Chapter 1. The field of f-block–transition metal hydride chemistry is introduced and summarized. Key properties of these compounds such as small molecule activation chemistry and H2 uptake and release are outlined. The dearth of actinide–transition metal species despite their potential for fundamental bonding insight and novel reactivity is highlighted, and the motivations for studying these compounds are stated.

Chapter 2. Reaction of K[Cp*IrH3] with actinide halides led to multimetallic actinide–transition metal hydrides U{(μ-H)3IrCp*}4 and Th{[(μ-H)2(H)IrCp*]2[(μ-H)3IrCp*]2}, respectively. These complexes feature an unexpected, significant discrepancy in hydride bonding modes; the uranium species contains twelve bridging hydrides while the thorium complex contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(III) starting material with the same potassium iridate resulted in the octanuclear complex {U[(μ2-H)3IrCp*]2[(μ3-H)2IrCp*]}2. Computational studies indicate significant bonding character between U/Th and Ir in the tetrairidate compounds, the first reported evidence of actinide-iridium covalency. In addition, these studies attribute the variation in hydride bonding between the tetrairidate complexes to differences in dispersion effects. This work establishes a novel route to synthesizing actinide–transition metal polyhydrides with close metal–metal contacts.

Chapter 3. Conversion of Cp*OsH5 to K[Cp*OsH4] with KBn, followed by reaction with tetravalent actinide halides results in the synthesis of uranium– and thorium–osmium heterometallic polyhydride complexes. Through these species, An–Os bonding and the reactivity of An–Os interactions are studied. These complexes are formally sixteen-coordinate, the highest observed coordination number for uranium and thorium. Computational studies suggest the presence of a significant bonding interaction between the actinide center and the four coordinated osmium centers, the first report of this behavior between osmium and an actinide. Upon photolysis, these complexes underwent intramolecular C–H activation with the formation of an Os–Os bond, while the thorium complex was able to activate an additional C–H bond of the benzene solvent, resulting in a μ-η1,η1 phenyl ligand across one Th–Os interaction. These results highlight the unique reactivity that can arise from actinide and transition metal centers in proximity, and expand the scope of actinide photolysis reactivity.

Chapter 4. The third Cp*-supported transition metal polyhydride – Cp*ReH6 – was shown to be a competent partner to actinide hydrides. The synthesis of actinide tetrarhenate complexes completed a series of iridate, osmate, and rhenate polyhydrides, allowing for structural and bonding comparisons. Computational studies examine the bonding interactions, particularly between metals, in these complexes. Several factors affect metal–metal distances and covalency for the actinide tetrametallates, including metal oxidation state, coordination number, and dispersion effects. The osmium and rhenium octametallic U2M6 clusters are reported as well, with similar analysis of structure and electronics.

Chapter 5. Reaction of the potassium iridate K[Cp*IrH3] with a bulky uranium(III) metallocene yielded a heterobimetallic U(III)–Ir species. Reactivity of this complex with CS2 is described, resulting in the novel ethanetetrathiolate fragment, as produced via hydride insertion and C–C coupling. This demonstrates the ability to combine the hydride insertion chemistry of transition metal hydrides with C–C coupling observed in U(III) compounds by bringing both metal centers in close proximity.

In this dissertation is presented the synthesis and reactivity of Nb(III) imido complexes supported by the BDI ligand (BDI = &beta-diketimate).

Chapter I. The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to Z-alkenes is reported and found to proceed through an unprecedented mechanism. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(NtBu)(CO)₂ to provide a Nb(V) metallacyclopropene, itself capable of &sigma-bond metathesis reactivity with H₂. The resulting alkenyl hydride species then undergoes reductive elimination to provide the Z-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from i) the dependence of product selectivity on the relative concentrations of CO and H₂, ii) the isolation of complexes closely related to those proposed to lie on the catalytic cycle, iii) H/D crossover experiments, and iv) DFT studies on multiple possible reaction pathways.

