Dinitrogen can be reduced by photochemical activation of the trivalent rare earth bis(pentamethylcyclopentadienyl) allyl complexes (η5-C5Me5)2Ln(η3-C3H4R) (Ln = Y, Lu; R = H, Me) to form the (N=N)2− complexes, [(C5Me5)2Ln]2(μ-η2:η2-N2). This demonstrates that productive organolanthanide photochemistry is not limited to complexes of the unusual (η3-C5Me4H)1- ligand found in the heterometallic complexes (C5Me5)2(C5Me4H)Ln and (C5Me5)(C5Me4H)2Ln (Ln = Y, Lu, Dy). Photolytic activation of (C5Me5)2Ln(C3H5) (Ln = Y, La) in the presence of isoprene provides a rare photopolymerization route to polyisoprene. Irradiation of (C5Me5)2Y(C3H4Me) in the absence of other substrates results in the isolation of the corresponding ‘tuckover’ complex, (C5Me5)2Y(μ-H)(μ-η1:η5-CH2C5Me4)Y(C5Me5).

This dissertation describes the synthesis and physical characterization of a variety of rare earth-containing molecules that have been targeted for the purpose of better understanding the roles that rare earth elements play in molecular magnetism. The complexes described here also contribute to fundamental understanding of rare earth coordination chemistry. Many of these results and conclusions have been developed in collaboration with the group of Professor Jeffrey R. Long at the University of California, Berkeley.

Chapters 1 through 4 of this dissertation focus on modifying different aspects of the known (N2)3−-bridged bimetallic single-molecule magnet (SMM) system {[(Me3Si)2N]2(THF)Ln}2(μ-η2:η2-N2)[K(18-crown-6)(THF)2], whose Tb analog had the highest blocking temperature ever observed for a SMM when it was published in 2011. Variations in the ancillary ligands, the metals, and the bridging unit were investigated to determine their effects on magnetic properties. In Chapter 1, the variation in the Lewis base L in the precursor to the (N2)3− complexes, the (N=N)2− complexes, {[(Me3Si)2N]2(L)Ln}2(μ-η2:η2-N2), was explored. Previously, only L = THF was known. It was found with the diamagnetic Ln = Y analog that L = pyridines, nitriles, and triphenylphosphine oxide can also support this reduced dinitrogen (N=N)2− system. However, further reduction to obtain a radical (N2)3− complex was not achieved. Density functional theory (DFT) calculations suggest this is due to the presence of lower lying orbitals based on the new neutral donors.

Chapter 2 analyzes how structural modifications in the previously reported SMMs {[(Me3Si)2N]2(THF)Ln}2(μ-η2:η2-N2)[K(18-crown-6)(THF)2] (Ln = Tb, Dy) can affect the molecular magnetism when the K+ counter cation is incorporated into the inner sphere of the magnetic core. The new series of SMMs that resulted, {[(Me3Si)2N]2(THF)Ln}2(μ3-η2:η2:η2-N2)K (Ln = Tb, Dy), displays blocking temperatures lower than the outer sphere K+ analogs. This result is thought to be due in part to the crystallographically-observed bending of the previously planar Ln2N2 core. In Chapter 3, a more drastic modification in the bridging unit of these bimetallic rare earth complexes was achieved: the (N2)2− anion in {[(Me3Si)2N]2(THF)Y}2(μ-η2:η2-N2) can reduce elemental sulfur and selenium to generate bridging E2− and (E2)2− chalcogenide complexes, where E = S, Se. Finally, Chapter 4 investigates the Tb analog of the first reported molecular example of an (NO)2−-containing complex, the radical bridged {[(Me3Si)2N]2(THF)Y}2(μ-η2:η2-NO), and demonstrates the importance of obtaining additional spectral characterization data to support X-ray crystallographic findings.

Chapter 5 describes synthetic aspects of complexes containing the [(C5Me5)2Ln]1+ moiety, which is an important component in rare earth starting materials, and Chapter 6 shows how this is used to design phenazine radical (phz)1−-containing SMMs. Although the targeted complexes, {[(C5Me5)2Ln]2(phz)}{BPh4} (Ln = Tb, Dy; phz = phenazine), could be synthesized and crystallographically characterized, magnetic data suggest the presence of multiple magnetic products in the crystalline sample. In Chapter 7, (C5Me5)1− metallocenes are investigated from another perspective. Attempts were made to obtain linear monometallic trivalent rare earth metallocene cations, as previously demonstrated for U4+, to determine their potential as single-ion magnets (SIMs).

