Metal-cyclam based Metal-Organic Frameworks for CO2 Chemical Transformations
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Designing new materials for CO2 capture and utilization is one of the most challenging research topics. Metal-organic frameworks (MOFs) are one of the most efficient CO2 adsorbents, as well as an emerging class of heterogeneous catalysts for CO2 chemical transformations. Highlighted by their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities, MOFs can serve as highly stable and reusable heterogeneous catalysts and provide a great platform to explore the structure-function relationships for transforming CO2 into useful chemicals. In this dissertation, we aim to develop a new class of metal-cyclam based robust MOFs as porous materials for CO2 uptake as well as efficient catalysts for CO2 chemical transformations, including CO2 chemical fixation, CO2 photo- and electroreduction. Chapter 1 introduces the concept and main challenges of CO2 capture and conversion. The potential of metal-cyclam complexes as molecular catalysts for CO2 conversion is also mentioned. The current state of the art in designing stable MOFs and azamacrocyclic-based MOFs is briefly discussed. Finally, the strategies, challenges and future outlook of using MOF as catalysts in CO2 chemical transformation are summarized. Metal-organic frameworks (MOFs) as highly ordered, tunable hybrid materials have shown great promise in photon collection, energy transfer and photocatalytic reactions. In Chapter 2, the fundamental principles of energy transfer in the condensed phase are summarized, and a series of studies in light-harvesting, excited state quenching and photo-excited reactivity occurring within ruthenium-polypyridyl-doped zirconium MOFs are reviewed. The application of MOFs in energy conversion devices such as dye-sensitized solar cells (DSSC) is also discussed. Chapter 3 reports two new robust 3D porous metal-cyclam based Zr-MOFs, VPI-100 (Cu) and VPI-100 (Ni) with potential as heterogeneous catalysts for CO2 chemical fixation. The frameworks are prepared by a modulated synthetic strategy and the structure highlighted by eight-connected Zr6 clusters and metallocyclams as organic linkers. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO2 with epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance. A follow-up study of CO2 chemical fixation using Hf analogs of VPI-100 is presented in Chapter 4. Structural characterization and catalytic performance of Hf-VPI-100 are summarized. Moreover, a detailed comparison of VPI-100 and Hf-VPI-100 is made. In situ powder X-ray diffraction (PXRD), quartz crystal microbalance (QCM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) have been used to probe the interaction between the guest molecules (CO2/epoxide) and Hf-VPI-100. For CO2, no specific chemical binding sites in MOFs has been observed and the uptake of CO2 does not change the crystal structure of Hf-VPI-100. Both QCM and DRIFTs revealed the irreversible binding between the framework and 1,2-epoxybutane. The epoxide uptake per unit cell of VPI-100 MOFs and diffusion coefficients have been calculated by QCM analysis. Transition metal complexes capable of visible light-triggered cytotoxicity are appealing potential candidates for photodynamic therapy (PDT) of cancer. In Chapter 5, two monometallic polyazine complexes, [(Ph2phen)2Ru(dpp)]2+ and [(Ph2phen)2Os(dpp)]2+ (Ph2phen = 4,7-diphenyl-1,10-phenanthroline; dpp =2,3-bis(2-pyridyl)pyrazine), were synthesized, characterized and studied as light activated drugs to kill rat malignant glioma F98 cells. Both compounds display strong absorption in visible spectrum, oxygen-mediated DNA and BSA photocleavage and significant photocytotoxicity under blue light irradiation along with negligible activity in the dark. The compounds show approximately five-fold higher cytotoxicity compared the traditional chemotherapeutic drug, cisplatin. Furthermore, [(Ph2phen)2Os(dpp)]2+ shows promising photocytotoxicity in F98 rat malignant glioma cells within the phototherapeutic window with an IC50 value of (86.07±8.48) µM under red light (625 nm) irradiation. In Chapter 6, the mixed-metal supramolecular complex, [(Ph2phen)2Ru-(dpp)PtCl2]2+, was found to display significant DNA modification, cell growth inhibition, and toxicity towards F98 malignant glioma cells following visible light irradiation. The design of this complex has a significantly higher potential for membrane permeability than three other FDA-approved anti-cancer agents, including cisplatin, and exhibited a dramatic ten-fold higher uptake by F98 cells than cisplatin in a two-hour window. Based on studies with a rat glioma cell line, the compound has very low cytotoxicity in the dark, but results in substantial cell death upon light treatment. The complex is thus among the first to exhibit all the hallmarks of a very promising new class of PDT agents.
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