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Scholarly Works (25 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

Chapter one discusses the synthesis and characterization of novel complexes of the uranyl(VI) ion with a substituted phenanthroline ligand ph2phen. The complexes are investigated by electronic and vibrational spectra and their solid-state structures are presented. Luminescence spectra and lifetimes are determined. Density functional theory (DFT) calculations are carried out to investigate the electronic structure of the complexes in the ground and excited states.

Chapter two discusses the photocatalytic reactivity of the new uranyl complexes in alkane fluorination, oxidation and C-C coupling reactions. Visible light mediated functionalization of the C-H bond in the substrate is achieved through hydrogen atom abstraction. The identity of the photocatalyst is investigated for the fluorination reactions. DFT calculations are performed to compare reaction energy barriers for different photocatalysts and gauge the interactions between the substrate and ph2phen ligand. Finally, Stern-Volmer analyses are performed to determine the nature of collisional quenching between the substrate and the photocatalyst.

    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2023)

    Chapter 1. To date, the exploration of ligands that can support cerium, or any other lanthanide (Ln), in photocatalysis has been limited to halides, pseudohalides, and simple nitrogen donors. No organometallic photocatalyst has yet been reported that combines the photoexcitable cerium cation with strongly binding, multifunctional, tunable ligands. This chapter conveys the synthesis and characterization of a range of cerium photocatalysts, alongside a new family of photoactive Ln(III) compounds (Ln = La, Nd, Sm) supported by a light-absorbing tunable aryloxide-tethered N-heterocyclic carbene (NHC), tetramethylcyclopentadienyl (CpMe4), and pseudohalide ligands. The synthesis and characterization of the trivalent uranium analogue of these species is also presented, alongside investigation into the photophysical properties of select complexes.

    Chapter 2. The complexes described in Chapter 1 are capable of the rare, selective,photocatalytic C–F bond functionalization of sp3 hybridized C–F bonds, when used in conjunction with magnesium alkyl reagents. The work described in this chapter is, to the best of our knowledge, the first example of catalytic C–F bond activation and functionalization mediated by a rare-earth metal complex. Ligand light-absorption and redox non-innocence allows for the photocatalysis to be accessed even by typically photo- and redox-inactive metals including lanthanum and magnesium. Through a combination of experiment and theory, this work provides unique insight into the cooperativity between metal and ligand in photocatalysis, and the significance of ligand redox-non innocence in radical mechanisms.

    Chapter 3. The studies described in Chapter 2 of this thesis conclude that specialized, light- absorbing aryloxide ligands are required for the efficient photocatalytic cleavage of strong C– F bonds by Ln complexes. In this chapter, trivalent Ln tetramethylcyclopentadienyl complexes (Ln = La, Ce, Pr, Nd, Sm) are shown to photochemically activate the weaker sp3 carbon– chlorine bond of chlorinated hydrocarbons via a reactive lanthanide alkyl species. Becauselight absorption by the CpMe4 ligand is efficient, photocatalytic reactivity is enhanced for cerium and possible for neighboring, normally photoinactive, Ln congeners. Calculations are utilized to further understand the mechanism of this process, and preliminary results in the catalytic dechlorination of polyvinyl chloride (PVC) are presented.

