The valorization of chemical feedstocks into higher-value commodity chemicals through bond activation and subsequent functionalization remains a pillar in the chemical sciences sector. Despite their low abundance, prohibitive cost, and toxicity, precious metals are often utilized in these transformations due to their high catalytic activity. Therefore, sustainable alternatives capable of supplanting them are of academic, environmental, and industrial importance. Redox-noninnocent metal-ligand frameworks comprised of inexpensive and abundant components have shown promise in this regard. Specifically, the cooperative interaction between the redox-active ligand and the Lewis-acidic metal functionality imbues these systems with “nobility,” enabling multi-electron bond activation and functionalization chemistry reminiscent of the 4d/5d metals. Typically, the base metal mediates the relevant bond making/breaking while the appended main-group support facilitates electron transfer during this process. The inherent electronic communication between the two sites is responsible for this reactivity profile. Therefore, we wondered if complementary modes of bond activation and functionalization chemistry are possible by shifting the reactivity locus onto the main-group site while encouraging the appended metal to shuttle the electrons. We are interested in generalizing this phenomenon by probing the electronic structure of tethered p- and d-block combinations. Herein, we present a library of compounds featuring main-group reaction nodes (ex. B, Al) tethered to encapsulated redox-active metals (ex. V, Fe) through linker atoms (ex. N) to investigate their electronic structure and examine their reactivity towards small molecules, which ultimately illuminates the effect of block pairing on the resulting electronic structure. In the first case to be discussed, we detail how tethering a redox-active vanadium imido onto boron results in redox-switchable borane vs. boron radical reactivity. We address the implications of the resulting hybrid electronic structure on the activation of small molecules bearing Group 14–16 element-hydrogen bonds using a suite of spectroscopic, electrochemical, crystallographic, and computational techniques. Building on these results, we adjust the vanadium to main-group stoichiometry, leading to bi- or tri-metallic molecular arrays with expanded electron storage capabilities to target multi-electron transformations of small molecules. Lastly, we explore how tuning the redox-active tether and main-group Lewis acidic or basic partner leads to markedly different bond activation chemistry. These results demonstrate that the prudent choice of main-group/metal pairing leads to interesting electronic structures with applications toward bond activation and functionalization chemistry.

The transition from fossil fuels towards renewable energy and energy storage is vital for reducing greenhouse gas emissions. Uranium, lithium, and cobalt are critical for nuclear power and battery storage. The demand for these minerals is increasing rapidly because of the shift towards clean energy. However, they are typically sourced from terrestrial mining, an environmentally taxing process. The mining industry accounts for 8% of global greenhouse gas emissions. Accordingly, it is crucial that we develop new strategies for sourcing critical minerals used in clean energy technology. Advancements in selective electrochemical metal capture chemistry could expand critical mineral extraction technologies to unconventional sources – spent nuclear fuel, seawater, end-of-life batteries.

In this thesis, we present new derivatized o-carborane (C2B10H12) clusters functionalized with donor (phosphine oxide, crown-ether, bipyridyl) appendages to target selective, electrochemical metal capture and release. Derivatized o-carboranes are redox switchable between the neutral (closo) and dianionic (nido) states. Herein, we observe metal binding in nido form and metal release in closo form. In Chapter 2, we discuss how tuning phosphine oxide carborane basicity may alter the donicity by installing electron withdrawing or electron donating groups on the carborane cage or aryl substituents. We further investigate these effects on uranyl (UO22+) ligation and biphasic capture/release chemistry and discover that hydrophobicity may be more important than tuning electronics. In Chapter 3, we extend carborane chemistry to crown-ether substituted derivatives for size-selective capture. We report interesting experimental and simulated voltammetry of the redox switch with alkali metals (Li+, Na+, K+) and utilize the observed phenomena to derive binding constants. In Chapter 4, we target sterically encumbered carboranes with bipyridyl appendages for transition metal (Mn2+, Fe2+, Co2+, Ni2+) binding. We observe interesting electrochemistry which suggests that increasingly Lewis acidic metals may bind more strongly. We begin to probe the metal coordination environments and reveal that square pyramidal geometries may be preferred. In Chapter 5, we explore additional donor groups (carboxylic acids, amides, sulfides, sulfoxides) adhered to o-carborane and discover that the ligating groups have a profound impact on carborane formal reduction potentials.

