The conversion of water to hydrogen and of carbon dioxide to industrially relevant chemical precursors are examples of reactions that can be used to store renewable energy as fuels or chemical building blocks for creating sustainable chemical manufacturing cycles. Unfortunately, current industrial catalysts for these transformations are reliant on relatively expensive and/or rare materials, such as platinum in the case of hydrogen generation, or lack selectivity towards producing a desired chemical product. Such drawbacks prevent global-scale applications. Although replacing such catalysts with more efficient and earth-abundant catalysts could improve this situation, the fundamental science required for this is lacking. In the first part of this dissertation, the synthesis and characterization of a novel binuclear iron fused porphyrin designed to break traditional scaling relationships in electrocatalysis is presented. Key features of the fused porphyrin include: 1) bimetallic sites, 2) a π-extended ligand that delocalizes electrons across the multimetallic scaffold, and 3) the ability to store up to six reducing equivalents. In the second part of this thesis, the electrochemical characterization of benzimidazole-phenols as “proton wires” is described. These bioinspired assemblies model the tyrosine-histidine pair of photosystem II, which serves as a redox mediator between the light-harvesting reaction center P680 and the oxygen evolution complex that enables production of molecular oxygen from water in cyanobacteria, algae, and higher plants. Results show that as the length of the hydrogen-bond network increases across a series of benzimidazole-phenols, the midpoint potential of the phenoxyl/phenol redox couple becomes less oxidizing. However, benzimidazole-phenols containing electron-withdrawing trifluoromethyl substituents enable access to potentials that are thermodynamically sufficient for oxidative processes relevant to artificial photosynthesis, including the oxidation of water, while translocating protons over ~11 Å.

Alkylphosphine- and alkylpyridine-substituted 2,6-bis(imino)pyridines (pyridine diimines, PDI) have recently been used as polydentate, redox non-innocent ligands that support the development of highly active catalysts. The alkyl phosphine-substituted ligand, Ph2PPrPDI, was added to (Ph3P)3CoCl and subsequent reduction using excess sodium amalgam yielded (Ph2PPrPDI)Co. Electronic structure analysis revealed a cobalt(I) complex that features a singly reduced PDI chelate. Additionally, low valent Ph2PPrPDI complexes of Fe and Ni were synthesized and structurally characterized. Furthermore, a series of Ph2PPrPDI Mn, Fe, Co, and Ni complexes were investigated to evaluate ligand denticity and redox activity. Finally, the catalytic hydrosilylation of carbonyls was investigated to compare the activity of the series and determine whether electron count plays a role in catalysis. An analogous ligand system featuring alkylpyridine substituents, PyEtPDI, was added to CoCl2, affording a cobalt dichloride complex with the formula [(PyEtPDI)CoCl][Cl]. Single crystal X-ray diffraction revealed a high-spin cobalt(II) center that possesses an octahedral geometry and an outer-sphere chloride ion. Further treatment using 2 equivalents of NaEt3BH resulted in the formation of (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co, which has been verified by multinuclear NMR spectroscopy and single crystal X-ray diffraction. (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co was then used in the catalytic hydroboration of nitriles at ambient conditions to yield the corresponding N,N-diborylamines, which were used as precursors for amide formation.

Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation.

Oxidoreductases catalyze transformations important in both bioenergetics and microbial technologies. Nonetheless, questions remain about how to tune them to modulate properties such as preference for catalysis in the oxidative or reductive direction, the potential range of activity, or coupling of multiple reactions. Using protein film electrochemistry, the features that control these properties are defined by comparing the activities of five [FeFe]-hydrogenases and two thiosulfate reductases. Although [FeFe]-hydrogenases are largely described as hydrogen evolution catalysts, the catalytic bias of [FeFe]-hydrogenases, i.e. the ratio of maximal reductive to oxidative activities, spans more than six orders of magnitude. At one extreme, two [FeFe]-hdyrogenases, Clostridium pasteuriaunum HydAII and Clostridium symbiosum HydY, are far more active for hydrogen oxidation than hydrogen evolution. On the other extreme, Clostridium pasteurianum HydAI and Clostridium acetobutylicum HydA1 have a neutral bias, in which both proton reduction and hydrogen oxidation are efficient. By investigating a collection of site-directed mutants, it is shown that the catalytic bias of [FeFe]-hydrogenases is not trivially correlated with the identities of residues in the primary or secondary coordination sphere. On the other hand, the catalytic bias of Clostridium acetobutylicum HydAI can be modulated via mutation of an amino acid residue coordinating the terminal [FeS] cluster. Simulations suggest that this change in catalytic bias may be linked to the reduction potential of the cluster.

