The Quest for Reduced Overpotentials: Derivatisation of Polypyridyl Cobalt Based Water Reducing Catalysts

Abstract

This thesis comprises three projects, aiming to reduce the overpotential of polypyridyl cobalt catalysts for the reductive half reaction of light-induced water splitting. In chapter 2, dinuclear cobalt complexes, also known as dualcores, are introduced. Dualcores are powerful catalysts. If their structures allow for intramolecular interactions of the two metal centers, the catalytic mechanisms may be changed and the overpotential can be reduced. In-depth analysis of physico-chemical parameters and performances with cyclic voltammetry, linear sweep voltammetry and chronoamperometric experiments particularly showed such beneficial interactions for a complex with a bridging pyrazinyl moiety between the two cobalt centers. The design of the ligand plays a key role in facilitating spatial proximity between the metal-ions. A bipyridyl-bridge between the Co-cores did not lead to synergies, neither did methylene moieties between the metal binding sites and the bridging pyrazinyl or pyridazyl linkers. The syntheses of ligands for dualcore-type molecules is challenging as they often require reactions between three individual molecules (two binding units and the linker). Modifications at the bridgehead methylene group of a series of polypyridyl water reducing catalysts are shown in chapter 3.3. Linear sweep voltammetry and photocatalytic experiments with inline H2-quantification were employed to investigate the impact of pendant N-heterocycles on the catalytic performance. Replacing pyridyl donors with pyrazinyls proved to be an effective strategy to improve photocatalytic performances and to anodically shift the redox potentials of the CoI/II couple, as is shown in chapter 4. Planar, anionic ligands with a pendant nitrile-group opened up new synthetic pathways, without a negative influence on the reducing potentials. The position of the pyrazinyl moiety within such ligands is of great importance, as photocatalytic experiments showed. Furthermore, replacing [Ru(bpy)3]Cl2 with a tris-rhenium complex as photosensitizer also led to improved photocatalytic performances, with the impact strongly depending on the structure of the catalyst.