Phthalocyanines are a well-studied class of organic macrocycles with interesting photochemical and electrochemical properties that have applications in chemical sensing, energy storage, and solar energy conversion. We aim to correlate the identity of the metal center with the electronic properties and redox behavior of phthalocyanine complexes. We first describe the synthesis and characterization of a series of soluble first-row transition metal phthalocyanines complexes. This work focused on elucidating how the identity of the metal center shapes the electrochemical behavior of these complexes. Particular attention was paid to the role of axial ligation and its effects the phthalocyanines complexes’ electrochemistry. It was found that axial ligation of the early transitions metal (V, Cr, Mn, and Fe) either through synthetic modification or solvent choice led to an increase in reversibility. While the late transition metals (Co, Ni, and Cu) demonstrated behavior consistent with adsorption onto the electrode surface which we attribute to their planarity. We proceeded to investigate in detail the interaction of a cobalt phthalocyanine complex with electrode surfaces using electrochemical techniques with the intention of forming electrochemically-modified surfaces. We found that the adsorptive behavior of our cobalt complex could be controlled through axial interaction with O2 and full surface modification was possible through repetitive cyclic voltammetry scanning.We then aimed to correlate the solution-state electrochemical behavior of our metal phthalocyanine complex with their ability to function as charge carriers in a redox flow battery. A redox flow battery architecture with a conductive carbon additive was then used to compare the utility of our soluble metal phthalocyanine complexes with commercially available phthalocyanines complexes that demonstrate low solubility in most common solvents. It was found that the use of a conductive carbon additive allowed for the relatively insoluble phthalocyanine complexes to behave as effective charge carriers and we demonstrated that commercially available PcM complexes (PcVO, PcFe, and Pc Ni) could be used in this architecture with good efficiency metrics. Finally, the optical and electronic properties of a manganese nitride phthalocyanine complex in both solution and thin films were investigated to determine its utility in organic electronics. It was found that while the complex displayed favorably red-shifted absorbance and energy levels however the excited-state lifetimes were short.  

With the growing threat of climate change, it is important that we rapidly transition from fossil fuels to renewable energy use. Renewable energy technologies have developed considerably making them cost competitive with fossil fuels; however, the key issue inhibiting our transition is storage. The United States has a 261 GW generation capacity from renewables, but only an energy storage capacity of 24.2 GW. Renewables, such as solar and wind, are intermittent, therefore, we cannot fully rely on renewables until we develop new technologies for storage.Ammonia (NH3) is a promising storage medium for excess electricity generated from renewables. Instead of curtailing renewable energy plants during peak hours to prevent grid overload, the excess electricity can be used to electrolyze water to produce H2 which can then be used to in the Haber Bosch process to produce NH3. NH3 meets the standards set by the United States Department of Energy for hydrogen storage and can be utilized in a direct ammonia fuel cell (DAFC) to generate electricity. However, the oxidation of NH3 to N2, nitrogen evolution reaction (NER), is kinetically challenging and incurs large overpotentials under ambient conditions. Developing robust catalysts to mediate the NER are therefore needed. Herein, we report a series of high-valent tetranuclear nickel clusters isolated from the chemical oxidation of an all Ni(II) ([Ni4]) neutral cluster. Electrochemical analysis of [Ni4] reveals three reversible sequential oxidations at 0.248 V (1e-), 0.678 V (1e-), and 0.991 V (2e-) vs. Fc+/Fc corresponding to mono-, di-, and tetra-oxidized species, [Ni4]+,[Ni4]2+, [Ni4]4+, respectively. Using spectroscopic, crystallographic, magnetometric, and computational techniques, we assign the primary loci of oxidations to the Ni centers in each case, thus resulting in the isolation of the first tetranuclear all-Ni(III) cluster,[Ni4]4+. The [Ni4]4+ contains the most high-valent nickel atoms to be reported in a molecule to be crystallographically characterized. Preliminary studies indicate the [Ni4] and its derivatives are promising candidates for investigating electrocatalytic NER and is described in this thesis. Additionally, we describe a transition from studying NER molecular catalysts in organic solvents to water. For smaller devices, aqueous DAFC show promise for industrial viability with volumetric energy densities comparable to or larger than compressed H2 and ~2000x higher than in organic solvents. Ruthenium Bipyridinedicarboxylate (RuBda) complexes are renowned for catalyzing water oxidation at rapid rates (~300 s-1) and were recently reported to catalyze NER in acetonitrile at sluggish rates (~0.5 s-1). We report a RuBda derivative capable of catalyzing the NER electrochemically in water achieving high faradaic efficiencies (>84%) and several turnovers. Under aqueous conditions, the complex exhibits the highest TOF (~4130 s-1) to be reported for a NER electrocatalyst and displays impressive stability in realistic commercial concentrations of NH3 (14,300 equivalents excess) without any indication of decomposition. Our kinetic analyses suggest that the catalyst operates via a unimolecular mechanism which is highly applicable for commercially viable fuel cells.

