Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
The activation of relatively inert C-H bonds has remained a challenge in organic chemistry for over a century. Metal catalysts have been deployed to overcome the labile reactivity of these bonds and through gas-phase studies, detailed mechanisms can be elucidated for this important class of chemistry. The powerful combination of ion-molecule reactions, collisional activation of ions, and density functional theory calculations allows for structural elucidation, intermediate determination, and as shown throughout this work, a complete picture of catalytic processes. Modified linear ion-trap mass spectrometers were used to trap and generate reactive intermediates of the form [(L)(M)(X)]+ (L= auxiliary ligand; M= Ni2+ , Pd2+, Pt2+; X= H, CH3) which can then be subjected to ion molecule reactions with various C-H bond containing neutrals. One of the most abundant hydrocarbons produced through fracking is ethane. With less economic importance than methane, ethane has typically suffered from poor usability. However, through dehydrogenation ethane can be converted into ethylene, an important polymerization precursor. The acceptorless dehydrogenation of ethane by ternary metal hydrides [(phen)M(H)]+ (where phen= 1,10-phenanthroline; M= Ni, Pd, and Pt) proceeded via a two-step mechanism. The initial C-H bond activation occurs through an ion-molecule reaction, forming the ion [(phen)M(C2H5)]+ and releasing neutral H2. A comparison of the three metals shows experimentally that the relative reaction rates are Pt > Ni >> Pd. Computational studies were in agreement with this relative reactivity order. The second step for the dehydrogenation of ethane occurs through collision-induced dissociation of the coordinated ethide species releasing ethylene and reforming the ternary metal hydride species, thus completing the catalytic cycle. Using a similar ternary complex [(phen)M(X)]+ (M=Ni, Pd, and Pt; X= H, CH3), the C-H bond activation of n-hexane led to a more complicated set of chemistry. An initial C-H bond activation step can occur at one of three unique C-H bonds: , , and . Despite the possibility of activation at these three C-H bonds, density functional theory calculations predicts that the C-H bond is most favorable for activation. However, upon collision-induced dissociation of the activated n-hexane ion, [(phen)M(C6H13)]+, products associated with each of activation sites are observed. This can be explained by the chain-walking phenomenon that has been described in polymerization catalysis, and through density functional theory calculations, the barrier for transition between , , and is well within the energy of collision-induced dissociations. A complex network of reactions is described with each pathway eventually producing the metal hydride or methide, thus closing the catalytic cycle. The transformation of cyclic hydrocarbons, often found in naphthalene, has not been studied in mechanistic detail for transition metal catalysts. After initial C-H bond activation, the collision-induced dissociation of the coordinated cyclohexyl species [(phen)M(c-C6H11)]+ affords products that are dependent on the identity of the metal used. For Ni and Pd complexes, the ring opening and “cracking” of the hydrocarbon chain is observed as the main pathway, however for Pt complexes, dehydrogenation is greatly favored. Utilizing density functional theory calculations, details pertaining to the mechanism are described as well as multiple catalytic cycles shown using ion-molecule reactions and collision-induced dissociation. Fatty acids found in biomass are a renewable source of C-H bonds, and through known decarboxylation chemistry described by O’Hair, complexes similar to those formed through C-H bond activation can be made. Fatty acids in general have been studied for their ability to produce diesel-like hydrocarbons without the use of petroleum products in solution by Crocker and others. In this work, the mechanism of deoxygenation is studied using a propionic acid model. Once deoxygenated ,the coordinated hydrocarbon chain can undergo further fragmentation through “cracking” chemistry similar to the chain-walking phenomenon described for n-hexane. The neutral products given off in this process are dependent on the identity of the metal used, as well as the degree of unsaturation in the fatty acid itself. Due to the stabilizing effects of allyl bonded metal complexes, saturated fatty acids undergo deoxygenation that is always paired with dehydrogenation. Experiments towards demonstrating a catalytic cycle are described as well as detailed mechanisms for using smaller carboxylic acid models. In addition to the breaking of C-C bonds through “cracking” chemistry, the formation of new C-C bonds can be formed in the gas-phase as shown by O’Hair and others. Also, of great importance in organic chemistry, the formation of C-N bonds is a required step in the synthesis of many novel molecules with applications in medicinal chemistry. This work details our work towards finding C-N coupling chemistry that can functionalize a wide range of new molecules.
Parker, Kevin, "Gas Phase C-H and C-C Bond Activation by Ternary Metal Complexes" (2022). Graduate Research Theses & Dissertations. 7527.
Northern Illinois University
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