Diversification and Functionalization of Organic Components in Metal-Organic Frameworks
Described in this dissertation are a range of methods for expanding the complexity of materials in the class of metal-organic frameworks (MOFs). From their discovery in the mid-1990’s until today, metal-organic frameworks have largely been built from a narrow set of building blocks: symmetric, aromatic, carboxylates and first row transition or rare earth metals. While much work has been devoted to investigating the scope of their possible applications, more fundamental understanding of their chemistry is needed for the full potential of this class of materials to be realized. In addition to their crystallinity and porosity, the primary reason for the success of metal-organic frameworks in fields ranging from gas storage to catalysis stems from their inherent tunability. Metal-organic frameworks, in contrast to other porous materials such as zeolites, are modular in that they are built from discrete organic and inorganic components and can therefore be tailored to specific purposes.
Increasing the attainable complexity of these materials allows for greater optimization toward existing applications and for exploring previously undiscovered areas. Complexity in solid-state materials is introduced through heterogeneity of composition or distribution. For metal-organic frameworks, this heterogeneity is manifested either in the backbone composing the underlying network or in the functionalities exposed to the pore space. Both approaches are investigated in this dissertation. Heterogeneity of the backbone rests in the diversity of the organic and inorganic building units. Heterogeneity of the pore space is provided by functionalization of organic and inorganic structural building units without altering their structural properties.
Chapter One presents an introduction to rational design of metal-organic frameworks encompassing the context and background for this work. The building block approach provides control of metal-organic framework structure, stability, and functionality. Both inorganic and organic building units are available for modification. Variations in linker length, geometry, and connectivity correlate with changes in the extended structure. Choice of coordinating group is another element of control. Much remains to be investigated in terms of linkage type in metal-organic frameworks by exploring new coordinating groups. Concerning the metal components, the multifarious clusters and chains serving as secondary building units (SBUs) have implications for the structure, stability, and function of these materials. The identity of the metal ions comprising these secondary building units impacts these aspects as well. Heterogeneity of metal-organic framework backbones has been achieved in mixed linker and mixed metal systems. Strategies to achieve pure phases of materials with mixed components include synthesis from a mixture of starting materials as well as post-synthetic modification. Inside heterogeneous pore spaces, desired functionalities coordinate to the metals of the frameworks or are sidechains of the organic linkers. An analysis of the structure and property implications of constructing metal-organic frameworks from heterotopic linkers, meaning those linkers with non-identical coordinating groups, had not been reported. The lack of investigation in this area was the impetus for the research presented in Chapters Two and Three.
Chapter Two describes the design, synthesis, and characterization of a heterotritopic linker for metal-organic frameworks. This compound bears a carboxylic acid, catechol, and pyridone and was not known in the literature. The original and optimized synthetic routes are given. The linker is synthesized reproducibly on gram scale in three steps with a single column chromatography purification. The analytical data for this linker are given, including the mass spectrometry, one-dimensional and two-dimensional nuclear magnetic resonance, and infrared spectra. The reasoning behind the choice of metrics and coordinating groups is described.
Chapter Three details the synthesis, structure elucidation and refinement, and properties of a metal-organic framework constructed from a heterotritopic linker and zinc(II), termed MOF-910. Despite the asymmetry of the linker, MOF-910 is both highly crystalline and symmetric. Synthetic conditions for crystallization of the heterotritopic linker with zinc(II) required an added base, such as triethylamine. The material is highly porous with a Brunauer-Emmett-Teller surface area of 2,120 m2 g-1and hexagonal channels 21 Å in diameter. The material is remarkably thermally and chemically stable for a zinc-based metal-organic framework. Integrity of the framework is maintained up to 320 °C and under acidic and basic aqueous conditions. The catechol moiety undergoes oxidation to the corresponding semiquinone during the metal-organic framework synthesis. The electron paramagnetic resonance spectrum indicates a ligand-centered radical.
Chapter Four concerns the applications and reticular chemistry insights uncovered by MOF-910. One focus is the prediction and control of structure of metal-organic frameworks through lower symmetry and heterotopic linkers. The process for reducing MOF-910 to its underlying topological network is explained. The tto (ttriangles. tetrahedra, octahedra) net, of which MOF-910 is the first representation, is described. The tendency of heterotopic linkers to form helical secondary building units is investigated. The dependence of helical pitch on the distance between the most proximal coordinating groups in asymmetric polytopic linkers is discussed. The contributions of these discoveries to the field of reticular chemistry are stated. The electrochromic and gas adsorption behavior of MOF-910 are described as well. Treatment with a mild oxidant converts colorless crystals to bright red with no change in structure discernible by single-crystal X-ray diffraction and no change in surface area. Exposure to a mild reductant causes red crystals to lose color.
Chapter Five focuses on functionalization of metal-organic frameworks constructed from tetrakis(4-carboxyphenyl)porphyrin and zirconium(IV) clusters. The introduction of functionality through the organic ligand is described in MOF-525 and MOF-545. Since the porphyrin core is not involved in structure formation of these metal-organic frameworks, additional reactive sites were installed in these frameworks by coordination to the porphyrin pyrroles. The synthetic conditions for constructing zirconium(IV) metal-organic frameworks with porphyrin linkers bound to aluminum(III), chromium(III), manganese(III), iron(III), cobalt(III), nickel(II), copper(II), and zinc(II) are reported. The synthesis of the aluminum(III) and chromium(III) linkers is given. The activation conditions for these generally difficult to evacuate mesoporous materials are listed. The ability of these metalated porphyrin materials to capture toxic gases is reported.