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Visible-light is a powerful “reagent” for sustainable synthetic organic chemistry. In particular, the combination of photo- and nickel catalysis (metallaphotocatalysis) has emerged as a valuable strategy for carbon–carbon and carbon–heteroatom cross-couplings. This research field is dominated by expensive homogeneous noble metal complexes that can only convert a small portion of visible light (<500 nm) into chemical energy. The highenergy photons that excite the photocatalyst can result in unwanted side reactions and the homogenous nature of these does not allow for straightforward catalyst recycling. Heterogeneous semiconductors that absorb visible light are a promising sustainable alternative to noble metal photocatalysts (Chapter 2). Their potential for metallaphotocatalytic C–N cross-couplings was demonstrated (Chapter 3). This transformation suffers from deactivation of the nickel catalyst using homogeneous photocatalysts. The broad absorption range (up to 700 nm) of an organic, heterogeneous carbon nitride photocatalyst (CN-OA-m) allows controlling the rate of the bond-forming step by carefully selecting the wavelength thereby preventing catalyst deactivation. This is not only crucial for the reproducibility of such reactions, but also expands the scope to substrates that were previously unsuitable. The redox potential of a carbon nitride photocatalyst can be tuned by changing the irradiation wavelength to generate electron holes with different oxidation potentials (Chapter 4). This was the key to design photo-chemo-enzymatic cascades that enable the synthesis of (S)- or (R)- 1-phenylethan-1-ol from ethylbenzene by choosing the irradiation wavelength and the enzyme co-catalyst. In contrast to common photocatalysts that can be only excited using short wavelengths, abundant organic dyes absorb broadly across the entire visible-light spectrum. Inspired by dye-sensitized solar cells, the short-lived excited singlet states of such dyes were harnessed for light-mediated cross-coupling reactions (Chapter 5). Immobilization of a nickel catalyst on dye-sensitized titanium dioxide results in a material that catalyzes carbon–heteroatom and carbon–carbon bond formations. The modular approach of dye-sensitized metallaphotocatalysts (DSMPs) accesses the entire visible light spectrum and allows tackling selectivity issues resulting from low-wavelengths strategically. The concept overcomes current limitations of metallaphotocatalysis by unlocking the potential of dyes that were previously unsuitable. However, recycling studies suffered from a gradual decrease of the yield due to leaching of the nickel catalyst and the dye from the surface of TiO2. This was rationalized by the weak interaction between carboxylic acid anchoring groups and titanium dioxide. Therefore, recyclable, bifunctional materials for metallaphotocatalytic C–S cross-couplings were developed (Chapter 6). Key to the success was the permanent immobilization through phosphonic acid anchor groups. The optimized catalyst harvests a broad range of the visible light spectrum and requires a nickel loading of only ~0.1 mol%. Another robust alternative to organic dyes that does not suffer from photobleaching was realized, by immobilizing carbon dots on titanium dioxid (Chapter 7). The potential of these sustainable materials was demonstrated for various carbon–heteroatom cross-couplings..
