Understanding weakly coordinating anions: tetrakis(pentafluorophenyl)borate paired with inorganic and organic cations.

Efficient design of ionic compounds requires a systematic understanding of cation-anion interactions. Weakening of electrostatic attraction is essential to increase the liquid range of the ionic compound and decrease its melting point. Here, we report simulations of the closest-approach cation-anion distances in a variety of ion pairs containing the tetrakis(pentafluorophenyl)borate (TFPB-) anion. Small alkali cations (Li+, Na+) penetrate the TFPB- core, whereas K+ and larger organic cations do not. In the latter case, the shortest possible distance from the cations to the boron atom of TFPB- ranges from 0.50 nm to 0.63 nm. TFPB- was shown to be substantially rigid, providing a steric hindrance to thermodynamically efficient cation-anion coordination. Our results prove that TFPB- is more efficient for electrostatic charge confinement than the tetraoctylammonium cation, whereas the perfluorophenyl group is more efficient than linear alkyl chains. These simulations will motivate development of TFPB--based ionic liquids with low phase transition points. Graphical Abstract Ionic configuration of the equilibrated "TFPB + K"system.