Following our original research plan, a large variety of multinuclear Ru(bda) architectures was synthesized and studied with regard to their activity for catalyzing chemical, photochemical and electrochemical water oxidation. After fundamental studies on the axial ligand exchange (WP1) a significant number of macrocyclic Ru(bda) oligomers were synthesized that in WP2 included rigid as well as foldable macrocycles. This allowed us to derive structure-property relationships with regard to the rate acceleration for water oxidation by proton-coupled electron transfer steps supported by water networks within the macrocycle’s cavity (so-called specific secondary coordination sphere effects). Different from macrocycles, due to the lack of reversibility for the axial Ru-pyridine bond, self-assembly of Ru(bda) precursors into larger polyhedra in WP3 proved to be difficult. For the few accomplished cage compounds based on tritopic pyridine ligands no improved WOC performance could be observed compared to the macrocyclic counterparts. More beneficial was the research on metallosupramolecular polymers in WP4. Here we managed to get a larger variety of metallosupramolecular oligomers and polymers with linear linker groups based on stiff linear aromatic linkers. Whilst the originally planned ring-opening polymerizations toward living metallosupramolecular polymers could not be achieved, this approach afforded single-site Ru(bda) water oxidation catalysts with unprecedented catalytic performance for the water nucleophilic attack (WNA) mechanism.
For obtaining porous crystalline solids (WP5) Ru(bda) units were embedded into building blocks that could be condensed by dynamic imine chemistry into Ru(bda) center containing covalent organic framework (COF) materials. Importantly, despite of the formation of dense interpenetrating networks with little porosity, a high catalytic activity for the Ru(bda) sites at the nanoparticle surfaces was observed. Recent research afforded also COFs with porosity and higher catalytic activity for oxidative water splitting. Further, Ru(bda) units could be non-covalently embedded into porous crystals of boron ester cage compounds, showing good catalytic performance. For the goals outlined in WP6 regarding functional characterization we were able to provide insights into both photocatalytic and electrocatalytic performance for a large number of Ru(bda) based water oxidation catalysts. Deposition of linear and macrocyclic Ru(bda) oligomers on multiwalled carbon nanotubes afforded composite materials with outstanding electrocatalytic activity for water oxidation (turn-over numbers > 1 million, turn over frequencies > 3000 s-1 at low overpotential).
The results of the SUPRAWOC project have so far been published in 15 peer-reviewed articles in renowned journals such as Nature Catalysis (1), Angewandte Chemie (3), Journal of the American Chemical Society (4) and Advanced Energy Materials (1). The major achievements have also been summarized in a recent article in Accounts of Chemical Research and presented at several conferences as invited or plenary talks, e.g. the IUPAC World Chemistry Congress in Den Haag 2023. In addition, our research topic has attracted the attention of popular science press such as Chemistry World, and been highlighted in non-scientific media.