Chapter II. Monometallic niobium arene complexes [Nb(BDI)(NtBu)(&mu-RC6H5)] (R = H and Me) were synthesized and were found to slowly converted into the diniobium inverted arene sandwich complexes [[(BDI)Nb(NtBu)]₂(&mu-RC6H5)] (R = H and Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy, and is in agreement with a dissociative mechanism. These compounds showed a lack of reactivity towards small molecules - even at elevated temperatures - which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt₂)][B(C6F5)₄], resulting in the mono-protonated cationic inverted sandwich complex [[(BDI#)Nb(NtBu)][(BDI)Nb(NtBu)](&mu-C6H5)][B(C6F5)₄] and the dicationic complex [[(BDI#)Nb(NtBu)]₂(&mu-RC6H5)][B(C6F5)₄]₂ (BDI# = (ArNC(Me))₂CH₂). NMR and UV-vis spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-visible spectra, arene ¹H NMR chemical shifts and large JCH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, the lack of sp³ hybridization of the arene carbon indicate that the Nb-arene donation is not accompanied by an increase in formal oxidation state, and suggest that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes.

Chapter III. Inverted sandwich complexes have seen interesting recent developments both in the nature of their bonding and in their use as chemical reactants. Although discussions concerning the electronic delocalization lend credit to their use as potential electronic and spintronic devices, mixed valent inverted sandwich complexes are rarely reported. We show in this work that the selective single electron oxidation of a neutral benzene inverted sandwich complex of niobium leads to an isolable cationic mixed valent benzene inverted sandwich complex. The latter complex shows unique structural features elucidated through studies with an arsenal of physical methods, including cyclic voltammetry, ¹H NMR, UV-Vis, magnetism, EPR spectroscopies, in addition to DFT calculations. These analyses indicate that although delocalization is allowed over the benzene ring from both niobium atoms, the single electron is unequally shared between the two metal centres. Under certain conditions, this complex reforms the neutral benzene complex along with a highly reactive Nb(IV) species, which is of great interest for potential chemical reactivity.

Chapter IV. All three C-F bonds in CF₃-substituted arenes are activated by a niobium imido complex, driven by the formation of strong Nb-F bonds. The mechanism of this transformation was studied by NMR spectroscopy which revealed the involvement of Nb(III). Attempts to extend this chemistry to non-aromatic CF₃ groups led to intramolecular reactivity.The mechanism of activation of C-F bonds in fluoroarenes using a well-defined niobium (III) imido complex has been investigated.

Chapter V. The Nb(III) arene species [BDI]Nb(NtBu)(C6H6), reacts stoichiometrically with fluoroarenes to yield niobium (V) aryl fluorides. Spectroscopic analysis supported by DFT calculations revealed the critical involvement of a Nb(III) fluoroarene-bound species. In contrast to previous reports of related reactivity, we found that perfluorinated arenes (i.e. those normally assumed to bear more `activated' C-F bonds) are, in the present system, much less reactive towards C-F bond cleavage than mono- or difluoro-substituted arenes. In addition to demonstrating stoichiometric hydrodefluorination reactions, we also describe an efficient and mild hydrodefluorination of mono- and di-fluoroarenes that is catalytic in niobium.

The ability to successfully sequester and separate trivalent lanthanides and actinides has applications to the nuclear fuel cycle, processing of irradiated target materials, and nuclear forensics. The challenge in separating the trivalent lanthanides and actinides is their similar size and charge. As a result of this, standard size exclusion or ion exchange techniques become much more challenging. Instead of focusing on size or charge for separation, their differences in interactions with complexing ligands can be utilized. Another challenge in trivalent lanthanide and actinide separations is that these metals are often dissolved in highly acidic matrices. Thus, the stability of the extractant ligand used to complex the metal is at risk of degradation.

The most common sequestration and separation routes based on complex formation for lanthanides and actinides use liquid-liquid separations, which can generate large volumes of hazardous organic waste. By moving to solid-liquid separations, in which the organic phase is replaced by a solid-phase extractant (SPE), the production of large volumes of waste can be minimized. The most common method of making SPEs for lanthanide and actinide separations is coating an organic extractant ligand on either a silica or polymer support. As a result of only coating the ligand rather than bonding it to the solid support, these materials can suffer from ligand degradation and detachment. Chapter 5 focuses on trivalent lanthanide and actinide interactions with a commercially available SPE. While those materials were found to have use for certain applications, their limitations motivated the organically-modified mesoporous silica (OMMS) development presented in Chapter 7.

