Functional metal-organic liquids

For decades, the study of coordination polymers (CPs) and metal-organic frameworks (MOFs) has been limited primarily to their behavior as crystalline solids. In recent years, there has been increasing evidence that they can undergo reversible crystal-to-liquid transitions. However, their "liquid" states have primarily been considered intermediate states, and their diverse properties and applications of the liquid itself have been overlooked. As we learn from organic polymers, ceramics, and metals, understanding the structures and properties of liquid states is essential for exploring new properties and functions that are not achievable in their crystalline state. This review presents state-of-the-art research on the liquid states of CPs and MOFs while discussing the fundamental concepts involved in controlling them. We consider the different types of crystal-to-liquid transitions found in CPs and MOFs while extending the interpretation toward other functional metal-organic liquids, such as metal-containing ionic liquids and porous liquids, and try to suggest the unique features of CP/MOF liquids. We highlight their potential applications and present an outlook for future opportunities.

Organometallic Ionic Liquids Containing Sandwich Complexes: Molecular Design, Physical Properties, and Chemical Reactivities

Ionic liquids (ILs) are salts with low melting points and are useful as electrolytes and solvents. We have developed ILs containing cationic metal complexes, which form a family of functional liquids that exhibit unique physical properties and chemical reactivities originating from metal complexes. Our study explores the liquid chemistry in the field of coordination chemistry, where solid-state chemistry is currently the main focus. This review describes the molecular design, physical properties, and reactivities of organometallic ILs containing sandwich or half-sandwich complexes. This paper mainly covers stimuli-responsive ILs, whose magnetic properties, solvent polarities, colors, or structures change by the application of external fields, such as light, heat, and magnetic fields, or by reaction with coordinating molecules.

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.