While most DES are formed by hydrogen bonding, Pennington’s group recently observed that grinding 1,3-dithiane with 1,2-F4DIB resulted in the formation of the first reported DES formed by halogen bonding (XB). This strong and highly directional interaction can be exploited to develop supramolecular synthons for rational crystal design and materials development. While these XB interactions are very similar to H-bonds, they have several significant advantages, including a higher degree of directionality due to the localization of the electrophilic site, which is exactly opposite to the covalent bond to the halogen atom. The strength of the interaction is easily tunable by varying the halogen atom (I>Br>Cl>>>F) and through modification of the electron-withdrawing ability of the moiety to which the halogen is bonded. X-bond donor sites are typically more hydrophobic than H-bond donor sites, and the addition of fluorine to the X-bond donor molecule, which is used to increase interaction strength, further increases its hydrophobicity. Recently, we have developed a new class of compounds, “X-panded polyiodides”, in which an iodide or triiodide anion serves as an XB-acceptor to organoiodine XB-donors. With variations of the cation and the organoiodine molecules, the interaction strength and associated lattice energy of these systems are essentially infinitely tunable. In cataloging the cocrystals formed as a function of the components, we have observed the formation of liquid phases with great promise as hydrophilic ionic DESs. Preliminary results revealed that some combinations form cocrystals (short-chain cations) and others form liquids (long-chain cations). Participants will further explore these compounds, generating solid-liquid phase diagrams using DSC/TGA to determine eutectic formation and then compare their liquid local structure to those of related cocrystals, utilizing Raman spectroscopy and Extended X-ray absorption Fine Structure (EXAFS). Finally, the stability of these complexes will be determined as a function of water content and temperature to determine their utility as inert phases for the extraction of organic and inorganic analytes from aqueous environments, with applications such as removal of halogenated molecules such as polychlorinated biphenyls (PCBs) or perfluoro alkyl substances (PFAS) from water, a system that could be used towards research and remediation of Clemson’s most extensive water reservoir, Lake Hartwell.
Dr. Bill Pennington, Clemson University
