Coordination Cages as a Scaffold for Permanently Porous Liquids
Porosity is a property that is conventionally associated with solids. While liquids can have transient pores, these cavities exist as temporary voids between the molecules of the liquid. The idea of permanently porous liquids combines the porosity of solids with the fluid properties of liquids by engineering intrinsic porosity into the liquid state. This class of materials shows potential in applications such as extractions and gas separations. We present the first adaptation of coordination cages into porous liquids by functionalizing a series coordination cages of different sizes and structures with alkylimidazolium and PEG-imidazolium moieties. The alkylimidazolium-functionalized cages showed increased solubility in similarly structured ionic liquids, yielding a Type II porous liquid. The PEG-imidazolium-functionalized cages were liquid at room temperature and acted as Type I porous liquids. Of these cages, a liquid Zn(II)4L4 tetrahedral capsule was chosen for additional characterization and applications. Rheological and thermal analysis of this liquid cage confirmed its fluidity. Positron annihilation lifetime spectroscopy indicated that the host pores remained empty under solvent-free conditions. This liquid cage was then used for host-guest experiments both in solution and in the neat state. The porous liquid was shown to encapsulate small alcohols shape-selectively, with a preference for alcohols with more branching. The liquid cage also dissolved and encapsulated three different chlorofluorocarbons. Additionally, we also present the first hetero-cage, which we define to be a coordination cage that is charge-balanced by a second, different cage of opposite charge. A hetero-cage was synthesized by charge-balancing a liquid coordination cage with +20 charge with 5 equivalents of a solid anionic cage with -4 charge. The resulting material was characterized by NMR, which confirmed that the cages were charge-balancing each other in the absence of additional counterions. Thermal analysis of hetero-cages indicated that the material was comprised of a solid suspension of the anionic cage in the liquid cationic cage, and that the phase change behaviors of the parent cages are preserved in the hetero-cage.