Although coordination polymers generally do not melt, several that do melt have been synthesized recently and have drawn much attention. In this study, two‐ and three‐dimensional coordination polymers that melt were synthesized, [Ru(Cp)(C6H5R)][M{C(CN)3}2] (R=H, Me, Et; M=K, Rb; Cp=C5H5), which are complex salts comprising M[C(CN)3] and organometallic ionic liquids [Ru(Cp)(C6H5R)][C(CN)3]. They have anionic [M{C(CN)3}2]n− coordination polymer frameworks, whose dimensionalities depend on the size of the organometallic cation inside. Their melting points decreased with increasing cation substituent length and size of the alkali metal ion (Tm=102–239 °C), and these low‐melting‐point coordination polymers exhibited incongruent melting, forming mixtures of solid M[C(CN)3] and ionic liquid upon melting. Using the same method, coordination polymers were synthesized with various bridging ligands, [Co(Cp)2][MX2] (X=B(CN)4, C(CN)3, N(CN)2; M=K, Na), as well as a paramagnetic coordination polymer, [Fe(Cp)2][K{C(CN)3}2].

Salts of sandwich complexes often exhibit a plastic phase at high temperatures. To determine a molecular design that can achieve a plastic phase at lower temperatures, we synthesized [CoCp2][X] and [Ru(Cp)(C6H6)][X] (Cp = C5H5) with various anions (X = monocarba-closododecaborate (CB11H12 −), B(CN)4 − , CF3BF3 − , OTf − , BF4 − , C(CN)3 − , and tris(pentafluoroethyl)trifluorophosphate (FAP −)) and investigated their phase behaviors. All of the salts except the C(CN)3 and FAP salts exhibited a plastic phase. The phase transition temperature to the plastic phase tended to decrease with decreasing anion size. This tendency contrasts that observed in octamethyl-and decamethylferrocenium salts. Although the phase transition temperatures of most salts were high, those of [CoCp2][CF3BF3], [Ru(Cp)(C6H6)][CF3BF3], and [Ru(Cp)(C6H6)][BF4] were below 300 K. The plastic phases of the salts had a CsCl-type structure. Crystal structure determinations at low temperatures revealed that the cations and anions were arranged alternately in most of the salts. However, the C(CN)3 salts exhibited a stacking arrangement of the cations, which is responsible for the absence of a plastic phase.

Salts of cationic sandwich complexes often exhibit an ionic plastic phase; however, only a few exhibit a plastic phase at room temperature. To explore the use of the CF3BF3 anion to lower the transition temperature to the plastic phase, we prepared salts of CF3BF3 with various ferrocene derivatives, [D][CF3BF3] (D=FeCp*2, Fe(C5Me4H)2, Fe(C5H4Me)2, FeCp(C5H4Me), FeCp2; Cp*=C5Me5, Cp=C5H5). Although [FeCp*2][CF3BF3] exhibited a plastic phase above 417 K, the other salts formed room‐temperature ionic plastic crystals with a phase transition to the plastic phase in the range 266–291 K. The crystal structure and thermal properties of [FeCp2][OTf] were elucidated for comparison. In addition, decamethylferrocenium salts with other anions were synthesized and structurally characterized: [FeCp*2][X] (X=N(SO2F)2 and B(CN)4) exhibited a phase transition to the plastic phase above 400 K, whereas carborane‐containing salts [FeCp*2]2[B12F12] and [FeCp*2][Co(C2B9H11)2] did not exhibit a plastic phase.

Quaternary ammonium salts with the carborane anion CB11H12 − ([cation][CB11H12]; cations : Me4N + , Et4N + , (C4H8)2N + , Bu4N + , and MeEt3(MeOCH2CH2)N +) were prepared. Each salt displayed several solid phase transitions and exhibited an ionic plastic phase at high temperatures. Crystallographic analysis revealed that the disorder of the cation conformation and anion orientation become more extensive in the higher temperature phases.

Several meltable coordination polymers (CPs) that possess substantial advantages attributable to their high flexibility and processability have been developed recently; however, the melting mechanism and vitrification conditions of these materials are not yet fully understood. In this study, we synthesized meltable CPs [A]­[K­(TCM)2] (A = onium cation, TCM = C­(CN)3 –) incorporating ionic liquid components and investigated their crystal structures and melting behaviors in detail. These CPs feature two- or three-dimensional anionic [K­(TCM)2] n – frameworks incorporating onium cations. Each CP was found to undergo incongruent melting at a temperature between 73 and 192 °C to produce a heterogeneous mixture of the ionic liquid ([A]­[TCM]) and microcrystalline K­[TCM]. Furthermore, they formed homogeneous liquids upon further heating to ∼240 °C. The melting points of these CPs were linearly correlated with those of their constituent ionic liquids. The vitrification of these materials upon rapid cooling from the molten state was further investigated. The cooling rates required for vitrification differed greatly between the CPs and were correlated with the cation flexibility.