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 Ka1 > Ka2 > Ka3, 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.