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Scholarly Works (11 results)

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UC Berkeley Electronic Theses and Dissertations (2023)

Chapter 1. Low-valent transition metalates—anionic, electron-rich organometallic complexes—comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M–E bond formation, among others. In this chapter, we briefly discuss the history and development of low-valent metalates, their fundamental chemical properties and ligand considerations, and highlight notable applications of metalates in the literature. From there, we introduce our group’s contribution to the field of low-valent metalates and describe the development of a rhenium(I) metalate. We then demonstrate the diverse utility of the highly reactive rhenium moiety through an account of four illustrative examples that highlight the metalates ability to react at multiple locations and as a reductant, nucleophile, and metalloligand.

Chapter 2. This chapter introduces the rhenium(I) metalates propensity to reversibly bind dinitrogen in solution. As a result, we are able to synthesize and characterize rhenium-group 9 heterobimetallic diazenido species that contain a non-linear end-on/end-on coordinated bridging dinitrogen fragment. By avoiding a dinitrogen reaction atmosphere, we are able to access non-diazenido heterobimetallic complexes using the same reagents, this time forming two different bridging hydride species. As for the diazenido complexes, computational investigations reveal that the unprecedented bonding arrangement primes the distal nitrogen for reactivity with electrophiles through a push-pull type activation mechanism. As a result, we are able to alkylate the nitrogen proximal to the group 9 metal, demonstrating the first example of N–C bond formation at a heterobimetallic dinitrogen complex.

Chapter 3. By synthesizing a series of rhenium-Group 14 complexes, this chapter reveals the strong π-donor properties of a rhenium(I) metalate. Using amidinate-supported tetrylene reagents, we observe that rhenium reacts with the silylene and germylene to form complexes with short, strong rhenium-tetrylene bonds. Through a combination of experimental and computational investigations, we reveal the significant σ and π contributions to the bonding orbitals, and compare these bonds with previously reported tetrylenes and tetrylynes. Additionally, we elaborate on the full reactivity between rhenium and the silylene reagent, revealing a bridging hydride minor product that likely forms upon decomposition of an isolable, diazenido-bridged rhenium-silylene intermediate. Finally, we note that analogous reactions with an amidinate-supported stannylene does not lead to π-type bonding with rhenium, indicating a break in reactivity within the Group 14 series as we get to the heaviest tetrylenes.

Chapter 4. Utilizing the rhenium(I) metalate, we access a full series of halide and chalcogenolate rhenium(III) complexes. Distinct trends in NMR chemical shifts down the halide and chalcogenolate series were noted, and attributed to the presence of temperature independent paramagnetism (TIP) in these Re(III) complexes, which was confirmed through magnetic susceptibility studies. While non-negligible TIP has been reported previously among octahedral d4 and square planar d6 metal complexes, this is one of the first studies to present a systematic series of complexes displaying TIP, allowing for direct observation of the role ligand identity (such as the halides) plays on TIP behavior. Additionally, we were able to perform multiconfigurational CASSCF calculations on the rhenium(III) halide series to investigate the unique coupling schemes each complex experiences between ground and low-lying excited states. These results expand the playbook of robust rhenium(III) chemistry, advance our understanding of TIP coupling schemes in related metal complexes, and help inform the role of ligand identity on electronic structure.

