Clathrate Formation in a Hydrogen-bonded Ionic Liquid

In contrast to crystalline solids, liquids possess no long-range order. Nonetheless, many liquids still display significant amounts of local structure which can strongly influence their properties. I will discuss my efforts to apply reticular chemistry—the use of rigid molecular building blocks with well-defined bonding geometries—to influence the properties of ionic liquids. Specifically, I will describe a strategy to convert a member of a well-known class of hydrogen-bonded organic frameworks (HOFs) into an ionic liquid and will show that many of the structural features present in the HOF are preserved in the liquid state. In the solid state, the guanidinium sulfonate HOFs form an extended layer structure due to hydrogen bonding between the guanidinium and sulfonate groups. Methylation of the guanidinium gives an analogous compound (MeG)2(PEDS) (MeG = methylguanidinum; PEDS = diphenylether disulfonate) which is a viscous ionic liquid above 43 °C and an amorphous glass below. Remarkably, the application of moderate pressures causes this liquid to crystallize as an analog to the original hydrogen-bonded framework structure with incorporation of gas. This process is similar to clathrate hydrate formation, which occurs in water under low temperature and high-pressure conditions. However, both the enthalpy and entropy of formation of the (MeG)2(PEDS) solid phase are unusually small relative to most clathrates, indicating that the ionic liquid phase already possess a significant degree of local structure prior to crystallization. I corroborate this finding with infrared spectroscopy and structural studies of the liquid state.