Synthesis of porous coordination cages and their application in porous salts
Date
2021
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Publisher
University of Delaware
Abstract
The work in this dissertation describes the design, synthesis, and characterization of porous coordination cages and their application towards novel porous charged-coordination cage-based salts. Specifically, metalorganic polyhedra featuring cuboctahedral, octahedral, and tetrahedral geometries were studied in depth with emphasis on speciation and gas adsorption properties. A wide variety of techniques and spectroscopic methods are covered, including single crystal and powder X-ray and neutron diffraction, gas adsorption, infrared and NMR spectroscopies, mass spectrometry, and thermal studies. Chapter one provides a brief introduction to supramolecular chemistry, supramolecular cages and porous coordination cages, as well as introducing the applications of the later. ☐ Chapter 2 is concerned with the expansion of the library of metalorganic polyhedra utilizing first-row transition metals. Described in detail is the design, synthesis and characterization of nickel(II) and cobalt(II) cages based on isophthalic acid derivatives and carbazole derived ligands. Here, a new metal-organic framework was synthesized from nickel(II) and isophthalic acid which features both dimeric and trimeric metal clusters. Additionally, functionalization of the 5-position in isophthalic acid with organic groups of varying length and bulk resulted in the formation of four tetragonal and hexagonal 2-dimensional frameworks. Finally, functionalization with solubilizing hydroxyl and isopropyl groups on isophthalic acid and the carbazole-based ligand, respectively, led to the successful synthesis of 0- dimensional cage compounds. While the cuboctahedral, nickelous cage loses porosity above 50 C, the carbazole-based cages maintained porosity above 200 C. ☐ In Chapter 3 the synthesis of a series of zirconium-based metal-organic cages is described. Initial studies concerned with the solubility of these cages resulted in the discovery of an isomerization phenomenon. Here, zirconium cages could adopt either a tetrahedral or cigar-like structure, or both, depending on the ratio of ligand width to ligand length. Ligands based on terephthalic acid, terphenyl-, and functionalized diacrylates were studied in depth. For the terephthalic acid species, it was found that functionalization with two methyl substituents was needed in order to maintain phase purity toward the tetrahedral side of equilibrium. Benzene-derived diacrylate ligands showed cigar-phase or mixed-phase products when using small functional groups (thiophene) or functionalizing only one-half of the benzene ring, respectively. Here, functionalization with two methoxy groups was needed to maintain the tetrahedral structure. Ultimately, for the terphenyl-derived ligands, diisobutoxide functional groups were installed in order to form a phase-pure tetrahedral product. A suite of characterization techniques including NMR, mass spectrometry, and XRD analysis, were used to elucidate the exact nature of these porous materials. ☐ Chapter 4 describes the preparation of a series of structurally related metal-organic frameworks to study the adsorption of small molecules, including methane and C2 hydrocarbons. A pillaring strategy was employed whereby cuboctahedral cages coordinated via 1,4-diazabicyclo[2.2.2]octane (dabco) to the axial metal sites in the paddlewheel-cluster form stable metalorganic frameworks comprised solely of cuboctahedral pores. Utilizing neutron diffraction techniques and subsequent modeling of the diffraction patterns, favorable adsorption sites of methane, ethane, and ethylene were found in the triangular and square pores of the cages. Here, the favorable interaction of these sites is ascribed to the deuterium-arene interactions. In an analogous manner, a metal-organic framework featuring the same octahedral cage geometry as those synthesized by carbazole-based ligands was shown to display porosity in excess of 4000 m2/g for the first time. This material was subsequently used in neutron diffraction studies to elucidate preferential methane binding within the octahedral pore. Interestingly, the methane first occupies the empty coordination sites of the metal-paddlewheel unit, followed by subsequent pore filling based on progressive formation of favorable binding pockets within the pore. Ultimately, the 0-dimensional cage analogues were shown to have comparable high-pressure methane uptakes to the MOF, rivaling that of HKUST-1 volumetrically. Additional experiments are described concerning the adsorption enthalpy of carbon dioxide in a series of mixed-metal framework materials. ☐ Finally, Chapter 5 describes the synthesis of novel cage-based porous salts. Here, a salt metathesis approach was adopted whereby an anionic sulfonate-based cuboctahderal cage was combined with a cationic zirconium-based cage. Upon addition of methanolic solutions of each cage, a light-blue powder precipitated from solution with concomitant formation of lithium triflate. Initially, both X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) spectroscopy confirmed the presence of both copper and zirconium in the product phase. Additional spectroscopies support the presence of both cages in a charge neutral manner where the ratio of cation: anion is best described as 6:1. Upon slow formation of single-crystal quality product, it was found that favorable charge-assisted hydrogen bonding between sulfonate groups and bridging hydroxo groups were responsible for the stability of the salt. Indeed, the cage-based salt, like metal-organic frameworks, is not soluble in common solvents. Importantly, it was shown that the surface area of the novel porous salt was higher than its constituent parts, with a BET surface area 500 m2/g.
Description
Keywords
Porous coordination cages, Porous salts