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Metal-Organic Cages for Catalysis Applications

Periodic Reporting for period 1 - MOCCA (Metal-Organic Cages for Catalysis Applications)

Reporting period: 2018-05-01 to 2020-04-30

The main focus of this project was the design, synthesis and characterization of metal-organic cages (MOCs) containing photosensitizing units and catalysts. These systems should be used as photocatalysts for light-driven reactions such as CO2-reduction.

The design of novel catalysts that are powered by light is of high importance for the society. Light is a clean reactant that is highly abundant and can be applied selectively by choosing the specific wavelength needed for the activation of a chemical bond. Metal-mediated assembly of coordination cages allows the construction supramolecular objects with multiple different functional building blocks. This way, the two components needed for photochemical conversions (photosensitizer + catalyst) are brought together, enabling more efficient cooperation between them.

The main objectives of MOCCA were:
- Synthesis of suitable organic and transition-metal-based building blocks
- Assembly of coordination cages with Pd(II) cations or other metals as mediators
- Testing the feasibility of the resulting systems as catalysts in photochemical reactions

In the section below it will be described what the project has contributed so far to these issues.
In a perspective article with the title: “Mixed-Ligand Metal–Organic Frameworks and Heteroleptic Coordination Cages as Multifunctional Scaffolds - A Comparison”, synthetic strategies to obtain heteroleptic coordination cages were compared with the preparation of multifunctional metal-organic frameworks. This article forms the basis for the experimental studies performed in MOCCA, where the preparation of coordination cages with photosensitizers and catalysts as ligands was tested. This perspective article was published here:

Pullen and G. H. Clever, Acc. Chem. Res., 2018, 51, 3052

In addition to this perspective, an review article as well a book chapter on similar topics have been produced and are currently under review.

A first experimental study has been published: An interpenetrated coordination cage of the formula [Pd4L8] containing organic photosensitizers as ligands (= L) that are assembled with Pd(II) cations was prepared. Its ability to photosensitize the generation of singlet oxygen was demonstrated. Singlet oxygen is used as a reagent in i.e. drug synthesis. This study has been published here:

S. Pullen, S. Löffler, A. Platzek, J. J. Holstein and G. H. Clever, Dalton Trans., 2020, DOI:10.1039/d0dt01674h

The main focus of MOCCA was the synthesis and assembly of ligands containing catalysts and photosensitizers that can be used for photochemical CO2 reduction.
Several of such ligands and cages have been prepared and functional studies are currently ongoing. We expect to be able to publish these results in near future.
So far MOCCA has resulted in two publications. In addition to this, a book chapter and a review article have been written and will be published soon.
Furthermore, several unpublished results have been produced. These studies will be continued and ultimately lead to at least two more publications.