Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item UNDERSTANDING THE SURFACE CHEMISTRY OF GAS PHASE ORGANOPHOSPHORUS CHEMICAL WARFARE AGENTS WITH SORBENT MATERIALS(2019) Holdren, Scott; Zachariah, Michael R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Chemical warfare agents (CWAs) pose a serious threat to civilians and warfighters due to their high toxicity and usage in recent attacks. Although existing filtration media (ASZM-TEDA) used in gas mask devices absorbs and decomposes a wide spectrum of CWAs, the filtration performance of this material can be compromised in the battlefield due to poorly understood mechanisms. The high toxicity of CWAs remains a barrier for most research institutions to study these compounds experimentally which hinders the search for improved filtration materials. To overcome this issue, studies are performed using relatively benign simulant compounds that have similar adsorption and decomposition properties as toxic CWAs. In this work, a report of experimental findings will be presented regarding how dimethyl methylphosphonate (DMMP), an organophosphorus CWA simulant, will adsorb and decompose on components that makeup ASZM-TEDA. The work presented in this dissertation deconstructs the components that makeup ASZM-TEDA in order to identify the role of specific metal oxides and the carbon support. This approach was facilitated using different analytical techniques including TGA, FTIR spectroscopy, and DFT modeling to gain a molecular understanding of how DMMP interacts with porous carbon (Chapter 3) and metal oxide nanoparticles/surfaces (Chapters 4 and 5). Lastly, a new method is described (Chapter 6) that overcomes many of the difficulties encountered in conventional measurements that monitor gas phase DMMP adsorption/desorption processes on sorbent materials. This method can be used to obtain reliable quantitative measurements and parameters (e.g. adsorption capacities, ∆Hads, and kads) of low vapor pressure adsorbate/sorbent systems making it particularly useful for CWAs/CWA simulants and new filtration materials (e.g. DMMP and porous carbon).Item NEW LIGAND SCAFFOLDS FOR COMBINING ARENE C-H ACTIVATION AND AEROBIC OXIDATION AT PLATINUM(2018) Watts, David; Vedernikov, Andrei N; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Extensive research over the past half-century has proven the utility of late transition metal complexes in the activation and functionalization of alkanes and arenes. Homogeneous platinum compounds have been particularly promising as they readily form air and water stable complexes that can react with some of the strongest C-H bonds (e.g., CH3-H, Ph-H) under relatively benign conditions. Yet, the development of methods for the application of O2 or air as the terminal oxidant in the oxidative functionalization of inert C-H bonds remains an elusive but important goal. The focus of this work is to enable the direct involvement of O2 with PtII-mediated C-H activation processes through computation directed intelligent ligand design, with the end goal of selective aerobic C-H functionalization. Prior experience with the hemi-labile tripodal ligand di-(2-pyridyl)methanesulfonate (dpms) lead us to develop a new class of sulfonated k3-CNN pincer pre-ligand, 6-phenyl-di-(2-pyridyl)methanesulfonate (ph-dpms). The ph-dpms derived PtII-aqua complex, (C6H4-dpms)PtII(H2O), was shown to be an especially active hydrogen/deuterium exchange catalyst with arene substrates. While facile, the arene C-H activation chemistry was also selective for aryl C(sp2)-H bonds over benzyl C(sp3)-H, despite severe steric protection of the former in some cases. Ph-dpms also supports the aerobic oxidation chemistry for which the earlier generation ligand, dpms, was engineered. An anionic [PtII(Ph)]- complex derived from ph-dpms undergoes relatively fast oxidation in trifluoroethanol (TFE) solvent resulting in oxidative C-C coupling between the phenyl substituent and ligand. Changing the solvent to MeOH allows for isolation of the PtIV-Ph intermediate. Furthermore, the ability to support both C-H and O2 activation was combined in the one-pot aerobic C-H oxidation of both electron rich and electron poor arenes to give (C6H4-dpms)PtIV(Aryl)(OH) complexes from (C6H4-dpms)PtII(H2O); a feat never accomplished before by a PtII complex without the use of co-catalysts or reagents to mediate the O2 chemistry. The limits of the new ligand scaffold were then explored through the reactivity of PtII chloro and aqua complexes derived from more rigid analogs of ph-dpms. The rigidity of the ligand was found to be intimately tied to both C-H and O2 activation chemistry as well as some detrimental bimolecular decomposition pathways.Item ELECTRONIC MODIFICATION WITHIN THE WELL-ESTABLISHED CPAM FRAMEWORK AS A MEANS TOWARD INCREASED REACTIVITY(2017) Thompson, Richard; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Early transition metals (group IV-VI) supported by the pentamethylcyclopentadienyl-amidinate mixed ligand set (CPAM) have been found to enable a number of important chemical transformations including (living) coordinative polymerization of alpha-olefins, fixation of dinitrogen and group transfer chemistry involving oxo, imido and sulfido ligands to unsaturated organic substrates, including carbon dioxide. A great deal of the allure and success associated with these complexes is their modularity, particularly as it concerns the amidinate component which is tunable at both the N-bound substituents as well as the distal position. Accordingly, a great deal of work has established that by reducing the sterics in all three positions engendered higher reactivity. There exists, however, a practical “steric wall” such that the size of substituents can only be contracted so much. Tuning of the electronic character of these well-established systems could prove to be a