Multidentate ligands for aluminium hydrides and halides
Creators: name: Levy, Abigail
Publisher: name: The University of Edinburgh
Year: 2022

description: Low-oxidation state organoaluminium compounds, in which aluminium is in the oxidation state +1, are highly reactive yet isolable species. The desire to synthesise and study aluminium(I) compounds stems from their potential reactivity, in particular oxidative addition, and the prospect of offering an alternate to rare and expensive transition metals. To access stable aluminium(I) species, suitable ligands and precursors are required. This thesis discusses the synthesis, properties and reactivity of multidentate ligand-supported aluminium halide and hydride complexes, which are explored as potential precursors to aluminium(I) complexes. Chapter 1 presents an overview of the synthesis and properties of reported aluminium hydride compounds including alane, (AlH3)n, and its adducts, as well as organoaluminium hydrides which are supported by group 14- and group 15-based ligands. The use of aluminium halides as precursors to low-oxidation state aluminium species is also discussed, along with a brief overview of aluminium(I) chemistry. In addition, the properties of bi- and tri-dentate ligands and the affects they may have on metal centres are also highlighted. Chapter 2 discusses the synthesis, reactivity and potential reduction chemistry of PNP-pincer ligand-supported aluminium hydrides and halides. The nitrogen and phosphorus-containing pincer ligand has a rigid carbazole backbone bearing two phosphine donors which allows for tridentate coordination to the aluminium centre. Varying the P-substituents from phenyl groups to tert-butyl groups, offers insight into how the steric and electronic properties of the ligand can affect the preparation and reactivity of the aluminium hydride and halide compounds, and whether PNP-type ligands are able to support neutral aluminium(I) centres. The reactivity of a PNP-aluminium dihydride towards anilines and isonitriles, along with magnesium and palladium complexes was explored. The reduction of PNP-aluminium halide complexes was also attempted. Chapter 3 discusses the use of carbon- and nitrogen-containing pincer ligands to support aluminium centres. The NCN-pincer ligand consists of a phenyl backbone with two dimethylamine donors. The small NCN-ligand offers different donating and steric properties compared to the ligand discussed in chapter 2. This work shows the synthesis of the aluminium dibromide, (MeNCN)AlBr2, which is then studied as a potential precursor to NCN-supported aluminium(I). The NCN-aluminium dibromide complex was reacted with a range of group 1 metal reducing agents, including potassium naphthalenide and sodium-potassium alloy. The magnesium(I) dimer [(MesNacNac)Mg]2 was also explored as a reducing agent. Chapter 4 explores the potential for diphosphide ligands to support an anionic aluminium(I) centre. Many reported aluminyl anions use nitrogen-based ligands, but phosphorus-containing aluminyl anions are yet to be reported. Two novel diphosphines, with a silicon-containing alkyl linker, are used as the ligand precursors. The preparation of the diphosphide-supported aluminium bromide, {TippSiP}AlBr(THF), is discussed. Reduction of the aluminium bromide complex is attempted with the aim to synthesise the aluminium(I) anion or the aluminium(II) dimer. Moreover, the contrast in properties of the diphosphine ligands compared to the diamine analogue is revealed in the synthesis of phosphine-aluminium adducts, in which P→Al dative bonds are formed.