The work presented here is a study on solid-phase extractants for lanthanide and actinide separations, particularly focused on OMMS. The OMMS materials are made by covalently binding the extractant ligand to the mesoporous silica, to increase the stability of these SPEs in the presence of acidic media. Chapters 6 and 7 discuss the synthesis and characterization of two types of OMMS materials as well as macroscopic and molecular level studies of these materials with trivalent lanthanides and actinides. Additionally, the stability of the OMMS materials in the presence of acid was probed using nuclear magnetic resonance (NMR) spectroscopy.

The first OMMS material discussed is a caprolactam (CA)-modified mesoporous silica. The behavior of Eu(III), Sc(III), and Al(III) was studied with the CA-modified mesoporous silica. Al and Sc were studied as comparative trivalent metals that had smaller ionic radii than Eu. The results of batch sorption experiments and mainly solid-state NMR spectroscopy studies are presented. The acid stability studies showed that the CA-modified mesoporous silica degraded in two ways in the presence of acid: 1) the 7-membered ring of the ligand opened, and 2) a portion of the isolated ligands on the surface were cleaved at the silane anchor. It was determined that while the CA materials were not effective sorbents for Eu(III) from acidic matrices, they did bind Al(III) and Sc(III). Through the NMR studies, both Al and Sc were found to complex to the CA ligand and the surface silanols simultaneously. This work was the first time solid-state NMR had been used to study the surface, ligand, and metals directly of OMMS materials via their NMR active nuclei (29Si, 13C, 1H, 27Al, and 45Sc).

The second OMMS material studied was a diglycolamide (DGA)-modified mesoporous silica. The main focus of this work was the interactions of the DGA-modified mesoporous silica with Eu(III) and Am(III), but also touched on other trivalent species such as La(III), Lu(III), and Y(III). Again, the stability of this material in the presence of acid was investigated using NMR spectroscopy and while the DGA ligand was found to withstand acid treatment, the isolated ligand cleavage at the silane anchor still occurred. The DGA-modified mesoporous silica was found to efficiently bind the trivalent species at high acid concentrations, particularly below pH 1. As a result of the mesoporous substrate and subsequently high ligand loading for the associated mass, the sorption capacity for Eu(III) (379 umol/g) was found to be higher than other diglycolamide SPEs. Through infrared spectroscopy and NMR spectroscopy experiments, the trivalent metals were found to bind to the carbonyl and ether oxygens of the DGA ligand. Fluorescence spectroscopy of Eu(III) and X-ray absorption spectroscopy studies elucidated that the trivalent metals are complexing as three ligands per metal center. Proof-of-concept column experiments demonstrate the use of DGA-modified mesoporous silica as a stationary phase to which Eu was both sorbed and then eluted.

A major focus of this work was the interplay of macroscopic and molecular level studies to best characterize metal interactions with OMMS materials. While both the bulk and molecular level investigations can provide information about these systems, neither gives the full picture in itself. In designing materials for separations, knowledge of the complete picture is necessary in order to best optimize methodologies and predict separation behavior.

Chapter 1. Reaction of the free base corrole (Mes2(p-OMePh)corrole)H3 with tantalum trialkyl precursors TaMe3Cl2 and TaBn3NtBu resulted in the formation of the tantalum dichloride and tantalum imido corrole complexes via alkane elimination. The X-ray crystal structures of these two compounds have been determined and the structural parameters are discussed. The Ta center of the dichloride species was found to sit out of the plane of the corrole ring by 0.903 Å and is cis-ligated, similarly to what has been reported for group 4 porphyrin complexes. The imido complex reacted with triphenylmethanol and 4-methylbenzyl alcohol, resulting in different extents of protonation of the imido group.