Chapter 5. We highlight the utility of the rhenium(I) metalate as an inverse sandwich type metalloligand in this chapter. First, we report the synthesis and characterization of of a series of tri-rhenium triple inverse sandwich complexes with the early lanthanides. However, while attempting to extend the series further, we experienced a gadolinium break wherein we synthesized two products for gadolinium, including a triple inverse sandwich complex as well as a diazenido bis-inverse sandwich complex. Analogues of the gadolinium diazenido were observed for other heavy lanthanides, suggesting that the heavier lanthanides prefer to adopt a diazenido structure whereas the lighter lanthanides form a triple inverse sandwich geometry. The end-on/end-on heterobimetallic diazenido moiety observed in the gadolinium complex was replicated by synthesizing other erbium and uranium diazenido complexes, representing rare examples of lanthanide and actinide end-on dinitrogen fragment coordination. Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing both a bis-inverse sandwich complex using a divalent lanthanide as well as a base-free, homoleptic rhenium-rare earth triple inverse sandwich complex.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2021)
    Salt metathesis reactions between a low-valent rhenium(i) complex, Na[Re(η5-Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), and a series of amidinate-supported tetrylenes of the form ECl[PhC(NtBu)2] (E = Si, Ge, Sn) led to rhenium metallotetrylenes Re(E[PhC(NtBu)2])(η5-Cp)(BDI) (E = Si (1a), Ge (2), Sn (4)) with varying extents of Re-E multiple bonding. Whereas the rhenium-stannylene 4 adopts a σ-metallotetrylene arrangement featuring a Re-E single bond, the rhenium-silylene (1a) and -germylene (2) both engage in π-interactions to form short Re-E multiple bonds. Temperature was found to play a crucial role in reactions between Na[Re(η5-Cp)(BDI)] and SiCl[PhC(NtBu)2], as manipulation of reaction conditions led to isolation of an unusual rhenium-silane, (BDI)Re(μ-η5:η1-C5H4)(SiH[PhC(NtBu)2]) (1b) and a dinitrogen bridged rhenium-silylene, (η5-Cp)(BDI)Re(μ-N2)Si[PhC(NtBu)2] (1c), in addition to 1a. Finally, the reaction of Na[Re(η5-Cp)(BDI)] with GeCl2·dioxane led to a rare μ2-tetrelido complex, μ2-Ge[Re(η5-Cp)(BDI)]2 (3). Bonding interactions within these complexes are discussed through the lens of various spectroscopic, structural, and computational investigations.
      Cover page: σ or π? Bonding interactions in a series of rhenium metallotetrylenes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2023)
      Simple lanthanide cyclopentadienyl (Cp) complexes can photochemically cleave the sp3 carbon-chlorine bond of unactivated chlorinated hydrocarbons including polyvinyl chloride (PVC). The excited state lifetimes of these simple complexes are among the longest observed for cerium complexes (175 ns for [(CpMe4)2Ce(μ-Cl)]2) and the light absorption by the Cp ligand is efficient, so photocatalytic reactivity is enhanced for cerium and now also made possible for neighboring, normally photoinactive, lanthanide congeners.
        Cover page: Photocatalytic dechlorination of unactivated chlorocarbons including PVC using organolanthanide complexesCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2023)
        Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H3] with UCl4 and ThCl4(DME)2 led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H)3IrCp*}4 (1) and Th{[(μ-H2)(H)IrCp*]2[(μ-H)3IrCp*]2} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI3(1,4-dioxane)1.5, resulted in the octanuclear complex {U[(μ2-H3)IrCp*]2[(μ3-H2)IrCp*]}2 (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.
          Cover page: A versatile strategy for the formation of hydride-bridged actinide–iridium multimetallicsCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2024)
          We expand upon the synthetic utility of anionic rhenium complex Na[(BDI)ReCp] (1, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex 1 reacts in a metathetical manner with chlorophosphines Ph2PCl, MeNHP-Cl, and OHP-Cl to generate XL-type phosphido complexes 2, 3, and 4, respectively (MeNHP-Cl = 2-chloro-1,3-dimethyl-1,3,2-diazaphospholidine; OHP-Cl = 2-chloro-1,3,2-dioxaphospholane). Crystallographic and computational investigations of phosphido triad 2, 3, and 4 reveal that increasing the electronegativity of the phosphorus substituent (C < N < O) results in a shortening and strengthening of the rhenium-phosphorus bond. Complex 1 reacts with iminophosphane Mes*NPCl (Mes* = 2,4,6-tritert-butylphenyl) to generate linear iminophosphanyl complex 5. In the presence of a suitable halide abstraction reagent, 1 reacts with the dichlorophosphine iPr2NPCl2 to afford cationic phosphinidene complex 6+. Complex 6+ may be reduced by one electron to form 6•, a rare example of a stable, paramagnetic phosphinidene complex. Spectroscopic and structural investigations, as well as computational analyses, are employed to elucidate the influence of the phosphorus substituent on the nature of the rhenium-phosphorus bond in 2 through 6. Furthermore, we examine several common analogies employed to understand metal phosphido, phosphinidene, and iminophosphanyl complexes.
            Cover page: Panoply of P: An Array of Rhenium–Phosphorus Complexes Generated from a Transition Metal AnionCreative Commons 'BY-ND' version 4.0 license
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2024)
