Unlocking the potential of post-synthetic modifications in covalent organic frameworks via the Ugi reaction

Field: Chemistry (Physical Sciences)

The present dissertation describes the development and applications of a new post-synthetic modification (PSM) strategy via the Ugi multicomponent reaction in crystalline, porous, organic solids known as Covalent Organic Frameworks (COFs). The novelty of this work includes the (1) development of a general strategy for the incorporation of functional group handles in COFs via the Ugi reaction, (2) production of tailored and recyclable COFs via the Ugi reaction for efficient capture and release of the rare-earth elements in aqueous solution, and (3) one-step preparation of COFs bearing isocyanide functional groups via the Ugi reaction. Chapter II describes the synthesis of COFs modified via the Ugi-multicomponent reaction (Ugi-COFs) to introduce functional group handles, including azide, alkyne, and vinyl groups, which significantly facilitates the development of any future customizations of the reticular architecture. To confirm the efficiency of the proposed transformation, thorough characterization of as-synthesized Ugi-COFs was acquired using Fourier-transform infrared spectroscopy (FT-IR), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance spectroscopy (SSNMR), isotherm measurements, and thermogravimetric analysis (TGA). Chapter III describes the application of the Ugi-COFs for the efficient capture and release of rare-earth elements in aqueous solution. The Ugi reaction was leveraged to decorate the COF template with functional groups that allowed for the introduction of diglycolic acid (DGA) moiety in the framework (Ugi-COF-DGA). Each modification step was thoroughly characterized using FT-IR, PXRD, SSNMR, TGA, and isotherm measurements. The performance of the Ugi-COF-DGA material was then evaluated for the capture of neodymium (Nd) in different conditions. Ugi-COF-DGA showed the best uptake performance at near neutral pH, with a maximum capacity of 570 mg of Nd per 1 g of COF, the highest reported adsorption capacity value for other DGA-functionalized solid adsorbents. The recyclability of the Ugi-COF-DGA was demonstrated over 3 consecutive runs, showing no significant deterioration in the performance of the material. A time study revealed that Ugi-COF-DGA reached its maximum adsorption capacity in about an hour, while the desorption happens almost instantaneously. Chapter IV describes the application of the Ugi PSM for one-step incorporation of isocyanide functional groups in the COF. To achieve the desired transformation, 1,4-cyclohexanediisocyanide was employed as a homo-bifunctional linker. One of the isocyanide groups of the linker was reacted with the COF backbone via the Ugi reaction (Ugi-COF-CyNC), while the other was preserved for future synthetic transformations. An excess of the homo-bifunctional linker was necessary to achieve sufficiently high loading of the free isocyanide groups in the Ugi-COF-CyNC. An array of characterization techniques, including FT-IR, PXRD, XPS, and SSNMR, were utilized to provide structural evidence for the post-synthetically modified materials. The isocyanide groups supported on Ugi-COF-CyNC were then coordinated to gold(I) chloride for efficient and recyclable hydration of alkynes in water at room temperature.