Doctor of Philosophy, The Ohio State University, 2018, Industrial and Systems Engineering

A novel class of gamma-prime strengthened cobalt-based superalloys may enable a significant temperature and efficiency capability improvement relative to nickel-base superalloys for future generation turbine engine hardware. However, little information exists regarding deformation processing of these novel Co-Al-W alloys into useable product forms with the necessary microstructure refinement at an industrially relevant scale with industrially relevant processes. To address this need, an ingot metallurgy thermomechanical processing sequence was demonstrated for a novel class of cobalt-base gamma-prime containing superalloys. From an as-cast ingot, the material was characterized and a homogenization heat treatment was developed and executed to reduce residual segregation from casting. Representative ingot conversion steps using extrusion were evaluated and performed followed by a recrystallization heat treatment to produce the desired fine-grain, wrought microstructure. Deformation processing of wrought material was completed at supersolvus hot-working temperatures using both cylindrical upset specimens to establish flow-stress behavior and custom-designed double-cone upset specimens to experimentally quantify the effect of strain, strain-rate, and temperature in microstructure evolution during hot-working, including the dynamic recrystallization and grain growth. All upset testing was completed at two supersolvus temperatures (1149 °C or 1204 °C) and one of three strain-rates (0.01/s, 0.1/s, or 1.0/s) depending on the type of testing completed. Required thermophysical and thermomechanical data was determined for material property inputs to a finite element model which was used to correlate observed microstructures to location-specific thermomechanical processing history. As part of this development, a significant effort was undertaken at each stage of processing to sufficiently characterize the microstructure through optical microscopy, electron microscopy, (open full item for complete abstract)

Committee: Rajiv Shivpuri (Advisor); Jerald Brevick (Committee Member); Hamish Fraser (Committee Member) Subjects: Aerospace Materials; Engineering; Industrial Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2014, Chemistry

Nature provides a way to transform the most abundant energetic source, sunlight, to chemical fuel. Artificial photosynthetic systems aim to convert solar energy into chemical fuels such as hydrogen through water splitting. Our group has proposed a zeolite-based system capable of mimicking the natural photosynthetic process such as light harvesting and charge separation. However, water splitting requires a catalyst that promotes the reactions. Catalyst that consist of earth abundant metals, such as cobalt, have been shown to catalyze water oxidation efficiently. We have developed two synthetic methods for the anchoring of cobalt catalyst onto zeolitic surfaces. The methods exploit the zeolite characteristic features such as pore size, high surface area and electronegative framework. The methods are optimized utilizing micron and nano-size zeolite crystals and later applied to zeolite membranes supported on polymeric supports. The first method consists of a three step hydrothermal synthetic route. The successful anchoring of 200 nm plate-like clusters onto the surface of micron-size zeolite is reported. Anchored ß-Co(OH)2 clusters undergo a topotactic transformation to Co3O4 at 125 0C. As a consequence, the catalytic activity of the material decreases due to the thermal oxidation. The hydrothermal method was performed using 40 nm zeolite Y particles. Characterization by Raman and XPS spectroscopy showed that the catalyst is an amorphous Co (II) hydroxide that “coats” the zeolite nanocrystals rather than separate clusters. A 5-fold increase in oxygen evolution yield over the micron-size catalyst was observed. The nano-supported clusters were found to be stable for more than one catalytic cycle. The second method exploits pore diameter as a way to control cobalt cluster size. The ambient temperature synthesis uses three bases which vary in cation size: NaOH, TMAOH and TBAOH. Successful size control was achieved as evidenced by “net-like” clusters in NaOH treated samples (open full item for complete abstract)

Committee: Prabir Dutta (Advisor); Claudia Turro (Committee Member); Ann Co (Committee Member); Andrea Wolfe (Committee Member) Subjects: Alternative Energy; Analytical Chemistry; Chemistry; Inorganic Chemistry; Materials Science

Doctor of Philosophy, Case Western Reserve University, 2021, Materials Science and Engineering

