Theses and Dissertations from UMD

Permanent URI for this communityhttp://hdl.handle.net/1903/2

New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

More information is available at Theses and Dissertations at University of Maryland Libraries.

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2015 - 20182

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    TOWARDS ENERGY EFFICIENT AND ATOM ECONOMICAL CHEMICAL CYCLES FOR NITROGEN FIXATION
    (2018) Duman, Leila Margaret; Sita, Lawrence R; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Converting the abundant, but largely inert, dinitrogen (N2) into the N-containing products upon which society relies (including proteins, explosives, and fertilizers) is a vital synthetic target as the worldwide population, and therefore demand for such commodities, continues to grow. The development of energetically and atomically efficient methods of cleaving, functionalizing, and releasing N2 as other valuable N-containing products (a process known as fixation) is of the utmost importance to sustainably meet the global demand. Transition-metal-mediated examples of N2 coordination, activation, cleavage, and N-atom product release are an attractive and active field of study in developing N2 fixation cycle that operate under mild conditions and consume few resources. Herein, the effects of a sterically reduced pentamethylcyclopentadienyl, amidinate (CPAM) ligand framework around group 6 metal centers (M = Mo, W) are investigated as a means of lowering barriers to reaction in order to complete a novel N2 fixation cycle. The design and implementation of this set of reactions are reported, including synthesis and characterization of new dinitrogen and dinitrogen-derived organometallic compounds, as are the groundbreaking thermally mediated N≡N cleavage from dinuclear M(II) (μ-N2) to M(V) (μ-N)2 dinuclear complexes, N atom functionalization via silylation to form terminal Mo(IV) trimethylsilyl imidos, and a controlled, “dry hydrolysis” with Me3SiCl and a readily available silanol or alcohol proton source (XOH) to releases hexamethyldisilazane (HN(SiMe3)2) and a silyl ether (XOSiMe3) while regenerating the M(IV) dichloride starting material. Reactivities within and modifications to this cycle are explored, including the introduction of innovative reagents, and the versatility of the CPAM system is demonstrated while making strides towards more targeted and sustainable means of synthetic N2 fixation.
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    Early Transition Metal Studies of Dinitrogen Cleavage and Metal-Nitrogen Bond Reactivity Towards Catalytic N2 Fixation
    (2015) Keane, Andrew John; Sita, Lawrence R.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The development of energy efficient catalysts that provide a range of commodity chemicals derived from the fixation of N2 is a highly attractive target due to the abundance of molecular nitrogen present in Earth’s atmosphere. Studies have focused on the systematic investigation of several key components of metal-catalyzed N2 fixation that detail molecularly discrete transformations involving the activation of N2, cleavage of the strong N≡N triple bond and N-atom functionalization to provide ammonia and other organic molecules of scientific and industrial interest. To this end, an evaluation of group 5 N2 cleavage using the pentamethylcyclopentadienyl, amidinate (CPAM) ligand framework that includes extensive kinetic and mechanistic investigations detailing the N-N cleavage reaction coordinate has been undertaken. Further studies conducted within group 5 include the synthesis and reactivity of tantalum imido complexes to elucidate metal-nitrogen bond reactivity and N-N cleavage relevant to the Chatt cycle. In group 6, photolytic N-N cleavage has been further investigated. Most notably, chemistry has been discovered for N2-derived metal nitride complexes of Mo and W that gives access to imido complexes capable of participating in nitrene group transfer (NGT) to carbon monoxide and isocyanides to provide isocyanates and carbodiimides respectively. For the first time, it has been demonstrated that imido complexes that participate in NGT chemistry can be derived from N2 to provide N-C based organic products. Collectively, these results serve to establish a new platform for studying the catalytic viability of the discovered transformations.