The origin of the research presented in Chapter 8 was the recent discovery that the +2 oxidation state is accessible for all the lanthanides in molecular species with formula [K(2.2.2-cryptand)][Cp′3Ln] (Cp′ = C5H4SiMe3) and that the Ho2+ analog of these complexes possesses the highest magnetic moment ever measured for a single metal ion, 11.4 μB. The synthesis and isolation of more molecular examples of these highly reactive divalent ions were pursued with the indenide ligand, (C9H7)1−. However, reduction of CpIn3Ln (CpIn = C9H7) resulted in C–H bond activation of an indenyl ligand and the first example of the indenyl dianion, (C9H6)2−.

Appendix A is a collection of results on miscellaneous projects not covered in the previous eight chapters. The results in this appendix are presented in chronological order and span a collection of ventures from dinitrogen reduction using lithium to inelastic neutron scattering (INS) experiments and finally a collaborative project with the group of Professor Alan F. Heyduk at the University of California, Irvine, presenting the mixed metal lanthanide/transition metal species (C5Me5)2Dy[M(SNS)2] (M = Mo, W). These latter complexes were synthesized to probe whether magnetic coupling of the mixed metal centers could occur. They truly exemplify the purpose of the research presented in this dissertation which is to explore the frontiers of molecular magnet design.

The chemistry presented in this thesis focuses on three projects involving the organometallic chemistry of uranium complexes. The first encompasses attempts to form isolable uranium metallocene phenyl species based on the previously synthesized (C5Me5)2UPh2, which was reported to undergo decomposition to a transient benzyne complex, (C5Me5)2U(C6H4), in solution. Initial attempts to produce the diphenyl complex for study led to the isolation of a new uranium benzyne complex, [(C5Me5)2UPh(μ2¬-C6H4)][Li(Et2O)]. Further studies with the [C5Me4(SiMe3)]− ligand led to the isolation of a metallated phenyl complex, [η5:η1-C5Me4(SiMe2CH2]U(C6H5), that is not a benzyne and [C5Me4(SiMe3)]2UBr(C6H5). The second project focuses on the development of the uranyl moiety, [UO2]2+, as a precursor for organometallic complexes of uranium. This study has led to the isolation of the new uranyl complexes UO2[N(SiMe3)2](NO3)(THF)2, [NEt4][(C5Me5)UO2(NO3)2], and [NEt4]2[UO2(η1-NO3)2(η2-NO3)2]. The third project involves the use of thermal analysis of organouranium compounds to test the viability of uranium nitride production from species dense in nitrogen atoms and to gain a better understanding of the thermal properties of organouranium species.

This dissertation describes synthetic, structural, spectroscopic, reactivity, and theoretical investigations into the factors that stabilize thorium and uranium compounds in the low oxidation states +2 and +3. Extensions of the actinide work to rare-earth metals and bismuth are also included. The importance of understanding this aspect of actinide metal chemistry is described in Chapter 1. Chapter 2 describes a theoretical study using density functional theory (DFT) on U(II) complexes analyzing how the local coordination geometry around the uranium center influences the electronic configuration. U(II) species with planar coordination geometries favor 5f36d1 electron configurations, while non-planer geometries appear to favor 5f4 configurations. Chapter 3 describes electrochemical determination of the Th(IV)/Th(III), Th(III)/Th(II), and U(III)/U(II) reduction potentials in tris(cyclopentadienyl) complexes with complementary spectroelectrochemical measurements. It was found that the reduction potentials trend with ligand donation strength. In addition, the study revealed that the Th(IV)/Th(III) and Th(III)/Th(II) reduction potentials were quite similar which led to the synthesis of Th(II) compounds directly from Th(IV) precursors.Chapters 4–6 detail reduction studies of heteroleptic U(III) complexes to prepare new U(II) compounds and understand how to stabilize this low oxidation state. Within these Chapters are UV-visible and EPR spectroscopic studies and DFT calculations on newly synthesized compounds to probe the electronic structure of the U(II) species. The heteroleptic species appear less stable than the homoleptic analogs despite having similar steric properties and reduction potentials. Chapter 7 describes the reaction of U(III) compounds with organoazides which led to the observation of a U(V) intermediate and a new mechanistic proposal for the formation of U(VI) imides. Such an intermediate had not been observed in this type of reaction and the results redefine the mechanism for the reduction of azides by uranium compounds. Chapters 8 and 9 are crystallographic studies on U(III) compounds containing cyclopentadienyl and iodide ligands, showing the many structural variations that can exist with simple organoactinide compositions. Chapters 10 and 11 describe the synthesis of new Th(III) compounds with the C5H4SiMe3 (Cp′) and C5H3(SiMe3)2 (Cp″) ligands and a new disproportionation route to Th(II) complexes that was discovered. Chapter 10 describes extensive efforts to characterize the elusive Cp′3Th. In Chapter 11, reactions of Cp″3Th with simple salts are described that lead to formation of Th(II) complexes by disproportionation. An unusual reduction of benzene by “[Cp″3ThMe]1−” is also reported. Chapter 12 extends the electrochemistry method described in Chapter 3 to the Cp′3Ln series and some (C5Me4H)3Ln compounds. It was found that the reduction potentials for the Cp′3Ln series were found to be practically identical across the Ln series (excluding Eu, Sm, Tm, and Yb). The remaining Chapters describe the coordination chemistry of the underutilized nitrogen heterocycles trimethyltriazacyclohexane and trimethyltriazacyclononane with the rare-earth metals (Chapter 13), the actinide metals (Chapter 14), and bismuth (Chapter 15). Appendix A describes attempts to form Th(II) complexes by reduction of C5Me4H-ligated Th(III) complexes. Appendix B describes DFT studies on a bimetallic yttrium system with benzoxazole-derived ligands. Appendices C and D summarize work with the bimetallic (CpAnYH)2 system [CpAn = (C5H3SiMe3)2SiMe2] and uranium borohydride coordination chemistry, respectively. Appendix E summarizes DFT studies on various complexes not yet mentioned in other sections of this dissertation. Finally, Appendix F contains a chronological list of crystal structures obtained throughout this work.