Two of the enzymes examined in this work, Clostridium pasteurianum HydAIII and Clostridium symbiosum HydY, display novel catalytic properties. HydY is exclusively a hydrogen oxidizing catalyst, and it couples this activity to peroxide reduction activity at a rubrerythrin center in the same enzyme. On the other hand, CpIII operates only in a narrow potential window, inactivating at oxidizing potentials. This suggests it plays a novel physiological role that has not yet been identified. Finally, the electrocatalytic properties of Pyrobaculum aerophilum thiosulfate reductase with either Mo or W in the active site are compared. In both cases, the onset of catalysis corresponds to reduction of the active site. Overall, the Mo enzyme is more active, and reduces thiosulfate with less overpotential.

The search for highly active, inexpensive, and earth abundant replacements for existing transition metal catalysts is ongoing. Our group has utilized several redox non-innocent ligands that feature flexible arms with donor substituents. These ligands allow for coordinative flexibility about the metal centre, while the redox non-innocent core helps to overcome the one electron chemistry that is prevalent in first row transition metals. This dissertation focuses on the use of Ph2PPrDI, which can adopt a κ4-configuration when bound to a metal. One reaction that is industrially useful is hydrosilylation, which allows for the preparation of silicones that are useful in the lubrication, adhesive, and cosmetics industries. Typically, this reaction relies on highly active, platinum-based catalysts. However, the high cost of this metal has inspired the search for base metal replacements. In Chapter One, an overview of existing alkene and carbonyl hydrosilylation catalysts is presented. Chapter Two focuses on exploring the reactivity of (Ph2PPrDI)Ni towards carbonyl hydrosilylation, as well as the development of the 2nd generation catalysts, (iPr2PPrDI)Ni and (tBu2PPrDI)Ni. Chapter Three presents a new C-O bond hydrosilylation reaction for the formation of silyl esters. It was found the (Ph2PPrDI)Ni is the most active catalyst in the literature for this transformation, with turnover frequencies of up to 900 h-1. Chapter Four explores the activity and selectivity of (Ph2PPrDI)Ni for alkene hydrosilylation, including the first large scope of gem-olefins for a nickel-based catalyst. Chapter Five explores the chemistry of (Ph2PPrDI)CoH, first through electronic structure determinations and crystallography, followed by an investigation of its reactivity towards alkyne hydroboration and nitrile dihydroboration. (Ph2PPrDI)CoH is the first reported cobalt nitrile dihydroboration catalyst.

New sol-gel routes based on peroxo complexes of early transition metals in a highly acidic medium were developed, to prepare metal oxide phosphates that feature structural protons. A sol-gel synthetic route was chosen because it allows atomic level mixing of precursors and lower heating temperatures, which are preferable in exploring metastable phases. Titanium and molybdenum sol-gel chemistries were the focus of the initial studies and the synthesis of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) type metal oxide phosphates were explored. For the synthesis of the metal oxide phosphates, hydrogen peroxide was employed to prepare the respective precursor solutions. The peroxide ligand suppressed the immediate precipitation of metal cations in aqueous medium, by coordinating to Ti4+ and Mo6+ ions, and produced a soft wet-gel following polycondensation. Phosphoric acid was used to acidify the reaction medium and to provide protons and phosphate ions as structural components. From this synthetic route, a series of Ti1-xMoxP2O7 (x = 0 – 0.5) and Mo1-yTiyP2O8-y (y = 0 – 0.4) crystalline compounds, with various degrees of purity, were synthesized. For x = 0 and y = 0, the crystalline compounds TiP2O7 and MoP2O8 were produced, respectively, after calcining at 600 °C.

In pursuit of new metastable molybdenum oxide phosphate compounds, peroxo-molybdenum precursor mixtures with different molar ratios were treated gently by low-temperature heating. After controlled drying in a lab oven, MoO2(H2O)(HPO4) crystals were obtained as a highly crystalline pure product instead of a gel. The dissolution of MoO2(H2O)(HPO4) in water and precipitation with a CsCl solution produced a new crystalline compound with a cubic unit cell (a = 11.8(2) Å). Further studies will lead to crystal structure determination and elucidation of the aqueous chemistry of MoO2(H2O)(HPO4).

Over the last few decades, homogeneous molybdenum catalysis has been a center of interest to inorganic, organic, and organometallic chemists. Interestingly, most of the important advancements in molybdenum chemistry such as non-classical dihydrogen coordination, dinitrogen reduction, olefin metathesis, and water reduction utilize diverse oxidation states of the metal. However, employment of redox non-innocent ligands to tune the stability and reactivity of such catalysts have been overlooked. With this in mind, the Trovitch group has developed a series of novel bis(imino)pyridine (or pyridine diimine, PDI) and diimine (DI) ligands that have coordinating phosphine or amine arms to exert coordination flexibility to the designed complexes. The research described in this dissertation is focused on the development of molybdenum catalysts that are supported by PDI and DI chelates and their application in small molecule activation.

