Abstract
This thesis consists of four chapters.
Chapter 1 introduces the process of self-assembly, with particular focus on a subset of self-assembly termed metallosupramolecular chemistry. The methods for the generation of metallosupramolecular architectures, in particular metallosupramolecular cages, are discussed, including the directional bonding, symmetry interaction, sub-component self-assembly, weak link, and post-assembly modification approaches. Some important applications of these structures are also highlighted, with focus placed on applications such as catalysis, drug delivery, spin cross-over, separation, stabilisation of reactive molecules and sequestration of environmental pollutants. The aims of the project are also laid out; the first being the synthesis of a family of monodentate, bidentate and tridentate ligands utilising imidazole and pyridine donors. The second aim of the project is the generation of palladium and platinum architectures of various sizes from these ligands. In particular, the construction of M2L4 cages from the bidentate ligands, and M6L8 and M3L2 cages from the tridentate ligands are a focus of this project. The third aim of this project involves the exploration of the applications of these architectures.
Chapter 2 discusses the design principles considered when synthesising ligands for the ultimate construction of supramolecular cages and macrocycles. A type of click chemistry, the copper catalysed azide-alkyne cycloaddition (CuAAC) reaction, is introduced and its uses discussed. Building on previous work by the Crowley group, a variety of ligands were synthesised using the CuAAC reaction. Starting from an appropriately substituted benzyl bromide starting material, the bromine substituent was replaced with an azide functional group, which was further reacted with either 3-ethynyl pyridine or 5-ethynyl-1-methyl-1H-imidazole via a CuAAC reaction. The resulting flexible monodentate, bidentate or tridentate ligands featured a benzene core connected via a methylene bridge to either triazole-pyridine or triazole-methylimidazole substituents. In an effort to pre-organise the larger tridentate ligands, three ethyl chains were incorporated into the structure, alternating with the three donor arms. This sterically geared the three donor arms to occupy one face of the benzene core while the ethyl chains occupied the other.
Chapter 3 details efforts to synthesise palladium and platinum cages and macrocycles. Recent work and applications of these structures are described in detail. Following this, attempts to construct platinum and palladium cages and macrocycles from the ligands described in chapter 2 are discussed. Initially, the synthesis of Pd2L4 cages from the four flexible bidentate ligands was explored. However, these efforts were somewhat hindered both by the insoluble and flexible nature of the ligands. Ultimately, only a PdL2 molecular bow tie was hypothesised to form from a meta-substituted pyridine-based ligand. Focus was then shifted to the synthesis of Pd6L8 cages using both tridentate ligands. Again, the flexible nature of the ligands most likely inhibited the success of these syntheses, resulting in mixtures of various architectures which were not able to be isolated. The project then pivoted to target M6L4 or M3L2 assemblies using the tridentate ligands. When an imidazole-based tridentate ligand was complexed with palladium(II)chloride in a 2:3 ratio, a single architecture was observed to form. X-ray crystallography data later confirmed the presence of a Pd3L2 cage. Further success was achieved using a tridentate pyridine-based ligand, with data consistent with the formation of another Pd3L2 cage. Additional syntheses using tridentate ligands resulted in the formation of a variety of architectures, including a Pt2L2 macrocycle and a Pt3L2 cage.
Chapter 4 explores carbon-carbon bond forming reactions, in particular the Suzuki-Miyaura cross-coupling reaction. Palladium-based catalysts used in the literature for this reaction are discussed, including palladium complexes and cages. Using the cross-coupling of phenylboronic acid with 4-bromoacetophenone as a model system, the catalytic abilities of a Pd3L2 cage were explored. The Pd3L2 cage (5 mol%) was able to catalyse the Suzuki-Miyaura cross-coupling reaction, performing well in water/ethanol mixtures and glycerol under inert conditions at 80 ℃. Further optimisation is still required, however, for the cage to be as efficient as other palladium cages already published in literature.