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Molecular mechanisms of C–C-coupling reactions: A microscopic view of on-surface chemical bond formation processes

Subject Area Solid State and Surface Chemistry, Material Synthesis
Experimental Condensed Matter Physics
Organic Molecular Chemistry - Synthesis and Characterisation
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 417197256
 
Final Report Year 2024

Final Report Abstract

On-surface synthesis has proven to be a powerful approach for fabricating various low-dimensional covalent nanostructures with atomic precision that could be challenging for conventional solution chemistry. In particular C–C coupling is one of the most popular on-surface reactions, of which the mechanisms, however, have not been well understood due to the lack of microscopic insights into the intermediates that are fleetingly existing under the harsh reaction conditions. Therefore, the main objective of this collaborative project was to study the stability and reactivity of aromatic radicals on the basis of a suitable on-surface model reaction. This involves the formation (cleavage of the C–leaving group bond), potential stabilization by the surface and/or an organic intermediate, and further reactions, such as insertion reactions, migration or rearrangement reactions. Bond-level scanning probe imaging and manipulation in combination with Density Functional Theory (DFT) calculations allowed for the identification of chemisorbed radicals, cyclized intermediates, and dehydrogenated products. These intermediates correspond to three main reaction steps, namely, debromination, cyclization (radical addition), and H elimination. In the first project part we generated and captured some of the intermediates at room temperature (RT) via the cyclodehydrobromination of 1-bromo-8-phenylnaphthalene (BPN) on a Cu(111) surface. We found that H elimination is the rate-determining step as evidenced by the predominant cyclized intermediates. Furthermore, we found a long-overlooked pathway of dehydrogenation, namely, atomic hydrogen-catalyzed H shift and elimination, based on the observation of intermediates for H shift and superhydrogenation and the proof of a self-amplifying effect of the reaction. Furthermore, we investigated the dehalogenative cyclization using 1-(2’-bromophenyl)-8-bromonaphthalene (BBPN) and 1-(2’-bromophenyl)-8-iodonaphthalene (BPIN) precursors to demonstrate how the order of the dehalogenation plays an important role to achieve the final product. For this part, we employed SPM tunneling current induced dehalogenation of individual molecules with high statistics. In the last project part, we explored the synthesis and imaging of 3D organic structures, using different/modified scanning probe methods. While the original intended bowl-shaped molecular architecture proved to be unstable, we could demonstrate how novel feedback mechanisms of the scanning tip allowed us to visualize non-planar molecular structures.

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