Global energy consumption continues to increase annually, leading to dangerous levels of atmospheric carbon dioxide. To avoid irreparable harm to our planet, we need to rapidly convert our grid from fossil fuel dependence to low carbon sources of energy such as nuclear, wind and solar power. These technologies, however, need further development before the full conversion of our grid can occur. In this dissertation, we present advancements in spent nuclear fuel recycling and redox flow batteries for energy storage.In the first half, we studied the use of electrochemically switchable carboranes (polyhedral boron-carbon cluster compounds) for selective actinide capture from simulated spent nuclear fuel. Initially utilizing a biphasic approach, an organic phase containing an electrochemically generated ortho-substituted nido-carborane anion, (POCb2-) was found to selectively extract uranyl (UO22+) from a mixed-metal alkali (Cs+), lanthanide (Nd3+, Sm3+), and actinide (Th4+, UO22+) aqueous solutions. Bulk electrolysis of the organic phase was used to re-generate the closo-carborane (POCb) and release the captured UO22+, which was later back-extracted into a fresh aqueous phase. In a follow up study, we heterogenized our molecular, electrochemically switchable carboranes onto carbon electrodes. These functionalized electrodes demonstrated carborane-based heterogeneous electrochemical behaviour enhanced by the inclusion of single-walled carbon nanotubes (CNTs). Charged functionalized electrodes selectively captured and released actinides (Th4+, UO22+) from a mixed solutions containing alkali (Cs+), lanthanide (Nd3+, Sm3+) and actinide (Th4+, UO22+) metal ions. In the second half of my thesis, we demonstrated the use of commercially available phthalocyanine metal complexes (PcM) as cheap charge carriers in RFBs for green energy storage. Our systems utilized slurries of conductive carbon Ketjenblack (KB) to create electronically conducting percolation networks that interfaced with undissolved PcM. We found no significant performance differences between the commercial unsubstituted PcM and 1,4,8,11,15,18,22,25-octaethoxyphthalocyanine metal complexes (EtOPcM) synthesized with peripheral solubilizing substituents despite their increased solubility. Electrochemical analyses of several commercial first-row variants (PcTiCl2, PcVO, PcMnCl, PcFe, PcCo, PcNi, and PcCu) as well as one heavy-metal analogue (PcPb) revealed most to be effective charge carriers for slurry-based PcM/KB RFB applications. Cells using PcCu proved to have the highest efficiencies, energy density (1.23 Wh/L), and stability (>99% capacity retention) of the series while also having the lowest cost by mass.  

