UC Irvine

This dissertation describes a variety of reduction techniques applied towards the synthesis and characterization of low oxidation state rare-earth metal compounds in order to determine the techniques’ efficacy and the ability of these methods to yield species with interesting properties different from those generated from the widely used potassium-based reduction systems. The research results described herein add to the fundamental knowledge base on the coordination and redox chemistry of the rare-earth metals, which are scandium, yttrium, and the lanthanides. In Chapter 1, an array of alkali metal (M) and organic chelate (L) combinations were used to generate nine [M(L)][Cp′3YII] (Cp′ = C5H4SiMe3) complexes in order to determine whether K/crypt (crypt = 2.2.2-cryptand) is indeed the most suitable for reducing Cp′3YIII and isolating Y(II) complexes. During the process, two new Y(II) complexes were crystallographically characterized, namely [(THF)Na2(18c6)2][Cp′3YII]2 and [Na(crypt)][Cp′3YII]. EPR, UV-visible absorbance, and IR spectroscopies revealed minimal differences in the spectroscopic characeristics of these complexes. Interestingly, the decomposition profiles of these species at room temperature show marked differences. This Chapter initiates a discussion of the factors that determine the stability of these low oxidation state-complexes. In Chapter 2, cesium metal smears were used to reduce Cp′3YbIII and Cp′3TmIII and generate a series of ytterbium(II) and thulium(II) complexes that display an extended structure by single crystal X-ray diffraction (XRD). The structures are composed of layers of hexagonal nets with alternating vertices of Cs(I) and Ln(II) ions. This structure is robust with respect to the level of THF-solvation, as compounds with 0, 1, and 2 bound THF molecules per monomer unit were characterized. The Tm-containing species, [(THF)Cs(µ–η5:η5–Cp′)3TmII]n, was synthesized in order to investigate whether the trigonal arrangement of S = ½ Tm(II) ions yielded spin frustration at low temperatures and whether single crystals were exfoliatable. By collaborating with Alexandre Vincent from the lab of Professor Jeffrey R. Long at the University of California, Berkeley, SQUID measurements were collected to show that the thulium ions are in the +2 oxidation state and non-interacting. And by collaborating with TJ McSorley from the lab of Professor Luis A. Jauregui in the Department of Physics at the University of California, Irvine, exfoliation of single crystals was successful down to approximately 50 layers. In Chapter 3, a method was adapted to induce the reduction of Ln(III) organometallic complexes (Ln = Sc, Y, La) by exposing them to γ-irradiation in a glassy matrix. The method was first utilized to reduce Ln(III) complexes to known [Ln(II)]1− anions, which were identified by EPR and UV-visible spectroscopies. That γ-irradiation method was extended to generate and characterize a new anion, namely [LaII(NR2)3]1− (R = SiMe3). Furthermore, the formation of [Cp′3YII]1− was monitored over time to show the low conversion rate of the method (<1% over 6.5 hours). This method may have broader implications for the generation and characerization of low oxidation state species otherwise unisolable by chemical means. In Chapter 4, an attempt to generate a bridging [C≡C]2− unit for reduction to the magnetically interesting, previously unknown [C≡C]3− bridge is described. The reaction of (C5Me5)2Y(μ–Ph)2BPh2 with NaC≡CH did not yield the desired (C5Me5)2Y(CC)Y(C5Me5)2 and instead forms an interesting C–C coupled product, namely (C5Me5)2Y(μ–η3:η1–CCCCH2)Y(C5Me5)2. This represents the second crystallographically characterized butatrienylidene dianion. The product was characterized by 1H, 13C, and HMQC NMR, UV-visible absorbance, and IR spectroscopies. Theoretical calculations were conducted to investigate the nature of the C–C bonding in the butatrienylidene bridge. In Chapter 5, results are reported on separate approaches to reduce organometallic rare-earth metal complexes using photoredox reactions and using barium powder. The photoredox approach focused on reducing Cp′3YIII in collaboration with Professor Harry B. Gray and his graduate student Dr. Javier Fajardo Jr. at the California Institute of Technology using a powerful photoreductant developed in their lab, namely W(CNdipp)6. Upon excitation with UV-visible light, the reduction potential is shifted approximately 2.3 V more negative {E1/2 of W(CNdipp)6 = −0.72 V vs Fc+/0, E1/2 of [W(CNdipp)6]* ≈ −3.0 V vs Fc+/0}. The reduction of Cp′3YIII by this species was attempted in toluene with the inclusion of a suite of sacrificial reductants. No isolation or characterization of Y(II) product was observed. Regarding the use of barium as a reductant, barium powder was generated in order to maximize its surface area and improve the speed at which a reduction reaction would occur. Reductions were attempted in the presence of crypt with Cp′3Ln (Ln = Y, Tm, Yb). Reductions were observed for thulium and ytterbium. Crystals isolated from a sample dissolved in wet acetonitrile demonstrate the electrostatic attraction of Ba(II) and anionic ligands, yielding the structure [Ba(crypt)Cp][Cp2Yb(μ–OSiMe3)2YbCp2].