Redox-Active Metal–Organic Cages and Frameworks for Electrochemical Applications

Abstract

With over thirty percent of industrial energy consumption in the United States stemming from chemical manufacturing, there is a pressing need to develop new methodologies to lower the energetic cost of chemical production. Industrial thermal catalysis strategies often require elevated temperatures and pressures, with the iron-catalyzed Haber–Bosch process operating at over 400 C and 200 atm. Electrocatalysis offers a greener, more sustainable, and critical alternative to traditional thermal catalysis, requiring less overall energy for chemical synthesis. This process is driven by redox–active molecules and materials that can reversibly donate or accept electrons to reduce or oxidize a substrate, triggering further chemical transformations into a desired product. While common electrocatalysts include redox–active organic molecules and supported metal nanoparticles, one particularly promising class comprises porous redox–active materials and molecules, such as coordination cages and metal–organic frameworks. This work describes the application of redox–active coordination cages and metal–organic frameworks in electrochemical applications, with a specific focus on their synthesis and characterization. Chapter 1 summarizes the current landscape of redox–active coordination cages and metal–organic frameworks, comparing these classes of materials with particular attention to their use in electrochemical methods. While coordination cages have been widely explored for traditional thermal catalysis, their loss of stability and solubility upon changes in charge state has limited their electrochemical applicability. Specific examples of coordination cage electrocatalysts, such as those used in carbon dioxide reduction, are discussed. Additionally, strategies for imparting conductivity to two-dimensional metal–organic frameworks are presented. Chapter 2 introduces an [Fe4L6]8+ coordination cage electrocatalyst, capable of being readily recycled via precipitation triggered by an overall reduction in charge state. This perylene diimide cage exhibits a wide range of charge states, spanning from +18 to −16, with multiple reversible redox events across a potential window greater than 2 V in acetonitrile. It serves as an effective catalyst for the electrochemical reduction of a series of vicinal dihalides, producing the corresponding alkenes with Faradaic efficiencies near unity. Furthermore, this cage is easily recyclable, leveraging the loss of solubility upon reduction to simplify post-catalytic separation. This work motivates further investigation into the role of cage charge, cavity size, and host–guest interactions in electrocatalysis. In Chapter 3, an analog of the perylene diimide cage based on a pyromellitic diimide ligand is introduced to modulate the redox properties of the resulting cage. Replacing the perylene diimide core with the pyromellitic diimide moiety shifts the cage’s reduction events to more strongly reducing potentials. The ability of this new cage variant to serve as an electrocatalyst for the reduction of a vicinal dibromide is evaluated. Additionally, evidence is presented for the role of ligand design in cage assembly: modification of the pyromellitic diimide ligand to include solubilizing methyl groups leads to the formation of the [Fe4L6]8+ cage as three discrete diastereomers, as opposed to the unmodified ligand, which yields both the tetrahedral cage and triple helicate species. This work expands the library of redox-active coordination cages and motivates the continued investigation into redox-active cage design. Chapter 4 marks a departure from the electrochemistry of coordination cages to investigate the impact of ligand oxidation state on the synthesis of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), a two-dimensional conductive metal–organic framework (MOF). The morphology of conductive MOFs strongly impacts their performance in applications such as energy storage and electrocatalysis. However, identifying the appropriate conditions to achieve a specific nanocrystal size and shape can be a time-consuming, empirical process. This work demonstrates how partial ligand oxidation dictates the morphology of Cu3(HHTP)2, a prototypical 2D conductive metal–organic framework. Using organic quinones as the chemical oxidant, we show that partial oxidation of the ligand prior to metal binding alters the nanocrystal aspect ratio by over 60-fold. Systematically varying the extent of initial ligand oxidation leads to distinct rod, block, and flake-like morphologies. These results represent an important advance in the rational control of Cu3(HHTP)2 morphology and motivate future studies of how ligand oxidation impacts the nucleation and growth of 2D conductive metal–organic frameworks.