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NAnoscience with SUrface MEtal CArbenes

Final Report Summary - NASUMECA (NAnoscience with SUrface MEtal CArbenes)

The project NASUMECA aimed to create and evaluate heterogeneous catalysts for metathesis reactions at the single molecule level by state of the art ultra high vacuum (UHV) surface science techniques. Given recent results in heterogeneous catalysis systems, which show more promise for isolated active metal atoms compared to extended active surfaces, we adjusted the phases of the project to give priority to the objectives related with the creation of single catalytically active Ru and Os sites isolated within porphyrin macrocycles.

Porphyrin molecules are versatile biological moieties with high thermal stability, which can host a wide variety of metal centres. Haem and chlorophyll are two famous examples of metalloporphyrins: the former is responsible for the oxygen transport in the blood stream whereas the latter plays a critical role in the photosynthesis process. The oxidation state of the porphyrin metal centres can be tuned additionally by axial ligands and Ru porphyrins have demonstrated excellent catalytic properties for delicate oxidation reactions as well as formation of metal carbene species.

We initiated a successful effort to metalate in vacuo free base porphyrins, as it was envisaged in our proposal. A thorough literature review pointed out that the most promising route to introduce Ru in the free base porphyrins was to use a metal carbonyl, Ru3(CO)12, instead of the more common UHV practise of atomic metal beam. Indeed synchrotron radiation experiments demonstrated that it is possible to metalate single layers of porphyrins very efficiently and without the formation of surface bound by-products. This study is expected to have a high impact in the field of porphyrinoid metal chemistry. The reaction products were visualised and their electronic properties probed by a combination of low temperature scanning tunnelling microscopy (STM) and ultraviolet and X-ray photoelectron spectroscopy. The image attached shows STM micrographs of a single porphyrin molecule without (left) and with a Ru centre (right) after exposure to the Ru precursor and thermal treatment. Further experiments elucidated the mechanism of this metalation and these results are currently in preparation for publication.

The discovery that Ru3(CO)12 can be used to create organometallic compounds in vacuo has raised additionally the following interesting question: Is it possible to use different metal carbonyls to introduce metal centres which are not accessible through solid metal sublimation? Therefore, we investigated the interaction of Os3(CO)12 with porphyrins, which we anticipate that will afford species with interesting optical properties. We found that the protocol of metalation needed to be adapted for the Os3(CO)12 but follows the same mechanism found for the Ru3(CO)12. This adaptation was enabled by our mechanistic understanding of the Ru3(CO)12 induced metalation.

In parallel, design of an extension to the existing UHV STM chamber has been implemented, providing the possibility for temperature programmed desorption experiments and a sample stage for annealing the sample to the temperatures required for the Ru single crystal preparation (1500 K).

Additionally the objectives defined in the training of the fellow as an independent have been fully achieved and can be summarised as following:

The researcher has been integrated in the E20 department from TUM, directed by Professor Johannes V. Barth. She is currently a lab supervisor in the variable temperature STM laboratory actively researching in the synchrotron campaigns. In this sense, the researcher enlarged her former experience in the development of the project and the running of the lab.

Through the duration of the reporting period the fellow has been a driving force in multiple research projects within the group, strengthening her experience in employing state-of-the-art surface science techniques for the characterisation of molecular overlayers and steering molecular self-assembly. Particularly she was trained in:
-Fundamental studies on the adsorption, self-assembly and thermal chemistry of biomolecules (including DNA/RNA bases, amino acids and porphyrins) on coinage metal surfaces
-Understanding surface catalyzed terminal alkyne homocoupling reactions for the fabrication of covalent molecular nanostructures
-Exploratory work of surface-confined metal-organic networks with lanthanides

These studies are directly relevant to the scientific training of the researcher as described in our proposal and have contributed in her management skills of R&D projects. The expansion of the fellow’s scientific exchange network and international co–operations have been fostered and indeed current ongoing collaborators include organic chemists (Prof. T. Bach, Technical University of Munich & S. Klyatskaya, M. Ruben, Karlsruhe Institute of Technology), theoretical chemists (A. P. Seitsonen, University of Zurich, Switzerland & J. Björk, Linköping University, Sweden) and inorganic chemists (Prof. C. Copéret, ETH Zürich, Switzerland).

Finally, the pedagogical competences of the fellow have been enhanced through the day-to-day supervision of two graduate, a master and two bachelor students.