Abstract
This thesis reports synthetic studies towards M6L8 octahedral metallosupramolecular cages constructed using hexa-peri-hexabenzocoronene (HBC)-based ligands. HBCs are a polycyclic aromatic hydrocarbon (PAH) consisting of a sixfold-symmetric, rigid, planar core of 42 carbon atoms, and have a number of interesting physical properties.
Benzene core ligands were synthesised as smaller analogues of the target HBC core ligands, used to develop synthetic routes to key building blocks and assess cage formation. M6L8 octahedral Pd(II) cages were successfully synthesised with the benzene core ligands, indicating that the HBC core M6L8 octahedral Pd(II) cages are within the realm of possibility. Attempts at encapsulation of organic/aromatic guests were not successful, although the benzene core cages display host-guest chemistry with anionic tosylate guests.
Two synthetic pathways to the target HBC core ligands were investigated. Attempts to synthesise target HBC core ligands by threefold palladium cross-coupling reactions between a triiodo-HBC core and unfunctionalised pyridine-linker units yielded an insoluble and inseparable mixture of di- and tri-substituted HBC core ligands. Organic solvent soluble mixture of di- and tri-substituted HBC core ligands, functionalised with solubilising hexyloxy chains, were synthesised using the same methods but could not be purified using chromatography.
An alternative pathway involving tris(biphenyl)benzene (TBB) intermediates bearing pyridyl groups provided a pure sample of a target hexyloxy functionalised HBC core ligand with phenylene linkers. Attempts at forming a HBC core cage with this ligand, however, were unsuccessful due to the low solubility of the HBC core ligand in the reaction solvent. This ligand synthesis pathway, involving oxidative cyclodehydrogenation as the final step, was less successful when applied to the related ethynylene linked pyridyl TBB precursor.
Both unsubstituted and hexyloxy functionalised tripyridyl TBB core ligands were synthesised with the aim of forming M6L8 octahedral Pd(II) cages. Preliminary studies towards the synthesis of the octahedral cages with TBB ligands did not provide clear evidence of the desired cage structures.
Having overcome a number of obstacles in developing a reliable and flexible synthetic route to TBB and HBC tripyridyl ligands, future efforts will focus on tailoring their solubility for cage formation and guest encapsulation.