The exfoliation of lithium cobalt oxide, a layered transition metal oxide, was investigated extensively using a two-step soft-chemical method to access cobalt oxide nanostructures. Through these studies, a deeper understanding of the exfoliation conditions necessary for the production of cobalt oxide nanosheets from protic lithium cobalt oxide was developed. These conditions involve aqueous reaction environments, hydroxide groups, bulky tetramethylammonium ions in solution, and high proton replacement in protic lithium cobalt oxide. Further reaction conditions, such as tetraalkylammonium ion concentration and ionic size, powder aging, reaction time, stir rate, and loading concentration gave deeper insights into exfoliation yield. Through these experimental results, exfoliation mechanisms of protic lithium cobalt oxide to form cobalt oxide nanosheets were determined. Furthermore, these insights led to new methods for producing cobalt oxide nanosheets, such as the exfoliation of large and ultrathin cobalt oxide nanosheets in pH neutral solutions, the stability of cobalt oxide nanosheets in non-aqueous solvents, and the production of high concentration cobalt oxide nanosheets solutions. Through these methods, the exfoliation, dispersion, and stability of cobalt oxide nanosheets were effectively controlled and understood yielding opportunities for new electronic applications and fundamental studies. Next, the effects of mechanical work (e.g., sonication and centrifugation) applied to cobalt oxide nanosheets demonstrated effects on UV-Vis absorption spectra related to the concentration, particle size, and electronic band structure. Then, advanced scanning probe microscopy techniques examined the fundamental electronic, optoelectronic, and electromechanical properties of cobalt oxide nanosheets; this revealed the work function of the cobalt oxide nanosheets to be 4.55 +/- 0.19 eV, with no light or thickness dependence. Lastly, the catalytic performance (e.g. photocatalyti (open full item for complete abstract)

Committee: Alp Sehirlioglu (Advisor); Emily Pentzer (Advisor); Roger French (Committee Member); Peter Lagerlof (Committee Member); Rigoberto Advincula (Committee Member) Subjects: Chemistry; Materials Science

Doctor of Philosophy, The Ohio State University, 1953, Graduate School

Committee: Not Provided (Other) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2015, Chemical Engineering

Ethanol steam reforming (ESR) is a promising way to produce hydrogen fuels from bio-renewable sources. Ethanol, as a liquid fuel, is convenient to transport and store. On the other hand, hydrogen is a clean and very efficient fuel if used in Proton Exchange Membrane (PEM) fuel cells. On site or on demand production of hydrogen from bio-ethanol can utilize the advantages of both ethanol and hydrogen fuels. In recent years, Co-based materials were found to be promising ESR catalysts because of their low cost and high efficiency. Ideal ESR catalysts should achieve fast conversion of ethanol, high selectivity to CO2, and long term stability without sintering, coking, or deactivation. Cobalt supported on reducible materials, such as CeO2 and ZnO, were found to have exceptional performance. On these catalyst materials, two oxidation states of Co, Co0 and Co2+, were found to be present. Mechanistic studied suggested that both Co0 and Co2+ have contributions to the catalytic activity of the Co catalyst, although their separate roles are still not clear. This study used a combination of first-principles methods, including density functional theory (DFT), microkinetic modeling, and ab initio thermodynamics, to study the ESR reactions on both the metallic and oxidized cobalt surfaces. Methanol steam reforming (MSR), as a simpler model of ESR, was also studied on the metallic cobalt surfaces. Through DFT calculations, the dominant reaction pathways of ESR and MSR have been identified on metallic and oxidized Co surfaces. The MSR reaction can be considered as a two-stage process, which consists of the methanol decomposition into CO, and the further conversion of CO into CO2 through water-gas shift (WGS) reaction. The ESR can be treated as a three-stage process, which includes ethanol decomposition into CH3 and CO, CH3 oxidation into CO, and CO conversion into CO2 through WGS. It was found that the metallic and oxidized Co phases have quite different catalytic activity in all st (open full item for complete abstract)

Committee: Aravind Asthagiri (Advisor); Umit Ozkan (Committee Member); Nicholas Brunelli (Committee Member); Anne Co (Committee Member) Subjects: Chemical Engineering

