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Towards Artificial Enzymes: Bio-inspired Oxidations in Photoactive Metal-Organic Frameworks

Periodic Reporting for period 3 - Supramol (Towards Artificial Enzymes: Bio-inspired Oxidations in Photoactive Metal-Organic Frameworks)

Reporting period: 2018-09-01 to 2020-02-29

Metal-organic frameworks (MOFs) are regarded as key compounds related to energy storage and conversion, as their unprecedented surface areas make them promising materials for gas storage and catalysis purposes. The proposed research activities explore the modular construction principles of MOFs or molecular coordination cages that may allow the replication of key features of natural enzymes thus demonstrating how cavity size, shape, charge and functional group availability influence the performances in catalytic reactions. The research activities address the question of how such novel, bio-inspired metallo-supramolecular systems can be prepared and exploited for sustainable energy applications. Thus, the investigations take direct inspiration from the key characteristics of the related natural enzyme photosystem II (PSII) which contains a tetranuclear manganese-based {Mn4CaO5} oxo-cluster. A scientific breakthrough that demonstrates the efficient conversion of light into chemical energy would be one of the greatest scientific achievements with unprecedented impact to future generations.

The research project focusses on the following aspects:

a) MOFs, clusters or coordination cages containing novel, catalytically active complexes with labile coordination sites are synthesised using rigid organic ligands that allow us to control the topologies, cavity sizes and surface areas of the materials. We incorporate photosensitizers to develop robust porous materials in which light-absorption initiates electron-transfer events that lead to the activation of a catalytic centre. In addition, photoactive molecules will serve as addressable ligands whereby reversible, photo-induced structural transformations impose changes to porosity and chemical attributes at the active sites on demand.

b) Catalytic studies focus on important oxidations of alkenes and alcohols. These reactions are relevant to H2-based energy concepts as the anodic liberation of protons and electrons can be coupled to their cathodic recombination to produce H2. The studies provide proof-of-concept for the development of photocatalytic systems for the highly endergonic H2O oxidation reaction that will be explored using most stable MOFs/cages. Further, gas storage and magnetic properties that may also be influenced by light-irradiation will be analyzed.
The proposed research activities explore the modular construction principles of metal-organic frameworks (MOFs) or molecular coordination cages that may allow the replication of key features of natural enzymes thus demonstrating how cavity size, shape, charge and functional group availability influence the performances in catalytic reactions. Key achievements in this period include:

- The synthesis of metal-organic frameworks that are characterised by exceptionally high surface areas;
- Synthesis of new MOFs containing light harvesting moieties and nano-sized pore diameters;
- The synthesis of MOFs that interface with photosensitizers and promote the photocatalytic oxidation of water;
- The synthesis of MOFs that electrochemically promote the oxidation of water;
- Synthesis of MOFs that can grown electrochemically on electrode surfaces;
- Synthesis of Mn cluster species that are composed of cubane-type motifs and resemble the structure of the Mn cluster in PS-II;
- Established the ability of selected Mn-clusters to be used as catalysts to promote the oxidation of organic substrates and water;
- Development of water oxidation catalysts containing earth-abundant metal ions;
- Discovery of giant molecular cages and investigation of host-guest chemistry.
- Development of supramolecular water oxidation catalysts (i.e. coordination cages as water oxidation catalysts);
- Investigation of magnetic properties of the new metallo-supramolecular systems;
- Development of light-active porous materials in which light-induced structural transition influence the uptake and release of guest molecules;
- Development of gas sorption materials, e.g. adsorption of CO2;
- Structural characterization of these materials/compounds using X-ray diffraction;
- Development of magnetic compounds and molecule-based materials that stabilize high spin ground states.
Based on the initial studies and our gained expertise in the synthesis of metal-organic frameworks, we will be able to prepare porous and photoactive compounds that are of the highest interest to researchers working in the area of catalysis. Moreover, these materials will be interesting for sensing, gas storage and separation applications. Unprecedented, porous photoactive systems and compounds that show reversible structural transitions on demand will be highly relevant to these areas of science.
Our current studies provide proof of concept for the development of efficient photocatalytic systems for the H2O oxidation reaction – the ultimate goal of our scientific endeavors. We are aware of the associated scientific challenge (i.e. stability requirements) but strongly believe that the developed bio-inspired supramolecular systems provide a new conceptional approach. Owing to its natural abundance, H2O represents the obvious source of reducing equivalents to produce H2 - thus storing solar energy in high-energy-density chemical bonds. However, scientific breakthroughs are hampered by the lack of efficient, abundant and cost-effective catalysts that promote the highly endergonic H2O oxidation half-reaction. To date, mainly noble metal oxides or related molecular species provide catalysts with satisfactory O2 conversion rates and/or stability under strong oxidizing conditions.
A scientific breakthrough that demonstrates that earth-abundant systems can act as efficient catalysts would arguably be one of the greatest scientific achievements ever and associated with unprecedented impact to Europe, mankind and future generations.
Giant cage containing 36 Cu(II) ions; Ball-stick, space-filling and topological representations.