diffraction, and Density Functional Theory (DFT)/Natural Localized Molecular Orbital (NLMO) calculations. The 125Te chemical shift tensor components correlate strongly with ChB strength and reveal a consistent increase in the δ11 component and chemical shift anisotropy (CSA) span (Ω) with stronger interactions. This contrasts with prior observations in Te(II) and Se(II) systems, underscoring the unique behavior of Te(IV) cations with three σ-holes. Octahedral distortion metrics (ζ, Δ, Σ) are linked to CSA trends, supporting a model in which ChB-induced asymmetry drives tensor orientation. DFT calculations with spin–orbit relativistic corrections reproduce the experimental trend, while NLMO analysis attributes δ11 shifts to specific bonding orbitals and their change in orientation. Our findings demonstrate the sensitivity of 125Te SSNMR to subtle structural and electronic variations in ChB systems and establish tensor analysis as a powerful probe of noncovalent interactions in chalcogen cocrystals.