Chapter 2. Incorporation of a Cp ligand into the Nb-BDI-imido system is shown to result in unusual κ1-coordination of the BDI ligand in the complex (κ1-N,BDI)Nb(NtBu)CpCl. Via chloride abstraction from this species, we develop synthetic routes to structurally analogous cationic and NbIV pseudo-four coordinate complexes where the BDI ligand returns to the κ2-coordination mode. We compare these to our previously reported tetrahedral niobium bis(imido) complexes to investigate the effects of the Cp ligand on this system. Substitution of the chloride with a hydride also causes return to bidentate binding of the BDI ligand. The X-ray crystal structures of these complexes have been determined, and the structural parameters reflecting the consequences of the electronic differences are discussed.

Chapter 3. Syntheses of high-valent dichloride and dimethyl niobium imido complexes bearing a bis(NHC)borate supporting ligand are described. Reaction of a dimethyl species with excess CO generates an equivalent of acetone, which inserts into a B-H bond of the bis(NHC)borate ligand to form a boryl-isopropoxide Nb(III) dicarbonyl complex. This mode of hydroboration reactivity also occurs readily upon treatment of the dichloride and dimethyl compounds with ketones, aldehydes, and isocyanates. Modification of the bis(carbene) ligand via hydroboration of benzophenone produces a dimethylniobium complex that undergoes intramolecular η6-arene coordination upon hydrogenation.

Chapter 4. A niobacyclopropane complex is shown to undergo insertion of ethylene and linear alkynes at the Nb-C bond of the metallacycle. This species also undergoes 1,2-addition reactions of tert-butylacetylene and H2 at the Nb-C bond, generating complexes bearing a secondary amide. The addition of dihydrogen generates a niobium-hydride, which upon heating inserts 2-butyne, and subsequently releases cis-2-butene. This reaction also generates a cyclometallated species, indicating that the intermediate generated upon alkyne insertion undergoes an intramolecular σ-bond metathesis. In the presence of H2, the hydride complex catalyzes the hydrogenation of 2-butyne generating both cis-2-butene and 1-butene. The results of a mechanistic study are presented, elucidating the role of intermediates involved in alkene isomerization.

Chapter 5. Carbodiimide cleavage by a Nb(III) synthon yields a series of bis(imido) isocyanide complexes. These species are shown to undergo [2+2] cycloaddition reactions with internal alkynes at one of the Nb-Nimido π-bonds. The initial metallacyclebutene intermediate undergoes insertion of the isocyanide ligand into the Nb-C bond, generating products containing an unsaturated iminoamine fragment. The bis(imido) complexes also undergo 1,2-additions with H2 and silanes. The outcome of these reactions differs for two bis(imido) species bearing different isocyanide ligands. An alkyl-isocyanide containing complex experiences nucleophilic attack of the carbene upon a formimidoyl intermediate generated upon reaction with either H2 or a silane. While the reaction of the aryl-isocyanide containing species with silane also likely generates a formimidoyl intermediate, this fragment couples to the silicon atom to generate a metallacyclopentane complex.

Chapter 6. The synthesis of a series of titanium imido-amido complexes supported by the bis(NHC)borate ligand is reported. Deprotonation of the compounds bearing an aryl(amide) generates anionic bis(imido) complexes charge-balanced by a potassium counterion. These complexes contain significantly elongated metal-nitrogen bonds, likely due to the π-loading effect of incorporating multiple imido groups on the same titanium center. Sequestration of the potassium atom leads to the isolation of a bis(imido) complex that undergoes cyclometallation in solution. This indicates that upon chelation of the cation, the nucleophilicity of the imido nitrogens increases.

Chapter 1. Previous work with group 5 imido β-diketiminates and rhenium terminal oxo complexes is summarized, which in combination provide the theoretical basis for the subsequent chapters. The overarching hypothesis and goals of this work are stated.

Chapter 2. Synthetic access to the first rhenium oxo β-diketiminate is achieved. Subsequently, the oxygen atom transfer (OAT) reactivity of this complex is gauged. It is determined that OAT from this complex is limited to trialkylphosphines of a moderate steric profile (e.g. PEt3), as small trialkylphosphines (e.g. PMe3) bind strongly to the metal center, and larger phosphines (e.g. PPh3) are kinetically limited due to the encumbering β-diketiminate ligand. Nitrene transfer from the β-diketiminate ligand to the metal is also observed, implying an ancillary limitation of this ligand in low valent rhenium complexes at high temperatures.