novel and potent method for affecting reactivity of these complexes within an already well understood steric environment.Item Small Molecule Activation and Atom and Group Transfer Reactions Mediated by Mid Valent Group 6 'CPAM' Complexes(2015) Farrell, Wesley Scott; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of organometallic compounds to activate small molecules (e.g. CO2, N2, N2O, O2, etc.) has long been of significant scientific interest. Described here is the synthesis and characterization of mid valent group 6 compounds supported by the pentamethylcyclopentadienyl, amidinate (CpAm) ligand framework, along with their ability to not only activate small molecules that are inexpensive, abundant, and/or hazardous, but use them to generate many value added products under mild conditions. Sulfur atom transfer (SAT) was employed to catalytically prepare carbonyl sulfide and isothiocyanates from elemental sulfur. In the case of carbonyl sulfide, this process was able to be performed in the presence of primary amines, allowing for the isolation of symmetric ureas, and in the case of isothiocyanates, the reaction was successful in the presence of benzhydrazide to allow for the isolation of aroylthiosemicarbazides in good yields. Molecular oxygen was found to afford high valent dioxo species which were inactive towards oxygen atom transfer (OAT). However, OAT was achieved for the catalytic deoxygenation of sulfoxides. Dinitrogen fixation has previously been discovered by our group to afford -ER3 (E = C, Si, Ge) derivatized isocyanates through [2+1] cycloaddition of CO. Reported here is an extension of this work to include N2 fixation with concomitant reduction of the greenhouse gas CO2 to prepare the same isocyanates via [2+2] cycloaddition of CO2. Furthermore, the completion of several efficient N2 fixation synthetic cycles through two distinct pathways is discussed. Additionally, given the tremendous impact of high valent group 6 alkylidene compounds to catalyze olefin metathesis reactions, the synthesis of mid valent CpAm group 6 alkylidenes was a challenging, yet attractive target. Attempts to isolate such compounds are presented, along with descriptions of the products obtained and their reactivity towards small molecules.Item Earth Abundant Bimetallic Nanoparticles for Heterogeneous Catalysis(2014) Senn Jr, Jonathan Fitzgerald; Eichhorn, Bryan; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer exchange membrane fuel cells have the potential to replace current fossil fuel-based technologies in terms of emissions and efficiency, but CO contamination of H2 fuel, which is derived from steam methane reforming, leads to system inefficiency or failure. Solutions currently under development are bimetallic nanoparticles comprised of earth-abundant metals in different architectures to reduce the concentration of CO by PROX during fuel cell operation. Chapter One introduces the Pt-Sn and Co-Ni bimetallic nanoparticle systems, and the intermetallic and core-shell architectures of interest for catalytic evaluation. Application, theory, and studies associated with the efficacy of these nanoparticles are briefly reviewed. Chapter Two describes the concepts of the synthetic and characterization methods used in this work. Chapter Three presents the synthetic, characterization, and catalytic findings of this research. Pt, PtSn, PtSn2, and Pt3Sn nanoparticles have been synthesized and supported on γ-Al2O3. Pt3Sn was shown to be an effective PROX catalyst in various gas feed conditions, such as the gas mixture incorporating 0.1% CO, which displayed a light-off temperatures of ~95°C. Co and Ni monometallic and CoNi bimetallic nanoparticles have been synthesized and characterized, ultimately leading to the development of target Co@Ni core-shell nanoparticles. Proposed studies of catalytic properties of these nanoparticles in preferential oxidation of CO (PROX) reactions will further elucidate the effects of different crystallographic phases, nanoparticle-support interactions, and architecture on catalysis, and provide fundamental understanding of catalysis with nanoparticles composed of earth abundant metals in different architectures.Item Applications of 2,3-Diketoesters in Organic Synthesis and Stereoselective Transformations(2014) Truong, Phong Minh; Doyle, Michael P; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dimethyldioxyrane oxidation of δ-hydroxy-α-diazo-β-ketoesters that are prepared by zinc triflate catalyzed Mukaiyama-aldol condensation of methyl diazoacetoacetates with aldehydes, occurred in quantitative yield to form dihydrofuranols that undergo acid catalyzed dehydration under mild conditions to generate 3-methoxyfuran-2-carboxylates in good yield. Oxidation of ζ-keto-α-diazo-β-ketoesters that are formed by zinc triflate catalyzed Mukaiyama-Michael condensation of methyl diazoacetoacetate enones procduced their 2,3,7-diketoester derivative in quantitative yield. The intramolecular acid catalyzed aldol cyclization of 2,3,7-triketoesters provides highly functionalized cyclopentanones with good diastereoselectivity in high overall yields via kinetically controlled and stereodivergent catalytic processes. Lewis acid catalysis gives high selectivity for the 1,3-anti tetrasubstituted cyclopentanones, whereas Brønsted acid catalysis produces the corresponding 1,3-syn diastereomer. The first enantioselective transformation of 2,3-diketoesters was demonstrated in carbonyl-ene reactions catalyzed by [Cu((S,S)-tert-Bu-box)](SbF6)2 generating chiral α-functionalized-α-hydroxy-β-ketoesters in up to 94% yield and 97% ee. The suggested mode of activation is bi-dentate coordination between copper and the oxygen atoms of the two keto-carbonyl groups. The 2,3-diketoesters are conveniently accessed from the corresponding α-diazo-β-ketoester, and catalyst loading as low as 1.0 mol % is achieved.