Master of Science, The Ohio State University, 2011, Chemistry

Zeolite-supported cobalt compounds were investigated as photocatalysts for water oxidation to O2. A three-step synthesis allows the deposition of cobalt-based catalyts onto the surface of zeolite Y. Cobalt-exchanged zeolite Y was treated with tetrabutylamonium hydroxide (TBAOH) or sodium hydroxide (NaOH) in order to precipitate the cobalt ions onto the surface. The resulting material is later heated in water or in NaH2PO4 solution. As a result of the synthesis two sets of samples are obtained for each base: zeolite-supported cobalt hydroxide and zeolite-supported cobalt phosphates. X-ray photoelectron spectroscopy (XPS) revealed the presence of Co (II) oxidation states in all samples. X-ray diffraction indicated amorphous materials for TBAOH samples whereas NaOH samples showed the crystalline phase of β-Co(OH)2. SEM images confirm the presence of approximately 0.3 μm particle on the zeolite surface. Utilizing non-supported β-Co(OH)2 as a model compound, the support's role was investigated. In addition, cobalt compounds on an aluminosilicate amorphous support were synthesized. Photochemical oxygen evolution curves ([O2] vs Time) showed higher rates as well as higher total number of μmoles of oxygen evolved for zeolite-supported catalysts. XPS data showed an increase of cobalt loading when samples were heated in a phosphate containing solution. Furthermore, the base used during the alkaline treatment controls the Co/Al ratio due to molecular sieving effects.

Committee: Prabir K. Dutta Ph.D (Advisor); Susan V. Olesik Ph.D (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2009, Chemical Engineering

Hydrogen is likely to play an important role in the energy portfolio of the future. Especially when it is used in fuel cells, it is an ideal energy carrier that can offer clean and efficient power generation. In the United States, ~95 % of hydrogen is produced using a steam reforming process [1]. Over 50% of world's hydrogen production relies on natural gas as the feedstock [2]. As the concern for a sustainable energy strategy grows, replacing natural gas and other fossil fuels with renewable sources is gaining new urgency. In this context, producing hydrogen from bio-derived liquids such as bio-ethanol has emerged as a promising technology due to the low toxicity, ease of handling and the availability from many different renewable sources (e.g., sugar cane, algae) that ethanol has to offer. An added advantage of producing hydrogen from bio-derived liquids is that it is quite suitable for a distributed production strategy. In this study, the effects of metal loading, preparation methods, synthesis parameters, cobalt precursors, impregnation medium, promoters and supports as well as reaction conditions are investigated for steam reforming of bioethanol and other bio-derived liquids over Co-based catalysts. In addition to these effects, the reaction networks and catalytic active sites are evaluated through steady state reaction using Gas Chromatography (GC)-Mass Spectrometer (MS) as analytical tools. Characterization studies have been performed by employing versatile characterization techniques such as Temperature Programmed Reaction (TPRxn), Temperature Programmed Reduction (TPR), Temperature Programmed Desorption (TPD), Temperature Programmed Oxidation (TPO), N2 Physisorption, Pulsed Chemisorption, X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Laser Raman Spectroscopy (LRS), Thermogravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), Isotopic Labeling, Scanning (open full item for complete abstract)

Committee: Umit Ozkan (Advisor); James Rathman (Committee Member); Jeffrey Chalmers (Committee Member) Subjects: Chemical Engineering; Chemistry; Energy; Engineering

Doctor of Philosophy, The Ohio State University, 2007, Chemical Engineering

The environmental applications of oxidation catalysis were examined as part of a two-stage strategy for the reduction of nitrogen oxides (NOx) and also for the removal of carbon monoxide from hydrogen streams. Cobalt-based catalysts were examined. Reduction of NOx, Co/TiO2 and Co/ZrO2 were studied for the oxidation of NO to NO2 in excess oxygen. NO oxidation was studied as the first step in a two-step catalytic scheme where NO is oxidized to NO2 and, in turn, NO2 is reduced with CH4 to N2 under lean conditions. Catalysts were prepared by sol-gel and incipient-wetness impregnation techniques. It was found that increasing the calcination temperature had an adverse effect on the activity of the IWI catalysts. Catalyst characterization showed higher activity for NO oxidation on Co/ZrO2 than on Co/TiO2 which was observed to correlate with the formation of the Co3O4 phase. For the preferential oxidation of CO, cobalt on several metal oxide supports were investigated and showed the highest activity on Co/ZrO2. A variety of reaction experiments were performed to examine the effects of reactant concentrations, residence time, and temperature on the CO conversion and O2 selectivity to CO2.

