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 NMe 3 C 2 H 3 + cation incorporates an unusually large anionic cage, [K 24 {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.