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Artificial Photosynthesis (AP) provides a promising method for the conversion of solar energy to chemical fuel in the form of H2 and O2. Development of heterogeneous systems in which H2 evolution catalysts are immobilized on metal oxide semiconductors is imperative for the large-scale implementation of AP systems. This research focuses on the immobilization of an active H2 evolution catalyst on large band gap semiconductors for the development and optimization of a highly active photocatalytic H2 generation system.

In energetic materials research, the main goal is to create novel structures with improved energetic properties compared to the current industry standards. Despite the abundance of new organic structures synthesized over the last 30 years, applications have been limited by this delicate balance of synthesis, safety, and standards. Based on this, purely organic structures appear to be reaching a limit of their explosive capacity, an idea that has been dubbed “the CHNO ceiling”. Thus, compounds synthesized decades ago remain as the industry standard. The research presented in this dissertation endeavored to mitigate this problem. Building on previous work on energetic metallic clusters, we investigated the hypothesis that an unstable, reducing, fuel metal center such as titanium(II) will lead to a large combustion exotherm. Data from bomb calorimetric measurements supports this hypothesis: for complexes of hydrotris(pyrazolyl)borate (Tp), the titanium metal center resulted in larger heat of combustion compared to iron and manganese analogs, as well as the ionic Mg(Tp)2 compound. The combustion analysis of TiTp2 also led to the creation of a new method of sample preparation for air-free calorimetry. With data supporting the Ti(II) hypothesis, the project moved toward incorporating Ti(II) into energetic materials. The first target was more nitrogen rich analog of Tp, hydrotris(1,2,4-triazolyl)borate (Ttri).. So far, this titanium complex has not been confirmed but reaction products still need to be characterized. We attempted to ligate the energetic tetrazole to a Ti(II) center. In this case, the reduction was unsuccessful, instead synthesizing a Ti(III)Cl3-tetrazole complex. Although not energetic, the products exhibited solvatochromism and different equivalents of tetrazole gave different results. This dissertation also examines reactions of Fe(II)Tp2 with chlorinated solvents and ethers. In the presence of sunlight, the iron complex reacted rapidly decomposed chloroform, ultimately forming [FeTp2]+, [FeCl4]-. In diethyl ether and dibutyl ether, the data suggested that Fe(II)Tp2 cleaved and oxidized. In the resulting diiron(III) complexes, each iron center maintained one original Tp ligand and were bridged by two carboxylate ions (acetate or butanoate) and one oxygen atom. These complexes highlight a previously unknown reaction of FeTp2, the mechanism of which is under investigation. / Chemistry

There are two main focuses to this thesis. The first is to explore interactions of weakly coordinating anions, specifically observing how they affect the cation they are paired with. Choosing the right anion for a reaction could be crucial to get high yields or good selectivity, so having an anion that could be tuned to different coordination strengths is useful. The previously synthesized imidazolyl phenyl (IMP) anions can be made with various functionalities that vary the coordination strength. The IMP anions, which have only been paired and studied with metal cations, were paired with triethylammonium to be able to investigate the interactions in an organic ion pair. IR and NMR spectroscopy as well as computational methods were used to explore the steric and electronic interactions between the ions. IR spectroscopy was used to attempt to see the triethylammonium N-H stretching frequency shift depending on which anion was paired with it. Similarly, 1H NMR spectroscopy saw a shift in the triethylammonium CH2 resonances when changing the coordinating anion. A scale was made using this data to see which anions are weakly coordinating anions and which are strongly coordinating anions. Computational approaches were used to supplement the experimental results we obtained. DFT calculations were done to calculate the energy of interaction between the anion and cation and compared to the experimental scale obtained using NMR spectroscopy. Using the web program SambVca 2.1, % buried volume calculations were performed to gain an understanding of the steric volume each anion had around the position it coordinated to the cation. Lastly, NBO calculations were performed to determine the charge on each atom of the ions before and after coordination to investigate how coordination affected the individual charges at each atom. The second focus is to find new catalysts for the Z-selective isomerization of terminal alkenes. Previous members in the Dobereiner lab have done extensive research on molybdenum catalyzed isomerization of terminal alkenes. It was proposed that the PCy3 ligand plays a large role in inducing Z-selectivity because of its large size. To test this, a smaller ligand, specifically 1,3-dimethyl-N-heterocyclic carbene, was used in place of PCy3. DFT experiments showed the energy barrier for the E mechanism is lower than the Z mechanism. Using the molybdenum catalyst as inspiration, DFT was used to test the same complex, but replacing Mo with W. The computational results suggest that the tungsten catalyst would also be Z-selective and would have better conversion because it has lower energy barriers compared to the Mo complex. These results are promising, but synthesizing the complex to test it experimentally would help be certain that the tungsten catalyst will perform better. / Chemistry

