Synthesis, modification, and utilization of calixarene-capped coordination cages
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Chapter 1 acts as an introductory chapter, discussing porous materials. More specifically, it explains the relationships between metal-organic frameworks and metal-organic cages while also discussing methods of modification in these materials both pre- and post-synthetically. Chapter 2 presents the modular synthesis of an isoreticular family of Ni(II)- and Co(II)- based calixarene-capped coordination cages. The octahedral and rectangular prismatic geometry cages obtained included various functional groups on their bridging ligand and were able to be synthesized in a modular fashion due to the use of calixarene molecules as a capping agent. The use of these molecules in cage synthesis also led to increased relative stability and reliable crystal packing directly resulting in ease of characterization and processability. ☐ Chapter 3 reports the targeting of low nuclearity calixarene-capped cages through the process of ligand tuning. Through tuning the orientation of the carboxylic acid on the bridging ligand through steric obstruction, Cluster2Ligand4 topology cages can be obtained. These utilize carboxylate based bridging ligands that bind with metal clusters at a smaller angle than previously reported, intrinsically porous, calixarene cages of this geometry. Furthermore, the presence of sterically hindering phenyl rings employed to alter carboxylate orientation lend to overall stability and porosity of the cage systems through various forms of pi-pi stacking. ☐ Chapter 4 describes the use of CuAAC “click” chemistry as a means to “post-synthetically modify” calixarene-capped cages of rectangular prism geometry. While functional groups can be used to optimize solubility, porosity, crystal packing, they can often interfere with the synthesis of a discrete cage resulting in undesirable phases of material. To avoid this occurring, modifying an already synthesized cage with a reactive small molecule containing a desired functionality can be a viable alternative. We show that CuAAC is a reliable method when conducting these modifications on calixarene-capped cages of this type functionalized with either azide or alkyne groups. Additionally, kinetic monitoring was able to be easily conducted on azide functionalized calixarene-capped cages using IR spectroscopy confirming the rapid nature of these modifications. ☐ In Chapter 5, we report further expansion into methods of post-synthetic modification for calixarene-capped cages of rectangular prism geometry. Through a myriad of characterization techniques, the viability of acylation as a method of PSM for this class of cage was demonstrated as well as presenting the first instance of palladium-catalyzed cross coupling as a means of PSM for a coordination cage. This was accomplished through collaboration with the High Throughput Experimentation (HTE) center at the University of Delaware where reaction conditions including solvent, catalyst system, and base were all optimized. It is significant to not that this is also the first instance of using HTE for this type of application. ☐ Finally, Chapter 6 discusses methods to increase stabilization between cages of varying types in the solid state as well as the ability to increase stability of the cages themselves through ligand tuning. Through inclusion of specific functional groups on the bridging ligands of coordination cages as well as the secondary building units (SBUs) used to construct them, interactions between cages can be achieved that stabilize these systems resulting in increased crystallinity and changes in porosity. By examination of these cage systems through single crystal x-ray diffraction (SCXRD), these interactions are easily observable through coordinative interactions between the functionalized ligand used to synthesize a given cage and a metal node of an adjacent cage. These interactions have significant influence over how these systems crystallize compared to non-functionalized analogues. Through tuning these interactions, new types of porous materials can be achieved such as porous salts, porous catalyst systems, pseudo-MOF systems. Furthermore, by changing the group that directly binds to the metal from carboxylate to triazole functionality, overall increases in thermal and chemical stability in metal organic polyhedra can be observed. This collection of work can directly lead to the reversible formation of MOF-like structures that have increased stability due to the increased stability of the cages of which they are comprised.
Description
Keywords
Organic frameworks, Metal-organic cages, Calixarene molecules, Crystal packing