Chalcogen Bonding with Telluronium Cations: toward Selective Population of Tellurium σ-Holes by Lewis Bases

The reaction of tris[3,5-bis(trifluoromethyl)phenyl]telluronium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF^-) salt with phosphine oxides OPR₃ in 1,2-dichloroethane is exothermic and leads to the population of the three Te σ-holes. Quite unprecedented, careful adjustment of the stoichiometry of reagents allowed the selective crystallization of the Lewis adducts containing one, two, or three Te-bonded OPR₃, according to X-ray diffraction. ¹H and ¹⁹F Diffusion-ordered NMR spectroscopy-monitored titrations of the telluronium salt by OPPh₃ reveal the growth of the hydrodynamic volume of the various cationic adducts, alongside a unique behavior of the BArF^- counterion that suggests its aggregation at high cationic molecular sizes. Isotherm titration calorimetry experiments show that the association equilibrium constants of OPR₃ binding to Te σ-holes follow the order <i>K</i>_a1 > <i>K</i>_a2 > <i>K</i>_a3, with respective values decreasing by 1 up to 2 orders of magnitude as the σ-holes are populated. Analysis of Te-O bonding by DFT methods confirms their dominant Coulombic character. The minor covalent character is nonetheless required to ensure telluronium-ligand cohesion. DFT calculations of the association thermochemistry raise the tangible contribution of ion "pairs" to the association enthalpy as a major issue for modeling: the allegedly "non-coordinating" BArF^- taking part in elusive multipolar ion-pairs is far from being an irrelevant thermochemical contributor.