Dissertation Defense: Gracia El Ayle
Name: Gracia El Ayle
Advisor: K. Travis Holman, Ph.D.
Title: Complexation of Air Gases in Small Cryptophanes
The molecule-within-molecule complexes of the container molecules constitute a
fascinating branch of supramolecular chemistry that has broad-reaching implications. Since 1981, cryptophanes, defined by the connection of two bowl-like cyclotribenzylenes (CTBs) so as to provide a container-like host cavity, have served to greatly advance our understanding of non-covalent and host-guest chemistries and offer much potential for applied technologies that stem from their exquisite molecular recognition properties. Cryptophanes have been extensively studied related to their ability to selectively encapsulate small molecules and gases. One of the smallest cryptophanes, known as cryptophane-111 (111), has a cavity size limited to ~ 73 Å3, thus allowing it to be an excellent host for small gases like H2, CH4, Xe, C2H4, and C2H6. In fact, 111 exhibits the highest affinity for Xe (VvdW = 42 Å3) in organic solvents with a binding constant of 1 × 104 M-1 at 293 K and a very slow decomplexation rate.
Developing hosts for the selective binding of the noble gas xenon and other small gases are of interest because of potential applications in sensing, separation, and storage. In recent years, cryptophane derivatives that effectively bind xenon have become very important in the context of burgeoning 129Xe NMR/MRI-based (bio)sensing technologies. On the other hand, gas separation occurs typically by cryogenic distillation, which is known to be very energetically costly. We envision that cryptophanes may have intriguing properties towards materials that exhibit porosity “without pores” or display extreme confinement of gaseous species purely by van der Waals forces.
This dissertation focuses on the synthesis, optimization, purification, and characterization of existing and new cryptophanes and CTBs. A functionalized derivative of 111, (±)-trisbromocryptophane-111, Br3-111 was successfully synthesized and its solution and solid-state properties towards gas encapsulation are outlined. Results show unprecedented kinetic confinement abilities of the smallest air gases, such as N2, O2, CO2, CH4, Ar, Kr, and Xe within the cryptophane cavity in the solid-state.
Moreover, the optimized synthesis of an important CTB precursor, as well as new CTBs with inherent C1-symmetry have been developed by alternative, more efficient direct cyclization of commercially available starting materials. It is envisioned that these procedures would facilitate the synthesis of cryptophanes on a larger scale.
To further explore the potential of cryptophanes as 129Xe-based sensors, water-soluble (Cp*Ru)-functionalized phenolic cryptophanes have been synthesized and characterized. A preliminary experiment shows tremendous potential for using these new molecules as sensitive 129Xe NMR pH sensors.
Wednesday, December 13 at 9:00am to 11:00am
Regents Hall, 209
3700 O St. NW