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.