            A single-source-precursor approach was developed to synthesize uranium-based materials outside of the typically-studied oxides. This approach allows for shorter reaction times, milder reaction conditions, and control over the chemicals present in synthesis. To this end, the first homoleptic uranium thioamidate complex was synthesized as a precursor for US2 materials. Pyrolysis of the thioamidate results in decomposition via an alkene elimination pathway and formation of γ-US2, which has historically been hard to access without the need for a secondary sulfur source. Despite the oxophilicity of uranium, the method successfully forms US2 without the inclusion of oxygen in the bulk final product. These findings are supported by simultaneous thermal analysis, elemental analysis, powder X-ray diffraction, and uranium L3-edge X-ray absorption fine-structure spectroscopy. This work represents the first example of a single-source precursor approach to target and synthesize actinide materials other than the oxides.
              Cover page: Formation of uranium disulfide from a uranium thioamidate single-source precursor
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              The low-valent rhenium(I) salt Na[Re(η5-Cp)(BDI)] (BDI = N,N′-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) was shown to react with various halide and chalcogenolate reagents, leading to the isolation of a series of rhenium(III)-halide and -pseudohalide complexes: Re(X)(η5-Cp)(BDI) (1-X, X = F, Cl, Br, I; 2, X = OTf) and Re(ER)(η5-Cp)(BDI) (3-ER, ER = SBn, SeBn, TePh). The 1H NMR spectra of these complexes displayed sharp resonances shifted several ppm from typical diamagnetic regions, as well as distinct chemical shift trends down both the halide (with the exception of F) and chalcogenolate series, with both Cp and BDI backbone peaks moving downfield with increasing atomic number. Subsequent magnetic susceptibility measurements of the rhenium(III) halides 1-Cl, 1-Br, and 1-I indicated that these complexes display temperature-independent paramagnetism (TIP), with χTIPvalues of 7.41(44) × 10-4to 1.50(51) × 10-3cm3mol-1. Multireference complete active space self-consistent field (CASSCF) computations incorporating spin-orbit state mixing revealed small energy separations (1.7-3.0 kcal/mol) between thermally isolated ground states and the first excited spin-orbit states for the rhenium halides, confirming that low-lying excited states are responsible for the observed TIP behavior.
                Cover page: Spectroscopic, Magnetic, and Computational Investigations on a Series of Rhenium(III) Cyclopentadienide β‑diketiminate Halide and Pseudohalide ComplexesCreative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2024)
                Amidate-based ligands (N-(tert-butyl)isobutyramide, ITA) bind κ2 to form homoleptic, 8-coordinate complexes with tetravalent 237Np (Np(ITA)4, 1-Np) and 242Pu (Pu(ITA)4, 1-Pu). These compounds complete an isostructural series from Th, U-Pu and allow for the direct comparison between many of the early actinides with stable tetravalent oxidation states by nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction (SCXRD). The molecular precursors are subjected to controlled thermolysis under mild conditions with the exclusion of exogenous air and moisture, facilitating the removal of the volatile organic ligands and ligand byproducts. The preformed metal-oxygen bond in the precursor, as well as the metal oxidation state, are maintained through the decomposition, forming fully stoichiometric, oxidation-state pure NpO2 and PuO2. Powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mapping supported the evaluation of these high-purity materials. This chemistry is applicable to a wide range of metals, including actinides, with accessible tetravalent oxidation states, and provides a consistent route to analytical standards of importance to the field of nuclear nonproliferation, forensics, and fundamental studies.
                  Cover page: Formation of Fully Stoichiometric, Oxidation-State Pure Neptunium and Plutonium Dioxides from Molecular Precursors
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2021)
                  Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)2]2(μ-N)(μ-κ1:κ1-BCMA) (7), [(U(BIMA)2)2(μ-N)(μ-NiPr)(K2(μ-η3:η3-CH2CHNiPr)]2 (8), and [U(BIMA)2]2(μ-N)(μ-κ1:κ1-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. 15N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N3)(BCMA)3 yielded the cationic U(V) species [U(N3)(BCMA)3][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)3, U(N3)(BCMA)3, and [U(N3)(BCMA)3][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.
                    Cover page: Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides
                    • Article
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                    UC Berkeley Previously Published Works (2021)
                    Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)3 (2) with KC8 yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)2] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I2 to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)2)2(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η7-C7H7)(TDA)2(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)4] (1-crypt) and U(COT)2, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)3]2(μ-η8:η3-C8H8) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)4 (4) with KC8 gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)3(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.
                      Cover page: A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon
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