Global concern about the long-term sustainability and environmental impact of burning fossil fuels has prompted widespread research on alternative fuels. Hydrogen is attractive as a chemical fuel because it is carbon-free and therefore its exothermic reaction with oxygen does not generate CO2 as a byproduct. Large scale implementation of hydrogen, however, depends on efficient and clean production of hydrogen from non-fossil sources. Splitting water with sunlight is often considered the “Holy Grail” of a sustainable hydrogen economy, however the greatest challenge towards achieving this goal is in lowering the high kinetic barrier of the water oxidation half reaction. With the right electronics and geometry of coordinated ligands, transition metals can effectively mediate the loss of 4 e- / 4 H+ required to oxidize water. This dissertation details the syntheses, characterization, and reactivities of three novel catalytic systems containing multi-functional ligands that appear to have a marked effect on catalysis.

Chapter 2 details the synthesis and reactivity of novel ruthenium complexes bearing 2,2’-bipyridine ligands with various pyrazolate groups in the 6,6’-positions. The structures of these complexes were characterized by NMR spectroscopy and x-ray crystallography and for one of the complexes, the unique solvatochromic and acid-base properties involving the uncoordinated nitrogen atoms of the ligand were investigated by non-aqueous and aqueous UV-visible spectroscopy and electrochemistry. Rapid O2 evolution was witnessed under acidic conditions in the presence of CeIV. While the apparent rates (between 3 and 4 s-1) are in the upper range of rates observed for ruthenium-based molecular catalysts, lag periods observed for 1 – 1.5 min prior to O2 production may be indications of slow catalyst dissolution or activation, or of catalyst modification. Alternatively, CeIV may have a non-innocent role during catalysis by interaction with the uncoordinated nitrogen.

Chapter 3 details the synthesis and catalysis-relevant properties of two novel ruthenium complexes, [Ru(2,2’-bipyridine-6,6’-diphosphonato)(pic)2] (pic=4-picoline) and [Ru(6,6’-diisopropyl-2,2’-bipyridine-6,6’-diphosphonato)(pic)2]. These complexes are the first examples of water oxidation catalysts deliberately bearing phosphonate groups positioned to serve as metal-bound ligands. They have been characterized by NMR spectroscopy, elemental analyses, x-ray crystallography, cyclic voltammetry, and UV-visible spectroscopy. From electrochemical data, we have discovered that charge states of P-OH / P-OR groups have a significant influence on redox and catalytic properties, features that are absent in much more commonly studied carboxylate analogs.

Chapter 4 details the synthesis and reactivity of a series of ruthenium complexes bearing novel 2,2’-biimidazole-4,4’-phosphonate ligands. Surprisingly, the new ligands did not participate in the intended tetradentate coordination, but instead bidentate N,N’-coordination, placing the phosphonate groups in the secondary coordination sphere to engage in hydrogen bonding interactions with two aquo ligands. The presented complexes have been characterized by NMR spectroscopy, x-ray crystallography, and cyclic voltammetry. Qualitative and quantitative oxygen evolution studies (from cyclic voltammetry and addition of excess CeIV, respectively) have shown a dramatic improvement in catalytic rates compared to those of previously reported Ru diaquo complexes. Our working hypothesis to explain the superior catalytic activity includes involvement of the phosphonates in the secondary coordination sphere potentially as basic proton-relaying functionalities.

Part 1 of this dissertation entails the synthesis and characterization of a new family of ruthenium complexes bearing remote NHC, 4-pyridylidene ligands for water oxidation. Catalytic water oxidation is important for the development of cost-effective and carbon-neutral methods to produce hydrogen fuel from water. We describe the Zincke pyridine synthesis of ligand precursors OCR, where OC = 3-carboxypyridinium and the nitrogen atom of the pyridinium has been quaternized has been functionalized such that R = methyl (Me), adamantyl (Ad), and 2,6-dimethylphenyl (Ar). All three ligand precursors were successfully cyclometallated to yield coordination complexes of the formula [RuII(OCR)(tpy)(L)]+[PF6]- (tpy = 2,2';6',2"-terpyridine, L = CH3CN or H2O). The 4-pyridylidene coordination of the OCR ligand was confirmed by NMR and x-ray diffraction studies. Ru(OCAd) exhibited a reversible RuII/RuIII with an E1/2 of 0.441 V at pH 1. Combined UV-vis, QTOF-MS, and TDDFT were used to study the stability of the complexes in solution. During catalytic screening for water oxidation activity with ceric ammonium nitrate as the sacrificial oxidant, Ru(OCR) appeared to exhibit an initial burst of O2 production followed by a plateau after 40-100 turnovers.Part 2 of this dissertation examines an alkene isomerization catalyst Ru(acn), that was previously reported by the Grotjahn lab to exhibit high activity for isomerizing terminal alkenes to their internal isomers. The catalyst was noteworthy for its fast reaction times, mild reaction conditions, and significantly, its unprecedented kinetic (E)-selectivity. We conduct the first combined experimental and theoretical study to unveil the mechanism of isomerization and the origin of the high (E)-selectivity exhibited by the catalyst. In-situ NMR reaction monitoring were used to acquire time-dependent kinetics data for the Ru(acn)-mediated isomerization of but-1-ene into its (E)-and (Z)-but-2-ene isomers, the latter of which required special reaction conditions. An extensive computational model was developed, and lowest energy pathways validated by experimental Arrhenius activation energies derived from the experimental data. The experimental activation energies compared well to the DFT energies for the isomerization pathway featuring an exo-coordinated η3-allyl, and the structures of key intermediates reveal inner-sphere steric interactions that favor the (E)-allyl intermediate over the (Z)-isomer.

