Supramolecular Approaches to Improve the Performance of Ruthenium-Based Water Oxidation Catalysts

Supramolecular principles have been widely applied to enhance the activity of homogeneous ruthenium-based water oxidation catalysts. For catalytic systems in which the O-O bond is formed via radical coupling of two metal oxyl subunits, self-assembly of mononuclear catalysts into vesicles or fibrous aggregates can be used to improve the interaction of two catalytic centers. Similarly, the catalytic subunits can be brought into spatial proximity by coordination chemistry, which allows the construction of highly efficient metallosupramolecular multinuclear structures. However, if the O-O bond is formed by the nucleophilic attack of water at a metal oxide species, the interaction of the catalytic center with substrate water molecules has to be improved. Here, by introducing tailored ligand systems, the second coordination sphere can be adjusted specifically to preorganize water for O-O bond formation whereas the implementation of proton accepting residues can be used to lower the activation barrier of crucial proton-coupled reaction steps. In a particularly outstanding example, both principles were combined in a multinuclear metallosupramolecular macrocycle.