Chapter 3. Attempts to incorporate a cyclopentadienyl ligand into the rhenium oxo β-diketiminate system using NaCp led to the isolation of an olefin-supported rhenium(III) terminal oxo complex. It was found that using SnCp2 rather than NaCp leads to the intended rhenium(V) oxo Cp complex. The reactivity of this rhenium(V) complex suggested that both the Cp ligand and oxo ligand are susceptible to nucleophilic attack, yielding a variety of rhenium(III) complexes. Computational studies suggest some degree of metal-ligand conjugation (“π-loading”) exists between the Cp and oxo ligands in the starting rhenium(V) complex, which supports the observations made during reactivity studies.

Chapter 4. The oxygen atom transfer reactivity of a rhenium(III) terminal oxo β-diketiminate complex is evaluated. It is shown that this compound will only transfer its oxo ligand to electrophilic reagents (e.g. isocyanides and CO) rather than nucleophilic ones (e.g. PMe3). A number of rhenium(I) isocyanide and carbonyl complexes are isolated as products of the oxygen atom transfer process. In all, the results suggest that the oxo moiety of the starting rhenium(III) complex behaves as a nucleophile, and has no demonstrable electrophilic character.

Chapter 5. Reduction of a cationic rhenium(V) oxo Cp β-diketiminate with metallic sodium yields an anionic rhenium(I) complex that is both strongly reducing and basic. Accordingly, the corresponding neutral rhenium(II) and rhenium(III) hydride complexes are isolated following oxidation and protonation, respectively, of the anionic compound. Combination of the anionic rhenium(I) compound with ZnCl2 formed a tetrametallic Re(I)-Zn(I)-Zn(I)-Re(I) complex where the four metal centers are arranged in a nearly linear fashion. This compound is the first example of a Zn(I) compound to be supported exclusively by bonding to other metal centers. Computational and reactivity studies of this tetrametallic compound are also presented.

Chapter 1. Previous collaborative work between the Arnold and Bergman groups on niobium and tantalum systems supported by N,N’-diaryl-β-diketiminate (BDI, aryl = 2,6-iPr2-C6H3) and tert-butylimido ligands is summarized. Routes to prepare high-valent niobium and tantalum halide and alkyl starting materials are described, and the reactions of these complexes with small molecule substrates are discussed. Reactions to access low-valent niobium systems are described, and several examples are provided in which low-valent niobium systems promote unusual stoichiometric and catalytic transformations. In addition, the chemistry of high-valent BDI niobium bis(imido) systems is discussed.

Chapter 2. The nitrene transfer reactivity of a series of BDI niobium bis(imido) complexes is reported. A novel metathesis process results in nitrene fragment exchange between alkylimido groups in Nb(V) bis(imido) complexes and arylisocyanide substrates. Calculations and experimental evidence indicate that these reactions likely proceed through cycloaddition and cycloreversion steps involving Nb(V) η2-carbodiimide intermediates, and occurs at a single metal center without elimination of carbodiimide. In contrast, reaction with unhindered alkylisocyanides results in extrusion of dialkylcarbodiimide, a process that was rendered catalytic in the presence of excess azide. This represents a rare example of oxidative nitrene transfer promoted by an early transition metal complex, in which reactivity across an imido group and two-electron metal-based redox chemistry are used in tandem to effect catalytic turnover.