This dissertation describes the synthesis, characterization, and reactivity of unique organometallic compounds of bismuth, samarium, uranium, and barium metal in efforts to expand the fundamental knowledge of their coordination and redox chemistry, particularly with respect to small molecule activation. This dissertation research involves the investigation of the NCN pincer ligand supported bismuth complex, Ar′BiCl2, to form bismuth benzyl, aryloxide, and dianionic ligand complexes, as well as studies of the reduction chemistry of (C5Me5)2Sm(THF)x in support of density functional theory research, along with projects with uranium and barium metallocenes.

Chapter 1 describes the synthesis, characterization, and structure of the first two bismuth-benzyl compounds, namely Ar′Bi(η1-CH2Ph)2 and Bi(η1-CH2Ph)3, synthesized by reaction of the analogous chloride salts with the Grignard reagent MgCl(CH2Ph).

Chapter 2 describes the reactivity of the unique oxyaryl bismuth compound Ar′Bi(C6H2tBu2-3,5-O-4), containing the unusual dianionic oxyaryl ligand (C6H2tBu2-3,5-O-4)2−. The first examples of carbon dioxide and carbonyl sulfide insertions into a bismuth-carbon bond are reported along with formal 1,5-addition of R3SiX (R = Me, Ph; X = Cl, CN, N3) reagents tothis complex. Chapter 3 further expands this chemistry by coupling the facile carbon dioxide insertion with protonation or silylation to functionalize CO2 to generate 3,5-di-tert-butyl-4-hydroxybenzoic acid or 3,5-di-tert-butyl-4-(trimethylsilyloxy)benzoate, respectively, in a cyclic process. This is a rare example of bismuth small molecule functionalization.

Chapter 4 describes the reactivity of Ar′Bi(C6H2tBu2-3,5-O-4) with nitric oxide (NO) gas and the NO delivery reagent Ph3CSNO. The first examples of NO insertion into a bismuth-carbon bond are reported with three spectroscopically and crystallographically characterized complexes. In each case, the monoanionic oximate [ON=(C6H2tBu2O)]1− is generated by formal oxidation of the dianionic ligand and concomitant reduction of nitric oxide to (NO)1−.

Chapter 5 describes the reactivity of bismuth with the tripodal ligands N,N′,N′′-[2,2′,2′′-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3−) along with the analogous tridentate bipodal ligand N,N'((methylazanediyl)bis(ethane-2,1-diyl))bis(4-methylbenzenesulfonamide) ([TSB]2−). Synthesis and crystallographic characterization of Bi[MST] and the polymetallic bismuth cluster, Bi5Na2O5[TSB]5, are reported.

