Doctor of Philosophy
Kim M. Baines
The work described in this thesis incorporates three main themes: the synthesis and reactivity of new coordination and organometallic gallium compounds, and the chemical state determination of molecular gallium complexes using XPS and XAS. The coordination chemistry of low valent gallium cations was explored using macrocyclic ethers as ligands. The experimental oxidation number, or chemical state, of newly synthesized low valent gallium cationic complexes was compared to known compounds to allow for the assessment of the electronic environment at gallium. The organometallic chemistry of gallium was examined using donor ligands to stabilize monomeric organogallium(III) compounds, demonstrating the ability to substitute the ligands on gallium and to generate a compound containing a gallium-carbon double bond.
Two multinuclear low valent gallium cations were synthesized using cryptand[2.2.2] as a stabilizing ligand and Ga2Cl4(THF)2 as a starting material. Conventional characterization techniques and computational methods were used to examine the structure and bonding of the cationic gallium cores contained within the cryptand ligand. These compounds are the first examples of binuclear cryptand[2.2.2] complexes, where two metal centres are located within the cryptand cavity.
The experimental chemical states, namely the experimentally determined electronic environments or partial charges, of the gallium centres in two gallium-cryptand[2.2.2] complexes were evaluated using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) as a means of probing the electronic environment of the gallium centres. The experimental XPS data of the gallium-cryptand complexes were compared to known gallium compounds with unambiguously assigned oxidation numbers to determine the electron density at the gallium centres and to allow for an assessment of their potential reactivity. To overcome the instrumental limitations of the XPS experiments, XAS studies of the synthesized gallium-cryptand[2.2.2] complexes and other low valent gallium compounds with multiple gallium atoms were performed to separate the signals originating from the individual gallium centres. The higher resolution of the XAS data allowed for the observation of multiple signals from gallium centres with different assigned oxidation numbers within a single complex and gave additional information on the electronic structure and bonding of the cryptand complexes in conjunction with computational studies.
The synthesis and reactivity of a gallium(I) cationic complex using 12-crown-4 as a stabilizing ligand was explored. The synthesis of [Ga(12-crown-4)][GaCl4] was achieved in one step from commercially available starting materials. Anion exchange reactions to replace the reactive tetrachlorogallate anion for the perfluorophenylborate were performed. [Ga(12- crown-4)][B(C6F5)4] was analyzed using XPS, which allowed for the classification of the gallium(I)-crown ether complex as electron deficient. Reactions of the gallium(I)-crown ether complex with Cp*K, cryptand[2.2.2], and DMAP demonstrated the facile synthesis of known gallium(I) compounds as well as the generation of novel gallium(I) cations, highlighting the use of the gallium(I)-crown ether complex as an effective starting material for new gallium(I) compounds.
The synthesis of a compound with a gallium-carbon double bond, a gallene, was explored. The synthetic route utilized was inspired by strategies reported for the synthesis of compounds containing main group element-carbon double bonds with the key step being a dehydrohalogenation of a gallium(III) fluoride. The precursor organogallium(III) fluorides were synthesized using an NHC and DMAP as donors ligands to stabilize neutral species. Dehydrohalogenation of the gallium(III) fluoride was examined in an attempt to generate a gallene. Tolualdehyde was used as a trapping agent in situ, resulting in the formation of a 2:1 cycloadduct, giving evidence for the generation of an intermediate gallene. The synthetic route presented highlights the use of donor stabilization to facilitate ligand substitution and exchange reactions for neutral organogallium compounds.
Bourque, Jeremy L., "Coordination and Organometallic Chemistry of Novel Gallium Complexes: Synthesis, Reactivity and Spectroscopy" (2017). Electronic Thesis and Dissertation Repository. 5026.