Chapter 3. BDI niobium bis(imido) complexes are shown to react with a variety of substrates across their Nb-N π-bonds by cycloaddition and 1,2-addition. Niobium bis(imido) intermediates generated by treatment of a Nb(III) precursor with azide substrates react further with azide substrates via [3+2] cycloaddition to give niobium tetrazene complexes. These compounds exist in equilibrium with the corresponding bis(imido) complexes, and are competent catalyst precursors for oxidative nitrene transfer to generate carbodiimides. Isolated bis(imido) complexes are also shown to engage in stoichiometric 1,2-addition reactions with dihydrogen, silanes, boranes, thiols, and terminal alkynes. These bis(imido) compounds also exhibit [2+2] cycloaddition and cycloreversion reactivity with carbon dioxide and carbon disulfide. Through these studies, the first example of an isolated early transition metal (group 3 to 5) complex bearing both a terminal imido and a terminal oxo ligand is accessed. Likely as a consequence of π-loading effects, this compound is reactive toward 1,2-addition of silane substrates across its oxo group.

Chapter 4. The cyclometallated tantalum(V) hydride complex {ArNC(Me)CHC(Me)N[2-(CHMeCH2)-6-iPr-C6H3]}Ta(NtBu)H was prepared from hydrogenolysis of (BDI)TaNtBuMe2 (BDI = N,N′-diaryl-β-diketiminate, aryl = 2,6-iPr2-C6H3). Based on mechanistic studies, formation of the hydride complex likely proceeds through a dihydride intermediate generated from successive σ-bond metathesis steps. Low-valent Ta(III) dicarbonyl derivatives are accessed by either introducing CO atmosphere to the DMAP adduct at room temperature, or by directly adding CO at low temperature. Instead of promoting reductive elimination, 2,6-dimethylphenylisocyanide reacts with the tantalum hydride complex via migratory insertion to give a product containing a unique metallaimidazole ring. This complex can be reversibly oxidized by one electron either electrochemically or chemically to give a tantalum radical cation species. DFT calculations, as well as X-ray crystallographic and EPR spectroscopic data are consistent with the electron being removed from an orbital primarily composed of Ta-C π-bonding character, but also delocalized over the tantalaimidazole ring.

Chapter 5. The synthesis and reactivity of niobium and tantalum complexes bearing tert-butylimido and N,N′-di-2,4,6-trimethylphenyl-β-diketiminate (BDIAr) or N,N′-di2,6-dichlorophenyl-β-diketiminate (BDICl) ligands is reported. Reaction of dimethyl complex (BDIAr)Nb(NtBu)Me2 with dihydrogen leads to generation of bis-μ-hydrido Nb(IV)-Nb(IV) complex {(BDIAr)Nb(NtBu)(μ-H)}2. One-electron oxidation of this complex results in release of dihydrogen to generate the imido-bridged mixed-valent Nb(III)-Nb(IV) dimer [{(BDIAr)Nb(μ-NtBu)}2][B(C6F5)4]. EPR spectroscopic measurements and DFT calculations are consistent with a symmetric structure containing a Nb-Nb single bond in which the unpaired electron is delocalized over both niobium centers.

Chapter 1:

A series of thorium and uranium tris-amidinate complexes featuring a rare O-bound terminal phosphaethynolate (OCP1-) ligand were synthesized and fully characterized. These are the first actinide complexes featuring the OCP1- moiety, and the coordination through the O-atom donor is unique compared to all previously reported metal complexes, which feature P-bound OCP1- ligands. The cyanate (OCN1-) and thiocyanate (SCN1-) analogs were prepared for structural comparison and feature preferential N-coordination to the metal center. Calculations suggest the binding in all these complexes is charge-driven. The thorium OCP complex reacts with Ni(COD)2 to yield a heterobimetallic adduct as addition across the C≡P triple bond is observed; this new species features an unprecedented reduced OCP1- bent fragment which bridges the two metal centers.

Chapter 2:

A new thorium monoalkyl complex, Th(CH2SiMe3)(BIMA)3 (2.2; BIMA = MeC(NiPr)2), undergoes insertion of chalcogen atoms resulting in a series of thorium chalcogenolate complexes, Th(ECH2SiMe3)(BIMA)3 (where E = S, SS, Se, Te; 2.5-2.8). Complex 2.6 represents the first alkyl disulfide thorium species and illustrates the ability of 2.2 to undergo controllable, stoichiometric atom insertion. All complexes have been characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, and melting point, and in the case of 2.1, 2.2, and 2.4-2.8, X-ray crystallography. X-ray diffraction studies reveal the η2-coordination mode of the sulfur atoms in 2.6, the acute nature of the Th-Se-C bond in 2.7, and the first molecular Th-Te single bond in 2.8. Insertion was achieved by balancing the thermodynamic driving force of chalcogenolate formation versus the BDE of the pnictogen-chalcogen bond in the transfer reagent. Utilizing Me3NO as an oxygen atom transfer reagent led to C-H activation and SiMe4 elimination rather than oxygen atom insertion, resulting in the alkoxide complex Th(OCH2NMe2)(BIMA)3 (2.4).

