Supramolecular Architectures for Ruthenium Water Oxidation Catalysis

Project Information

SUPRAWOC

Grant agreement ID: 787937

DOI

10.3030/787937

Project closed

EC signature date 2 May 2018

Start date 1 July 2018

End date 30 June 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 490 934,00

EU contribution

€ 2 490 934,00

Coordinated by

JULIUS-MAXIMILIANS-UNIVERSITAT WURZBURG
Germany

Periodic Reporting for period 4 - SUPRAWOC (Supramolecular Architectures for Ruthenium Water Oxidation Catalysis)

Reporting period: 2023-01-01 to 2024-06-30

Summary of the context and overall objectives of the project

The search for renewable alternatives for finite fossil energy sources is one of the most urgent challenges for modern society because of both the need for a sustainable energy supply and the need for a reversal of the environmental problems due to the climate change. Water splitting by renewable energy sources, e.g. wind or solar energy, into oxygen and hydrogen or other chemical fuels provide an intriguing solution for a future clean energy production. In this regard, water oxidation is the most demanding half reaction of water splitting as it requires the accumulation of four electrons at one catalytic center to facilitate the formation of dioxygen by O-O bond formation from two water molecules. Ruthenium complexes with 2,2'-bipyridine-6,6'-dicarboxylate (bda) as equatorial ligand and pyridines as axial ligands are among the most promising molecular water oxidation catalysts (WOCs) and have therefore great potential to achieve practical artificial photosynthesis. Based on earlier reports from our group on the outstanding WOC performance of macrocycles bearing three Ru(bda) units, the SUPRAWOC project explored a wider variety of multinuclear metallosupramolecular architectures with different size, shape and dimensionality. Precise control of structure and size were achieved through a directional bonding approach with suitable nodes and linkers, e.g. for macrocyclic, cage-type, linear oligomeric and polymeric architectures as well as porous three-dimensional covalent metalorganic frameworks solids. All new multinuclear WOC systems were characterized by state-of-the-art analytical techniques and the catalytic activity was evaluated in water oxidation reactions by means of chemical, photochemical or electrochemical stimulation. Detailed mechanistic investigations and supporting quantum chemical calculations gave an in-depth understanding of the underlying chemical and physical principles. Based on an increasing depth of our mechanistic understanding, at the end of the project novel catalytic systems with outstanding performance for water oxidation were accomplished at single Ru(bda) centers surrounded by an enzyme-like pocket that supports fast proton-coupled electron transfers as required for the water nucleophilic attack pathway.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

In this project, we have extended the scope of Ru-based WOCs from molecular single-site catalysts towards multinuclear yet still structurally precise metallosupramolecular assemblies of various size, shape and dimensionality. These systems are obtained in modular synthetic protocols, in which the geometrical outcome of the assemblies is governed by a directional bonding approach, thus greatly enhancing the scope and chemical space of molecularly precise Ru-based WOCs.

In addition to these structural accomplishments that established an entirely new class of metallosupramolecular architectures, the SUPRAWOC project provided important insights into the role of water networks around and between the catalytically active Ru(bda) units for rapid proton-coupled electron transfer steps as required for the WNA mechanistic pathway. These insights enabled us to accomplish a major breakthrough, the design of a single-site Ru(bda) catalyst surrounded by an enzyme-like pocket that pre-organizes water molecules for rapid proton-coupled electron transfers, matching the performance of the tetramanganese cluster utilized by plants in natural photosynthesis (Nature Catalysis 2022).

A second originally not envisioned breakthrough originated from collaborative research with the group of Antoni Llobet at ICIQ Tarragona. Here we discovered that the deposition of trinuclear macrocycles (Adv. Energy Mater. 2020) as well as multinuclear metallosupramolecular polymers (J. Am. Chem. Soc. 2021) on multiwalled carbon nanotubes affords electrodes with outstanding turn-over numbers and frequencies for electrocatalytic water oxidation. This result might enable the development of robust and high-performing catalysts for the water oxidation reaction that might be ultimately implemented into water splitting devices.

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