Alternative methods for the synthesis of porous materials
Date
2022
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Publisher
University of Delaware
Abstract
Chapter 2 presents the electrochemically mediated synthesis of TiIII-MIL-101 from the inexpensive Ti4+ precursor TiCl4. The framework obtained via electrosynthesis is identical to that prepared from the significantly more expensive and air-sensitive starting material TiCl3. The above electrosynthetic strategy was also extended to prepare TiIII-MIL-100 and two high-quality extended TiIII-MIL structures, for the first time. Several physical methods demonstrate that these materials are superior in quality to samples of the analogous MOFs prepared via conventional routes from starting exogenous TiCl3. ☐ Chapter 3 reports the electrochemical synthesis of functionalized Cu(II) porous coordination cages utilizing anodic dissolution, which has previously only been used for MOF formation. Via UV-vis analysis, the cage can be formed in as little as 9 minutes in solution. Gas adsorption, powder X-ray diffraction, IR, x-ray photoelectron spectroscopy and SEM experiments show that the material is analogous to the cages prepared via typical solvothermal routes involving different Cu(II) salts. ☐ Chapter 4 describes utilization of a mixed-ligand strategy to tune the properties of cuboctahedral porous coordination cages. While functional groups can be used to optimize solubility, porosity, crystal packing, thermal stability toward desolvation, reactivity, or optical activity, optimization of multiple properties can be challenging given their interconnected nature. For example, installation of functional groups to increase the solubility of porous cages typically has the effect of decreasing their porosity and stability toward thermal activation. Here we show that mixed-ligand cages can potentially be used to address these issues as the benefits of various functional groups can be combined into one mixed-ligand cage. We further show that although ligand exchange reactions can be employed to obtain mixed ligand copper(II)-based cages, direct synthesis of mixed-ligand products is necessary for molybdenum(II) paddlewheel-based cages as these substitutionally inert clusters are resistant to ligand exchange. We ultimately show that highly soluble, highly porous, thermally stable cuboctahedral cages are isolable by this strategy. ☐ In Chapter 5, we report the design and synthesis of surface functionalized permanently microporous coordination cages and their use in the isolation of mixed metal solids. Judicious alkoxide-based ligand functionalization was utilized to tune the solubility of starting copper(II)-based cages and their resulting compatibility with the mixed-cage approach described here. We further prepared a family of isostructural molybdenum(II) cages for a subset of the ligands. The preparation of mixed-metal cage solids proceeds under facile conditions where solutions of parent cages are mixed and product phases isolated. A suite of spectroscopic and characterization tools confirm the starting cages are intact in the amorphous product. Finally, we show that utilization of precise ligand functional groups can be used to prepare mixed cage solids that can be easily and cleanly separated into their constituent components through simple solvent washing or solvent extraction techniques. ☐ Finally, Chapter 6 shows that the facile reaction of TBAX (TBA+ = tetra-n-butylammonium; X =F– and Cl¬–) with molybdenum paddlewheel-based coordination cages of the M4L4 and M24L24 lantern and cuboctahedra structure types, respectively, affords charged cages by virtue of coordination of halide anions to the internal and/or external metal sites on these structures, as confirmed by single crystal X-ray diffraction, X-ray photoelectrons spectroscopy, and NMR spectroscopy. At a practical level the TBAX/cage reactions, which are fully reversible upon isolation of the cage with the appropriate solvent, solubilize otherwise rigorously insoluble cages. This method significantly increases the solution processability of these highly porous solids. Toward the formation of new porous salts, halide binding also serves to incorporate charge on neutral cages and make them amenable to simple salt metathesis reactions to afford new porous salts based on anions and cations with intrinsic porosity. A combination of diffraction methods and a suite of spectroscopic tools confirms speciation of the isolated solids which represent a new class of highly tunable porous salts. Ultimately, this work represents a roadmap for the preparation of new porous solids and showcases the utility and broad applicability of anion binding as a strategy for the synthesis of porous salts.
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Keywords
Double salt, Electrochemistry, Porous, Porous coordination cage, Post-synthetic modification, Pre-synthetic modification