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Synthesis and Spectroscopy of Aluminum Molecules and Materials for Investigating Electronic Structure in f-Element-Aluminum Bimetallics

  • Author(s): Altman, Alison Beth
  • Advisor(s): Arnold, John
  • et al.

Chapter 1:

Polarized aluminum K-edge X-ray Absorption Spectroscopy (XAS) and first principle calculations were used to probe electronic structure in a series of (BDI)Al, (BDI)AlX2, and (BDI)AlR2 coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-β-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XAS measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal–ligand bonding. Qualitative trends in core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)AlX2 systems with more electronegative X1- ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)AlH2 the experimental Al K-edge XAS spectra and spectra calculated using the eXcited electron and Core-Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)AlH2 are similar relative to the (BDI)AlX2 and (BDI)AlMe2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b1 symmetry that is localized on Al and orthogonal to the (BDI)Al plane. The presence of this low energy unoccupied molecular orbital on electron-rich (BDI)Al constitutes a more distinguishing aspect of its valence electronic structure relative to the formally trivalent compounds (BDI)AlX2 and (BDI)AlR2. The work shows that Al K-edge XAS can be used to provide valuable insight into electronic-structure to reactivity relationships for main-group coordination compounds.

Chapter 2:

Oxygen and aluminum K-edge XAS, imaging from a scanning transmission X-ray microscope (STXM), and first principles calculations were used to probe the composition and morphology of bulk aluminum metal, α- and γ-Al2O3, and several types of aluminum nanoparticles. The imaging results agreed with earlier transmission electron microscopy studies that showed a 2 to 5 nm thick layer of Al2O3 on all the Al surfaces. Spectral interpretations were guided by examination of the calculated transition energies, which agreed well with the spectroscopic measurements. Features observed in the experimental O and Al K-edge XAS were used to determine the chemical structure and phase of the Al2O3 on the aluminum surfaces. For unprotected 18 and 100 nm Al nanoparticles, this analysis revealed an oxide layer that was similar to γ-Al2O3 and comprised of both tetrahedral and octahedral Al coordination sites. For oleic-acid protected Al nanoparticles, only tetrahedral Al oxide coordination sites were observed. The results were correlated to trends in the reactivity of the different materials, which suggests that the structures of different Al2O3 layers have an important role in the accessibility of the underlying Al metal towards further oxidation. Combined, the Al K-edge XAS and STXM results provided detailed chemical information that was not obtained from powder X-ray diffraction or imaging from a transmission electron microscope.

Chapter 3:

Correlated electron phenomena in lanthanide and actinide materials are driven by a complex interplay between the f and d orbitals. In this study, aluminum K-edge XAS and Density Functional Theory calculations were used to evaluate the electronic structure of the dialuminides, MAl2 (M = Ce, Sm, Eu, Yb, Lu, U, and Pu). The results showed how the energy and occupancy of the 4f or 5f orbitals impacted mixing of Al 3p character into the 5d or 6d conduction bands, which has implications for understanding the magnetic and structural properties of correlated electron systems.

Chapter 4:

The synthesis and reactivity of paramagnetic heterometallic complexes containing a Ti(III)-µH-Al(III) moiety are presented. Combining different stoichiometries of Cp2TiCl and KH3AlC(SiMe3)3 (Cp = cyclopentadienyl) resulted in the formation of either bimetallic Cp2Ti(µ-H)2(H)AlC(SiMe3)3 or trimetallic (Cp2Ti)2(µ-H)3(H)AlC(SiMe3)3 via salt metathesis pathways. While these complexes were indefinitely stable at room temperature, the bridging hydrides were readily activated upon exposure to heteroallenes, heating, or electrochemical oxidation. In each case, formal hydride oxidation occurred, but the isolated product maintained the +3 oxidation state at both metal centers. The nature of this reactivity was explored using deuterium labelling experiments and Density Functional Theory (DFT) calculations. It was found that while C-H activation from the Ti(III) bimetallic may occur through a -bond metathesis pathway, chemical oxidation to Ti(IV) promotes bimolecular reductive elimination of dihydrogen to form a Ti(III) product.

Chapter 5:

Here, we present the synthesis of a low-valent thorium heterobimetallic containing the Th(III)-µH-Al(III) motif via reduction of a Th(IV)Cl precursor, as well as the direct synthesis by salt metathesis of a uranium(III) aluminum heterobimetallic analogue. These complexes were structurally characterized and compared using electron paramagnetic resonance techniques to a related titanium(III) system, and evidence was found for significant aluminum participation in the orbital occupied by the unpaired electron in the thorium system. DFT calculations confirmed the ThAl character of this interaction.

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