Committee: Umit Ozkan (Advisor) Subjects: Engineering, Chemical

Master of Science in Engineering, University of Akron, 2012, Chemical Engineering

The objective of this study was to evaluate the effect of titanium oxide, lanthanum oxide, and zirconium oxide on alumina supported cobalt catalysts. The hypothesis was that the presence of lanthanum oxide, titanium oxide, and zirconium oxide would reduce the interaction between cobalt and the alumina support. This was of interest because an optimized weakened interaction could lead to the most advantageous cobalt dispersion, particle size, and reducibility. The presence of these oxides on the support were investigated using a wide range of characterization techniques such as SEM, nitrogen adsorption, x-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed reduction after reduction (TPR-AR), and hydrogen chemisorptions/pulse reoxidation. Results indicated that both La2O3 and TiO2 doped supports facilitated the reduction of cobalt oxide species in reference to pure alumina supported cobalt catalysts, however further investigation is needed to determine the effect of ZrO2 on the reduction profile. Results showed an increased corrected cluster size for all three doped supported catalysts in comparison to their reference catalysts. The increase in reduction and an increase in the cluster size led to the conclusion that the support-metal interaction weakened by the addition of TiO2 and La2O3. It is also likely that the interaction decreased upon presence of ZrO2 on the alumina, but further research is necessary. Preliminary results have indicated that the alumina-supported catalysts with titanium oxide and lanthanum oxide present are of interest because of the weakened cobalt support interaction. These catalysts showed an increased extent of reduction, therefore more metallic cobalt is present on the support. However, whether or not there is more cobalt available to participate in the Fischer-Tropsch synthesis reaction (cobalt surface atoms) depends also on the cluster size. On one hand, increasing cluster size alone tends to decrease the (open full item for complete abstract)

Committee: Steven Chuang Dr. (Advisor); George Chase Dr. (Committee Member); Bi-min Zhang Newby Dr. (Committee Member) Subjects: Chemical Engineering

Doctor of Philosophy, Miami University, 2025, Chemistry and Biochemistry

The purpose of this dissertation is to observe physical properties of molecules in solution. Structural dynamics information is provided for three systems: a cobalt-centered complex converting between three coordination states at low temperature, a lanthanide complex adopting two NMR-active enantiomers according to identity of the metal center, and a chaperone protein interaction determining binding symmetry. These systems are investigated using a variety of analytical methods – including crystallography, NMR spectroscopy, Density Functional Theory calculations, and Molecular Dynamics simulations. Chapter 2 examines the dynamics of TpPh,Me Cobalt (II) NO3 (TpPh,MeCoNO3) [TpPh,Me = tris-3-phenyl-5-methylpyrazolylborate]. Solid-state XRD structure of TpPh,MeCoNO3 is presented for the first time, showing a five-coordinate Co (II) complex with TpPh,Me with NO3 bound as a bidentate ligand. Variable temperature NMR spectra are complicated at low temperature, with signals coalescing as temperature is increased. The high temperature NMR spectra indicate a four-coordinate structure above room temperature. Spectral analysis demonstrates the TpPh,MeCoNO3 complex occupies three concurrent structures at low temperatures. These three structures are analyzed using Density Functional Theory (DFT) calculations of four- and five-coordinate structures generated in silica from the crystal structure. In Chapter 3, the conformational interconversion of two NMR-active LnDOTAM structures (Ln=La-Lu; DOTAM=1,4,7,10-tetrakis(acetamido)-1,4,7,10-tetraaza-cyclododecane) are examined using a series of 13 lanthanide ions. Variable-temperature 1H NMR spectra demonstrate the concentration of the two identifiable conformations in solution depends on the identity of the metal ion. At low temperature, early LnDOTAM (Ce-Nd) have a high concentration of the twisted square antiprismatic geometry (TSAP), and later LnDOTAM (Sm, Eu, Tb-Yb) have a higher concentration of the square antiprismatic geomet (open full item for complete abstract)

Committee: David Tierney (Advisor); Rick Page (Committee Chair); Michael Crowder (Committee Member); Dominik Konkolewicz (Committee Member); Luis Actis (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Molecular Biology; Physical Chemistry

Master of Science, The Ohio State University, 2006, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 1950, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 1961, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 1969, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, Miami University, 2024, Chemistry and Biochemistry

The bulky ligand, 2,6-bis-(3',5'-diphenylpyrazolyl)pyridine (bdppp), was attached to Co(II) to produce the two complexes, [Co(bdppp)Cl2] and [Co(bdppp)2](BF4)2. Both complexes were studied by X-ray crystallography, UV-vis, EPR, Evans (NMR) susceptibility, and variable temperature NMR to discover how the bulky phenyls on the 3 and 5 positions of the pyrazole rings change the structure of the complex. [Co(bdppp)Cl2] was found to have a trigonal bipyramidal geometry and [Co(bdppp)2](BF4)2 had a pseudo-octahedral geometry. The two structures were found to be surprisingly similar electronically, with the magnetic z-axis in the direction of the Co-Npy bonds in both complexes, which were high-spin in solution at all tested temperatures.