Molecules with reversible, redox-switchable properties have potential application in many fields, including catalysis, electronics, waste remediation, and energy storage. To further this field, a detailed understanding of the relationship between chemical structure and electronic properties must be developed for different systems. Here, we first examine this relationship for phthalocyanines, a well-studied class of macrocyclic ligand. We first discuss generalized synthesis and characterization of a library of soluble, metal phthalocyanine species, and then proceed to investigate the electronic character of this macrocyclic ligand in extreme detail with modern experimental and computational methods, and discuss a fundamental discovery regarding a terminal manganese nitride phthalocyanine (EtOPcMnN) that is the first reported molecular species capable of reversibly switching between aromatic, non-aromatic, and anti-aromatic states. We then discuss possible reasons for the extreme stability of EtOPcMnN, and discuss more generalized conclusions pertaining to the electronic character of other phthalocyanines and related aromatic macrocycles.

Further studies leverage the unusual stability of EtOPcMnN for energy storage applications, revealing a potential use as a symmetric redox-flow battery charge carrier. Electrochemical kinetics and battery measurements are performed, revealing that EtOPcMnN can function as a 2-electron symmetric charge carrier capable of functioning as both anolyte and catholyte in a redox flow battery. A new architecture for redox-flow batteries is also developed, in which a redox-active “sediment” is utilized in lieu of a traditional solution-state charge carrier, and represents a possible strategy for increasing charge carrier loading beyond the solubility limit while preventing the high viscosity typically associated with slurry-based systems.

Finally, the redox-switchable properties of another class of ligand, carboranes, are examined for application as reversible chelating agents. It is demonstrated with a carborane phosphine species that redox events can be accessed to induce a C-C bond scission which results in a structural distortion of the carborane that can be utilized as a “molecular switch” capable of changing the bond angle of the phosphine ligands, resulting in a convenient electrochemical switch for the capture and release of metal ions. We demonstrate a proof-of-concept system in which uranyl can be electrochemically captured and released from biphasic systems in a reversible manner, and discuss the application of such systems for new nuclear waste remediation strategies.

For the first part of this defense, we wanted to study new abundant-metal homogeneous ammonia oxidation catalysts in order to gain important mechanistic understandings of these molecular-level transformations. Our catalyst of choice was an Fe-phthalocyanine complex, PcFe (Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), which we synthesized and with which we subsequently generated PcFe−amine complexes with ammonia and related alkylamines. Treatment of PcFe with an excess hydrogen atom abstractor, 2,4,6-tri-tert-butylphenoxyl radical (tBuArO•), in the presence of NH3 resulted in catalytic H atom abstraction (HAA) and C−N coupling to produce 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one and tBuArOH. Exposing FePc to an excess of the trityl (CPh3) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (TrArO•), and NH3 did not lead to catalytic ammonia oxidation as similarly reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N−H and β C−H in the PcFe-alkylamine congeners, led to multiple HAA events in the presence of tBuArO•, yielding a cyanide or an imine complex depending on which alkylamine was used. Detailed computational studies were also performed to elucidate the mechanisms involved in these transformations.

For the second half of this defense, we explored the selective, electrochemical capture and release of Li+ from seawater using redox-switchable substituted ortho-carboranes (Cb). Reduction of the neutral “closo-Cb” to the 2 e- reduced “nido-Cb” resulted in C–C bond cleavage and subsequent cage opening, increasing the bite angle of the Cb-appended donor groups. This structural change can be reversed by oxidation of nido-Cb back to closo-Cb. To capture Li+ ions, we synthesized and fully characterized a series of Cb-substituted crown ether compounds, which are known to have high affinity and selectivity to Li+. Li+ binding affinity and selectivity was monitored via 1H, 7Li, 23Na NMR spectroscopy, as well as with cyclic voltammetry. These measurements point to significant observed selectivity for Li+ vs. other alkali metals in the nido-Cb form as described in this thesis. Furthermore, we also extended our studies to group 2 metals, specifically cesium and strontium capture/release, for nuclear waste treatment purposes using larger Cb-crown species.

