<br> <p>The interest in understanding the <i>f</i>-block elements has been increasing because of the large applications of these elements across all fields of science and technology. The lanthanides are used in various technologies like car batteries and phone screens. The actinides are the basis of current nuclear fuel processes. The <i>f</i> -block has many interesting properties and has been proven to be fruitful in inorganic chemistry. Neodymium is redox inactive and was studied with a redox active ligand pyridine diimine to see if multielectron chemistry was viable. The neodymium chemistry is still in the preliminary stages of research, but there is possibility of fruitful reactivity. Recently neptunium chemistry was introduced to the Bart lab to study its rich redox chemistry. Neptunium’s fundamental properties have been investigated for the last 80 years with new bonding properties and behavior still being discovered today. Studies of neptunium began with investigating the trivalent oxidation state. Synthesis of new low valent trans-uranic starting materials is important because the fundamental chemistry of these trivalent compounds is not well studied. By creating Neptunium materials that are analogous to known uranium and lanthanide starting materials, <i>f</i>-block chemists will be able to apply their previously studied syntheses to a new element. </p>

<p>Uranium imido compounds have been traditionally studied due to their analogous nature to uranyl compounds. The investigation of uranium imido bonds can open the door to the activation of uranium oxygen bonds, which is important for the recyclization of spent nuclear fuel. This research encompasses further characterization of imido compounds utilizing electrochemical techniques which will better the understanding of these compounds. Utilizing what we already know about uranium imido compounds, further reactivity studies were conducted. Additionally, uranium is a suitable candidate for modeling synthesis of transuranic elements, specifically neptunium. Therefore, additional pathways to synthesize uranium tris(imido) and uranium tertrakis(imido) complexes were explored as a modeling system.  </p>

<p dir="ltr">This work describes the electronic and geometric structure of molecular metal complexes involving different ligand environments. These include the Cu-redox active ligand reduction series, Tp*₂U imido and anilido transformations, Lewis base activation by Tp*₂U, silyluranium synthesis and reactivity, and electrochemistry of plutonyl.</p>

<p>Metal alkyls have a variety of uses including important intermediates in a variety of  processes. In this research, a thorium bis-alkyl species was fully characterized and explored for its potential reactivity, specifically for the formation of thorium bis-amide complexes. A series of three thorium bis-amide complexes was synthesized and characterized in this work. Additionally, several pathways have been attempted to synthesize an actinide alkylidene within this project including the use of a homoleptic tetrabenzyl complex, the use of diazoalkanes through the loss of dinitrogen, deprotonation of alkyls, and reducing a metallacycle complex. However, many of these did not result in products that suggest that an alkylidene was formed. These reactions ranged from being thermally unstable, decomposing, not reacting, or forming multiple products and being unable to discern one major product.<br> </p>

<p>While crucial advancements have been made in understanding transition metal−nitrogen interactions, the actinides have not been studied in such depth as their transition metal counterparts. Uranium has shown to catalyze the Haber−Bosch process to produce NH₃ but more attention has turned to transition metals such as iron due to their low cost and accessibility. It is thought that transition metal imido species are essential intermediates to this process; therefore, it is critical to understand NH bond cleavage and formation on the metal. To study the potential that uranium has, it is important to bridge the knowledge gap of uranium with its transition metal counterparts and further understand NH bond cleavage and formation on the metal to make the suspected imido intermediate.</p> <p>Redox neutral methods have been popular and effective for synthesizing uranium imido complexes such as starting with a uranium(IV) amide and deprotonating it with a base to yield its respective uranium(IV) imido. It was of interest to understand if the bisTp* uranium(III) system would be amenable to a deprotonation pathway. To test this, the reactivity of Tp*₂UBn with bulky 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)benzenamine (terpy-aniline) and sterically smaller p-toluidine (ptol-aniline) was explored to first synthesize uranium(III) anilido species. Following successful synthesis, their reactivity is explored to yield respective uranium(IV) imido species by oxidative deprotonation.</p> <p>In addition to redox neutral methods, synthetic processes that rely on redox reactions at the uranium center have also been successful but are less common since the starting material must be a stable, low-valent uranium species. Our group has explored this method to make uranium(IV) imido species where the addition of 1 equivalent of organic azide to trivalent Tp*₂UBn or one equivalent of organic azide and potassium graphite to Tp*₂UI results in the formation of uranium(IV) imido species. The downside to this is azides are explosive and their synthesis could inhibit synthesis of diverse complexes. A redox method that eliminated usage of explosive azides is of interest so the reactivity of hydrogen atom transfer (HAT) reagents, Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*), with uranium(III) anilido complexes of varying steric bulk and electronic profile was explored. Conversion to their respective uranium(IV) imido species was achieved and this method was also explored with uranium(III) amides smaller than a phenyl since their respective azide are too dangerous to synthesize.</p> <p>Following isolation of uranium(III) anilido complexes and exploring reactivity it was of interest to understand how they compare to phosphorus analogues and how reactivity and interactions might be similar. Reactivity of Tp*₂UBn with phosphines of various steric bulk and electronic profile allowed for the isolation of uranium(III) phosphido complexes and their reactivity showed to be different than previously explored uranium(III) anilido counterparts. The electronic differences of the pnictogens were also observed in the crystal structures.</p> <p>With the differences in reactivity and electronic effects between the nitrogen and phosphorous complexes having been observed, our curiosity expanded to explore more uranium-pnictogen interactions. Therefore, synthesis of bis-substituted arsine and bis-substituted phosphine ligands were conducted for reactivity with Tp*₂UBn. Preliminary data reveals these bonds are more unstable and reactive relative to uranium(III) anilido species, likely due to the electronic mismatch between oxophilic uranium and soft pnictogens. Where applicable, compounds were characterized by multinuclear NMR spectroscopy, infrared spectroscopy, electronic absorption spectroscopy, single crystal X-ray crystallography, and quantum chemical calculations.</p>

