Incongruent Melting and Vitrification Behaviors of Anionic Coordination Polymers Incorporating Ionic Liquid Cations.

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.

Anion-Ordering Phase Transitions in Biferrocenium and Ferrocenium Salts with the Bis(trifluoromethanesulfonyl)amide Anion.

The bis(trifluoromethanesulfonyl)amide anion (Tf2N), which is a common component of ionic liquids, often exhibits disorder in the solid state. In this study, the phase transitions and crystal structures of the Tf2N salts of 1,1‴-dineopentyl-1',1″-biferrocene (=npBifc), 1',1″-biferrocene (=Bifc), ferrocene, and cobaltocene (1-4, respectively) were compared. All the salts exhibited phase transitions at low temperatures, which are accompanied by anion ordering, though the ordering was not complete in 2 and 3. X-ray crystallographic investigation revealed that the cations and anions in 1 and 2 adopted alternating arrangements and segregated columnar arrangements, respectively. The cation in 1 exhibited a symmetrical, average-valence structure in the room-temperature phase owing to rapid valence tautomerization, whereas the cation exhibited an unsymmetrical structure in the low-temperature phase. The cation in 2 exhibited an unsymmetrical, trapped-valence structure in both phases. The cation valence states in these salts were accounted for by the electrostatic interactions between the cations and anions. The crystal structures and phase behavior of the ferrocenium salt 3 were very different from those of 4.

Crystal Structures and Melting Behaviors of 2D and 3D Anionic Coordination Polymers Containing Organometallic Ionic Liquid Components.

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)(C6 H5 R)][M{C(CN)3 }2 ] (R=H, Me, Et; M=K, Rb; Cp=C5 H5 ), which are complex salts comprising M[C(CN)3 ] and organometallic ionic liquids [Ru(Cp)(C6 H5 R)][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 ].