      Cover page: Synthesis of low-valent lanthanide organometallics for photocatalysis and electronic structure studies
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2017)
      Neptunium complexes in the formal oxidation states II, III, and IV supported by cyclopentadienyl ligands are explored, and significant differences between Np and U highlighted as a result. A series of neptunium(iii) cyclopentadienyl (Cp) complexes [Np(Cp)3], its bis-acetonitrile adduct [Np(Cp)3(NCMe)2], and its KCp adduct K[Np(Cp)4] and [Np(Cp')3] (Cp' = C5H4SiMe3) have been made and characterised providing the first single crystal X-ray analyses of NpIII Cp complexes. In all NpCp3 derivatives there are three Cp rings in η5-coordination around the NpIII centre; additionally in [Np(Cp)3] and K[Np(Cp)4] one Cp ring establishes a μ-η1-interaction to one C atom of a neighbouring Np(Cp)3 unit. The solid state structure of K[Np(Cp)4] is unique in containing two different types of metal-Cp coordination geometries in the same crystal. NpIII(Cp)4 units are found exhibiting four units of η5-coordinated Cp rings like in the known complex [NpIV(Cp)4], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of ca. -8 pm associated with change in oxidation state between NpIII and NpIV. The rich redox chemistry associated with the syntheses is augmented by the reduction of [Np(Cp')3] by KC8 in the presence of 2.2.2-cryptand to afford a neptunium(ii) complex that is thermally unstable above -10 °C like the UII and ThII complexes K(2.2.2-cryptand)[Th/U(Cp')3]. Together, these spontaneous and controlled redox reactions of organo-neptunium complexes, along with information from structural characterisation, show the relevance of organometallic Np chemistry to understanding fundamental structure and bonding in the minor actinides.
        Cover page: Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2016)
        Heterobimetallic complexes containing short uranium-group 10 metal bonds have been prepared from monometallic IU(IV)(OAr(P)-κ(2)O,P)3 (2) {[Ar(P)O](-) = 2-tert-butyl-4-methyl-6-(diphenylphosphino)phenolate}. The U-M bond in IU(IV)(μ-OAr(P)-1κ(1)O,2κ(1)P)3M(0), M = Ni (3-Ni), Pd (3-Pd), and Pt (3-Pt), has been investigated by experimental and DFT computational methods. Comparisons of 3-Ni with two further U-Ni complexes XU(IV)(μ-OAr(P)-1κ(1)O,2κ(1)P)3Ni(0), X = Me3SiO (4) and F (5), was also possible via iodide substitution. All complexes were characterized by variable-temperature NMR spectroscopy, electrochemistry, and single crystal X-ray diffraction. The U-M bonds are significantly shorter than any other crystallographically characterized d-f-block bimetallic, even though the ligand flexes to allow a variable U-M separation. Excellent agreement is found between the experimental and computed structures for 3-Ni and 3-Pd. Natural population analysis and natural localized molecular orbital (NLMO) compositions indicate that U employs both 5f and 6d orbitals in covalent bonding to a significant extent. Quantum theory of atoms-in-molecules analysis reveals U-M bond critical point properties typical of metallic bonding and a larger delocalization index (bond order) for the less polar U-Ni bond than U-Pd. Electrochemical studies agree with the computational analyses and the X-ray structural data for the U-X adducts 3-Ni, 4, and 5. The data show a trend in uranium-metal bond strength that decreases from 3-Ni down to 3-Pt and suggest that exchanging the iodide for a fluoride strengthens the metal-metal bond. Despite short U-TM (transition metal) distances, four other computational approaches also suggest low U-TM bond orders, reflecting highly transition metal localized valence NLMOs. These are more so for 3-Pd than 3-Ni, consistent with slightly larger U-TM bond orders in the latter. Computational studies of the model systems (PH3)3MU(OH)3I (M = Ni, Pd) reveal longer and weaker unsupported U-TM bonds vs 3.
          Cover page: Metal–Metal Bonding in Uranium–Group 10 Complexes
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2015)
          The diuranium(III) compound [UN''2]2(μ-η(6):η(6)-C6H6) (N''=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory . In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal-CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal-metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.
            Cover page: Characterizing Pressure‐Induced Uranium CH Agostic Bonds
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2013)
            The new binuclear chromium Pacman complex [Cr2(L)] of the Schiff base pyrrole macrocycle H4L has been synthesized and structurally characterized. Addition of isocyanide, C≡NR (R = xylyl, tBu), or triphenylphosphine oxide donors to [Cr2(L)] gives contrasting chemistry with the formation of the new coordination compounds [Cr2(μ-CNR)(L)], in which the isocyanides bridge the two Cr(II) centers, and [Cr2(OPPh3)2(L)], a Cr(II) phosphine oxide adduct with the ligands exogenous to the cleft.
              Cover page: Isocyanide and Phosphine Oxide Coordination in Binuclear Chromium Pacman Complexes
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2020)
