Quaternary Ammonium Salts with B(CN) ₄ or C(CN) ₃ Anions: From Ionic Plastic Crystals and Ionic Liquids to Meltable Coordination Polymers

The effect of the anion shape (tetrahedral B(CN)4– or planar C(CN)3–) on the phase behavior of six synthesized quaternary ammonium salts containing double bonds was investigated. All three B(CN)4 salts formed ionic plastic crystals (IPCs), whereas two of the three C(CN)3 salts were ionic liquids. The reaction of these salts with their corresponding potassium salts formed meltable coordination polymers (CPs), comprising two- or three-dimensional anionic frameworks bridged by polycyano anions with incorporated onium cations. Notably, the B(CN)4-based CP with the NMe3C2H3+ cation incorporates an unusually large anionic cage, [K24{B(CN)4}62]38–, which accommodates 13 cations inside. All CPs exhibited incongruent melting behavior, in which the potassium salt of the anion is deposited upon melting. The C(CN)3-based CPs melted 50–90 K above their parent onium salts, whereas the melting points of the B(CN)4-based CPs and onium salts were comparable, reflecting the high melting points of the B(CN)4 salts, which form the IPC phase.