Committee: David Tierney (Committee Chair); Rock Mancini (Committee Member); Michael Crowder (Committee Member); Carole Dabney-Smith (Committee Member) Subjects: Chemistry; Inorganic Chemistry

Doctor of Philosophy, Miami University, 2024, Chemistry and Biochemistry

This dissertation investigates the magnetic properties of several series of structurally-related molecules. Distinctions in their magnetic properties are studied using multiple spectroscopic techniques, coupled with structural determinations and theoretical calculations. In Chapter 2, a tripodal ligand tris-(2-pyridylmethyl)amine (tpa) and the corresponding series of air-sensitive, high-spin Co(II) complexes are developed by altering one monodentate ligand. Variable temperature paramagnetic relaxation measurements using NMR and structural parameter determination by SC-XRD are presented. Deviations in electronic transitions are studied using EPR and UV-vis. DFT calculations are carried out using ORCA to extensively explore and correlate the data between multiple techniques, enabling a comprehensive analysis of molecular motions. Chapter 3 explores unique examples of a six-coordinate Co(II) complex crystallized in four distinct packing motifs. Using SC-XRD and EPR, the influence of the non-covalent interactions (T-stacking) and the spatial arrangement of a chemical moiety within a molecule are studied on the spin-switch behavior. Multiple solvent VT-NMR, Evan's NMR, UV-vis, and MS were utilized to substantiate the analyses. Chapter 4 investigates structural dynamics in two novel pentacoordinate Co(II) complexes based on the tripodal ligand tris((1-benzyl-4-triazolyl)methyl)amine (tbta). Utilizing UV-vis, EPR, and VT-NMR, these complexes, featuring an anionic monodentate ligand and tbta were comprehensively investigated in solution. Chapter 5 presents the synthesis and detailed crystallographic structural examination of a unique five-coordinated Co(II) complex. The complex showcases a first-of-its-kind combination of the tripodal ligand tbta and the neutral ligand imidazole.

Committee: Michael Crowder (Committee Chair); David Tierney (Advisor); Scott Hartley (Committee Member); Robert McCarrick (Committee Member); Jason Berberich (Committee Member) Subjects: Chemistry

Doctor of Philosophy, The Ohio State University, 2024, Chemistry

The advancement of new catalytic processes using base-metal catalysts, particularly those employing earth-abundant metals, has been pivotal in shaping our modern way of life. Their impact spans across vital sectors such as agriculture, transportation, energy, and pharmaceuticals. Embracing sustainable sources for fine chemical synthesis could further enhance the benefits of base-metal catalysis. However, activating thermodynamically stable precursors like ethylene, propylene, CO2, H2, etc. and further incorporating them with high stereoselectivity into other commonly available substrates such as 1,3-dienes, alkynes, and enynes is a very challenging task. My dissertation work is centered on developing scalable, atom-efficient, and cost-effective catalytic methods for producing value-added products. The primary goals are to utilize sustainable feedstocks or readily available precursors to promote environmentally friendly chemistry. To achieve these objectives, two efficient catalytic methods have been devised using cobalt complexes with ligands derived from naturally occurring amino acids or commercially available bis-phosphine ligands. A systematic investigation of ligands was crucial in designing and synthesizing novel ligands to achieve high levels of chemo-, regio-, and enantioselectivity. iii The first method focuses on a broadly applicable approach that facilitates ligand dependent chemoselective [4+2] and [2+2] cycloaddition between various alkynes and 1,3-dienes to form 1,4-cyclohexadiene and cyclobutenes from exact same precursor. This has led to the synthesis of a library of over 22 enantiopure 1,4-cyclohexadiene and 20 nearly enantiopure cyclobutenes with a vinyl handle which are essential motifs in bioactive compounds, in excellent yields. The second method involves regio-and stereoselective synthesis of tetrasubstituted alkenes containing a vinyl and a boronate as functional group (over 20 examples) from readily available ethylene and boron reagent alo (open full item for complete abstract)

Committee: T.V. RajanBabu (Advisor) Subjects: Chemistry

MS, University of Cincinnati, 2023, Engineering and Applied Science: Materials Science