<p>Thorium complexes primarily exist in the thermodynamically stable (IV) oxidation state with only a few low-valent thorium(III) and thorium(II) complexes having been isolated. As a result, redox chemistry with thorium at the metal center is synthetically challenging without carefully selected ligand systems. This redox-restricted nature of thorium(IV) makes redox-active ligands (RALs) an attractive option to facilitate multi-electron redox chemistry with thorium. In this work, first, a series of thorium(IV) complexes featuring the redox-active iminoquinone ligand and its derivatives, including the iminosemiquinone and amidophenolate species, were synthesized and characterized. Rare thorium oxygen multiple bonds were then accessed by exploiting the RALs on the thorium center and using dioxygen in dry air. Other oxidation chemistry was attempted with the thorium amidophenolate complexes as well. Second, armed with the knowledge of synthesizing multiple bonds with thorium(IV) complexes, similar chemistry was explored with cerium as it is in the same group as thorium. A series of cerium(III) and cerium(IV) complexes featuring the redox-active iminoquinone ligand and its derivatives were synthesized. Oxidation chemistry was explored with the cerium amidophenolate complexes and a rare cerium oxo was isolated. Finally, with interest in expanding and addressing a gap in the literature related to the synthesis, characterization, and utility of thorium alkyls, several tetrabenzylthorium complexes were synthesized, characterized, and some reactivity was explored. A highlight of this work involved the isolation of the first crystal structure of ligand and solvent free tetrabenzylthorium since its first synthesis in 1974. Full spectroscopic and structural characterization of the complexes was performed via ¹H NMR spectroscopy, X-ray crystallography, EPR spectroscopy, electronic absorption spectroscopy, and SQUID magnetometry, which all confirmed the identity and electronic structure of these complexes. </p>

<div><div><div><p>The chemistry of the f-block is relatively unknown when compared to the rest of the periodic table. Transition metals and main group elements have enjoyed thorough study and development over the last 200 years, while many of the lanthanides and actinides weren’t even discovered until the 1940’s. This is troublesome, as knowledge of these elements is critical for environmental, industrial and technological advances. Understanding bonding motifs and reactivity pathways is fundamental to advancing the field of f-block chemistry. The use of redox- active ligands has aided in the construction of new bonding modes and discovery of new reaction pathways by providing electrons for these transformations. A particularly successful partnership is formed when redox-active ligands are combined with lanthanides, as these elements are usually considered redox-restricted. A series of lanthanide complexes featuring the iminoquinone ligand in three oxidation states will be discussed. The use of the ligands as a source of electrons for reactivity is also described, with new bonding motifs for lanthanides being realized. The iminoquinone ligand can also serve to break bonds. The uranyl (UO22+) ion is notoriously difficult to handle due to its strong U-O multiple bonds. To overcome this, we developed a series of uranyl complexes and studied the ability of the iminoquinone ligand to serve as an electron source for reduction of uranium, with concomitant U-O bond cleavage.</p></div></div></div>

