Fluorinated Boroxin-Based Anion Receptors for Lithium Ion Batteries: Fluoride Anion Binding, Ab Initio Calculations, and Ionic Conductivity Studies

Novel fluorinated boroxines, tris(2,6-difluorophenyl)boroxin (DF), tris(2,4,6-trifluorophenyl)boroxin (TF), and tris(pentafluorophenyl)boroxin (PF), have been investigated for potential applications in lithium ion batteries through fluoride anion binding, ab initio calculations, and ionic conductivity measurements. Structures of the fluorinated boroxines and boroxin-fluoride complexes have been confirmed by comparing their ¹⁹ F and ¹¹ B NMR chemical shifts with those obtained by the DFT-GIAO method. The stoichiometry of the fluoride anion binding to these boroxines has been shown to be 1:1 using ¹⁹ F NMR and UV-vis spectroscopy. UV-vis spectroscopic studies show the coexistence of more than one complex, in addition to the 1:1 complex, for perfluorinated boroxin, PF. DFT calculations (B3LYP/6-311G**) show that the fluoride ion complex of DF prefers unsymmetrical, covalently bound structure (7) over the symmetrically bridged species (10) by 12.5 kcal/mol. Rapid equilibration of the fluoride anion among the three borons in these boroxines results in a single ¹⁹ F NMR absorption for all of the aromatic ortho-or para-fluorines at ambient temperature. The effect of these anion receptors on lithium ion conductivities was also explored for potential applications in dual ion intercalating lithium batteries.