Recent progress in the development of transition-metal based photoredox catalysts

hotoredox catalysis is an old concept that has experienced a remarkable revival in the past decade, leading to important progress in organic synthesis. Many of the current photoredox applications are based on well-known RuII polypyridine and cyclometalated IrIII complexes, but there is now growing interest in the development of new photoredox catalysts. We review recent examples of new coordination compounds, which were synthesized for combined applications in photophysics and photoredox catalysis. Emphasis is on conceptually new approaches in ligand and metal complex design to obtain favorable excited-state properties, whilst the discovery of new organic photoredox reactions is less in focus. Specific examples include our own Mo0 complexes with isocyanide chelate ligands, new types of CrIII polypyridines, as well as novel ReI, IrIII, PtII, and CeIII/IV complexes. Most of this work has been disclosed over the last 3 years, and parts of it were presented at the 23rd International Symposium on the Photophysics and Photochemistry of Coordination Compounds (ISPPCC) in Hong Kong.