For the first part of this work, we explored the potential of repetitively cycling the orthosubstituted carborane, 1,2-(Ph2PO)2-1,2-C2B10H10 (POCb), by galvanostatic bulk electrolysis (GBE) to capture and release the uranyl cation, UO2 2+ . The metal capturing process worked successfully for the first cycle of GBE and captured UO2 2+ successfully in the form of [UO2( POCb)2] . However, the second cycle of the GBE using the same POCb, does not show a significant amount of reduced POCb. The results show different side-products form after the second reduction step indicating that the second charge cycle to generate nidoPOCb, the reduced form of POCb was unsuccessful. By treating with different methods of extraction during the cycle of GBE, we were able to gain some understanding about what is hindering the second reduction. From series of experiments, we determined that a water adduct forms with closoPOCb and this hinders the ability of closoPOCb to be reduced properly, and therefore we could not capture much uranyl in the second cycle. These results are presented herein. The second part of the thesis investigates the capture of different lanthanide metals using nidoPOCb2- . Surprisingly, each lanthanide with nidoPOCb shows different coordination even though we added the same equivalents, 2.1:1. X-ray diffraction studies confirmed the different coordination environments. While the nature of this coordination remains under investigation, v we suspect that the high reduction potential of the lanthanide metal is likely at play. Further experiments were conducted to back up the idea.

Alkanes, or saturated hydrocarbons, have long been zealously exploited for their energy content through combustion processes; however, practical processes for converting them directly to higher value commodity chemicals remain limited. This stems from the inherent inertness of alkanes and the subsequent difficulty in activating their strong and localized C–C and C–H bonds. The introduction of functionality into unactivated C–H bonds incites many practical advantages – from offering new methodologies for fine chemical synthesis, to far-reaching implications of replacing our current alkane petrochemical feedstocks. For the past several decades, there has been tremendous work in utilizing transition-metal complexes to activate inert C–H ¬bonds and install functionality under mild conditions. While profitable practical applications remain a long-term goal, our mechanistic understanding of these transition-metal mediated transformations has advanced tremendously, and has provided a framework to recognize new strategies for useful C–H bond activation.

For example, recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst – used for the partial oxidation of butane to maleic anhydride – may play a critical role in initiating butane C–H bond activation through a mechanism termed reduction-coupled oxo activation (ROA), similar to proton-coupled electron transfer (PCET); however, there has been a general lack of experimental evidence to support this mechanism. Herein, we present a wide library of molecular model compounds to examine the validity of the proposed ROA mechanism, which incites C–H bond activation through a main-group/transition metal cooperative mechanism. We report the synthesis, characterization, and reactivity of a series of mono/multi-metallic vanadium phosphate complexes, including the first experimental evidence supporting the proposed ROA mechanism using 1,4-(bistrimethylsilyl)-pyrazine as a “bulky hydrogen” surrogate. Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential step-wise intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values are described. Additionally, ongoing work has shown that careful electronic tuning of these vanadium phosphate complexes can enable enhanced reactivity towards weak C–H bonds.

The synthesis of two substituted phthalocyanine ligands for synthesis of metal phthalocyanine complexes is reported. Both phthalocyanine ligands contain ethereal substitutions of differing size. One substitution is an ethoxy phthalocyanine species and the other is an isopropoxy phthalocyanine.

The synthesis and characterization of two vanadyl (IV) phthalocyanine species is reported. EPR data, mass spectrometry data and x-ray diffraction are included to characterize the vanadyl ethoxyphthalocyanine. The vanadyl isopropoxyphthalocyanine is characterized by mass spectrometry.

The synthesis and characterization of a chromium (II) phthalocyanine is reported. The species is characterized by NMR, mass spectrometry and x-ray diffraction. This species and its reactions with O atom donors are reported and the results are examined.

The synthesis and characterization of a manganese (III) chloride and a “naked” manganese (III) ethoxyphthalocyanine are reported. Mass spectrometry and NMR are used to characterize these two species. The reactions of these species with O atom donors are reported and some preliminary data is examined to explore the potential formation of a manganese oxo phthalocyanine species.