Chapter 6 describes the ligand transfer chemistry of Ar′Bi[O2C(C6H2tBu2-3-5-O-4)-κ2O,O′], with the dianionic ligand [O2C(C6H2tBu2-3,5-O-4)]2−, with oxophilic metal compounds, namely (C5Me5)2UCl2, SmI2(THF)2, and (C5H5)2TiCl2. Successful ligand transfer was accomplished and the product, (C5Me5)2UCl[O2C(C6H2tBu2-3-5-OH-4)-κ2O,O′] was structurally and spectroscopically characterized. Chapter 7 explores the magnetic properties of uranium complexes in detail. Over 500 literature examples from the last 50 years are compiled and examined to find trends dependent on metal oxidation state and ligand identity.

Chapter 8 describes the reaction chemistry of the samarium oxide [(C5Me5)2Sm]2(μ-O) with CO2 to test DFT rationales for the formation of the oxalate [(C5Me5)2Sm]2(μ-η2:η2-O2CCO2)from (C5Me5)2Sm(THF)x with CO2. Analogously, the reaction of (C5Me5)2Sm(THF)x with pyridine and 1-hexyne in support of DFT work on the affects of ligand donation strength on Sm2+ reactivity and carbon-carbon coupling of alkynes is reported.

Appendix A describes the exploration of the reduction chemistry of the alkaline earth metallocenes Cp2M and [Cp3M][K(chelate)] (M = Ba, Ca; Cp = C5H4SiMe3, C5H3(SiMe3)2; chelate = 18-crown-6, 2.2.2.-cryptand) in efforts to isolate monovalent alkaline earth species. Spectroscopic analysis of the reduced barium species along with initial reactivity is reported.

Rare-earth metal complexes are of interest in the use of new technologies such as single-molecule magnets (SMMs). New SMMs containing rare-earth metals can expand the limits of modern technology, but must first be made and studied before implementation. The synthesis of the new complexes (C5Me5)2Ln(μ-S)2M(μ-S)2Ln(C5Me5)2 (Ln = Y, Gd, Tb, Dy; M = Mo, W), Ln2M, and the subsequent reduction of the molybdenum compounds with Co(C5Me5)2 to form radical-bridged species [(C5Me5)2Ln(μ-S)2Mo(μ-S)2Ln(C5Me5)2][Co(C5Me5)2], (Ln2Mo)1− is described. An undetected ammonia impurity in the starting material led to the synthesis of asymmetric complexes [(C5Me5)2Ln(NH3)](μ-S)2M(μ-S)2Ln(C5Me5)2, Ln2MNH3, and the reduction of these complexes with KC8 in the presence of 18-crown-6 yielded [K(18-crown-6)](μ-S)2M(μ-S)2Ln(C5Me5)2, KMLn. Further exploration of the ammonia impurity led to the synthesis of the new yttrium metallocenes, (C5Me5)2Y(NH2)(THF), YNH2, and [(C5Me5)2Y(NH3)(THF)][BPh4], YNH3. Ammonia reactions with other lanthanide metallocenes (C5Me5)2Ln(THF)x (Ln = Sm, Yb; x = 0–2) and Cp′′3Ce (Cp′′ = C5H3(SiMe3)2) were pursued to create ammonia-containing complexes which could not be definitively characterized due to instability. In situ addition of the 2,4,6,-tri-tBu-phenoxyl radical yielded evidence for oxidation and hydrogen abstraction, but the complexes have not yet been crystallographically characterized. Finally, 89Y NMR spectroscopy was employed to obtain another method of characterization for yttrium compounds and the advantages of 1H89Y HMBC experiments over traditional 89Y NMR experiments are highlighted.

This dissertation describes efforts to expand the understanding of Ln(II) chemistry, particularly that of the non-traditional 4fn5d1 ions, through the synthesis and isolation of new Ln(II) species supported by bis(trimethylsilyl)amide ligands and exploration of their reactivity. Described herein is the isolation of a new series of Ln(II) complexes ligated by NR2 (R = SiMe3) ligands. These ligands were previously not thought to be capable of supporting the nontraditional lanthanide ions in the 2+ oxidation state. Following their isolation, the reaction chemistry of these [LnIINR2)3]1− complexes was investigated to provide insight into the unique reaction chemistry of non-traditional Ln(II) ions.

