Isocyanoborato Complexes of d6 Metals: Photophysics, Electronic Structures and Challenging Applications in Photocatalysis

Considering the steadily growing world population and the accompanying increase in energy consumption, the change from fossil fuels as the primary energy source to more environmentally friendly alternatives is unavoidable. Some of the most promising sustainable energy sources are based on solar energy conversion, which is increasingly popular and seems indispensable to enable a greener future.
The use of light as an energy source has furthermore been identified as a useful concept in synthetic organic chemistry, because it can enable new reaction types, allows for mild reaction conditions and often provides good selectivity. As the light-absorbing and photoactive species in these reactions, photocatalysts, often based on transition metals, are employed. Even though the selection of available photocatalysts is considerable, there is still a significant potential for the development of novel compounds that could enable more challenging reactions. In this thesis, the borylation of cyanido complexes of d6 metals is identified as a useful concept in order to obtain potent photocatalysts for photoredox- and energy transfer catalysis.
In the first part (Chapter 2), a general introduction containing the necessary theoretical background is provided. This includes photophysical principles as well as important literature reports to put the topics discussed in this thesis into a broader perspective.
In the main part (Chapters 3 to 5), new isocyanoborato complexes with different metal centers are investigated. In the first project (Chapter 3), the borylation of two well-known ruthenium(II) cyanido complexes is identified as a useful approach to boost their photocatalytic performance. These results are complemented by in-depth photophysical studies, providing insight into the changes of the electronic structure accompanying borylation. This study is then extended in the second and third project to complexes of iridium(III) (Chapter 4) and iron(II) (Chapter 5). In Chapter 4, the focus lies on the possible applications of an IrIII isocyanoborato complex for photochemical transformations. This new luminophore exhibits an exceptionally high triplet energy and therefore enables challenging energy-transfer catalyzed reactions as well as photochemical upconversion deep into the UV. In contrast, Chapter 5 focuses on the electronic structures and excited-state dynamics of two Fe(II) isocyanoborato complexes by combining different experimental techniques, thereby allowing to paint a conclusive picture of the changes associated with borylation.
In summary, this thesis provides detailed insight into the photophysics and electronic structures of isocyanoborato complexes based on Ru(II), Ir(III) and Fe(II). Furthermore, the Ru(II) and Ir(III) luminophores are shown to be applicable to several challenging photocatalyzed reactions in synthetic organic chemistry.