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The aim of this work is to synthesize and study novel multinuclear manganese systems to model the structure and function of the oxygen-evolving complex (OEC). With the synthesis and study of these model complexes, a greater understanding of nature’s OEC mechanism and processes will come to fruition as well as a viable homogenous water-oxidation catalyst. In addition, small molecule activation was investigated using an FeI precursor. A tetramanganese “pinned-butterfly” cluster with the chemical formula Mn4(µ3-N2Ph2)2(µ-N2Ph2)(µ-NHPh)2(L)4, was synthesized via self-assembly with the addition of N, N’-diphenylhydrazine to Mn(N(SiMe3)2)2). The reaction proceeds over the course of a few hours with a visible color change pale yellow to yellow, black and finally red. The self-assembly mechanism was elucidated with methods such as ligand labeling, kinetic isotope effect, IR spectroscopy, X-ray diffractometry (single crystal and powder), UV/vis kinetic studies and absorbance studies, hydrogen-atom transfer (HAT) competition studies, NMR studies, GC studies and freezing point depression studies. The “twisted basket” cluster is a two-hydrogen-atom reduced analogue of the aforementioned “pinned-butterfly” cluster with a chemical formula of Mn4(µ-NHPh)4(µ-PhNNPh-2N,N’)2(py)4. Conversion between the clusters was investigated and achieved with the addition of an equivalent of N,N’-diphenylhydrazine and heat to a solution of the “pinned-butterfly” complex. This conversion between the clusters displays similarities to the OEC in the sense that it is undergoing proton-coupled electron transfer (PCET), cluster rearrangement and N-N bond formation. While these novel tetramanganese clusters provide us unique, reactive, and flexible clusters, they are far too sensitive to air and water to perform any useful catalysis. Due to the ligands’ lack of stabilization, alternate ligand platforms were investigated that would be able to form more rigid complexes, but retain lability. Bi- and tridentate ligands were investigated that resulted in the synthesis of several novel multinuclear homo- and hetero- metallic complexes. The ligands include polyoligimeric silsesquioxanes and substituted pyridines. These multinuclear Mn clusters show similarities to the OEC in their composition and structures. Upon exposure to air, a color change is observed without the precipitation of a manganese oxide insoluble species. This observation supports the increased stability, yet retained reactivity of the chelated clusters. Lastly, an FeI precursor was reacted with CS2 in attempts to isolate an Fe-S carbide complex and model the iron-molybdenum cofactor (FeMoco). Instead, a CS2 bridging dimer was formed and isolated. The activation of CS2 led us to attempt the reaction of the FeI precursor with other analogues such as CO2, diisopropylcarbodiimide, methylisothiocyanate, and phenylacetylene. CO2 and acetylene have been shown to be reactive substrates to the native FeMoco. These small molecule activated Fe complexes were characterized using X-ray diffraction technqiues, UV/visible spectroscopy, electron paramagnetic resonance spectroscopy and infrared spectroscopy. / Chemistry