Electron-poor ligands that also contain a pendant basic or acidic functional group should provide improved conditions and catalytic performance in a variety of reactions, such as anti-Markovnikov addition of O-H compounds such as water, alcohols, and carboxylic acids to terminal alkenes, a highly-sought reaction for which there is no good general solution. A second application is alkoxycarbonylation reactions of aryl halides to make esters. A strategy to synthesize electron-poor monophosphines or (bis)phosphines was developed. The analysis of the electron-poor phosphines using coordinating-ion-spray high resolution mass spectrometry was developed due to the inability of the ligands (or water- and air-sensitive precursors) alone was often not successful. Initial results in testing electron-poor phosphines with either pendant OH or OCH3 functionality as part of a metal complex for alkoxycarbonylation catalysis is reported.

It is imperative that we improve clean and renewable energy technology in the face of growing energy demand. Solar energy is potentially abundant, but we need to capture, convert and store solar energy in usable forms. Hydrogen could be ideal for energy storage, if made using sunlight energy to split water by oxidizing water to dioxygen and protons and using the released electrons to reduce protons to hydrogen in a net transfer of electrons. In Chapter 2, we explored water oxidation which is believed to be the more challenging of the two steps. We found evidence for catalyst degradation while using iridium complexes with ceric ammonium nitrate (CAN) as a sacrificial oxidant, possibly forming iridium oxide nanoparticles. Chapter 3 describes studies of analogs of well-known ruthenium water oxidation catalysts with bipyridine ligands that were designed to be bifunctional, by adding potential pendant bases such as hydroxy, alkoxy, amino, and heterocyclic amines, and some combinations of these groups. One successful catalyst was the methoxy complex which proved to be more robust than the parent unsubstituted complex while using CAN. Extended π systems, e.g. naphthyridine were found to lead to less active catalysts; therefore, we attempted to slow degradation and increase duration of catalysis by protecting the complex with sterically hindered groups, but we suspect that ligand loss interfered. Chapter 4 focused on ligands that would support formation of binuclear complexes which could improve catalysis, taking into account one of the proposed mechanisms for water oxidation involving two metal complexes

Work presented in Chapter 1 involves synthesis (by direct metalation of C-H bonds) and characterization of bis protic N-heterocylic carbene (PNHC) pincer complexes of platinum, palladium, and even nickel, in addition to their reactivity with alkynes and under basic conditions. Alkynes were converted to aldehydes as the major anti-Markovnikov addition products. The complexes were treated with different bases to form proposed trifunctional complexes containing one PNHC and one basic imidazolyl ligand. Addition of sodium alkoxide yielded dimeric species and lithium bases gave LiCl-imidazolyl adducts, which proved to be moisture sensitive but did not lose LiCl and undergo ligand exchange with substrates.

Avid consumption of fossil fuels by humans has led to record high levels of the greenhouse gas carbon dioxide (CO2). An energy storage medium to replace dwindling and finite fossil fuel supplies must be found. Hydrogen gas (H2) is an attractive alternative to hydrocarbon fuels but presently natural gas is the primary feedstock for H2 production globally. Therefore, an alternative process for hydrogen production must be developed. Photoelectrochemical electrolysis of water (H2O) presents a promising method for the clean generation of H2. The oxidation of water to oxygen (O2), four protons (H+), and four electrons (e-) must precede the reduction of H+ to H2 and is the more demanding reaction both kinetically and thermodynamically. Transition metal catalysis can realize this challenging transformation. Careful choice of metal and ligand design can facilitate each stage of the oxidation of water; in particular, the management of H+ during catalysis can help improve both the speed and the durability of the catalyst. This dissertation describes the synthesis, characterization, and water oxidation activity of two new water oxidation catalysts featuring heteroatom H+ relays and the development of instrumentation to better quantify the O2 produced during H2O oxidation.