Chapter 1 outlines the synthesis and characterization of a new series of Ln(II) complexes [M(crypt)][Ln(NR2)3] (M = K or Rb) and describes their spectroscopic properties as they relate to electron configuration. Chapter 2 discusses the synthesis of the elusive [YII(NR2)3]1− complex in addition to reactions of [Ln(NR2)3]1− with CO. Chapter 3 describes new examples of uranium in the +2 oxidation state isolated by reduction of Cptet3U (Cptet = C5Me4H) and U(NR2)3 (R = SiMe3) in the presence of 2.2.2-cryptand to produce [K(crypt)][Cptet3U] and [K(crypt)][U(NR2)3], respectively. Chapter 4 expands the understanding of lanthanide-based dinitrogen reduction chemistry through discovery of the first end-on Ln2(μ-η1:η1-N2) complexes. The formation of end-on versus the more common side-on Ln2(μ-η2:η2-N2) complexes was possible by using the Ln(II) complexes detailed in Chapters 1 and 2. Chapter 5 details the reaction chemistry of the [Gd(NR2)3]1− complexes with toluene and the isolation of a methylcyclohexadienyl dianion. Chapter 6 discusses the reaction chemistry of [Gd(NR2)3]1− with BiPh3 and PPh3 and the formation of a rare example of a complex containing a Ln−Bi bond.

This dissertation describes the Ln(II) reduction reactivity of both isolated Ln(II) complexes containing (NR2)1− (R = SiMe3) amide ligands and in situ Ln(NR2)3/M reactions. The results show the diversity of products obtained by amide-based Ln(II) reduction due to subtle changes in reaction conditions. Chapter 1 outlines the reactions of isolated [GdII(NR2)3]1− with N,N-dicyclohexylcarbodiimide, CS2, S8, pyridine, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), MeCN, BiPh3, and SnPh4 in order to determine if the 4f75d1 configuration in Gd(II) in an isolated compound could reveal new reaction pathways. Chapter 2 outlines solvent decomposition reactions by Ln(NR2)3/K reactions with the smaller rare earth metals, Ln = Y, Ho, Er, and Lu. This Chapter provides more information on solvent reactivity in these reductive systems by describing a series of C–O cleavage products isolated from Ln(NR2)3/K reactions with ethers. Chapter 3 outlines the investigation of the reductive chemistry of cerium with in-situ Ce(NR2)3/M/N2 reactions. In this Chapter it is shown that crypt can function as a bidentate ligand to rare-earth ions. Chapter 4 outlines the reductive chemistry of neodymium with in-situ Nd(NR2)3/M/N2 reactions. Examination of the in-situ reduction of Nd(NR2)3 at low temperature in Et2O in the presence of 18-crown-6 provides a crystal system that demonstrates a solid-state isomerization of an end-on bound (N=N)2− moiety to side-on. This Chapter also describes a possible connection between the Ln(NR2)3/M reductions of N2 and reductions by isolated Ln(II) complexes.

This dissertation focuses on efforts to expand upon the discovery that the +2 oxidation state is available in molecular forms for all the rare earth metals, i.e. Sc, Y, and the lanthanides. In addition to the previously isolated +2 ion complexes of La, Ce, Nd, Sm, Eu, Dy, Tm, Yb, and Sc, the first complexes containing +2 ions of Y, Pr, Gd, Tb, Ho, Er, and Lu had been prepared under strongly reducing conditions with the (C5H4SiMe3)33− ligand set. This dissertation describes efforts to broaden the ligand sets available to isolate new +2 rare-earth metal ions and to study their reactivity and physical properties. Chapter 1 describes the use of (C5H5)33− and (C5H4Me)33− ligand sets to isolate more examples of Y2+ complexes, and compares their stability and properties with those of other ligand sets. For (C5H4Me)33−, reduction reactions with other rare-earth metals as well as some reactivity studies are also described. Chapter 2 describes the dinitrogen reactivity of the Y2+ species generated from the reductions of (C5H5)3Y and (C5H4Me)3Y. Chapter 3 describes the synthesis and reactivity of +2 ion complexes from the reduction of ( (C5Me4H)3Ln complexes. Chapter 4 describes the synthesis of the first crystallographically characterizable Sc2+ complex, {Sc[N(SiMe3)2]3}− including physical characterization by EPR and UV-vis spectroscopy. Chapter 5 describes the reactivity of the Sc2+ complex, [Sc[N(SiMe3)2]3}−, with dinitrogen to generate the first end-on bridging dinitrogen complex of a rare-earth metal, {[Sc[N(SiMe3)2]3]2(μ-η2:η2-N2)}2−. Chapter 6 describes additional reactivity studies of [Sc[N(SiMe3)2]3}− with carbon monoxide, carbon dioxide, and acetonitrile. Chapter 7 describes the mechanochemical synthesis of the sterically crowded complexes, (C5Me5)3Ln (Ln = Tb, Dy, Ho, Er).