Photoredox Catalysis Driven by Visible Light and Iron N-Heterocyclic Carbene Complexes

Abstract: Photoredox catalysis is a rapidly expanding field, in which organic transformations are catalysed using light. The most commonly used photoredox catalysts (PCs) are transition metal complexes which absorb in the visible region of the electromagnetic spectrum. Traditionally, such PCs are based on scarce and expensive metals, such as ruthenium or iridium, in combination with polypyridyl ligands. Research has been dedicated to replacing these metals with the Earth-abundant metal iron. However, iron polypyridyl complexes exhibit poor excited state (ES) lifetimes on the sub-picosecond timescale, which significantly limits their use as PCs.Replacing the polypyridyl ligands with strongly σ-donating N-heterocyclic carbene (NHC) ligands, has led to Fe-NHC complexes which posess charge-transfer ESs with longer lifetimes on the picosecond to nanosecond timescale. Furthermore, these complexes exhibit beneficial ES reduction and oxidation potentials. An additional benefit of these Fe-NHCs, is that they absorb in the green region of the visible spectrum, which is of lower energy than the commonly employed blue light. The use of such low energy light not only requires less energy, but can also potentially avoid undesireable side-reactions.The applicability of three different Fe-NHC complexes as PCs utilising green light has been probed by their application on two different organic reactions: the atom transfer radical addition (ATRA) reaction, and the base-promoted homolytic aromatic substitution (BHAS) reaction.Firstly, the application of Fe-NHC complexes on the ATRA reaction was investigated, in which several photoreactors were assessed. Reaction optimisations were performed and extensive mechanistic investigations were conducted, which included a variety of methods grounded in organic synthetic chemistry and physical chemistry. Interestingly, an Fe-NHC complex was found to efficiently catalyse the ATRA reaction via a reductive quenching cycle, utilising a consecutive photoinduced electron transfer in which both the Fe(III) and Fe(II) oxidation states of the complex were involved. Furthermore, the reaction was also found to proceed in absence of a sacrificial electron donor, utilising the Fe(III)-NHC PC in an oxidative quenching cycle. Lastly, a broad substrate scope was established.Similarly, the use of Fe-NHC complexes to catalyse the BHAS reaction was examined. This reaction was found to be catalysed by an Fe(III)-NHC PC under green light irradiation. The reaction conditions were optimised and a substrate scope was established. Also in this case, mechanistic investigations were conducted.This thesis aims to contribute to the field of iron photoredox catalysis, employing charge-transfer ESs to efficiently catalyse organic transformations under irradiation with green light.