Preparation of Brand New Ligand Architectures Incorporating N-Heterocyclic Carbene Complexes with Carboranyl Substituents: Coordination Chemistry, Catalytic Evaluation and Anticancer Properties

In this thesis two very different types of ligand classes are brought together, namely N-heterocyclic carbenes (NHCs) and carboranes. Chapter one discusses the fundamental structure and bonding involved in these two types of compound. An account of the synthetic strategies employed in the literature for the synthesis of NHCs and carboranes, and the techniques employed to coordinate these ligands to various metal centres is discussed.
Chapter two discusses the synthetic strategies explored towards a new hybrid ligand scaffold that fuses N-heterocyclic carbenes (NHCs) and o-carborane. A focal point of this work was to demonstrate the tailorability of this novel ligand system. A detailed account of the techniques employed for the characterisation of these novel ligands is given.
Chapter three discusses the synthetic challenges associated with deprotonation of the zwitterionic ligand precursors reported in Chapter 2. A deprotonation study revealed that the ligand must possess an ethyl linker or longer between the imidazolium and the carborane for a free carbene to be isolated. Subsequent coordination to various metal centres, including titanium, iron and rhodium, is explored.
The synthesis and full characterisation of RhI-NHC complexes that exhibit closo-carboranyl substituents is described in Chapter four. These complexes display unprecedented chemistry with a rare example of a RhI-metallacycle coordinating through both the NHC and the carbon vertex of the carborane. The catalytic activity of these complexes was probed in the hydrosilylation of acetophenone.
Chapter five investigates the application of our NHC-carborane ligands to the higher oxidation states of RhIII and IrIII as well as RuII. We have discovered that the nature of the α-substituent of the NHC, as well as reaction conditions, can determine the vertex at which cyclometallation occurs. A series of IrIII complexes were successfully cyclometallated through the NHC and either a carbon atom or the B3/B6 vertex of the cage. Examination of the Ir complexes in the transfer hydrogenation of acetophenone revealed that cyclometallation through the carborane has a profound effect on the catalytic activity, indicating a bifunctional mechanism and involvement of the carborane moiety.
Chapter six describes the diverse coordination modes of NHC-carborane ligands at AgI. The steric nature of the ligand precursor plays a crucial role in the overall stability and the type of silver complex formed, with the first structurally elucidated example of AgI directly bound through the carborane reported. In addition, a series of carboranyl complexes derived from the natural product theobromine were synthesised. The antiproliferative properties of these complexes against the HCT116 (colon) cancer cell line was evaluated.