Synthesis and Reactivity of an Aluminyl Anion
The work detailed in this thesis describes the synthesis and reactivity of an N-heterocyclic aluminyl anion ([Al(NONDipp)]–, NONDipp = [O(SiMe2NDipp)2]2–, Dipp = 2,6-iPr2-C6H3) charge-balanced by an alkali metal cation. As a low-valent aluminium(I) nucleophile, the chemistry of the aluminyl anions is versatile and remains a rapidly developing area of chemistry.
Chapter One provides a comprehensive literature review on the different aluminyl anion systems and their general structure and reactivity. Commonly isolated as potassium salts, the aluminyl anions can be used to access a range of other bimetallic systems that contain novel Al–M bonds that have demonstrated the reduction of carbon dioxide and other heteroallenes. The potassium aluminyls are by far the most well studied, displaying a rich chemistry that exploits the reduction potential and nucleophilicity of the Al(I) centre. Several modes of reactivity have been identified, including oxidative addition, reductive coupling, cycloaddition, and oxidation reactions. A key finding is the synergistic interactions between the Al and K metal centres which influence the outcome of a reaction.
Chapter Two extends the chemistry of the potassium aluminyls to the other alkali metals via reduction of the iodide precursor, Al(NONDipp)I. Three main structural classes were identified, depending on the coordination sphere of the alkali metal. Contacted dimeric pairs were isolated in absence of coordinating solvents, where cations were held between flanking aryl interactions of the Dipp substituents. Monomeric ion pairs could be accessed from the addition of monodentate or bidentate ligands to the corresponding contacted dimeric pairs, rendering discrete Al–M bonds. Separated ion pairs were formed on the addition of suitable polydentate donors to the corresponding contacted dimeric pairs, giving a naked aluminyl with no interactions between the cationic and anionic components.
Chapter Three investigates the oxidative addition chemistry of the alkali metal aluminyls towards polar acidic, polar basic, and non-polar E–H bonds. In each instance, the E–H bonds add across the Al centre to give the corresponding anionic aluminium(III) hydride complexes. The installed Al–H bonds are highly polar and were shown to reduce carbon dioxide under ambient conditions to give formate complexes. Hydroboration of these formate complexes was attempted and shown to yield the corresponding anionic aluminium(III) alkoxide complexes.
Chapter Four details the reduction of ethene and propene by a potassium aluminyl complex. For ethene, a highly strained metallacycle was formed that was shown to engage in C–H activation and ring expansion reactions. Of note is the carbonylation of ethene, which successfully installs the desired C=O functionality to grow a C3-chain. Clear synergistic effects were observed in this reaction, where the contacted dimeric pair and monomeric ion pair formed different products. For propene, only C–H activation was observed due to the acidity of the allylic proton.
Chapter Five explores the homologation and reductive coupling of CO and isonitriles by the alkali metal aluminyls. A range of chain-growth products were formed in these reactions, from the C3 → C4 → C5 homologation of CO, which was found to be dependent on the alkali metal cation. To understand this process in more detail, the isonitriles were reacted with the potassium aluminyl and shown to give C2 → C3 products. The results in this chapter further highlight cooperation between the aluminium and alkali metals in these systems, where the aluminyl complexes are capable of cleaving extremely strong bonds.