Establishing dinuclear ruthenium complexes based on the bis(bipyridyl)pyrazolate ligand system for catalytic water oxidation

The implementation of an energy supply based on renewable sources is one of the future‘s great challenges. The production of solar fuels by artificial photosynthesis is considered as a potential solution to this problem. In this context, water oxidation, i. e. the oxidation of water into molecular dioxygen, was identified as bottleneck. This work contributes to meet this challenge by establishing a new family of dinuclear ruthenium complexes as catalysts for water oxidation. These complexes are based on the rigid bis(bipyridyl) pyrazolate ligand system and were prepared via a synthetic key intermediate. Members of this family proved to be efficient catalysts for water oxidation. The influence of different substitutents on both the electrochemistry and catalytic activity was systematically investigated. The synthesis of a highly water soluble complex allowed detailed mechanistic investigations. They include the characterisation of different oxidation states by a bundle of spectroscopic methods and the construction of a Pourbaix diagram. Labelling experiments revealed that the oxygen-oxygen bond formation – the key step of the water oxidation process – takes place via a water nucleophilic attack. Finally, a mechanistic scenario for the water oxidation process for this family of dinuclear complexes was developed. Thereby, complexes based on the bis(bipyridyl)pyrazolate ligand system found their way into the illustrious circle of in-depth analysed water oxidation catalysts.