Chapter 3:

The reactivity of the thorium monoalkyl complex Th(CH2SiMe3)(BIMA)3 (2.2; BIMA = MeC(NiPr)2) with various small molecules is presented. While steric congestion prohibits the insertion of N,N’-diisopropylcarbodiimide into the Th-C bond in 2.2, the first thorium tetrakis(amidinate) complex, Th(BIMA)4 (3.1), is synthesized via an alternative salt metathesis route. Insertion of p-tolyl azide leads to the triazenido complex Th[(p-tolyl)NNN(CH2SiMe3)-κ2N1,2](BIMA)3 (3.2), which undergoes thermal decomposition to the amido species Th[(p-tolyl)N(SiMe)3](BIMA)3 (3.3). The reaction of 2.2 with 2,6-dimethylphenylisocyanide results in the thorium iminoacyl complex Th[η2-(C=N)-2,6-Me2-C6H3(CH2SiMe3)](BIMA)3 (3.4), while the reaction with isoelectronic CO leads to the products Th[OC(=CH2)SiMe3](BIMA)3 (3.5) and Th[OC(NiPr)C(CH2SiMe3)(C(Me)N(iPr))O-κ2O,O′](BIMA)2 (3.6), the latter being the result of CO coupling and insertion into an amidinate ligand. Protonolysis is achieved with several substrates, producing amido (3.8), aryloxide (3.9), phosphide (3.10, 3.11), acetylide (3.12), and cationic (3.13) complexes. Ligand exchange with 9-borabicyclo[3.3.1]nonane (9-BBN) results in the formation of the thorium borohydride complex (BIMA)3Th(μ-H)2[B(C8H14)] (3.14). Complex 2.2 also reacts under photolytic conditions to eliminate SiMe4 and produce Th(BIMA)2(BIMA*) [3.15, BIMA* = (iPr)NC(CH2)N(iPr)], featuring a rare example of a dianionic amidinate ligand. Complexes 3.1, 3.2, 3.4, 3.5, and 3.11-3.15 were characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, melting point and X-ray crystallography. All other complexes were identified by one or more of these spectroscopic techniques.

Chapter 4:

A series of uranium tris-guanidinate complexes is presented, including the first homoleptic U(III) guanidinate species (4.2). Reaction of 4.2 with diphenyldiazomethane results in the two-electron oxidation of U(III) to U(V), producing the first isolable U(V) diphenyldiazomethane complex (4.3), featuring a short U-Nimido bond. Corresponding U(V) imido (4.4), U(V) oxo (4.5), and U(IV) azido (4.6) complexes were also synthesized for spectroscopic comparison. All complexes were characterized by 1H NMR spectroscopy, FTIR, EA, melting point, and X-ray crystallography.

Chapter 5:

Thorium dialkyl complexes featuring bis-amidinate and bis-guanidinate frameworks are reported, and their ability to insert molecular oxygen is evaluated. The bis-amidinate system Th(BTBA)2(CH2SiMe3)2 (5.2; BTBA = PhC(NSiMe3)2) is able to undergo instantaneous oxygen insertion to form the corresponding dialkoxide complex (5.3), as well as selenium atom insertion to form the diselenolate (5.4). The bis-guanidinate system Th(TIG)2(CH2SiMe3)2 (5.6; TIG = iPr2NC(NiPr)2) undergoes oxygen insertion at a significantly slower rate, and preliminary mechanistic studies suggest a radical-type mechanism is at play. All complexes were characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, melting point, and X-ray crystallography.