Aluminum and its alloys have been researched on a wide scale for different applications due to their unique properties and potential applications in different industries. Though Al-Co system is well-established, it still lacks research in the Al-rich side (<5 wt.% Co) for elevated temperature stability. This study evaluates the effect of Co and Sc additions to pure Al in terms of high temperature stability and improved mechanical properties through an integrated experimental and computational approach. This work has been carried out in the eutectic region which studies the binary Al-0.75 wt.% Co, Al-1 wt.% Co and Al-1.25 wt.% Co and the ternary Al-1 wt.% Co-0.1 wt.% Sc systems. These alloys were first investigated using the Calculation of Phase Diagrams (CALPHAD) methodology to study the phases present and their volume fractions by plotting phase diagrams. Solidification behavior was analyzed through Scheil calculations. Nucleation, growth and coarsening of precipitates was studied using the TC-PRISMA module of Thermo-Calc. The obtained data was then used to compare with experimental results. In the experimental part, these alloys were fabricated by vacuum arc melting (VAM). Cast alloys were heat treated at 300°C from the as-cast condition with different ageing times up to 1500 hours. Optical microscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis were carried out to characterize the alloys, examine the eutectic structure and verify the present phases. Mechanical properties (strength) of the alloy in as-cast and aged conditions were evaluated using Vickers microhardness tests. Obtained data was used to evaluate the high temperature stability of these alloys. Prepared alloys demonstrate different eutectic microstructures depending on the Co content, consisting of hypoeutectic to complete eutectic nature. As per the predictions by Thermo-Calc, presence of Al-Co intermet (open full item for complete abstract)

Committee: Dinc Erdeniz Ph.D. (Committee Chair); Matthew Steiner Ph.D. (Committee Member); Eric Payton Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2022, Chemistry

Two projects are presented here. The first project is a cationic cobalt(I)- catalyzed intramolecular Diels-Alder reaction. The cationic cobalt catalyst generated in-situ from a cobalt (II) precatalyst, which is reduced by activated Zn and is activated by NaBArF was shown to catalyze the reaction of several diene-enes and diene-ynes to afford the Diels-Alder products in moderate to good yields and in some cases high enantioselectivities. In the second project we studied the mechanism of the reaction of Cp2TiCl with trisubstituted epoxides. We found that the proposed intermediate in these reactions Cp2Ti(H)Cl is thermally unstable and spontaneously decomposes to Cp2TiCl and H2 gas.

Committee: T.V. RajanBabu (Advisor); Psaras McGrier (Committee Member); Jovica Badjic (Committee Member) Subjects: Chemistry

Master of Science, The Ohio State University, 2022, Chemistry

Many commercially valuable chemical transformations are facilitated using catalysts based on “noble metals,” such as palladium, platinum, and iridium. These metals are scarce resources on Earth and require more energy to refine relative to the first-row transition metals of the periodic table. However, noble metal catalysts prevail due to their predilection for facilitating the two-electron redox processes upon which most catalytic processes rely. Coaxing first-row transition metals to facilitate two-electron, rather than one-electron, redox processes is a challenging task in catalyst design. Work within the Thomas lab facilitates two-electron redox processes at first-row transition metals by incorporating them into a heterobimetallic motif held together by a bifunctional phosphinoamide ligand framework. The resulting heterobimetallic complexes pair an electron-rich, Lewis basic “late” first-row transition metal with an electron-poor, Lewis acidic “early” transition metal of the 1st, 2nd, or 3rd row. The “early” metal helps stabilize the “late” metal by forming highly polarized metal-metal bonds, which stabilizes formally lower oxidation states of the late metal. Furthermore, the metal-metal bond(s) facilitate catalysis by donating or receiving electron pairs, acting as an “electron reservoir” that the substrate can access via either of the two metal centers. The most useful metal pairing uncovered in the Thomas lab is zirconium as the “early” metal and cobalt as the “late” metal, held together by a bis(phosphinoamide) ligand framework. Yet-to-be published results from this system suggest utility in catalytic chemical dinitrogen fixation to ammonia and in stabilizing novel Co(-I)-(η6-arene) interactions. This thesis reports efforts to improve performance of the former and provide complimentary data for the latter through the synthesis of analogous complexes containing hafnium instead of zirconium.

Committee: Christine Thomas (Advisor); Claudia Turro (Committee Member) Subjects: Chemistry