Chapter 2 describes the incorporation of phosphonate monoester and sulfonate and pendant bases into the first coordination sphere of the well-studied water oxidation platform [(2,2';6',2"- terpyridine)(2,2’-bipyridine)Ru(OH2)]2+. The complexes were characterized by combustion analysis, NMR spectroscopy, and x-ray crystallography. The catalytic performance of the complexes was evaluated for water oxidation catalysis using ceric ammonium nitrate (CAN) as a sacrificial oxidant via manometry and by square wave and cyclic voltammetry in a buffered aqueous milieu. The phosphonate monoester was found to perform poorly under chemical oxidation conditions but did show electrocatalytic behavior by cyclic voltammetry. The sulfonate system performed very well with CAN as the oxidant demonstrating a turnover frequency of 0.88 s-1 and turnover number of 7402. The sulfonate system also demonstrated electrocatalytic behavior suggesting homogenous electrocatalysis is maintained. Pourbaix analysis and a computational study suggest the intermediacy of a unprecedented ruthenium (III) oxyl, with the sulfonate acting as a pendant base late in the catalytic cycle.

Chapter 3 describes the development of two pieces of instrumentation for the detection of oxygen produced by our water oxidation catalysts. The first instrument is a dual manometry/optical oxygen sensing cell which was constructed for evaluating catalyst performance. Problems with the fluorescent oxygen sensor including drift and sensitivity to humidity led us to focus on pressure as the primary indication of oxygen production. The cell is a robust and easy to use system that provides excellent repeatability and reliability, with <2% drift at more than twice typical operating pressure over 60 h. The second piece of instrumentation was an automated system for sampling headspace gas in a bulk electrolysis (BE) cell. The design and construction of a panel that interfaces a custom BE cell, potentiostat, and gas chromatograph/mass spectrometer (GC/MS) is described.

While classical alkene transformations have been known since the 1800s, the last 10-20 years have seen a renewed interest in providing transition metal catalysts to expand the diversity of alkene reaction partners. One important goal is selective isomerization of alkenes, to produce unique isomeric compounds that expand the scope of other synthetic transformations. In the past 11 years, the Grotjahn group has been at the forefront of the development of highly (E)-selective alkene isomerization catalysts; one that began with the discovery of the efficient catalyst 1.1 and continued with the development of more positionally selective catalyst 1.2 (& 1.2a). Catalyst 1.1 is currently sold commercially by Strem Chemicals, and has been used in a number of tandem and sequential processes to produce high-value compounds.

This dissertation details applications and modifications of existing Grotjahn catalysts (cyclopentadienyl)[2-(di-i-propylphosphino)-4-(t-butyl)-1-methyl-1H-imidazole]acetonitrile ruthenium(II) hexafluorophosphate (1.1) and (pentamethylcyclopentadienyl)[2-(di-i-propylphosphino)-4-(t-butyl)-1-methyl-1H-imidazole]acetonitrile ruthenium(II) hexafluorophosphate (1.2), along with its bisacetonitrile analogue 1.2a. The major goal that drives the direction of the dissertation include increasing the efficiency of 1.2 + 1.2a by developing a new version of the catalyst (discussed in chapters 3 and 4) that allows for dramatically increased reaction rates, and observation of the interactions of alkenes with this catalyst to understand more about the origins of selectivity and efficiency of this new catalyst. Other goals include determining the degree of the difference in selectivity between the new catalyst and existing Grotjahn catalysts, and increasing the scope of use of existing catalyst 1.1.

CHAPTER 1. This chapter introduces the field of study and motivations steering the direction of development. The topics of catalysis and bifunctional catalysis are discussed.CHAPTER 2. This chapter describes the development of biaryl phosphines containing functionalization with a pendent base for use in gold catalyzed organic reactions. This chapter starts with development of synthetic methods for high efficiency in the synthesis of new iterations on the chemical structure. The use of high throughput screening for reaction discovery is discussed and the result from our trials is discussed. Evaluation of the different ligand candidates is done through NMR spectroscopic studies on two reactions. The first reaction studied is carboxylic acid addition to terminal alkynes. The second reaction studied is isomerization of alkynes. CHAPTER 3. This chapter describes the synthesis of hydrogen-bond-donor-functionalized ferrocenyl bisphosphines, and studies on the coordination chemistry of the novel bisphosphines with platinum and nickel. Development of the synthetic route to yielding the target compounds was necessary, and the major points of concern are discussed. Studies on the coordination chemistry of the ferrocenyl bisphosphines with platinum and nickel are discussed

This dissertation includes exploring two scientific challenges, where both water oxidation catalysis and NH addition to alkynes will benefit greatly from the rational design of ligands. Chapter 1: The long-term goal for scientists is to use sunlight for the energy to split water. The main target of our research is the oxidation half reaction. Water oxidation catalysts have been reported by groups of Sun and Llobet but sustainable catalysis is still impaired by catalyst deactivation. This dissertation attempts to design robust catalysts through manipulating of the apical coordination sites. Chapter 2 uses phosphorus in the primary coordination sphere to create stronger axial ligand-ruthenium bonds; Chapter 3 uses a chelating macrocyclic ligand to reduce the chance of ligand loss. Chapter 4, focuses on the hydroamination of C-C multiple bonds as the most atom-efficient method for the introducing of nitrogen atoms into organic molecules.