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Expanding Synthetic Aspects of Redox and Mechanical Organometallic Chemistry of Rare Earth Complexes

  • Author(s): Kotyk, Christopher Matthew
  • Advisor(s): Evans, William J
  • et al.

This dissertation describes the synthesis, characterization, and reactivity of organometallic complexes of yttrium and the lanthanides in an effort to more completely understand the nature of a recently-discovered class of +2 ions of these rare earth metals. The reactivity of complexes of a new set of Ln2+ ions (Ln = rare earth metal) with unprecedented 4fn5d1 electron configurations has been explored to expand the unique chemistry possible with the rare earth elements. The isolation of unexpected reaction products is described as well as the discovery of a new divalent lanthanide system and the utilization of solvent-free mechanochemical synthesis for established rare earth organometallic species.

In Chapter 1, the reactivity of the highly-reducing, air-, moisture-, and temperature-sensitive divalent lanthanide complexes [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Dy) is characterized by examining reactions with aromatic organic substrates of known reduction potential. Complexes of the 4fn5d1 Ln2+ ions reduce naphthalene and biphenyl within minutes to form a new class of reduced aromatic complexes, [K(2.2.2-cryptand)][Cp'2Ln(η4-C10H8)] (Ln = Y, La, Ce, Dy) and [K(2.2.2-cryptand)][Cp'2Y(η6-C6H5Ph)], respectively. The naphthalene reactions also produced the previously unobserved ligand redistribution products [K(2.2.2-cryptand)][Cp'4Ln] (Ln = Y, La), which show the effect of the lanthanide contraction on structure as the lanthanum complex has four η5-Cp' rings while yttrium has three η5-Cp' rings and one η1-Cp' ring.

Chapter 2 describes the synthesis and characterization of a new class of 4fn5d1 Ln2+ complexes in which a reduced benzene dianion bridges two metal centers: [K(2.2.2-cryptand)]2[(Cp'2Ln)2(μ-η6:η6-C6H6)] (Ln = La, Ce). In these complexes, the (C6H6)2− dianion formally takes the place of a (Cp')1− anion in the [K(2.2.2-cryptand)][Cp'3Ln] complexes above and suggests a pattern that three anionic carbocyclic rings can stabilize the new Ln2+ ions. In accordance with the presence of two metal centers of 4fn5d1 electron configurations and (C6H6)2− dianions, these complexes react as four electron reductants to reduce two equiv of naphthalene to two equiv of [K(2.2.2-cryptand)][Cp'2Ln(η4-C10H8)] (Ln = La, Ce). This constitutes a better synthesis of these complexes than the one above since no (Cp'4Ln)1− byproducts are formed.

Chapter 3 describes the solvent-free synthesis of the previously established complexes (C5Me5)3Y, Cp'3Y, [Cp'2Y(μ-Cl)]2, Y[N(SiMe3)2]3, and U[N(SiMe3)2]3 by mechanochemical ball milling. Not only do these reactions show the feasibility of ball milling as an alternative method to synthesizing organometallic rare earth complexes, but these preliminary results also show promising routes that shorten reaction times and limit the generation of organic solvent waste. This new method of solid-state synthesis is especially important for the isolation of (C5Me5)3Y as the established synthetic route must avoid aromatic hydrocarbons.

Chapter 4 describes a study of the reactivity of organometallic rare earth non-classical carbocation complexes (C5Me5)2Ln[(OC)2C5Me5-κ2-O,O'] (Ln = La, Sm) obtained from sterically crowded (C5Me5)3Ln and carbon monoxide. Investigation of the reactivity is consistent with DFT analysis that suggests the LUMO of the La complex is the 5dz2 orbital of the metal center rather than an orbital centered on the carbocation.

Also described in this dissertation are the synthesis and structural and spectroscopic characterization of two previously unreported rare earth amide complexes, Y(NCy2)3(THF) and Y(NPh2)3(THF)2 (Chapter 5), the insertion of CO2 into a Y–C bond for the first time with the Cp' ligand to form [Cp'2Y{μ-O2C[3-(Me3Si)C5H4]-κ2O,O'}]2 (Appendix A), the synthesis and structural characterization Cp'2Ln metallocene complexes of unsubstituted phenyl ligands, Cp'2LnPh(THF) (Ln = Y, Dy) (Appendix B), and the attempted synthesis of a bimetallic bridging monoanionic benzene rare earth complexes with the aim of developing a new class of single molecule magnets (Appendix C).

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