<p>Uranium imido complexes are highly sought after for their analogous nature to the uranyl moiety. Because of the highly reactive characteristics of uranium imido bonds compared to uranyl oxygen bonds these complexes have been used to investigate chemistry that can be used to activate the uranyl moiety. The activation of the <em>trans</em>-oxo groups of the uranyl moiety would open the door for the recycling of spent nuclear waste, diverting these chemicals from long term storage to a second life beyond nuclear fission. A suitable analog to the uranyl moiety has been discovered with the uranium bis(imido) family of complexes, these complexes can participate in chemistry that is similar if not, exactly the same as uranyl complexes. Studies with the uranium bis(imido) complex have been used to probe uranyl reactivity because the analogous nature of the two moieties. With that a uranium(IV) <em>cis</em>-bis(imido) complex was synthesized demonstrating how electron donation to the metal center can disrupt the Inverse Trans Influence (ITI) can as a result activate the <em>trans</em>-ligands on uranium. This complex is the first reported U(IV) bis(imido) with <em>trans</em> imido groups and achieved this geometry without large steric ligands to facilitate the <em>cis</em>- geometry. Computational analysis of this complex shows the stable nature of the geometry and how the fundamental electronics of this complex are the leading factor in the resultant geometry. When reactivity of the <em>cis</em>-bis(imido) was explored via protonation experiments a unique U(V) complex was isolated.</p> <p>Additional protonation reactivity was explored using UO2(tBubpy)(NTSA)2 with a variety of anilines to synthesize uranyl imido complexes. These experiments showed that the electronic environment—not the steric profile—of the anilines has a much greater effect on the stability of the resulting uranyl imido. The resulting uranyl imido complexes demonstrate the analogous nature of uranyl and uranium imido chemistry.</p> <p>Activation of the <em>trans</em>-imido groups on uranium bis(imido) complexes has also been shown with the synthesis of the uranium tris- and tetrakis(imido) complexes. These later complexes have shown that increased electron donation to the uranium metal center weakens and elongates the imido bonds, exposing these compounds to reactivity previously unavailable to uranium compounds with fewer multiply bound groups. </p>

<p>  </p> <p>Actinide complexes have been targeted for their potential in group transfer applications. The study of these metals, such as thorium and uranium, is essential to better understand the reactions these metals are capable of facilitating.  Hydrotris(pyrazolyl)borates such as hydrotris(3,5-dimethylpyrazolyl)-borate (Tp*) and hydrotris(pyrazolyl)-borate (Tp) are superbulky, scorpionate ligands that have previously been used to synthesize novel uranium complexes and probe the reactivity of these materials. Similar thorium analogs have also been synthesized, but their reactivity has yet to be explored in great depth. Tp*ThCl3(THF) and Tp2ThCl2 have been reproduced and investigated as possible starting materials for such reactivity studies. While the former was found to be largely unreactive, the latter presents promising reactivity for the synthesis of thorium-element multiple bonds, and a novel thorium imido, Tp2Th(NDipp)(THF), has been synthesized and characterized using this scaffold. </p> <p><br></p> <p>The reactivity of a uranium tetrakis(imido), [U(NDipp)4][K2], has also been investigated to probe the prospect of group transfer reactions for potential catalysis applications in the future. An isocyanate, PhNCO, was reacted with this compound; the observed product showed that group transfer was incomplete, and a four-membered metallocycle product is likely formed instead. The synthesis of a novel thorium tris(imido) has also been targeted, and preliminary results are outlined. </p>

The synthesis of metal-redox active ligand complexes is described, along with reactivity studies aimed at facilitating novel C-N bond forming reactions. A copper bis(iminosemiquinone) structure is characterized, analyzed and its reduction series are characterized and the reactivity of the Cu(II) bis(amidophenolate) analog is investigated with tosyl azide. The identification of the major reaction product and its characterization is detailed, with reaction sensitivities and heavily distorted x-ray diffraction single crystal structure generating a complex data set. The characterization of the isolated product is ongoing, with EPR studies aimed at identifying the radical nature of the complex. Unusual solvent effects and solubility issues have been noted with these initial EPR studies and more data is necessary before analysis can be properly attempted. An ytterbium bis(amidophenolate) complex was synthesized and its reactivity studied with aryl azides. Initial reactivities generate the first documented lanthanide tetrazenes in-lieu of the targeted ytterbium imido. Reactivities and characterization of these complexes support a stable, heavily ionic tetrazene-metal complex with no observed redox nature, UV light sensitivities, or imido azide-tetrazene equilibrium observed in various tetrazene transition metal complexes. Synthesis of a sterically blocked ytterbium imido was attempted, utilizing DMAP. Initial isolation was achieved with characterization and reactivity studies supporting the imido nature of the complex. The weak coordinating of the DMAP provided instability that proved in opposition to crystallization, however, so the imido could not be confirmed. Initial reactions using alternative steric hinderance from triphenylphosphine oxide and pyridine N-oxide prove promising to increasing the stability of the presumed ytterbium imido. Organic synthesis was performed generating a potential antibacterial agent. The synthesis of cyclopropenes was initiated as antagonists for ETR proteins in fruits and plants. The intermediates proved highly sensitive to harsh chemical conditions, which was overcome utilizing a tin-mediated Barbier allylation. The cyclopropene alcohol synthon was synthesized, though protecting group optimization is necessary.