Using the phosphine containing PDI chelate, Ph2PPrPDI, several low-valent molybdenum complexes have been synthesized and characterized. While the zerovalent monocarbonyl complex, (Ph2PPrPDI)MoCO, catalyzes the reduction of aldehyde C=O bonds, the C-H activated Mo(II) complex, (6-P,N,N,N,C,P-Ph2PPrPDI)MoH was found to be the first well-defined molybdenum catalyst for reducing carbon dioxide to methanol. Along with low- oxidation state compounds, a Mo(IV) complex, [(Ph2PPrPDI)MoO][PF6]2 was also synthesized and utilized in electrocatalytic hydrogen production from neutral water. Moreover, with the proper choice of reductant, an uncommon Mo(I) oxidation state was stabilized and characterized by electron paramagnetic resonance spectroscopy and single crystal X-ray diffraction.

While the synthesized (PDI)Mo complexes unveiled versatile reduction chemistry, varying the ligand backbone to DI uncovered completely different reactivity when bound to molybdenum. Unlike PDI, no chelate-arm C-H activation was observed with the propyl phosphine DI, Ph2PPrDI; instead, a bis(dinitrogen) Mo(0) complex, (Ph2PPrDI)Mo(N2)2 was isolated. Surprisingly, this complex was found to convert carbon dioxide into dioxygen and carbon monoxide under ambient conditions through a novel tail-to-tail CO2 reductive coupling pathway. Detailed experimental and theoretical studies are underway to gain further information about the possible mechanism of Mo mediated direct conversion of CO2 to O2.

The late first row transition metals, being inexpensive and environmentally benign, have become very attractive for sustainable catalyst development. However, to overcome the detrimental one electron redox processes exhibited by these metals, the employment of redox non-innocent chelates turned out to be very useful. The Trovitch group has designed a series of pentadentate bis(imino)pyridine ligands (pyridine diimine, PDI) that are capable of binding the metal center beyond their 3-N,N,N core and also possess coordination flexibility. My research is focused on developing PDI-supported manganese catalysts for organic transformations and renewable fuel production.

The thesis presents synthesis and characterization of a family of low valent (PDI)Mn complexes. Detailed electronic structure evaluation from spectroscopic and crystallographic data revealed electron transfer from the reduced metal center to the accessible ligand orbitals. One particular (PDI)Mn variant, (5-Ph2PPrPDI)Mn has been found to be the most efficient carbonyl hydrosilylation catalyst reported till date, achieving a maximum turnover frequency of up to 4950 min-1. This observation demanded a thorough investigation of the operative mechanism. A series of controlled stoichiometric reactions, detailed kinetic analysis, and relevant intermediate isolation suggest a mechanism that involves oxidative addition, carbonyl insertion, and reductive elimination. Noticing such remarkable efficiency of the (PDI)Mn system, it has been tested for application in renewable fuel generation. A modest efficiency for H2 production at an apparent pH of 8.4 have been achieved using a cationic Mn complex, [(Ph2PPrPDI)Mn(CO)]Br. Although, a detailed mechanistic investigation remained challenging due to complex instability, a set of relevant Mn(-I) intermediates have been isolated and characterized thoroughly.

The dissertation also includes synthesis, characterization, and electronic structure evaluation of a series of Triphos supported iron complexes. Using this pincer chelate and either 2,2’-bipyridine (bpy) or 1,3,5,7-cyclooctatetraene (COT), a set of electronically interesting complexes have been isolated. Detailed electronic structure investigation using spectroscopy, magnetometry, crystallography, and DFT calculations revealed redox non-innocent behavior in the Bpy and COT ligands. Additionally, CO binding to the (Triphos)Fe system followed by reaction with borohydride reagents allowed for the isolation of some catalytically relevant and reactive iron hydride complexes.

The addition of aminoalkyl-substituted 2,6-bis(imino)pyridine (or pyridine diimine, PDI) ligands to [(COD)RhCl]2 (COD = 1,5-cyclooctadiene) resulted in the formation of rhodium monochloride complexes with the general formula (NPDI)RhCl (NPDI = iPr2NEtPDI or Me2NPrPDI). The investigation of (iPr2NEtPDI)RhCl and (Me2NPrPDI)RhCl by single crystal X-ray diffraction verified the absence of amine arm coordination and a pseudo square planar geometry about rhodium. Replacement of the chloride ligand with an outer-sphere anion was achieved by adding AgBF4 directly to (iPr2NEtPDI)RhCl to form [(iPr2NEtPDI)Rh][BF4]. Alternatively, this complex was prepared upon chelate addition following the salt metathesis reaction between AgBF4 and [(COD)RhCl]2. Using the latter method, both [(NPDI)Rh][BF4] complexes were isolated and found to exhibit κ4-N,N,N,N-PDI coordination regardless of arm length or steric bulk. In contrast, the metallation of PPDI chelates featuring alkylphosphine imine substituents (PPDI = Ph2PEtPDI or Ph2PPrPDI) resulted in the formation of cationic complexes featuring κ5-N,N,N,P,P-PDI coordination in all instances, [(PPDI)Rh][X] (X = Cl, BF4). Adjusting the metallation stoichiometry allowed the preparation of [(Ph2PPrPDI)Rh][(COD)RhCl2], which was characterized by multinuclear NMR spectroscopy and single crystal X-ray diffraction.