Investigation of Iron-N-Heterocyclic Carbene Complexes for Photocatalytic Applications

Humanity’s ever-growing energy demands require the implementation of more sustainable ways to harvest and store energy. At the same time, the reduction of greenhouse gas emission and thus global warming remains of paramount importance. To this end, solar energy harvesting presents an attractive route towards satisfying these requirements—be that by conversion to thermal energy, electricity or to chemical energy. Molecular systems based on iron could prove particularly interesting in this area, due to their advantageous properties such as appreciable visible light-absorption as well as the high availability and low cost of iron. As of yet, they have not been extensively studied though, which can be attributed to the inherently poor photophysical and -chemical properties associated with these systems. However, well thought-out structural design can be used to combat said issues.
The ligand motif used in Fe-N-heterocyclic carbene (NHC) complexes aids in mitigating the formerly mentioned unfavourable properties, allowing for bimolecular quenching with electron donors and acceptors to occur, which is crucial for photocatalysis. In this thesis, the application of hexa-NHC complexes based on Earth-abundant iron as light-harvesters for solar-to-chemical energy conversion is demonstrated. The utility of Fe(III)-NHC complexes for sensitisation of artificial photosynthesis reactions as well as their performance as photoredox catalysts in organic transformations has been studied and the underlying mechanisms have been investigated, giving insight into the prospects and limitations associated with these systems. For the hydrogen evolution and aminomethylation reactions investigated here, conventional single-photon mechanisms were observed. Meanwhile, in the case of the atom transfer radical addition reactions, sequential photon absorption of two different oxidation states of the catalyst within the same catalytic turnover was shown to be operative. Furthermore, a study featuring the Fe(II)-congener of an Fe(III)-NHC complex with demonstrated photocatalytic activity revealed a less than favourable excited state lifetime compared to the Fe(III)-parent complex, showcasing that the preparation of Fe(II)-NHC complexes with synthetically useful photophysical and chemical properties is not necessarily facile. Early investigations of a photocatalytic reaction driven by heterogenisation of Fe(III)-NHC complexes are also shown. There the goal was to improve upon the sustainability of these catalysts by facilitating their recyclability. This thesis thus showcases the potential of Fe-NHC complexes for photocatalytic applications, whilst also revealing their shortcomings.