Transition metal complexes have a remarkable diversity of uses, with applications in fields varying from medicine to catalysis and materials science. The development of precious metal catalysis has dramatically altered the field of organometallic chemistry. The most well established and consequently most frequently used catalysts contain palladium, gold, iridium and rhodium. While these catalysts are frequently employed in organic synthesis, they suffer from the drawbacks of being some of the scarcest elements available on Earth and therefore also some of the most expensive. These realities have motivated scientists to search for more abundant and cheaper alternatives. In contrast to this reliance on precious metals, Nature is able to effectively harness the reactivity of Earth-abundant metals. Exploiting the reactivity of metals utilised in biological systems will allow the development of environmentally and biologically benign catalysts from metals such as iron. Accordingly this will revolutionise ways of constructing new carbon-carbon and carbon-heteroatom bonds, processes that are fundamental to synthetic organic chemistry. Reliable methods that exhibit good functional group tolerance are highly desirable and cycloisomerisation reactions, which construct carbocyclic and heterocyclic rings, are key to this work. This proposal will develop robust methodology utilising redox active iron-based complexes as catalysts that are both efficient and convenient, holding great importance in organic chemistry for the reliable construction of a diverse range of heterocycle motifs. Through this project novel iron-based catalysts will be developed that have the potential of improving the cost effectiveness of important synthetic processes. This in turn will impact positively on industries that rely on heterocyclic molecules (e.g. pharmaceuticals).