Chapter 1:

Polarized aluminum K-edge X-ray Absorption Spectroscopy (XAS) and first principle calculations were used to probe electronic structure in a series of (BDI)Al, (BDI)AlX2, and (BDI)AlR2 coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XAS measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal–ligand bonding. Qualitative trends in core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)AlX2 systems with more electronegative X1- ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)AlH2 the experimental Al K-edge XAS spectra and spectra calculated using the eXcited electron and Core-Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)AlH2 are similar relative to the (BDI)AlX2 and (BDI)AlMe2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b1 symmetry that is localized on Al and orthogonal to the (BDI)Al plane. The presence of this low energy unoccupied molecular orbital on electron-rich (BDI)Al constitutes a more distinguishing aspect of its valence electronic structure relative to the formally trivalent compounds (BDI)AlX2 and (BDI)AlR2. The work shows that Al K-edge XAS can be used to provide valuable insight into electronic-structure to reactivity relationships for main-group coordination compounds.

Chapter 2:

Oxygen and aluminum K-edge XAS, imaging from a scanning transmission X-ray microscope (STXM), and first principles calculations were used to probe the composition and morphology of bulk aluminum metal, α- and γ-Al2O3, and several types of aluminum nanoparticles. The imaging results agreed with earlier transmission electron microscopy studies that showed a 2 to 5 nm thick layer of Al2O3 on all the Al surfaces. Spectral interpretations were guided by examination of the calculated transition energies, which agreed well with the spectroscopic measurements. Features observed in the experimental O and Al K-edge XAS were used to determine the chemical structure and phase of the Al2O3 on the aluminum surfaces. For unprotected 18 and 100 nm Al nanoparticles, this analysis revealed an oxide layer that was similar to γ-Al2O3 and comprised of both tetrahedral and octahedral Al coordination sites. For oleic-acid protected Al nanoparticles, only tetrahedral Al oxide coordination sites were observed. The results were correlated to trends in the reactivity of the different materials, which suggests that the structures of different Al2O3 layers have an important role in the accessibility of the underlying Al metal towards further oxidation. Combined, the Al K-edge XAS and STXM results provided detailed chemical information that was not obtained from powder X-ray diffraction or imaging from a transmission electron microscope.

Chapter 3:

Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the f and d orbitals. In this study, aluminum K-edge XAS and Density Functional Theory calculations were used to evaluate the electronic structure of the dialuminides, MAl2 (M = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results showed how the energy and occupancy of the 4f or 5f orbitals impacted mixing of Al 3p character into the 5d or 6d conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

Chapter 4:

The synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(III)-µH-Al(III) moiety are presented. Combining different stoichiometries of Cp2TiCl and KH3AlC(SiMe3)3 (Cp = cyclopentadienyl) resulted in the formation of either bimetallic Cp2Ti(µ-H)2(H)AlC(SiMe3)3 or trimetallic (Cp2Ti)2(µ-H)3(H)AlC(SiMe3)3 via salt metathesis pathways. While these complexes were indefinitely stable at room temperature, the bridging hydrides were readily activated upon exposure to heteroallenes, heating, or electrochemical oxidation. In each case, formal hydride oxidation occurred, but the isolated product maintained the +3 oxidation state at both metal centers. The nature of this reactivity was explored using deuterium labelling experiments and Density Functional Theory (DFT) calculations. It was found that while C-H activation from the Ti(III) bimetallic may occur through a -bond metathesis pathway, chemical oxidation to Ti(IV) promotes bimolecular reductive elimination of dihydrogen to form a Ti(III) product.

Chapter 5:

Here, we present the synthesis of a low-valent thorium heterobimetallic containing the Th(III)-µH-Al(III) motif via reduction of a Th(IV)Cl precursor, as well as the direct synthesis by salt metathesis of a uranium(III) aluminum heterobimetallic analogue. These complexes were structurally characterized and compared using electron paramagnetic resonance techniques to a related titanium(III) system, and evidence was found for significant aluminum participation in the orbital occupied by the unpaired electron in the thorium system. DFT calculations confirmed the ThAl character of this interaction.