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Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Irvine Electronic Theses and Dissertations (2015)

This dissertation describes the use of spectroscopic and computational methods to understand new classes of rare earth and actinide coordination complexes. In Chapters 1 and 2, the use of UV-vis spectroscopy and density functional theory (DFT) to understand a rare form of photochemical activation of rare earth mixed-ligand tris(cyclopentadienyl) complexes, (C5Me5)3-x(C5Me4H)xLn, and metallocene allyl complexes, (C5Me5)2Ln(C3H5) (Ln = Lu, Y) is described. The photochemistry involves a ligand-based reduction in a trivalent rare earth complex that generates a reducing system powerful enough to reduce dinitrogen. Chapter 3 describes the use of Raman spectroscopy to understand bond lengths in reduced dinitrogen rare earth complexes, [(C5Me5)2Ln](μ-η2:η2-N2), and analyze the degree of dinitrogen reduction based on the ancillary ligands. Chapter 4 describes the power of NMR spectroscopy to characterize complicated mixtures of heterobimetallic bridging hydride complexes, (C5Me5)2Ln(H)2Ln′(C5Me5)2, and tuckover hydride complexes, (C5Me5)2Ln(μ-H)(μ-η1:η5-CH2C5Me4)Ln′(C5Me5) (Ln, Ln′ = La, Y, Lu). DFT was used to investigate the metal site preferences in these complexes.

Chapters 5 through 10 describe different techniques to understand the first examples of molecular rare earth and actinide tris(cyclopentadienyl) complexes in the formal +2 oxidation state, [K(2.2.2-cryptant)][(C5R5)3Ln] (Ln = Y, lanthanides, Th, U). DFT is used to describe the configuration of these complexes as 4d1 for Y, [Ln3+]5d1 for 10 lanthanides and [An3+]6d1 for Th and U (Chapter 5 and 6). UV-vis spectroscopy was used to distinguish between different electron configurations of Ln2+ complexes (Chapter 7). Magnetic susceptibility measurements characterize two Ln2+ complexes to have record high single-ion magnetic moments (Chapter 8). Ligand and metal edge X-ray absorption spectroscopy were used to analyze the oxidation state of the metals (Chapter 9). Reactivity of cyclooctatetraene with Ln2+ complexes is described in Chapter 10. Chapter 11 presents the synthesis of new Ln3+ and Ln2+ complexes, using a tris(aryloxide)arene coordination environment. Chapter 12 describes the use of DFT to predict new coordination environments that could allow the stabilization of the +2 oxidation state for the rare earths and actinides.

    Cover page: Spectroscopic and Computational Analysis of Rare Earth and Actinide Complexes in Unusual Coordination Environments and Oxidation States
    • Article
    • Peer Reviewed
    UC Irvine Previously Published Works (2015)
    Reduction of the Th3+ complex Cp''3Th, 1 [Cp'' = C5H3(SiMe3)2], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp''3Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC8 in the presence of 18-crown-6 generates the analogous Th2+ compound, [K(18-crown-6)(THF)2][Cp''3Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M-1 cm-1, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 Å Th-(Cp'' ring centroid) distances, respectively, equivalent to the 2.520 Å distance measured in 1. Density functional theory analysis of (Cp''3Th)1- is consistent with a 6d2 ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp''2Th(C8H8), 4, and [K(18-crown-6)]Cp''.
      Cover page: Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C 5 H 3 (SiMe 3 ) 2 ] 3 Th} 1− anion containing thorium in the formal +2 oxidation stateCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2015)
      The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 μB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets.
        Cover page: Record High Single-Ion Magnetic Moments Through 4f n 5d1 Electron Configurations in the Divalent Lanthanide Complexes [(C5H4SiMe3)3Ln]−
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2017)
        A new series of Ln3+ and Ln2+ complexes has been synthesized using the tris(aryloxide)arene ligand system, ((Ad,MeArO)3mes)3-, recently used to isolate a complex of U2+. The triphenol precursor, (Ad,MeArOH)3mes, reacts with the Ln3+ amides, Ln(NR2)3 (R = SiMe3), to form a series of [((Ad,MeArO)3mes)Ln] complexes, 1-Ln. Crystallographic characterization was achieved for Ln = Nd, Gd, Dy, and Er. The complexes 1-Ln can be reduced with potassium graphite in the presence of 2.2.2-cryptand (crypt) to form highly absorbing solutions with properties consistent with Ln2+ complexes, [K(crypt)][((Ad,MeArO)3mes)Ln], 2-Ln. The synthesis of the Nd2+ complex [K(crypt)][((Ad,MeArO)3mes)Nd], 2-Nd, was unambiguously confirmed by X-ray crystallography. In the case of the other lanthanides, crystals were found to contain mixtures of 2-Ln co-crystallized with either a Ln3+ hydride complex, [K(crypt)][((Ad,MeArO)3mes)LnH], 3-Ln, for Ln = Gd, Dy, and Er, or a hydroxide complex, [K(crypt)][((Ad,MeArO)3mes)Ln(OH)], 4-Ln, for Ln = Dy. A Dy2+ complex with 18-crown-6 as the potassium chelator, [K(18-crown-6)(THF)2][((Ad,MeArO)3mes)Dy], 5-Dy, was isolated as a co-crystallized mixture with the Dy3+ hydride complex, [K(18-crown-6)(THF)2][((Ad,MeArO)3mes)DyH], 6-Dy. Structural comparisons of 1-Ln and 2-Ln are presented with respect to their uranium analogs and correlated with density functional theory calculations on their electronic structures.
          Cover page: Comparisons of lanthanide/actinide +2 ions in a tris(aryloxide)arene coordination environmentCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2015)
          A new option for stabilizing unusual Ln2+ ions has been identified in the reaction of Cp'3Ln, 1-Ln (Ln = La, Ce; Cp' = C5H4SiMe3), with potassium graphite (KC8) in benzene in the presence of 2.2.2-cryptand. This generates [K(2.2.2-cryptand)]2[(Cp'2Ln)2(μ-η6:η6-C6H6)], 2-Ln, complexes that contain La and Ce in the formal +2 oxidation state. These complexes expand the range of coordination environments known for these ions beyond the previously established examples, (Cp''3Ln)1- and (Cp'3Ln)1- (Cp'' = C5H3(SiMe3)2-1,3), and generalize the viability of using three anionic carbocyclic rings to stabilize highly reactive Ln2+ ions. In 2-Ln, a non-planar bridging (C6H6)2- ligand shared between two metals takes the place of a cyclopentadienyl ligand in (Cp'3Ln)1-. The intensely colored (ε = ∼8000 M-1 cm-1) 2-Ln complexes react as four electron reductants with two equiv. of naphthalene to produce two equiv. of the reduced naphthalenide complex, [K(2.2.2-cryptand)][Cp'2Ln(η4-C10H8)].
            Cover page: Isolation of +2 rare earth metal ions with three anionic carbocyclic rings: bimetallic bis(cyclopentadienyl) reduced arene complexes of La2+ and Ce2+ are four electron reductants.
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2017)
            The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f n 5d1 configurations (not 4f n+1 5d0). In these 4f n 5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f n 5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.
              Cover page: Evaluating the electronic structure of formal Ln II ions in Ln II (C 5 H 4 SiMe 3 ) 3 1− using XANES spectroscopy and DFT calculationsCreative Commons 'BY' version 4.0 license
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