              Chemists have spent over a hundred years trying to make ambient temperature/pressure catalytic systems that can convert atmospheric dinitrogen into ammonia or directly into amines. A handful of successful d-block metal catalysts have been developed in recent years, but even binding of dinitrogen to an f-block metal cation is extremely rare. Here we report f-block complexes that can catalyse the reduction and functionalization of molecular dinitrogen, including the catalytic conversion of molecular dinitrogen to a secondary silylamine. Simple bridging ligands assemble two actinide metal cations into narrow dinuclear metallacycles that can trap the diatom while electrons from an externally bound group 1 metal, and protons or silanes, are added, enabling dinitrogen to be functionalized with modest but catalytic yields of six equivalents of secondary silylamine per molecule at ambient temperature and pressure.
                Cover page: Metallacyclic actinide catalysts for dinitrogen conversion to ammonia and secondary amines
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2020)
                The oxo- and catecholate-bridged UIV/UIV Pacman complex [{(py)UIVOUIV(μ-O2C6H4)(py)}(LA)] A (LA = a macrocyclic "Pacman" ligand; anthracenylene hinge between N4-donor pockets, ethyl substituents on meso-carbon atom of each N4-donor pocket) featuring a bent UIV-O-UIV oxo bridge readily reacts with small molecule substrates to undergo either oxo-atom functionalisation or substitution. Complex A reacts with H2O or MeOH to afford [{(py)UIV(μ-OH)2UIV(μ-O2C6H4)(py)}(LA)] (1) and [{(py)UIV(μ-OH)(μ-OMe)UIV(μ-O2C6H4)(py)}(LA)] (2), respectively, in which the bridging oxo ligand in A is substituted for two bridging hydroxo ligands or one bridging hydroxo and one bridging methoxy ligand, respectively. Alternatively, A reacts with either 0.5 equiv. of S8 or 4 equiv. of Se to provide [{(py)UIV(μ-η2:η2-E2)UIV(μ-O2C6H4)(py)}(LA)] (E = S (3), Se (4)) respectively, in which the [E2]2- ion bridges the two UIV centres. To the best of our knowledge, complex A is the first example of either a d- or f-block bimetallic μ-oxo complex that activates elemental chalcogens. Complex A also reacts with XeF2 or 2 equiv. of Me3SiCl to provide [{(py)UIV(μ-X)2UIV(μ-O2C6H4)(py)}(LA)] (X = F (5), Cl (6)), in which the oxo ligand has been substituted for two bridging halido ligands. Reacting A with either XeF2 or Me3SiCl in the presence of O(Bcat)2 at room temperature forms [{(py)UIV(μ-X)(μ-OBcat)UIV(μ-O2C6H4)(py)}(LA)] (X = F (5A), Cl (6A)), which upon heating to 80 °C is converted to 5 and 6, respectively. In order to probe the importance of the bent UIV-O-UIV motif in A on the observed reactivity, the bis(boroxido)-UIV/UIV complex, [{(py)(pinBO)UIVOUIV(OBpin)(py)}(LA)] (B), featuring a linear UIV-O-UIV bond angle was treated with H2O and Me3SiCl. Complex B reacts with two equiv. of either H2O or Me3SiCl to provide [{(py)HOUIVOUIVOH(py)}(LA)] (7) and [{(py)ClUIVOUIVCl(py)}(LA)] (8), respectively, in which reactions occur preferentially at the boroxido ligands, with the μ-oxo ligand unchanged. The formal UIV oxidation state is retained in all of the products 1-8, and selective reactions at the bridging oxo ligand in A is facilitated by: (1) its highly nucleophilic character which is a result of a non-linear UIV-O-UIV bond angle causing an increase in U-O bond covalency and localisation of the lone pairs of electrons on the μ-oxo group, and (2) the presence of the bridging catecholate ligand, which destabilises a linear oxo-bridging geometry and stabilises the resulting products.
                  Cover page: Selective oxo ligand functionalisation and substitution reactivity in an oxo/catecholate-bridged U IV /U IV Pacman complex
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2021)
                  A library of new dinuclear CeIV complexes of the type [NEt4]2[Ce2X6(TP)(sol)2] (X = Cl, ODipp, OSiMe3; sol = py, THF), where TP represents a family of tetraphenolate ligands that control intermetallic distance, are readily made in good yields. The ligands strongly stabilize the cerium +4 oxidation state and allow the incorporation of alkylammonium cocations in an "ate"complex formulation that enables them to be used as soluble, single-component catalysts for the ring-opening copolymerization (ROCOP) of a variety of anhydrides and epoxides. High turnover frequencies (TOFs) are achieved with high ester linkage selectivity, low dispersities, and rates that are highly tunable by the intermetallic distance enforced by the TP ligand, demonstrating that a closely coupled di-CeIV unit provides excellent rates of ROCOP catalysis and that, more generally, rare-earth complexes deserve further attention as ROCOP initiators.
                    Cover page: Dinuclear Ce(IV) Aryloxides: Highly Active Catalysts for Anhydride/Epoxide Ring-Opening Copolymerization
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2021)
                    Neutral arenes such as benzene have never been considered suitable ligands for electropositive actinide cations, yet we find that even simple UIII UX3 aryloxide complexes such as U(ODipp)3 bind and reduce arenes spontaneously at room temperature, forming inverse arene sandwich (IAS) complexes XnU(μ-C6D6)UXm (X = ODipp, n=2, m=3; X = OBMes2 n=m=2 or 3) (ODipp = OC6H3iPr2-2,6; Mes = 2,4,6-Me3-C6H2). In some of these cases, further arene reduction has occured as a result of X ligand redistribution. These unexpected spontaneous reactions explain the anomalous spectra and reported lack of further reactivity of strongly reducing UIII centers of U(ODipp)3. Phosphines that are not considered suitable ligands for actinides can catalyze the formation of the IAS complexes. This enables otherwise inaccessible asymmetric and less congested IAS complexes to be isolated and the bonding in this series compared.
                      Cover page: Instantaneous and Phosphine-Catalyzed Arene Binding and Reduction by U(III) ComplexesCreative Commons 'BY-ND' version 4.0 license
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