Publication Date

2024

Document Type

Dissertation/Thesis

First Advisor

Ryzhov, Victor

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

In the wake of climate change from rising CO2 emissions and with peak oil looming, the dive into sustainable fuels and energy is becoming increasingly essential. This dissertation investigates ternary ligand-metal cationic complexes for potential catalytic applications in the decarboxylation of carboxylic acids to produce bioavailable and carbon-neutral fuel sources. By utilizing gas-phase reactivity studies via mass spectrometry, namely, collision-induced dissociation and ion-molecule reactions, mechanisms can be elucidated in a regulated chemical environment. When paired with density functional theory calculations for molecular geometries, chemical interactions, charge densities, and energetics, system efficiencies can be further clarified presenting a more complete mechanistic and catalytic analysis. Previous work by our group demonstrated the strength of the catalytic production of hydrogen from formic acid with [(L)ZnH]+, where L = 1,10-phenanthroline (phen) or 2,2′:6′,2″-terpyridine (terpy), 2,2-bipyridine (bipy). Similar chelates and multidentate nitrogenous ligands were attempted using [(terpy)ZnH]+ as a comparative standard. The ion [(L)Zn(OOCH)]+ (where L = tris(pyridin-2-ylmethyl)amine (TPMA), N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine (TPEN), 1,4,7-triazonane (9-N-3), tri(pyridin-2-yl)phosphine (TPP), tri(pyridin-2-yl)phosphine oxide (TPPO), di-(2-picolyl)amine (DPA)) is formed in solution, introduced into the mass spectrometer, and subjected to collisional activation to generate the [(L)ZnH]+ species. The zinc hydride ion is then reacted with neutral formic acid inside the ion trap of the mass spectrometer to reproduce the [(L)Zn(OOCH)]+ ion through the loss of neutral H2 and thereby completing the formal catalytic cycle. All the ligands performed kinetically slower than the terpy standard, except for DPA which has the geometry most similar to terpy thus providing rationale for its similar reactivity. A secondary study compared the [(terpy)ZnH]+ system to other metals in the 1st transition row demonstrated the versatility of the terpy ligand as Fe(II) and Co(II) operated at similar reaction efficiencies and kinetics as Zn. Published studies by Boddien and co-workers on a cage iron-phospholigand complex’s ability to decarboxylate formic acid into hydrogen piqued the interest of our group. Our collaborators from the University of Portland synthesized several ligands with structural variances from the published work to produce comparable ternary complexes [(L’)M(OOCH)]+, where the synthetic ligands (L’) were tris(2-(diphenylphosphino)phenyl)phosphine (DPPP), tris((diphenylphosphino)methyl)phosphite (DPMP), and (((2-(methylthio)phenyl)phosphinediyl)bis(2,1 phenylene))bis (diphenylphosphine) (MPPD). The DPPP ligand did not form any complex ionic species of the form [(L’)MH]+ capable of ion-molecule reaction to generate hydrogen. Successful complexes were seen with Mn(II) and Zn and the MPPD ligand, and Fe(II) with the DPMP ligand. Ion-molecule reaction of the metal hydride [(L’)MH]+ with formic acid thereby providing a full formal catalytic cycle producing the [(L’)M(OOCH)]+ ion with molecular hydrogen had the trend of: [(DPMP)FeH]+ > [(MPPD)MnH]+ >> [(MPPD)ZnH]+. Theoretical calculations demonstrated molecular geometry and energetic calculations demonstrated the favorable nature of these systems. Crown ethers are known chelating agents with typically strong electron donating groups amongst their cyclic molecule cavity that form host-guest pairings with cationic species. Crown ethers therefore appeared to be an ideal choice in the hydrogen generation studies from formic acid. Herein, ions formed of the general ion [(crown)M(OOCH)]+ (where crown = 12-crown-4 (12c4), 15-crown-5 (15c5), and 18-crown-6 (18c6)) were activated through collision induced dissociation to form the metal hydride species [(crown)MH]+. Fragmentation in these systems produced very little side reactions, i.e., competitive reaction channels to the metal-hydride. Ion molecule reaction with formic acid with the Zn systems to establish the formal catalytic cycle and were shown to be kinetically slow with all iterations of crown tested, resulting in the following reactivity trend: 12c4 > 15c5 = 18c6 (with no ligand resulting in >1% reaction efficiency). Smaller metals in the first-row transition series were tested with the 12c4 ligand and ion-molecule reaction of the [(12c4)MH]+ species demonstrating that the overall reaction trend loosely followed periodic table atomic size trends: Mn=Co=Ni>Fe>>Cu>>Zn. Energy diagrams produced by theoretical calculations confirmed the reactivity seen in the experiments. The last chapter of the dissertation deals with the novel gas-phase catalytic systems from production of hydrocarbons from bioavailable fatty acid sources. The ion [(terpy/phen)Zn(OOCR)]+ after collisional activation demonstrated decarboxylation to form the alkylated (organozinc) cation [(terpy/phen)Zn(R)]+ (where R = C3H7, C6H13, C7H15, C17H35, C17H33, C17H31). From this ion two pathways could proceed: i.) a gas-phase ion-molecule reaction with a neutral carboxylic acid R’COOH resulting in the ion [(terpy/phen)Zn(OOCR’)]+ and the alkane RH; ii.) a second activation of the [(terpy/phen)Zn(R)]+ ion producing an alkene and the ion [(terpy/phen)Zn(H)]+ which underwent ion-molecule reaction with R’COOH to form [(terpy/phen)Zn(OOCR’)]+ and the accompanied H2 loss. When R=R’ the reaction is presented as two formal catalytic cycles as seen with butyric acid resulting in the overall equations: C3H7COOH = C3H8+ CO2 or C3H7COOH = C3H6 + H2+ CO2. This work presents Zn/N-ligand complexes as a potential catalyst for fatty acid deoxygenation with no fragmentation of the hydrocarbon chain, producing alkanes or terminal alkenes.

Extent

199 pages

Language

en

Publisher

Northern Illinois University

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In Copyright

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