Covalent Post-Assembly Modification of Metal-Organic Capsules
Self-assembled molecular containers have been used for a range of applications, including molecular sensing, catalysis, and chemical separations. Post-assembly modification of these supramolecular structures has proven a useful strategy to impart the new functionality needed for these applications, circumventing the need to design and synthesize additional cage components. This thesis will detail the reactivity of anthracene-edged MII4L6 (MII = CoII, FeII, ZnII) tetrahedra with various reaction partners in hetero-Diels-Alder reactions and explore potential applications following these reactions. The first work demonstrates that iron(II) cages incorporate between one and five equivalents of singlet oxygen (1O2), whereas cobalt(II) capsules react fully and incorporate six equivalents of 1O2 . Structural changes following these reactions impacted the host-guest chemistry of the cobalt(II) cages, and binding studies using isothermal titration calorimetry are discussed. Specifically, the modified cages were found to bind the target guest molecules (pyrene, phenanthrene, β-endosulfan) more weakly than the parent cages. The cobalt(II) hosts could partially revert back to the precursor structures when subjected to high temperatures in the solid state, which suggested that such a mechanism might be used in the future to encapsulate and release molecular cargo on demand. The second work focuses on the reactivity of the anthracene and naphthalene moieties of the cages with a range of electron-deficient traizolinedione (TAD) derivatives in hetero-Diels-Alder reactions, and explores the potential for the reversibility of the transformation. Butyl-TAD was found to irreversibly react with the anthracene-edged cages, while the naphthalene- based cages remained untouched. This reduced reactivity of the naphthalene hosts was attributed to the increased degree of aromaticity of naphthalene compared to anthracene, allowing the anthracene cages to react with dienophiles. Unfortunately, the reaction was not found to be reversible under any of the tested conditions. Overall, this work presents the hetero-Diels-Alder reaction as a method for inducing covalent post-assembly modification on imine-based metal-organic cages. This is an important step toward developing transformations in which a single external stimulus (for instance singlet oxygen or an electrophile such as TAD) may be used to reversibly fine tune the binding of industrially-relevant molecules. This work could thus enable the creation of new materials that take up, and then release, specific molecules from mixtures.